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The Global Positioning System: A Shared National Asset (1995)

Chapter: 3 Performance Improvements to the Existing GPS Configuration

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Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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3
Performance Improvements to the Existing GPS Configuration

Introduction

As pointed out in the previous section, civil users of the GPS have accommodated themselves to the currently available SPS (Standard Positioning Service) in attempting to meet their individual performance requirements, and a number of innovative uses of GPS have been demonstrated with the existing system. An even more capable system would likely result in a larger number of applications. Improved accuracy, integrity, availability, and reliability of the signal could provide improved results at significantly lower cost. For example, if the stand-alone GPS could provide an accuracy approaching 5 meters (2 drms), the need for many of the existing or planned differential systems could be avoided.

In accordance with the committee's statement of task, this chapter will recommend a sequence of enhancements to the GPS that will serve to improve the accuracy of the system for civilian, commercial, and military users. After a discussion of the current performance achievable from the basic GPS, the subsequent sections address specific accuracy improvements focused on enhancing civilian, commercial, and military use of the system. Many of the suggested improvements also will have benefits other than better accuracy, such as increased integrity, improved availability, and enhanced resistance to RF (radio frequency) interference. These improved characteristics are discussed where appropriate. The final section of this chapter presents an overall strategy for implementing the recommended improvements. As noted throughout the text, some of the improvements are meant to be applied to the current GPS satellite constellation and others to the Block IIR and Block IIF constellations. When available, the approximate cost of each improvement also is given.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Current GPS Performance

Accuracy

As can be seen from Table 3-1, the contributors to civilian SPS signal accuracy errors are SA (Selective Availability), the atmospheric error, the clock and ephemeris errors, the receiver noise error, and the multipath error. For the military PPS (Precise Positioning Service) signal, the largest error contributors are the clock and ephemeris errors, the receiver noise, and multipath errors, since the PPS signal is not degraded by SA. The ionospheric error for the PPS signal is small relative to that for the SPS signal since the military has access to both the L1 and L2 frequencies and can correct for the ionospheric error.

Table 3-1 Observed GPS Positioning Errors with Typical SPS and PPS Receiversa

Error Source

Typical Range Error Magnitude (meters, 1σ)

 

SPS with II/IIA satellites

PPS with II/IIA satellites

Selective Availabilityb

24.0

0.0

Atmospheric Error

 

 

Ionosphericc

7.0

0.01

Troposphericd

0.7

0.7

Clock and Ephemeris Errore

3.6

3.6

Receiver Noisef

1.5

0.6

Multipathg

1.2

1.8

Total User Equivalent Range Error (UERE)

25.3

4.1

Typical Horizontal DOP (HDOP)h

2.0

2.0

Total Stand-Alone Horizontal Accuracy, 2 drmsi

101.2

16.4

a. It is assumed here that a ''typical" SPS and PPS receiver has a four-satellite position solution.

b. J. F. Zumberge and W. I. Bertiger, "Ephemeris and Clock Navigation Message Accuracy in the Global Positioning System," Vol. I, Chap. 16. Edited by B. W. Parkinson, J. J. Spilker, P. Axelrad, and P. Enge (To be published by AIAA, in press 1995). This error is manifested as increased clock and ephemeris errors when SA is on.

c. For the SPS signal, the ionospheric content is quite variable, with large diurnal variations, and large variations over the 11-year solar cycle. Depending on the Total Electron Content (TEC), a delay at L, ranging from less than 1 meter to 70 meters can result. A typical SPS receiver has an algorithm that can remove about 50 percent of the ionospheric error, leading to an error ranging from less than 1 meter to 35 meters. For the above table, an error of 7 meters was used, which is typical for a daytime mid-latitude ionospheric error near the maximum of the 11-year solar cycle, after correction by the standard algorithm. Because the ionospheric error is not independent between satellites, it should not strictly be considered a range error to be multiplied by HDOP (Horizontal Dilution of Precision). When the ionospheric content is uniform above the receiver, such as during the pre-sunrise morning, it contributes little to horizontal error, but maps into errors in the vertical position and receiver clock. When there are significant gradients in the ionospheric content, however, such as exist at local dawn and dusk, errors are induced into the horizontal position. Therefore, the use of 7 meters for a range error, which is multiplied by HDOP, is a somewhat conservative choice. For the PPS signal the ionospheric error is removed by a linear combination of the L1 and L2 observables. This correction leaves residual ionospheric error of 1 centimeter or less.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

d. For a typical SPS or PPS receiver, software models correct for all but around 0.7 meters (la) of the tropospheric error. The tropospheric error is even more highly correlated than the ionospheric error, due to its uniform distribution. The errors introduced by the troposphere normally map into the vertical position and receiver clock errors. As for the ionospheric error, the multiplication of this error by HDOP to obtain the horizontal error is a conservative calculation.

e. This value is based on observed data as noted in "Ephemeris and Clock Navigation Message Accuracy in the Global Positioning System." (See note a above). The combined clock and ephemeris error does not contain SA epsilon error in the broadcast ephemeris nor the SA dither error in the broadcast time.

f. For a SPS receiver, the receiver noise for independent 1-second measurements can actually range from around 0.25 to 2.0 meters, depending on its design. For a PPS receiver, the single-frequency pseudorange noise error is less because the ten times faster Y-code chip rate overcomes the 3 dB to 6 dB signal-to-noise ratio penalty relative to the C/A code. In forming the linear combination required to removed the ionospheric error, Y-code corrected = 2.55(Y)L1-1.55(Y)L2, the noise error of the Y-code is effectively multiplied by the root sum square of 2.55 and 155, which is approximately 3. (A single-frequency PPS receiver like the Plugger would have a receiver range error smaller by a factor of three, but at the cost of retaining a 7-meter error due to the ionosphere). The PPS receiver noise error can range from 0.1 to 0.8 meters (1s), for independent 1-second measurements.

g. For a SPS receiver, multipath can typically range from 0.4 to 5 meters (1s), depending on the antenna, antenna surroundings, and receiver design. For a PPS receiver, the single-frequency multipath error is somewhat less, typically by a factor of 0.5, because of the faster chip rate. In forming the linear combination required to remove the ionospheric error, Y-code corrected = 2.55(Y)L1-1.55(Y)L2, the Y-code multipath error is effectively multiplied by the root sum square of 2.55 and 1.55, which is approximately 3. This explains why the PPS multipath error exceeds the SPS multipath error. (A single-frequency PPS receiver like the Plugger would have a multipath error smaller by a factor of three, but at the cost of retaining a 7-meter error due to the ionosphere). The PPS multipath error can range from 0.3 to 2 meters.

h. HDOP can vary depending on the geometry of the satellites. For a typical SPS or PPS receiver, the geometric strength of a four-satellite solution is limited, so a conservative HDOP of 2.0 was used.

i. These values are based on observation and differ from the accuracy values specified by the DOD (Department of Defense), shown in Figure C-7, Appendix C.

Specific technical modifications to GPS to reduce the errors discussed above and improve the accuracy for both the military and civilian communities are discussed in detail below. As explained in the table notes above, the exact numbers in the tables can vary. If all of the recommendations are implemented, the committee believes that the stand-alone horizontal GPS accuracy will approach 5 meters (2 drms).

Greater stand-alone accuracy could take the place of differential GPS systems for some users who require accuracies of a few meters (2 drms). For example, greater standalone GPS accuracy would allow many vehicle positioning and navigation requirements to be met without the use of DGPS. To use a military example, precision weapons, such as missiles and smart bombs that have been equipped with GPS, presently require expensive

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

terminal guidance packages or access to differential corrections to meet accuracy requirements of a few meters (CEP). In order to take advantage of GPS accuracy, accurate knowledge of the target location is essential. Various target-determination techniques are being developed, but until they are available, munitions delivery, even with GPS, will still require highly accurate terminal guidance systems. Using an enhanced GPS with greater accuracy for guidance would provide two levels of benefit. First, the requirements on an inertial navigation device can be relaxed because more accurate GPS determination of position and velocity will be possible. Second, under some conditions where jamming is not likely, GPS could be used to provide a very economical and accurate stand-alone munitions guidance system.

Integrity and Availability

In general, improving the system ranging accuracy also improves integrity and availability. As noted in Appendix C, availability is the percentage of time that a user's positioning errors lie within a specified accuracy. If the ranging errors are decreased, then positioning errors will remain inside the accuracy bounds for higher DOP (Dilution of Precision) values. As a consequence, the amount of time that the system is available increases, especially in the presence of satellite outages. Improvements in availability for the SPS as ranging accuracy improves will be shown throughout this chapter by comparing availability values for Chicago, Illinois.1

Integrity checking algorithms also benefit from improved ranging accuracy. Most integrity algorithms, such as RAIM (Receiver Autonomous Integrity Monitoring), are based on consistency checks among redundant sets of measurements.2 Poor consistency indicates the possibility of a position solution error exceeding the protection limit. However, when the range errors are large, the consistency checks are not reliable except under very favorable satellites geometries. With improved range measurement accuracy, consistency can be reliably measured even under poor user-satellite geometries.

Selective Availability And Anti-Spoofing

The GPS was designed to provide our military forces with an advantage when engaged with other military forces, while still providing a reasonable positioning service to

1  

Chicago, Illinois, was randomly chosen by the MITRE Corporation, which determined the availability values presented in this chapter using a GPS availability model developed for the FAA (Federal Aviation Administration). The analytical model accounts for individual satellite short-term and long-term failures and restorations for the 24-satellite constellation and assumes a conservative serial restoration strategy (that is, only one satellite can be replaced at a time). The GPS receiver was assumed to have an elevation mask angle of 5 degrees.

2  

RAIM, which utilizes receiver software algorithms to detect unreliable satellites or position solutions, is defined in Appendix C.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

the civil community. Two features were incorporated into the GPS to provide this advantage. The first, SA, degrades the GPS signal so that the unencrypted signal on L1 will provide a stand-alone horizontal accuracy of 100 meters (2 drms).3 The second, A-S (Anti-Spoofing), provides encryption of the P-code on L1, and L2 in order to deny the signal to the adversary and to increase resistance to spoofing.

Selective Availability

Currently, the full accuracy of GPS is denied to stand-alone non-PPS users of GPS for both navigation and time transfer through the implementation of SA. SA comprises two functions: (1) fluctuation of the GPS satellite clock frequency, known as dither, and (2) transmission of incorrect ephemeris parameters in the navigation message, termed epsilon. SA affects all GPS observables, which include the C/A-code and P-code pseudorange measurements and the L1 and L2 carrier phase measurements. SA is discussed in greater detail in Appendix C. The DOD has stated that the degradation produced by SA will be limited to a value that maintains the 100-meter (2 drms) specified stand-alone horizontal accuracy of the SPS. Furthermore, at a recent meeting of the DOD/DOT (Department of Transportation) Signal Specification Issues Technical and Policy Groups, additional specifications were discussed and agreed upon for limits on the individual satellite range rate and acceleration errors, shown in Table 3-2.4

Table 3-2 SA Errors from DOD/DOT Signal Specification Issues Technical Group

Type of Error

Specification

Range Rate Bound

Not to exceed 2 m/s

Range Acceleration Bound

Not to exceed 19 mm/s2

Range Acceleration

8 mm/s2 (2a)

Under special circumstances, the level of SA errors can be set to zero or increased to a larger value, but only by the National Command Authority. For example, SA was set at a very low level during the Persian Gulf War and during the initial occupation of Haiti

3  

The Under Secretary of Defense for Research and Engineering officially established the 100-meter (2 drms) accuracy level for the SPS on June 28, 1983. This policy is reiterated in each biannual publication of the Federal Radionavigation Plan.

4  

Report of the DOD/DOT Signal Specification Issues Technical Group to the Policy Group, Washington, D.C., 13 December 1994.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

(it was then set back to 100 meters, 2 drms), because of the lack of PPS equipment fielded by the U.S. military at those times.

PPS receivers are able to completely remove the effects of both SA dither and epsilon from their observations through the use of a security module. SPS receivers can eliminate the effects of SA through the use of local or wide-area DGPS broadcasts of differential corrections. DGPS reference stations typically broadcast observed range and range-rate errors. The level of SA-induced range acceleration determines the rate at which the corrections must be updated to keep the user error within acceptable bounds. Satellite position errors produced by the epsilon technique will decorrelate as the separation between the reference station and the user increases. Wide-area DGPS will provide orbit corrections for each satellite to compensate for this effect.

Military Utility of SA

The DOD has stated that SA is an important security feature because it prevents a potential enemy from directly obtaining positioning and navigation accuracy of approximately 12.5 meters CEP (30 meters, 2 drms) from the C/A-code. Since the military has access to a specified accuracy of 8 meters CEP (21 meters, 2 drms), they believe U.S. forces have a distinct strategic and tactical advantage. With SA at its current level, a potential enemy has access only to the 42-meter CEP (100 meters, 2 drms) accuracy available from the SPS. The DOD believes that obtaining accuracies better than 42 meters CEP requires a substantial amount of effort. DOD representatives have expressed their belief that our adversaries are much more likely to exploit the GPS C/A-code, rather than DGPS, because its use requires less effort and technical sophistication than is required to use DGPS. In addition, some DOD representatives contend that local-area DGPS broadcasts do not diminish the military advantage of SA because they could be rendered inoperative, if warranted, through detection and destruction or by jamming.

It is the opinion of the NRC committee however, that meter-level accuracies are readily obtainable, even in the presence of SA set at its current level or even at higher levels. As shown in Figure 3-1, several DGPS systems, operated by both commercial and government entities, routinely provide position accuracies approaching 1 meter (2 drms) in the United States and in most of the populated areas of the world. Further information on commercially available systems is provided in Appendix C. Even within the U.S. government, civilian agencies such as the Federal Aviation Administration, the Coast Guard, and the Army Corp of Engineers are planning to operate systems that will, in combination, cover the entire United States and beyond, as shown in Figure 3-2. Furthermore, if the full GLONASS constellation is completed in 1995 as currently planned, this system also will provide properly equipped users with an additional source of highly accurate positioning data, as shown in Figure 3-3.5

5  

Unlike GPS, GLONASS does not deny accuracy to some users through the use of SA or a similar technique.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Figure 3-1

DGPS coverage provided by commercially available systems, including Skyfix and Sercel. (Courtesy o National Air Intelligence Center)

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Figure 3-2

DGPS coverage provided by the planned FAA WAAS (Wide-Area Augmentation System). Source: Innovative  Solutions International, Inc., presentation at the National Technical Meeting of the Institute of Navigation Meeting, California, January 1995.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Figure 3-3

Position estimates from GPS and GLONASS obtained from measurement snapshots taken 1 minute apart over an entire  day. Position from (a) GPS with SA off, (b) GPS with SA on, (c) GLONASS, and (d) GPS plus GLONASS.  (Courtesy of MIT Lincoln Laboratory)

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Even if potential adversaries are not taking advantage of DGPS at this time, the NRC committee believes that it would be prudent for the DOD to recognize the potential capability that currently exists. In addition, the establishment of a low-cost, militarily controlled local-area DGPS network for use by an adversary in a theater of conflict is an even more likely possibility. Local-area differential systems are easy to build or buy and are inexpensive. Furthermore, the NRC committee believes that the detection and elimination of these military local-area DGPS stations, either in wartime or peacetime, would be difficult. Local-area DGPS reference stations are small and can be installed in less than an hour. Signals from such systems are difficult to detect because they can be broadcast at low power and at spread-spectrum frequencies or in rapid on/off cycles, with very short transmission times. Therefore, they are not easy to detect electronically or visually.

The NRC committee expects that any enemy of the United States sophisticated enough to operate GPS-guided weapons will be sophisticated enough to acquire and install local-area differential system or take advantage of an existing commercial system. These systems can have the capability to provide velocity and position corrections to cruise and ballistic missiles with accuracies that are equal to or superior to those available from an undegraded C/A-code. Even if the level of SA is increased, DGPS methods could still be used to provide an enemy with accurate signals. Further, as previously mentioned, if the full GLONASS constellation is completed in 1995 as currently planned, this system also will provide properly equipped users with an additional source of highly accurate positioning data.

The unencrypted C/A-code, which is degraded with SA, still provides our adversaries with an accuracy of 100 meters, 2 drms (42 meter CEP), which would still be more than adequate to deliver chemical, biological, or even explosive weapons, if creating terror in a city is the enemy's objective (see Figure 3-4). Further, any enemy encountered is not likely to share the U.S. military's interest in limiting collateral damage. With SA set at zero, the stand-alone accuracy improves to 30 meters, 2 drms (approximately 13 meters CEP) or better, depending on the solar cycle and user equipment capabilities. While this improvement enhances the ability of an adversary to successfully attack high-value point targets, significant damage also can be inflicted with accuracies of 100 meters, 2 drms. Therefore, in either case (30-meter or 100-meter accuracy, 2 drms) the NRC committee believes that the risk is sufficiently high to justify denial of the L1 signal by jamming. The jamming strategy has the additional benefit of denying an adversary all radionavigation capability including the even more accurate DGPS threat.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Figure 3-4

Horizontal scatter plot of 42 meters CEP (100 meters, 2 drms) with SA at its current level and horizontal scatter  plot of approximately 10 meters CEP (24 meters, 2 drms) without SA (Figure Courtesy of Mr. Jules McNeff, Office of the Assistant Secretary of Defense, C3I)

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

The NRC committee strongly believes that preservation of our military advantage with regard to radionavigation systems should focus on electronic denial of all useful signals to an opponent, for example, by jamming and spoofing, while improving the ability of civil and friendly military users to employ GPS in a jamming and spoofing environment. Continued effort to deny the accuracy of GPS to all users except the U.S. military via SA appears to be a strategy that ultimately will fail. Thus, the NRC committee recommends that the military employ jamming techniques in a theater of conflict to fully deny an enemy the use of GPS or other radionavigation systems.

The NRC committee believes that the principal shortcoming in a jamming strategy, regardless of the level of SA, is the difficulty military GPS receivers currently have acquiring the Y-code during periods when the C/A-code is unavailable due to jamming of the L1 signal.6 The implementation of direct Y-code acquisition capability, as recommended later in this chapter, would provide the optimal solution to this problem. In the interim, various operating disciplines, also discussed in this chapter, can minimize the impact of L1 C/A-code jamming on the ability to acquire the Y-code. The committee believes that a focused, high-priority effort by the DOD to develop and deploy direct Y-code user equipment, backed by forceful political will from both the legislative and executive branches, can bring about the desired result in a relatively short period of time. The technology for developing direct Y-code receivers is available today.

Impact of SA on GPS User Equipment Manufacturers and U.S. Competitiveness

It has been argued that SA provides a competitive advantage to U.S. manufacturers of GPS and DGPS user equipment, and DGPS service providers. This has apparently been true in the past and to some extent currently. However, the advantage is at best temporary, as indicated by growing foreign competition, especially from Japan. Foreign manufacturers already possess the technology to achieve results equivalent to those of U.S. manufacturers. Within 1 to 2 years, any competitive advantage for U.S. manufacturers will disappear.

One market analysis has shown that if SA is eliminated, the number of GPS and DGPS users in North America is expected to increase substantially. The market for GPS receivers and systems is estimated to be around $64 billion by the year 2004, as compared to $42 billion with SA at its current level.7

There is considerable concern within the U.S. civil user community, and even more concern among the international community, regarding the reliability of a navigation system under the control of the U.S. military. Removal of the SA signal degradation is likely to be viewed as a good faith gesture by the civil community and could substantially improve international acceptance and potentially forestall the development of rival satellite navigation systems.

6  

The C/A-code is normally used initially to acquire the Y-code.

7  

The analysis by Michael Dyment, Booz·Allen & Hamilton, 1 May 1995, is shown in Appendix E.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Impact of SA on Civil Users

Turning SA to zero, or completely eliminating SA, would have an immediate positive impact on civil GPS users. The following benefits can be expected:

  • improved stand-alone navigation, positioning, and timing accuracy;
  • improved availability for any given positioning accuracy;
  • enhanced ability to perform RAIM;
  • reduced data rate requirements for DGPS corrections;
  • enable system modifications that further improve accuracy; and
  • improved WAAS.

Each of these benefits is discussed further below.

Increased Stand-Alone Navigation, Positioning, and Timing Accuracy. The stand-alone accuracy for SPS users would immediately increase from 100 meters (2 drms) to around 30 meters (2 drms) if SA were turned to zero, as shown in Table 3-3.8 For many users currently employing DGPS techniques, such as emergency response vehicles, accident data collection, and vehicle command and control, stand-alone horizontal accuracy of approximately 30 meters (2 drms) is sufficient. Currently, DGPS-equipped receivers cost substantially more (several hundred dollars) than a stand-alone receiver. Savings would result from the elimination of the need for a DGPS receiver and electronics to insert the messages to the GPS receiver. Savings also will result from elimination of the user fee imposed by private DGPS providers.

8  

Recent measurements with SA off have ranged from 5 meters to 10 meters (2 drms). However, the accuracy without SA greatly depends on the condition of the ionosphere at the time of observation and user equipment capabilities.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 3-3 The Effect of Eliminating SA on GPS SPS Stand-Alone Horizontal Accuracya

Error Source

Typical Range Error Magnitude (meters, 1σ)

 

SPS with SA (II/IIA Satellites)

SPS with No SA (II/IIA Satellites)

Selective Availability

24.0

0.0

Atmospheric Delay

 

 

Ionospheric

7.0

7.0

Tropospheric

0.7

0.7

Clock and Ephemeris Error

3.6

3.6

Receiver Noise

1.5

1.5

Multipath

1.2

1.2

Total User Equivalent Range Error (UERE)

25.3

8.1

Typical Horizontal DOP (HDOP)

2.0

2.0

Total Stand-Alone Horizontal Accuracy, 2 drms

101.2:

32.5

a All footnotes to Table 3-1 also apply to Table 3-4.

Improved Availability. As explained earlier in this chapter, GPS availability is directly related to accuracy. When the stand-alone horizontal accuracy of the system improves to around 30 meters (2 drms), the availability of any accuracy greater than 30 meters will increase. For example, the average observed availability of the 100-meter (2 drms) SPS for a receiver located in Chicago, Illinois is currently 99.2 percent. For the same 100-meter accuracy level with SA removed, the availability would increase to approximately 99.94 percent.9

Enhanced Integrity Monitoring. The ability of a receiver to detect invalid GPS pseudorange measurements autonomously also would be greatly enhanced if SA were turned to zero. RAIM is generally possible if six or more satellites are visible and are providing pseudorange accuracies that allow the easy detection of an inaccurate signal. With SA set at its current level, each satellite range may be in error by 25 meters (ls) or more, as shown in Table 3-3. This makes it difficult to distinguish a failure. Without SA, pseudorange accuracy improves to almost 8 meters (la), dramatically improving the ability to isolate specific satellite faults, as well as signal tracking problems within the receiver itself. An analysis of the impact on RAIM with the elimination of SA was conducted for this study by the MITRE Corporation. The improved RAIM capability has been quantified in terms of

9  

Based on analysis conducted by the MITRE Corporation for the Memorandum from the MITRE Corporation to the NRC committee, 7 February 1995. For more details, see footnote 1 earlier in this chapter.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

the availability of six useable satellites for three phases of aircraft flight. These results are shown in Table 3-4 and discussed further in Appendix F.

Table 3-4 Effect of SA Removal on RAIM Availability for Aviation Applicationsa

Aviation Application

Availability With SA at its Current Level

Availability With SA Turned to Zero

Phase of Flight

Protection Limit

21 Satellitesb

24 Satellitesc

21 Satellites

24 Satellites

En Route

2.0 nautical miles

93.16%

99.89%

96.34%

99.98%

Terminal Area

1.0 nautical miles

89.96%

94.39%

94.39%

99.95%

Non-Precision Approach

03 nautical miles

80.89%

98.88%d

91.10%

100.00%d

a. This analysis has been made for a single-frequency C/A-code receiver aided by a barometric altimeter (required for aviation supplemental navigation use of GPS) with a visibility mask angle of 5 degrees.

b. The probability of having 21 satellites operating is assumed to be 98 percent.

c. The probability of having 24 satellites operating is assumed to be only 70 percent. However, the values in this table reflect the fact that if 24 satellites are fully operational, an incremental improvement in availability exists.

d. Although these values would intuitively be lower than the 1 nautical mile terminal area protection limit value, availability improves for the 0.3 nautical mile non-precision protection limit because the barometric altimeter inputs provide extra information in this phase of flight.

Reduced Data Rate Requirements for DGPS Corrections. In addition to reduced receiver costs and DGPS provider fees, a stand-alone horizontal positioning accuracy of approximately 30 meters (2 drms) would allow users to avoid the complexity and expense of receiving differential corrections or post-processing their data. Users requiring accuracies from around 1 meter to 30 meters could still use DGPS, but at a much reduced update rate.10

10  

The required update rates are derived below, assuming 0.2 meters is allotted to the clock portion of the differential correction for SA at its present nominal level and for SA turned to zero. In addition, this analysis is only valid assuming that precise range-rate information is provided in the navigation message. The result is that the update rate is about two orders of magnitude lower when SA is turned to zero. This advantage would be less for lower accuracy requirements. Other requirements may force higher update rates for specific differential users.

Example with SA at current level:

The l¬ SA range acceleration is 0.004 m/s2 from Table 3-2. In order to calculate the update rate required for differential corrections, set 0.2 m = 0.5(a)(t2), where a = 0.004 m/s2. Solving for t results in a required update period of t = 10 seconds.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Enable System Modifications that Further Improve Accuracy. If SA is turned to zero, then accuracy is limited by ionospheric errors, clock and ephemeris errors, multipath errors, and receiver noise errors, as Table 3-3 illustrates. As discussed later, technical modifications can reduce these errors. However, with SA set at its current level, any modifications to reduce other errors and improve accuracy will be overwhelmed by the degrading effects of SA.

Improved WAAS. When SA dithering of the GPS signals is employed, the DGPS corrections required to circumvent SA accuracy degradation must keep up with the dithering rate. Since WAAS will broadcast its differential corrections as part of the navigation message data carried by a GPS-like L1 signal, a high-data rate for the differential correction is required, which constrains the flexibility of providing additional information on the navigation message. If SA were eliminated, the data rate requirement could be relaxed and more information, such as GPS integrity information and other safety or air traffic control related information, could be sent to the user. As noted above, integrity also would improve if SA were eliminated, However, even if SA were removed, the FAA's integrity and availability requirements would still not be met with the basic GPS. Some type of augmentation, such as WAAS, would still be required.

Findings and Recommendations

The NRC committee finds that in view of the rapid proliferation of both local and wide-area DGPS systems worldwide and the ease with which local DGPS stations can be deployed, the current effectiveness of SA in deterring precision attack by adversary forces is severely limited and will essentially be ineffective in the near future.

The NRC committee also found that effective countermeasures to adversary use of GPS and DGPS are currently inadequate. The NRC committee believes that future military strategy should focus on electronic denial of all useful signals to our enemies, for example, by jamming and spoofing, while improving U.S. military ability to use GPS in a jamming and spoofing environment.

The principal shortcoming in this strategy, regardless of the level of SA, is the difficulty military GPS receivers currently have in acquiring the Y-code during periods when the C/A-code is unavailable due to jamming of the L1 signal. The implementation of direct Y-code acquisition capability, as recommended later in this chapter, would provide the optimal solution to this problem. Based on information from receiver manufacturers, the committee believes that the technology for developing direct Y-code receivers is available

Example with SA turned to zero:

In this case, the error in the differential correction due to the satellite clock does not include any clock dithering, and so is dominated by the satellite oscillator stability, which is Δf/f = 5 x 10-13. Using the formula: 0.2 m = t(c)( Δf)/f to calculate the range error, where c is the speed of light = 3x 108 m/sec, gives a required update period of t = 1,333 seconds (22.2 minutes).

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

today. The committee believes that a focused high priority effort by the DOD to develop and deploy direct Y-code user equipment, backed by forceful political will from both the legislative and executive branches, can bring about the desired result in a relatively short period of time. However, in the interim time before direct Y-code receivers are fielded by the military, various operating disciplines also discussed in this chapter, can minimize the impact of L1 C/A-code jamming on the ability to acquire the Y-code.

The committee also has taken cognizance of the DOD belief that exploitation of the GPS C/A-code is more likely in the near term than exploitation of DGPS signals. Even if potential adversaries are not taking advantage of DGPS at this time, the NRC committee believes that it would be prudent for the DOD to recognize the potential capability that currently exists.

The NRC committee believes that continued reliance on SA as a means of denying precise GPS position location to all non-military users over a wide area is a strategy that will ultimately fail. In addition, the removal of SA and the subsequent increase in accuracy obtainable by civil and commercial GPS users would have substantial benefits, as previously discussed. If the use of SA is eliminated, the NRC committee also expects that the market for GPS receivers and systems would increase substantially, as discussed further in Appendix E.

The six most important findings of the NRC committee regarding the impact of SA on the various classes of civilian users and on meeting its intended purpose are

(1)  

The military effectiveness of SA is significantly undermined by the existence and widespread proliferation of DGPS augmentations as well as the potential availability of GLONASS signals.

(2)  

Turning SA to zero would have an immediate positive impact on civil GPS users. Without SA, the use of DGPS would no longer be necessary for many applications. System modifications that would further improve civilian accuracy also would be possible without SA.

(3)  

Deactivation of SA would likely be viewed as a good faith gesture by the civil community and could substantially improve international acceptance and potentially forestall the development of rival satellite navigation systems. Without SA, the committee believes that the number of GPS and DGPS users in North America would increase substantially. 11

(4)  

It is the opinion of the committee that the military should be able to develop doctrine, establish procedures, and train troops to operate in an L1 jamming environment in less than three years.

11  

The analysis by Michael Dyment, Booz · Allen & Hamilton, 1 May 1995, is shown in Appendix E.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

(5)  

The technology for developing direct Y-code receivers is currently available and the development and initial deployment of these receivers could be accomplished in a short period of time if adequately funded.

(6)  

The FAA's WAAS, the Coast Guard's differential system, and GLONASS are expected to be fully operational in the next 1 to 3 years. The Coast Guard's DGPS network and the WAAS will provide accuracies greater than that available from GPS with SA turned to zero and GLONASS provides accuracies that are comparable to GPS without SA. At the same time, other local DGPS capabilities are likely to continue to proliferate.

Selective Availability should be turned to zero immediately and deactivated after three years. In the interim, the prerogative to reintroduce SA at its current level should be retained by the National Command Authority.

Anti-Spoofing

The purpose of A-S is to protect military receivers from an adversary transmitting a spoofed P-code signal and to deny the precision to an adversary through encryption.12 When A-S is turned on, the P-code modulation on both the L1 and L2 carriers is replaced with a classified known as the Y-code that has the same chipping rate and correlation properties as the P-code. (C/A-code is not affected by Y-code transmission.) Except for special arrangements to turn off A-S for specific requirements, it has remained on continuously since January 31, 1994.

Impact of A-S on Military Users

PPS receivers are able to track the Y-code through the use of a security module that employs National Security Agency cryptographic techniques, and requires the manual distribution of encryption keys.

There are compelling reasons to retain the A-S feature. If the recommendation to remove SA is implemented and potential adversaries have access to the resulting more accurate C/A-code on the L1 frequency, the reasons to retain A-S become still more compelling. In addition to its anti-spoofing feature, A-S forces adversaries to use the C/A-code on the L1 frequency, which can be denied by jamming techniques (without impacting L2). The NRC committee believes that denying L1 to an enemy through jamming, while employing only L2 for its own forces, should be the basis of a new military doctrine for the use of GPS. However, this doctrine will require U.S. military receivers to acquire the Y-code rapidly without the C/A-code. Military receivers also should be able to provide accurate

12  

The process of sending incorrect information to an adversary's radio equipment (in this case a GPS receiver) without their knowledge, using mimicked signals, is known as spoofing.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

ionospheric corrections in the absence of L1. Modifications to military receivers to accomplish this are discussed later in this report.

The current manual distribution of decryption keys is laborious and time consuming. The DOD is currently developing the means to distribute the keys electronically. Such a capability would greatly enhance the use of the encrypted L2 Y-code. The committee also believes that technology is available to upgrade the current P-code encryption method and suggests that the Air Force should explore the necessity of utilizing this technology. Modifications to the Block IIR satellites and the Block IIF request for proposal may be required if upgraded encryption methods are necessary. Changes to military receivers also will be required.

Impact of A-S on Civil Users

SPS receivers cannot directly track Y-code, which significantly limits civil access to measurements of the L2 signal for correction of ionospheric errors. Several Y-codeless approaches have been developed to overcome this problem.13 These techniques, however, have a lower signal-to-noise ratio than dual-frequency tracking. This creates difficulties in situations where the receiver is moving, is subject to multipath signals, or is operating in areas where signal attenuation exists, such as in an urban area or under foliage. Despite these limitations, less approaches are still being used for many surveying and scientific applications. SPS users also would benefit from access to an unencrypted L2 signal, because its bandwidth is approximately ten times as wide as the L1 signal. The wider bandwidth would improve resistance to interference and reduce vulnerability to multipath.

Findings and Recommendations

A-S is critically important to the military because it forces potential adversaries to use the C/A-code on L1, which can be jammed if necessary without inhibiting the U.S. military's use of the encrypted Y-code on L2. Further, encryption provides resistance to spoofing of the military .

Although many civil users could benefit if A-S is turned off, as discussed in the previous chapter and above, their requirements can be met with other enhancements described in subsequent pages.

A-S should remain on and the electronic distribution of keys should be implemented at the earliest possible date. In addition, the Air Force should explore the necessity of upgrading the current encryption method. Required receiver enhancements should be incorporated in future planned upgrades.

13  

Some codeless approaches include (1) delay and multiply to recover the carrier and code phases, (2) squaring to recover the carrier phase, (3) cross-correlation of the L1 and L2 signals to measure the differential carrier phase and code pseudorange, and (4) P-code enhanced versions of these techniques.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Signal Structure Modifications To Reduce Atmospheric Delay Error

If SA is turned to zero, as recommended above, the next largest contributor to the civilian SPS error budget is the atmospheric error consisting mainly of ionospheric delay as discussed in Appendix C and as shown in Table 3-1. Since the military normally has access to two frequencies, military users can correct the ionospheric error,14 but civilian users cannot.15 In order to compensate for the ionospheric error, the civilian community has been able to develop innovative techniques for recovering components of the encrypted Y-code signal. The chief limitation on the use of these somewhat expensive receivers is that to function effectively, the signal-to-noise ratio required for the L2 signal must be considerably higher than that required by a military PPS receiver. While this is achievable in stationary situations, there are many circumstances in which these conditions do not apply. For example, when the receiver is in a moving vehicle and/or there is ionospheric scintillation present (which corrupts the phase of the received signals), the receiver can lose lock. Several minutes may be required to recover the tracking ambiguity cycle needed for precise positioning.16 The same is true when the receiver must view some of the satellites through foliage or in the presence of multipath signals.

Unfortunately, an ideal signal reception environment is the exception rather than the rule. As the number of more demanding real-time civil applications increases, users are seeking ways to improve GPS performance. With the current GPS signal structure, civilian designers and users must confront impediments such as non-trivial levels of electrical interference and strong and rapidly changing multipath reflections from buildings and nearby vehicles. Civilian access to an additional frequency would enable improved accuracy through ionospheric corrections, multipath rejection, and single-frequency operation when interference jams one of the two civilian frequencies.

14  

As mentioned in Appendix C, the tropospheric portion of atmospheric delay cannot be eliminated through the use of two frequencies.

15  

Because the ionosphere is a dispersive medium, the ionospheric delay is frequency dependent. The existence of two frequencies allows the time of arrival of each to be compared by a receiver, calibrating the error caused by signal delay through the Earth's ionosphere. PPS users have access to both L1 and L2, whereas SPS users have access only to L1.

16  

Ionospheric scintillation of the GPS signals occurs when two or more paths are taken between the satellite and the receiver. This is caused by fluctuations in the free electron content and therefore, the refractivity of the ionosphere. When these paths carry signals of about the same amplitude, they cancel as the differential delay of the paths vary by integer plus one-half wavelengths, or they add as the differential delay of the paths vary by integer wavelengths. This scintillation is analogous to optical delays in the neutral atmosphere, which cause stars to twinkle in the visible spectrum.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Guidelines and Technical Considerations

In studying possible options for the addition of another civilian frequency, a set of guidelines and technical considerations was developed as follows:

(1)  

The signal must not interfere with the military's jamming techniques for denial of GPS signals.

Any signal enhancement should preserve and maximize the ability of the military to deny the GPS signal to adversaries through local jamming of any unencrypted s without adversely impacting the L2 Y-code signal. The use of encryption on the Y-code effectively denies its use to unauthorized parties.

(2)  

The signal must be backward compatible.

A significant investment has been made in receiver purchases and existing receiver performance must not be degraded; although existing receivers may not be able to take advantage of the new signal.

(3)  

The frequency allocation for the signal must be considered.

The signal should be assigned a frequency in the L-band spectrum that has a reasonable chance of receiving an official allocation from the Federal Communications Commission and, in some cases, the International Telecommunications Union as well. By using an L-band frequency, the cost of receiver modifications should not increase substantially.17 Unfortunately however, because many of the proposed mobile satellite communication services (Iridium, Globalstar, and others) plan to use L-band frequencies, L-band frequency allocation is difficult to attain. In light of this potential problem, a preliminary assessment was undertaken to identify possible L-band frequencies that could be used for transmission of an additional GPS signal.18 Based on this preliminary assessment, it appears that several sub-bands have promise for the proposed signal, and several frequencies were selected as potential candidates. Although these frequencies are included in Table 3-5, in-depth investigation and coordination will be required before a specific frequency band, wide or narrow, can be selected.

17  

The addition of an L4 signal would not affect the operation of existing receivers, but manufacturers would have to modify future receivers (add another channel, and change the correlator and processor) to take advantage of a new L-band signal. If a frequency much greater than L-band is used, additional antennas would have to be added to the receivers, and the satellite transmitted power would have to increase.

18  

A preliminary analysis of the L-band spectrum allocation that was conducted by Mr. Melvin Barmat, Jansky/Barmat Telecommunications Inc., Washington D.C., January 1994, is shown in Appendix I.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

(4)  

The signal should optimally be spaced for ionospheric correction and wide lane ambiguity resolution.

The NRC committee determined that ideally, the new GPS signal should be on an L-band frequency sufficiently offset from L1 to permit user correction of ionospheric delay, which would improve user accuracy yet be close enough to L1 to allow fast, wide-lane cycle ambiguity resolution, also termed wide-laning.19 For adequate ionospheric correction, the separation between L1 and a new frequency should be at least 200 MHz.20 For optimal wide-lane ambiguity resolution, the frequency difference between L1 should be no greater than 350 MHz.

(5)  

The signal should occupy a wide frequency band

The signal should occupy a wide frequency band, that is, around 10 MHz, to reduce the effects of multipath and improve resistance to unintentional RF interference. A wide-band signal has two main advantages over a narrow-band signal.21 First, use of a wide-band signal allows about a 10-dB improvement in interference rejection over a narrow-band signal. This is significant for both stand-alone and differential users needing improved availability in the presence of wide-band or continuous wave interference. The second advantage is that upon signal reacquisition, a wide-band signal can recover submeter pseudorange accuracy faster than a narrow-band signal in both low- and high-multipath environments. For example, as discussed in Appendix G, in a high multipath environment, such as around buildings, a narrow-band signal will have an error larger than a wide-band signal after signal reacquisition. Many important real-time vehicular applications, such as aircraft precision approach and land vehicle guidance, would benefit from the faster accuracy recovery obtained with a wide-band, faster chipping-rate signal.

New Signal Structure Options

Ten signal structure enhancements to the current GPS signal structure were considered and are described in Appendix H. Each option involved possible changes to L1 or L2, as well as possible signal transmissions on a new frequency. Using the previously

19  

Wide-lane ambiguity resolution (wide-laning) is a processing technique developed by civilian DGPS users to process carrier phase data. With wide-laning, the two carrier frequencies are mixed to provide a difference frequency of about 45 times longer wavelength, improving the speed and reliability of cycle ambiguity resolution. The wide-laning technique is available to cross-correlation types of receivers today, but at a serious loss in effective carrier-to-noise ratio as compared with a dual-frequency code-tracking receiver.

20  

Letter from J. A. Klobuchar, U.S. Air Force Geophysics Laboratory, 22 December 1994.

21  

A wide-band signal is generally defined to be around 20 MHz wide; a narrow-band signal around 2 MHz wide.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

discussed guidelines, the NRC committee determined that 2 of the 10 options should be seriously considered. These two options are discussed below in order of preference.

Option 1: Wide-Band L4 Signal

The optimal scenario for an enhanced civilian GPS signal would entail the provision of a new wide-band frequency, termed L4, that would be broadcast unencrypted to allow for universal access. The wide bandwidth sufficiently offset from the current L1 signal would allow for ionospheric delay correction, wide-lane ambiguity resolution, improved interference rejection, and faster accuracy recovery in multipath environments.

The pseudorandom noise chosen for the L4 wide-band signal should have a bandwidth similar to the present P-code, but with a sequence length chosen for rapid acquisition by low-cost civilian receivers. 22 Although not needed for acquisition purposes, the signal could have C/A-code in phase quadrature, which would allow manufacturers to get the most benefit from the new signal without significant changes to their investment in application-specific integrated circuit (ASIC) correlators.23

Based on the previously mentioned frequency allocation analysis, it appears that several options may exist for a wide-band L4 signal. The first option would be to place the center of the wide-band L4 signal at 1258.29 MHz. If the Russian Federation follows through on plans to move GLONASS L2 transmissions to the lower portion of their frequency allocation (1242.9-1251.7 MHz by 1998 and 1242.9-1248.6 by 2005), even a wide-band signal placed at 1258.29 MHz would cause little frequency overlap. Therefore, the possibility of interference with GLONASS would be low. The second option would be to place the wideband L4 signal at 1841.40 MHz. Again, the feasibility of receiving a frequency allocation in this area of the spectrum would require further investigation.

Option 2: Narrow-Band L4 Signal

If a wide-band frequency allocation proves impossible to obtain for L4, a narrow-band signal should be considered as the second best option. Several potential frequencies have been identified that have sufficient spacing from L1 to allow for the correction of ionospheric delay. These include 1237.83 MHz (which is the upper null of the existing L2 frequency); 1258.29 MHz; and 1841.40 MHz. A narrow-band signal placed at any of these frequencies would carry a C/A-type code.

22  

The code sequence length for the current P-code is 1 week.

23  

A dual-frequency L1/L4 receiver would still need an additional RF/IF (intermediate frequency) section and synthesizers. For current dual-frequency receiver manufacturers, hardware changes would not be difficult.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Additional Considerations

Regardless of which frequency, bandwidth, and type is chosen for the new L4 signal, its relative utility to a number of different user communities also will be affected by the type of data superimposed on the signal. For example, the inclusion of integrity information in a data message would be useful to aviation, maritime, and land transportation users concerned with safety. A navigation message also would be useful because it would allow the L4 signal to be used for navigation without access to an additional frequency (that is, L1 or L2). Users employing codeless techniques, who are interested in improved correlation between the L4 signal and the L1 signal, would benefit from having the same data transmitted on each signal. However, if a navigation message were broadcast unencrypted, potential adversaries of the United States also could take advantage of an L4 signal in a theater of war, unless L4 is jammed along with other radionavigation signals. Thus, an L4 signal with no data would probably be most acceptable from a military perspective.

The rate at which data are broadcast on the L4 signal also is important. A high data rate would increase the amount of information that could be sent to a user and would allow the information to be sent very quickly. High data rates, however, generally make a signal more susceptible to jamming. Conversely, a signal with a low data rate is more jam resistant, but also is limited in its ability to get information to a user in a timely manner. Data rate also may have an impact on the power level required for a new L4 signal, which is an important consideration because of its effect on required satellite power.

Because of these many considerations, the committee believes that it is premature to suggest a specific data message or broadcast rate for the L4 signal, but believes that it should be designed with the flexibility to add the data considered most critical to the GPS user community when the first L4-capable satellite is launched.

Improvements Anticipated from Adding L4

Increased Accuracy

The new L4 signal, which would be available to civilian users, would reduce the typical ionospheric error of 7.0 meters to 0.01 meters (la), regardless of the option selected, as shown in Table 3-5. This would result in a stand-alone accuracy as low as 21.2 meters (2 drms) compared with approximately 30 meters (2 drms) with L1 alone. With the addition of the L4 signal, several DPGS accuracy requirements could be met with the stand-alone GPS accuracy, including those for surface surveillance and autonomous vehicle location and interrogation. The addition of an L4 signal also assists short- and long-baseline differential users (e.g., Category III approach and landing, mapping, surveying, precision farming, and Earth science applications) by calibrating the spatially uncorrelated components of the ionosphere seen across the baseline, and by speeding up ambiguity resolution to get accuracies of a decimeter or better. Even in the presence of SA, dual-frequency civil receivers that operate in a codeless mode would benefit from an additional, unencrypted, signal.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Additional Benefits of L4

The existence of an unencrypted L4 signal greatly reduces a civilian receiver's probability of RF interference by providing a second frequency, which can be used in the event that L1 is subject to interference. The wide-band L4 signal also would aid in commercially important emerging markets where reception is less than ideal, since GPS must operate in applications subject to strong and intermittent multipath and signal blockage. The success or failure of GPS in those applications depends upon quick recovery of accurate pseudorange measurements once the signal is restored.

From the military perspective, the addition of the L signal retains A-S on both L1 and L2 and is quite flexible with respect to selective denial of civilian service. Of all the frequencies mentioned above, 1237.83 MHz would be the most difficult to jam because it is the closest to L2. However, based on an analysis described in Appendix J, this frequency could be selectively jammed without affecting the use of the Y-code on L2. In order to selectively deny civilian service, broadband jamming of L1 and L4 could be used. Note that even if no navigation message is broadcast on the L4 signal, it should be jammed because the last ephemeris information could be used in combination with L4 ranging data to locate a target. It also should be noted that broadband jamming of both L1 and L4 would eliminate the capability for dual-frequency ionospheric corrections. This would reduce PPS accuracy and force the U.S. military to rely on other methods of obtaining ionospheric corrections. As discussed later in this chapter, ionospheric correction models broadcast on the navigation message remove only about 50 percent of the ionospheric error. However, by using receivers with the capability to store the last known ionospheric correction and updating that information with a process called Differential Ranging Versus Integrated Doppler (DRVID), ionospheric corrections can be improved further over the 50 percent correction obtained in the L2 broadcast models.

Reduction of Receiver Noise and Multipath Errors

As shown in Table 3-5, when using a typical SPS receiver, the receiver noise and multipath actually increase when another frequency is added because of the noise and multipath from the additional frequency. As a result, the beneficial effects of adding another frequency to reduce the ionospheric error are diminished. If more advanced receivers are used, reductions in the receiver noise and multipath errors can be achieved, and the HDOP can be reduced to around 1.5.24 The error reductions achieved by using a more advanced receiver results in stand-alone SPS performance ranging from 11.3 meters to 13.1 meters (2

24  

The characteristics of a more advanced, dual-frequency SPS and PPS receivers (as compared to the typical receiver described previously) include: (1) use of more satellite signals in the solution (typically six to eight satellites), (2) lower noise amplifier, (3) better tropospheric model, (4) on-board multipath processing capability and low-multipath antenna, and (5) lower C/A-code measurement noise due to narrow correlator spacing. For an all-in-view receiver and a elevation mask angle of 5 degrees, an HDOP of 1.5 is predicted 95 percent of the time. Source: Analysis completed by Mr. Tom Hsiao of the MITRE Corporation, 15 February 1995.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

drms), depending on the L4 signal bandwidth and frequency, as shown in Table 3-6. These accuracies would satisfy the accuracy requirements for aviation traffic alert/collision avoidance systems (TCAS). The PPS performance would improve to 11.1 meters (2 drms) or 4.6 meters (CEP), as shown in Table 3-7.

With accuracy levels of 11.3 to 13.1 meters (2 drms), GPS availability also is enhanced, and RAIM is improved as well. For example, for a stand-alone horizontal accuracy of 100 meters, the availability of four satellites would increase from the previous value of 99.94 percent to approximately 99.96 percent. RAIM availability, which is dependent on the presence of six useable satellite signals, is shown in Table 3-8.

Although not shown in Tables 3-6 or 3-7, even further improvements to the receiver noise and multipath errors can be made through use of the most advanced receivers that have improved receiver signal processing, are integrated with auxiliary sensors, and have multi-element antenna arrays.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 3-5 Elimination of Ionospheric Error by the Addition of Another Frequency.

Error Source

Typical Range Error Magnitude (meters, 1σ)

 

SPS With II/IIA Satellites

SPS Improved (no SA, additional narrow L-band signal)

SPS Improved (no SA additional wide L-band

 

 

1237.83

1258.29

1841.40

1258.29

1841.40

 

 

Narrow-band, C/A-type code

Narrow-band, C/A-type code

Narrow-band, C/A-type code

Wide-band, P-type code

Wide-band P-type

Selective Availability

24.0

0.0

0.0

0.0

0.0

0.0

Atmospheric Error

 

 

 

 

 

 

Ionospheric

7.0

0.01

0.01

0.01

0.01

0.01

Tropospheric

0.7

0.7

0.7

0.7

0.7

0.7

Clock and Ephemeris Error

3.6

3.6

3.6

3.6

3.6

3.6

Receiver Noise

1.5

4.6

4.9

6.9

2.7

5.6

Multipath

1.2

3.7

3.9

5.6

2.7

4.8

Total User Equivalent Range Error (UERE)

25.3

6.9

7.3

9.6

5.3

8.2

Typical Horizontal DOP (HDOP)

2.0

2.0

2.0

2.0

2.0

2.0

Total Stand-Alone Horizontal Accuracy (2 drms)

101.2

27.8

29.0

38.5

21.2

32.9

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 3-6 Effect of Reduced Ionospheric Error by the Addition of Another Frequency and Additional Improvements with Using a More Advanced SPS Receivera

Error Source

Typical Range Error Magnitude (meters, 1σ)

 

SPS With II/IIA Satellites

SPS Improved (no SA, additional narrow L-band signal)

SPS Improved (no SA, additional wide L-band signal

 

 

1237.83

1258.29

1841.40

1258.29

1841.40

 

 

Narrow-band, C/A-type code

Narrow-band, C/A-type code

Narrow-band, C/A-type code

Wide-band, P-type code

Wide-band P-type code

Selective Availability

24.0

0.0

0.0

0.0

0.0

0.0

Atmospheric Error

 

 

 

 

 

 

Ionosphericb

7.0

0.01

0.01

0.01

0.01

0.01

Troposhericc

0.7

0.2

0.2

0.2

0.2

0.2

Clock and Ephemeris Error

3.6

3.6

3.6

3.6

3.6

3.6

Receiver Noised

1.5

0.6

0.7

0.9

0.5

0.8

Multipathe

1.2

1.5

1.6

2.3

1.0

1.9

Total User Equivalent Range Error (UERE)

25.3

3.9

4.0

4.3

3.8

4.2

Typical Horizontal DOP (HDOP)f

1.5

1.5

1.5

1.5

1.5

1.5

Total Stand-Alone Horizontal Accuracy (2 drms)

76.0

11.9

12.0

13.1

11.3

12.5

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

a. The characteristics of a more advanced, dual-frequency SPS receiver (as compared to the typical receiver described previously) include: (1) use of more satellite signals in the solution (typically six to eight satellites), (2) lower noise amplifier, (3) better tropospheric model, (4) on-board multipath processing capability and low multipath antenna, and (5) lower C/A-code measurement noise due to narrow correlator spacing.

b. With the addition of an unencrypted, coded signal, the SPS ionospheric error is removed by a linear combination of the L1 and L4 observables. This correction leaves residual ionospheric error of 1 centimeter or less.

c. For improved receivers, software models correct for all but around 0.2 meters (1σ) of the tropospheric error.

d. For an improved SPS receiver, the receiver noise for independent 1-second measurements can be as low as 0.2 m for the narrow-band signal, and 0.1 meter for the wide-band signal. These are the single-frequency errors and must be increased to account for the linear combination used to calibrate ionospheric errors. For example, the narrow-band error must be multiplied by a factor of 3.1 when 1237.83 MHz and 1575.42 MHz (L1) frequencies are used.

e. For an SPS receiver with a low-multipath antenna and on-board multipath reduction processing, the multipath can be as low as 05 meters (1σ) for the narrow-band signal, and 0.2 meters (1σ) for the wide-band signal. These errors are very dependent on the number of reflective objects near the antenna. These are the single-frequency errors and must be increased to account for the linear combination used to calibrate ionospheric errors. For example, the narrow-band error must be multiplied by a factor of 3.1 when 1237.83 MHz and 1575.42 MHz (L1) frequencies are used.

f. For an all-in-view receiver and a elevation mask angle of 5 degrees, an HDOP of 1.5 or less was predicted 95 percent of the time. Source: Analysis completed by Mr. Tom Hsiao, the MITRE Corporation, 15 February 1995.

Table 3-7 Effect of Using a More Advanced PPS Receiver on Stand-Alone Accuracya

Error Source

Typical Range Error Magnitude (meters, 1σ)

 

PPS with Typical Receiver

PPS with Advanced Receiver

Selective Availability

0.0

0.0

Atmospheric Error

 

 

Ionosphericb

0.01

0.01

Troposphericc

0.7

0.2

Clock and Ephemeris Error

3.6

3.6

Receiver Noised

0.6

0.3

Multipathe

1.8

0.6

Total User Equivalent Range Error (UERE)

4.1

3.7

Typical Horizontal DOP (HDOP)f

2.0

1.5

Total Stand-Alone Horizontal Accuracy, 2 drms

16.4

11.1

a. The characteristics of a more advanced, dual-frequency PPS receiver (as compared to the typical receiver described previously) include: (1) use of more satellite signals in the solution (typically six to eight satellites), (2) lower noise amplifier, (3) better tropospheric model, and (4) on-board multipath processing capability and low-multipath antenna.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

b. For a PPS receiver, the ionospheric error is removed by a linear combination of the L1 and L2 observables. This correction leaves residual ionospheric error of 1 centimeter or less.

c. For improved PPS receivers, software models correct for all but around 0.2 meters (lσ) of the tropospheric error.

d. For an improved PPS receiver, the receiver noise for independent 1-second measurements can be as low as 0.1 meters (1σ). These are the single-frequency errors and must be increased to account for the linear combination used to calibrate ionospheric errors. The single-frequency error of 0.1 meters must be multiplied by a factor of 3 when the standard L2= 1227.6 MHz and L2= 1575.42 MHz frequencies are used.

e. For an improved PPS receiver with a low-multipath antenna and on-board multipath reduction processing, the multipath can be as low as 0.2 meters (1σ). These errors are very dependent on the amount of reflective objects near the antenna. These are single-frequency errors and must be increased to account for the linear combination used to calibrate ionospheric errors. For example, the single-frequency error of 0.2 m must be multiplied by a factor of 3 when the standard L2 = 1227.6 MHz and L1 = 1575.42 MHz frequencies are used.

f. For an all-in-view receiver and a elevation mask angle of 5 degrees, an HDOP of 1.5 or less was predicted 95 percent of the time. Source: Analysis completed by Mr. Tom Hsiao, the MITRE Corporation, 15 February 1995.

Table 3-8 Effect of SA Removal and Dual-Frequency Capability on RAIM Availability for Aviation Applicationsa

Aviation Application

Availability With SA Set to Zero

Availability With SA Turned to Zero and L4Added

Phase of Flight

Protection Limit

21 Satellitesb

24 Satellitesc

21 Satellites

24 Satellites

En Route

2.0 nautical miles

96.34%

99.98%

96.80%

100.00%

Terminal Area

1.0 nautical miles

94.39%

99.95%

95.19%

99.98%

Non-precision Approach

03 nautical miles

91.10%

100.00%d

93.12%

100.00%d

a. This analysis has been made for a single frequency C/A-code receiver aided by a barometric altimeter (required for aviation supplemental navigation use of GPS) with a visibility mask angle of 5 degrees.

b. The probability of having 21 satellites operating is assumed to be 98 percent.

c. The probability of having 24 satellites operating is assumed to be only 70 percent. However, the values in this table reflect the fact that if 24 satellites are fully operational, an incremental improvement in availability exists.

d. Although these values would intuitively be lower than the 1 nautical mile terminal area protection limit value, availability improves for the 03 nautical mile non-precision protection limit because the barometric altimeter inputs provide extra information in this phase of flight.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Findings and Recommendations

The NRC committee determined that the addition of a new, L-band signal, L4, offers civilian users much improved precision in many reception environments as well as preserving selective denial options for the military. The NRC committee anticipates that domestic suppliers of commercial GPS receivers, who also are the suppliers of dual-frequency military receivers, would enjoy some advantage over foreign competitors in providing dual-frequency civilian receivers.

The NRC committee believes that the L4 signal could be added to several Block IIR spacecraft using the existing volume and power on the Block IIR spacecraft. If it is assumed that the L4 signal transmits at a radiated power similar to the L1 or L2 signals, then approximately 180 watts of DC power is required.25 The exact amount of power however, will depend on the specific frequency selected for L4. Since the current Block IIR L-band (L1, L2, and L3) navigation payloads and harnesses weigh around 160 kilograms (353 lbs), the L4 signal generation system is expected to weigh approximately one-fourth to one-fifth that amount.26 Based on information provided to the NRC committee through various presentations, it is believed that the sufficient power for an additional frequency can be made available on the Block IIR spacecraft by utilizing the currently unused Reserve Auxiliary Payload power margin, and by re-definition and re-allocation of other existing margins.

In order to add a new signal, several Block IIR hardware modifications are required, including the addition of a frequency synthesizer, modulator/intermediate power amplifier, a high-power amplifier, and a payload processor.27 The NRC committee believes that adequate space for this additional hardware currently exists on the Block IIR spacecraft. Based on cost information for the current Block IIR L-band navigation package, the committee believes that the addition of another, unencrypted L-band signal would cost approximately $1.3 million per Block IIR satellite.28

Immediate steps should be taken to obtain authorization to use an L-band frequency for an additional GPS signal, and the new signal should be added to GPS Block IIR satellites at the earliest opportunity.

25  

Information provided by Martin Marietta Astro Space Division of Lockheed-Martin, 6 February 1995.

26  

Information provided by Martin Marietta Astro Space Division of Lockheed-Martin, 12 April 1995.

27  

Information provided by Martin Marietta Astro Space Division of Lockheed-Martin, 6 February 1995 and by ITT Corporation, 13 March 1995.

28  

It is estimated that the non-recurring design and development costs for each of the existing Block IIR L-band signals are $11 million, and the unit price for each existing L-band signal is around $500,000 per satellite. It is estimated that the cost for each L4 signal payload processor would be $100,000, and the non-recurring costs for deliverable test equipment would be $3 million. Information provided by ITT Corporation, 13 March 1995.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Performance Improvements to the GPS Operational Control Segment and Satellite Constellation

Current Status of the Operational Control Segment and Planned Upgrades

The current operational control segment (OCS) consists of a master control station (MCS) plus four additional monitor sites that collect GPS Y-code measurements from a maximum of 11 satellites each.29 All but one of these sites are capable of sending uploads to the GPS satellites.

There are plans to award an OCS consolidated contract in July 1995 to provide maintenance until the year 2000, to make improvements to the existing software architecture and user interfaces, and to support the deployment and operation of the Block IIR satellites. There is an option in the contract to replace the operational control software, but the winning contractor can choose to upgrade existing software rather than replace it.

In February 1996, the Air Force plans to award another OCS contract, which will be effective beginning in the year 2000. This contractor will assume responsibility for the operational control segment, the Block IIR constellation, and the development and deployment of the Block IIF satellites. However, neither of these contracts address critical upgrades that would enhance the operation of the OCS and thus enhance the performance of GPS.

The NRC committee recommends changes below that will enhance the overall GPS operation and improve performance. Most of these changes focus on the OCS and can be implemented immediately. Some improvements, however, focus on the operation of the Block IIR constellation and cannot be introduced until several Block IIR satellites are in orbit.

Recommended Upgrades to the Operational Control Segment

In addition to other operational functions, such as satellite health monitoring and routine maintenance, the GPS control segment is responsible for determining the ephemeris 30 and clock parameters and uploading them to the satellites. A partitioned Kalman Filter31 at the master control station estimates the orbits and clock errors for each

29  

Information provided by Air Force Space Command, 1 December 1994.

30  

Ephemeris is defined as a satellite's position as a function of time.

31  

A Kalman Filter incorporates both observations and mathematical models of the system dynamics to produce an estimate of the current state of a system. By using knowledge of how the system state can change over time, the Kalman Filter allows the contributions of individual measurement errors to be averaged. In the MCS filter, the system state includes satellite orbital parameters, clock parameters, and numerous other elements.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

of the GPS satellites as well as the clock errors for the monitor site receivers. Updated orbit and clock corrections are uploaded to each satellite once a day.

With the current GPS constellation, the clock and ephemeris errors contribute approximately 3.4 and 1.4 meters (1σ), respectively, to the SPS and PPS error budget, for a combined error of 3.6 meters (1σ)32, as shown in Table 3-1.33 Once SA, the atmospheric, receiver noise, and multipath errors have been eliminated or reduced, ephemeris and clock errors become the largest contributors to the UERE. As shown below, several methods can be used to reduce combined clock and ephemeris errors to increase accuracy and improve overall performance.

Accuracy Improvements

Planned Experiments Involving Correction Updates and Additional Monitoring Stations. An innovative, near-term strategy for improving PPS accuracy and integrity has been investigated by the Air Force, and an experiment to test the strategy is expected to begin in the spring of 1995. The experiment involves uploading pseudorange corrections for all satellites with each scheduled, individual satellite upload.34 These corrections would be made available to PPS users in the navigation message. A PPS receiver can decode the messages from all satellites it is tracking and apply the most recent correction set. The Air Force expects that this will improve the combined error contribution of clock and ephemeris for PPS users by half, to approximately 2 meters (1σ). If SA is turned to zero as previously recommended, SPS users will not receive the same benefit from this experiment as PPS users unless current security classification policies are changed to allow the most recent clock and ephemeris parameters to be broadcast from each satellite unencrypted.35

In conjunction with the above experiment, the Air Force is investigating another enhancement that could provide further reduction in the combined PPS clock and ephemeris error. This enhancement involves the integration of data from five Defense Mapping Agency (DMA) GPS monitoring sites with the existing Air Force operational control segment in a simulated Kalman Filter. By including additional data from the DMA sites, which are located at higher latitudes than the Air Force sites, an additional 15 percent improvement in combined clock and ephemeris accuracy can be anticipated, based on tests previously

32  

The error of 3.6 meters (1σ) was obtained by taking the square root of the sum of the squares of 3.4 and 1.4 meters (1σ).

33  

J. F. Zumberge and W. I. Bertiger, ''Ephemeris and Clock Navigation Message Accuracy in the Global Positioning System," Volume I, Chapter 16. Edited by B. W. Parkinson, J. J. Spilker, P. Axelrad, and P. Enge. To be published by AIAA, in press, 1995.

34  

Satellites are normally uploaded once per day.

35  

Currently the most recent clock and ephemeris updates are broadcast in an encrypted portion of the navigation message. Clock and ephemeris parameters less than 48 hours old are classified.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

conducted by the DMA.36 It should be emphasized that this experiment will be conducted as a software simulation only, so PPS users will not actually observe the additional 15 percent simulated improvement.

Recommended Implementation of More Frequent Updates and Additional Monitoring Stations. Full operational implementation of the first experiment, which involves uploading of clock and ephemeris corrections for all satellites with each scheduled, individual satellite upload, should not be difficult to accomplish and would appear to reduce the combined clock and ephemeris error to half of its current value.

Operational implementation of the second planned experiment, which involves the incorporation of DMA monitor site data, is more difficult to achieve. While well-distributed geometrically, DMA GPS monitor stations do not have secure communications data links to the master control station. Existing Air Force sites, which are used for other purposes, have secure data links to Air Force Space Command (co-located with the GPS master control station), but are not well distributed in latitude for GPS monitoring and do not have GPS receivers. Additional GPS monitoring sites are expected to improve stand-alone GPS accuracy. More importantly, a well-distributed set of monitor sites would allow continuous tracking of each satellite, enabling the prompt detection of satellite failures. An estimated cost of $9 million for using DMA data in real-time and an estimated cost of co-locating Air Force monitor stations at DMA sites was provided to the committee.37

The DOD's more frequent satellite navigation correction update strategy should be fully implemented as soon as possible following the successful test demonstration of its effectiveness. In addition, the current security classification policy should be examined to determine the feasibility of relaxing the 48-hour embargo on the clock and ephemeris parameters to civilian users.

Additional GPS monitoring stations should be added to the existing operational control segment. Comparison studies between cost and location should be completed to determine if Defense Mapping Agency or Air Force sites should be used.

Recommended Use of a Non-Partitioned Kalman Filter with Improved Dynamic Models. The original computer hardware used for the OCS was not capable of processing all satellites in a single Kalman Filter. The existing software was written with this limitation as well. The hardware has since been upgraded, leaving only the software to restrict full processing of all satellite clock and ephemeris data simultaneously. Unfortunately, there currently are no definite plans to upgrade the Kalman Filter software, including the dynamic

36  

Stephen Malys, DMA, Viewgraphs from presentation at the PAWG 1993, Colorado Springs, Colorado.

37  

Information provided by the Aerospace Corporation, 21 February 1995.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

model. It is possible that the winning contractor of the 1995 contract may choose to eliminate the partitions, but there is not a specified requirement to do so.

Based on recent improvements to the DMA's Kalman Filter, which originally had a configuration similar to the GPS Kalman Filter, use of an updated, non-partitioned GPS Kalman Filter is expected to reduce the combined clock and ephemeris error by 15 percent.38 Furthermore, an additional 5 percent improvement can be achieved by using improved dynamic models in the Kalman Filter, which would allow better predictions of satellite behavior 1 day ahead.39 An estimated cost of $7.5 for upgrading the Kalman Filter and improving its dynamic models was provided to the committee.40

The operational control segment Kalman Filter should be improved to solve for all GPS satellites' clock and ephemeris errors simultaneously through the elimination of partitioning and the inclusion of more accurate dynamic models. These changes should be implemented in the 1995 OCS upgrade request for proposal.

The combined clock and ephemeris improvement obtained with each of the above upgrades is shown in Table 3-9. If all three of the recommendations above are implemented, the combined clock and ephemeris error is expected to be approximately 1.2 meters (1s). As shown in Table 3-10 and Figure 3-5, if: (1) SA is turned to zero; (2) an additional GPS L-band signal is added; (3) more advanced receivers are utilized; and (4) each of the clock and ephemeris accuracy improvements are implemented, then a stand-alone GPS SPS accuracy of 5.4 meters (2 drms) with a narrow, L-band signal should be obtainable, and a stand-alone GPS SPS accuracy of 4.9 meters (2 drms) with a wide-band signal should be obtainable.41 In addition, as shown in Table 3-11, a PPS accuracy of 4.2 meters (2 drms) (1.8 meters CEP) also would be obtainable.

With stand-alone accuracies at this level, many civilian and military accuracy requirements, such as the following will be met:

  • Aviation—Category I approach and landing.
  • Maritime—Recreational boating, vessel-tracking services, and harbor/harbor approach requirements.

38  

Stephen Malys, DMA, Viewgraphs from presentation at the PAWG 1993 meeting, Colorado Springs, Colorado.

39  

Stephen Malys, DMA, Viewgraphs from presentation at the PAWG 1993 meeting, Colorado Springs, Colorado.

40  

Information provided by the Aerospace Corporation, 21 February 1995.

41  

Civil users would have access to this level of accuracy only if the 48-hour embargo on clock and ephemeris parameters is lifted.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
  • ITS—Infrastructure management, highway navigation and guidance, mayday incident and alert, automated bus/railstop annunciation, collision avoidance (hazardous situation), and vehicle or cargo location (hazardous material transport).
  • Earth Science—Oceanographic navigation and real-time positioning.
  • Spacecraft—Real-time satellite orbit determination.
  • Military—Precision-guided munitions.

Table 3-9 Reduction of Combined Clock and Ephemeris Errors

Enhancement

Anticipated Combined Clock and Ephemeris Error Improvement over Existing Combined Error of 3.6 meters (1σ)

Correction Updates (50% reduction)

1.8 meters

Additional Monitor Stations

(additional 15% reduction)

1.5 meters

Non-partitioned Kalman Filter

(additional 15% reduction)

1.3 meters

Improved Dynamic Model

(additional 5% reduction)

1.2 meters

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Figure 3-5

Approximate stand-alone horizontal SPS accuracy, 2 drms, resulting from recommended improvements and enhancements.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 3-10 Impact of Reduced Clock and Ephemeris Error on SPS Stand-Alone Accuracy

Error Source

Typical Range Error Magnitude (meters, 1σ)

 

SPS With II/IIA Satellites

SPS Improved (no SA, additional narrow L-band signal)

SPS Improved (no SA, additional wide L-band signal)

 

 

1237.83

1258.29

1841.40

1258.29

1841.40

 

 

Narrow-band, C/A-type code

Narrow-band, C/A-type code

Narrow-band, C/A-type code

Wide-band,

P-type code

Wide-band,

P-type cc

Selective Availability

24.0

0.0

0.0

0.0

0.0

0

Atmospheric Error

 

 

 

 

 

 

Ionospheric

7.0

0.01

0.01

0.01

0.01

0.01

Tropospheric

0.7

0.2

0.2

0.2

0.2

0.2

Clock and Ephemeris Error

3.6

1.2

1.2

1.2

1.2

1.2

Receiver Noise

1.5

0.6

0.7

0.9

0.5

0.8

Multipath

1.2

1.2

1.6

2.3

1.0

1.9

Total User Equivalent Range Error (UERE)

253

1.8

2.1

2.8

1.7

2.4

Typical Horizontal DOP (HDOP)

1.5

1.5

1.5

1.5

1.5

1.5

Total Stand-Alone Horizontal Accuracy (2 drms)

76.0

5.4

6.4

8.3

4.9

7.1

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 3-11 Impact of Reduced Clock and Ephemeris Error on PPS Stand-Alone Accuracy

Error Source

Typical Range Error Magnitude (meters, 1σ)

 

PPS with II/IIA satellites

PPS Improved

Selective Availability

0.0

0.0

Atmospheric Error

 

 

Ionospheric

0.01

0.01

Tropospheric

0.2

0.2

Clock and Ephemeris Error

3.6

1.2

Receiver Noise

0.3

0.3

Multipath

0.6

0.6

Total User Equivalent Range Error (UERE)

3.7

1.4

Typical Horizontal DOP (HDOP)

1.5

1.5

Total Stand-Alone Horizontal Accuracy,2

11.1

4.2

As with the previous performance improvements, the increased positioning accuracy achieved by reducing clock and ephemeris errors also enhances availability. For example, for a stand-alone horizontal accuracy of 100 meters, the availability of four satellites would increase from the previous value of 99.96 percent to 99.97 percent. The improved RAIM availability is shown in Table 3-12.42

Table 3-12 Effect of SA Removal, Dual-Frequency Capability and Reduced Clock and Ephemeris Errors on RAIM Availability for Aviation Applications a

Aviation Application

Availability With SA Turned to Zero and L4 Added

Availability With SA Turned to Zero, L4 Added, and Reduced Clock and Ephemeris Error

Phase of Flight

Protection Limit

21 Satellitesb

24 Satellitesc

21 Satellites

24 Satellites

En Route

2.0 nautical miles

96.80%

100.00%

97.08%

100.00%

Terminal Area

1.0 nautical miles

95.19%

99.98%

95.70%

100.00%

Non-Precision

03 nautical

93.12%

100.00%d

94.36%

100.00%d

42  

Based on analysis conducted by the MITRE Corporation for the NRC committee, 7 February 1995. For more details, see footnote 1 earlier in this chapter.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

a. This analysis has been made for a single-frequency C/A-code receiver aided by a barometric altimeter (required for aviation supplemental navigation use of GPS) with a visibility mask angle of 5 degrees.

b. The probability of having 21 satellites operating is assumed to be 98 percent.

c. The probability of having 24 satellites operating is assumed to be only 70 percent. However, the values in this table reflect the fact that if 24 satellites are fully operational, an incremental improvement in availability exists.

d. Although these values would intuitively be lower than the 1 nautical miles terminal area protection limit value, availability improves for the 0.3 nautical miles non-precision protection limit because the barometric altimeter inputs improve in this phase of flight.

Overall System Improvements

Improved Monitor Station Receivers. The receivers currently used at the monitor stations are outdated compared with currently available commercial receivers. The receivers at the monitor stations can track only 11 satellites at a time, and the tracking schedules cannot easily be revised or priority given as to which 11 satellites to track.43 This results in a tracking gap of 3 to 4 hours per satellite per day. In addition, these receivers do not take full advantage of high-precision carrier phase data, which could be used to reduce multipath error contributions to the monitor station observables. Since some of the monitor sites suffer from very poor multipath environments, reduction of multipath errors is important. The main deficiency with current receivers, is that they can track only the Y-code and not the C/A-code, which is currently used by both civilians and the military. If there is a problem with the C/A-code, the MCS usually finds out only when C/A-code users call in to complain.

By upgrading the monitor stations with a high-quality, all-in-view receiver with C/A code, Y-code, L1, L2, (and L4) observables, OCS performance would be improved as follows: (1) the integrity of the C/A-code could be monitored, which would allow faster detection and correction of a problem by the OCS; (2) the high-precision carrier phase data could be used to reduce multipath error to the monitor station observables, thereby improving overall GPS accuracy; and (3) all satellites in view could be monitored, which would eliminate existing individual satellite tracking gaps of 3 to 4 hours per day and allow prioritized monitoring of any failing satellite signals.

Improvements to the monitor station facilities would require both software and hardware upgrades. Currently, the Air Force plans to award a $5 million contract to replace the monitor station receivers via a competitive bid in the summer of 1995. However, computer and software modifications required to take advantage of the improved receivers will not be upgraded at the same time. There also is a requirement in the 1995 OCS contract to replace the monitor station computers in order to take advantage of the new receivers, but there appears to be little coordination between the two procurements and little attention paid to the interfaces needed to optimize the system. The cost of replacing

43  

As many as 14 satellites can be in view of a monitor station at one time.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

the monitor station computers and the software could not be obtained at the time of this report, since the contract had not been awarded. 44

Procurements for the replacement of the monitor station receivers, computers, and software should be carefully coordinated. The new receivers should be capable of tracking all satellites in view and providing C/A-code, Y-code, and L1, and L carrier observables to the OCS. Upgradability to track a new L4 signal also should be considered. OCS software also should be made capable of processing this additional data.

Backup Master Control Station. In view of the rapidly expanding use of GPS for both the military and civilians, it is critically important that the GPS be capable of continuous operation in all foreseeable contingencies. Currently, a considerable degree of redundancy exists in the space segment. However, very little if any redundancy exists in the operational control segment. Presently, a backup MCS is in place at the current OCS contractor's facility, but there are no firm, long-term plans to maintain such a facility. It is possible that the eventual implementation of the Block IIR autonomous navigation operation capability could remove some of the urgency for a backup system, but even so, such a capability will not be operational until near the year 2000 or later and will not completely eliminate the need for a backup MCS.45 Air Force representatives have estimated that a backup MCS will cost around $14.4 million.46

Firm plans should be made to ensure the continuous availability of a backup master control station.

Operational Control Segment Simulator. Presently, there is no dedicated capability to test and prove out system hardware and software modifications or to train personnel in any new operational procedures resulting from the changes. Instead, the operational control segment and the space segment currently are used for testing and training purposes. This procedure not only imposes some degree of risk on the operational system and interferes with operational performance. Tests and training activities could be effectively performed in a facility that functionally simulates the operational system. This is a particularly critical issue in the near future because of the planned OCS upgrades and the deployment and

44  

Information provided by Capt. Earl Pilloud, Chief, GPS Control Segment, Air Force Space Command, 23 February 1995.

45  

Block IIR satellites have a military requirement to maintain a specified position accuracy for up to 180 days without clock and ephemeris updates from the MCS. This mode of operation is called autonomous navigation, or autonav. Autonav is accomplished by making inter-satellite pseudorange measurements using UHF (ultra high frequency) crosslinks and on-board processing to determine each satellite's ephemeris and clock offset.

46  

Memorandum from Col. Bruce M. Roang to the NRC committee, 23 December 1994.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

operation of the Block IIR satellites. Also, if the recommendations of this report are implemented, a simulation facility would enable prompt and effective testing of the proposed modifications prior to their incorporation in the operational system. An Air Force estimate for the cost of an operational control segment simulator is $14.4 million.47

A simulator for the space and ground segment should be provided as soon as possible to test software and train personnel.

Operational Control Segment Software. The current OCS system software was written several years ago. The hardware has since been upgraded, and over the years some software revisions have been made. However, the various upgrades have been written in different programming languages. This has produced a system that is lacking in modularity and is both difficult and expensive to maintain and upgrade. Because of this, an increasingly large percentage of the OCS budget is used to make relatively small changes to the system.

Since the original software was designed, significant improvements have been made in software development and management technology. Today, a system can be designed and implemented that would have improved reliability, longevity, and ease of enhancements through modular software engineering practice. Given the current state of the OCS software, the DOD's planned changes, and the recommendations contained in this report, the most economical and effective solution to this problem is to develop a new OCS software suite using current technology and methods. There is an option in the 1995 OCS upgrade procurement to either upgrade the existing software or to replace it with improved software that is easier to maintain and upgrade, but the choice is left up to the winning contractor.

The operational control segment software should be updated using modern software engineering methods in order to permit easy and cost-effective updating of the system and to enhance system integrity. This should be specified in the 1995 OCS upgrade request for proposal.

Planned Block IIR Operation

Currently, each Block II/IIA satellite is updated once a day from the OCS with clock and ephemeris corrections generated by the MCS's Kalman Filter. As a result of military requirements, each Block IIR satellite will have a Kalman Filter on board and will be able to autonomously determine clock and ephemeris corrections independent of the OCS.48 By

47  

Memorandum from Col. Bruce M. Roang to the NRC committee, 23 December 1994.

48  

Block IIR satellites have a military requirement to maintain a specified position accuracy for up to 180 days without clock and ephemeris updates from the MCS. This mode of operation is called autonomous navigation, or autonav. Autonav is accomplished by making inter-satellite pseudorange measurements using UHF crosslinks and on-board processing to determine each satellite's ephemeris and clock offset.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

exchanging clock and ephemeris information every 15 minutes via UHF communications crosslinks, which will connect each satellite in the constellation to all of the other satellites in view, each satellite will have knowledge of the ephemeris and clock information of all the satellites in the constellation. Based on the 15-minute ranging data exchanged, the Block IIR satellites can autonomously update the navigation message being broadcast to users.

The current plan for testing the autonomous ranging capability is initially to download the 15-minute ranging data from each satellite's Kalman Filter once per day to the OCS so that it can be compared with the ground-based data derived from the MCS's Kalman Filter. After successful testing of autonomous satellite ranging capability is completed, clock and ephemeris corrections will be determined with the on-board Kalman Filter, and the satellites will automatically update the navigation message every hour. However, even with autonomous generation of clock and ephemeris corrections, the Air Force plans to continue daily uploads of the satellites' clock offset relative to UTC.49 After 24 hours, the combined clock and ephemeris error for the Block IIR satellite constellation is expected to be 1.9 meters (ls).50

Suggested Improvements Using the Autonomous Ranging and Crosslink Communication Capability

Current plans call for the use of the Block IIR satellite crosslink capability only for specific commands related to SA and autonomous navigation. Further improvements in accuracy, system reliability, and integrity could be obtained by exploiting the satellite ranging data obtained during the 15-minute autonomous ranging cycles and by more effectively utilizing the communication crosslinks. These improvements are discussed below.

Accuracy Improvements by Incorporating Satellite Ranging Data into Ground Solution

Since the satellite ranging data will initially be sent to the OCS for comparison with the ground-based data, the space-based measurements also could be incorporated into the MCS's Kalman Filter. By uploading these integrated corrections to the satellites, an incremental improvement in accuracy can be achieved over the initial planned Block IIR operational procedure, where the satellites will be uploaded with only ground-based clock and ephemeris corrections.

When autonomous satellite ranging capability has been activated, further accuracy improvements could be achieved if the integrated corrections were sent to satellites at least once per day. Ideally, one satellite could be sent the corrected data every hour and the crosslinks could be used to relay the information to all the other satellites. These integrated

49  

Source: Input provided to the NRC committee by Capt. Christopher Shank and Capt. Earl Pilloud, Air Force Space Command, January and February 1995.

50  

Response from Martin Marietta Astro Space Division of Lockheed-Martin, 6 February 1995.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

corrections, instead of corrections generated autonomously on the satellites, could be used to update the navigation message every hour. In order to operate in this manner, the data rate of the Block IIR UHF communication crosslinks may have to be modified. The exact improvement to the combined clock and ephemeris error is not known, because a complete analysis was not conducted. However, more frequent uploads of integrated space-based and ground-based clock and ephemeris information should result in errors no greater than 1.2 meters (1s).

The planned Block IIR operation should be reexamined and compared to the accuracy advantages gained by incorporating inter-satellite ranging data in the ground-based Kalman Filter and uploading data at some optimal time interval, such as every hour, to all GPS satellites.

Satellite Health Monitoring to Improve System Reliability and Availability

Since the Block IIR satellites will have a UHF communications crosslink capability, satellite health monitoring could be implemented that could improve overall system reliability and availability. For example, if a satellite detected an anomalous on-board health reading, but was not in contact with a ground station, it could relay the information through the crosslinks, enabling another satellite that was in contact with a ground station at that instant to download the information. The MCS in turn, could upload commands to the failing satellite via the crosslinks. This would improve the reliability of each individual satellite by minimizing out-of-service time, thus improving the percentage of time that a full 24-satellite constellation would be available to users.

Block IIR satellite communication crosslinks should be used to the extent possible with the existing crosslink data rate to support on-board satellite health monitoring for improved reliability and availability and in order to permit a more rapid response time by the operational control segment.

Ground-Based Integrity Improvements

The Block IIR communications crosslinks also could be used to improve GPS signal integrity for all users. For example, if an anomalous pseudorange signal was detected at a monitoring station, the MCS could upload a command to the satellite broadcasting the anomalous signal by relaying this command through the crosslinks. The faulty satellite could be commanded to broadcast a code that could not be tracked by a user's receiver, and would therefore, be dropped from the users' positioning solution.

To use the crosslinks to improve GPS integrity for PPS and SPS users, the receivers at the monitor stations must be upgraded to monitor the C/A-code. The data rates on the crosslinks must be able to support commands sent from the MCS.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

The Block IIR inter-satellite communications crosslinks should be used to relay integrity information determined through ground-based monitoring.

Performance Improvements To Enhance The Military Use Of GPS

From its very inception, the force enhancement capabilities of GPS for both U.S. and allied armed forces has been one of the system's most important capabilities. This remains true today, despite the fact that civilian and commercial use of the system has grown rapidly. The current DOD policy is to secure for both the United States and its allies the full accuracy of GPS by using the encrypted Y-code on both L1 and L2, while denying that accuracy via SA to a potential enemy who, like most civilians, will have the C/A-code available only on L1. However, with the widespread use and proliferation of DGPS, the accuracy degradation produced by SA is routinely eliminated and, in many cases, civilians have access to more accurate signals than the military. As the cost of DGPS equipment decreases, differential technology and capability will proliferate. Differential systems will therefore become difficult to identify and render inoperative in a conflict situation. Furthermore, because adversarial forces are far less likely to be concerned with collateral damage, the 100-meter (2 drms) stand-alone accuracy of the SPS already poses a risk for our forces operating in a theater of war.

Earlier in this chapter the NRC committee recommended that the DOD concentrate future efforts towards the denial of GPS capability to an enemy by jamming the L1 signal, the L4 signal (if added), and other frequencies that may be employed by enemy forces to broadcast differential corrections. This strategy implies that U.S. forces must be properly equipped to operate in a high jamming environment generated by both U.S. military and enemy jammers. Based on this objective, the remainder of this section recommends several near-term technical enhancements to improve the overall performance of military GPS user equipment operating in the presence of spoofing, jamming, and interference. The greatest improvement in user equipment performance will result from the combined implementation of all five recommended enhancements in a single integrated system. Possible operational procedures that could be used prior to the availability of each recommended enhancement also are discussed.

Recommended Technical Improvements to Military User Equipment

Rapid, Direct Y-Code Acquisition

Current military receivers are designed to first acquire the more powerful C/A-code before handing over to the encrypted Y-code. Upon receiver restart, or following a loss of signal lock, a PPS receiver must go through acquisition in which a two-dimensional time-frequency search is carried out by trial correlations. With current receivers, this search conventionally is done serially, resulting in seconds to minutes of acquisition time for the C/A-code prior to Y-code hand-over, depending upon the amount of signal blockage

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

experienced and the movement of the vehicle carrying the receiver. If the L1 signal is jammed, the current receivers cannot acquire the C/A-code and as a result are denied access to the encrypted Y-code as well as dual-frequency ionospheric corrections. One receiver improvement that would enhance military access to the encrypted Y-code in a jamming environment would be the ability to acquire the Y-code rapidly without first acquiring the C/A-code. A method for improving L2 ionospheric corrections in an L1 jamming environment is addressed later in this chapter.

In order to obtain direct Y-code access, the signal acquisition processing capability of current PPS receivers must be improved through the use of massively parallel correlators, built using application-specific integrated circuit (ASIC) technology.51 The technology is now available that would allow the incorporation of at least 1,000 parallel correlators per receiver at a reasonable cost. This would allow direct Y-code acquisition within 2 seconds in a non-jamming environment, without prior acquisition of the C/A-code. This would allow faster receiver time-to-first-fix after power-down and would enhance signal availability after a blackout interval.52 The ability to directly acquire the Y-code on L2 would ensure that the selective denial of the L1 signal and the C/A-code through spoofing and jamming would eliminate or seriously degrade an enemy's use of GPS without impacting U.S. capabilities. According to experts in the field of military receiver technology, the technology for direct Y-code acquisition is in hand and in fact, the current military ''Plugger" receivers do try to directly reacquire the Y-code after signal loss.53 A military receiver with the capability to initially acquire the Y-code directly could be developed in 9-15 months depending on: (1) the amount of input received from the military regarding specifications; (2) the level of trade-off accepted between jamming-to-signal ratio versus the amount of time for direct Y-code acquisition; and (3) the ASIC development.54

The development of receivers that can rapidly lock onto the Y-coded signals in the absence of the C/A-code should be completed. The deployment of direct Y-code receivers should be given high priority by the DOD.

51  

Massively parallel correlators using ASIC technology, permit the receiver to compare, at very fast speeds, the internally generated pseudorandom noise codes to the received codes, which contain data about the satellite's position and time the code was transmitted.

52  

See Appendix K for calculations showing a direct Y-code acquisition time of 2 seconds with current ASIC technology.

53  

Personal conversation with Mr. Tyler Trickey, Rockwell-Collins, February 1995.

54  

Information provided to the NRC committee by Mr. Charles Trimble of Trimble Navigation Ltd., 31 March 1995.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Antenna Subsystem Improvements

The jamming analysis shown in Appendix L assessed three candidate techniques for improved jamming resistance: aiding of tracking loops with inertial sensors, increased processing gain via wider signal bandwidth, and nulling/directive antenna systems. Nulling antennas were found to provide the single biggest improvement in jamming performance on the order of 25-35 dB.

Both military and civilian users have deployed multi-element antenna structures for several years. In the late 1970s, work on the multi-element military AE-1 began. This antenna system was designed to effectively null a single jammer. In both this unit and its derivative design, the AE-1A, element phase shifting and combining is carried out in the radio frequency analog domain. These units have been deployed on a number of aircraft, but have not yet been widely utilized on other military weapons systems, primarily due to cost and size considerations.

Recently, more effective techniques have been developed wherein element phase shifting and amplitude weighting is done after spread-spectrum signal correlation, eliminating the RF phase shift components in favor of lower-cost correlator ASIC's and signal processing at baseband. With processing gain applied before nulling and beam forming algorithms, much improved jamming-to-signal margins are available.55 These and future developments aimed at reducing the size and cost of antenna structures should be actively pursued.

Nulling antennas and antenna electronics should be employed whenever feasible and cost effective. Research and development focused on reducing the size and cost of this hardware should actively be supported.

Inertial Aiding Improvements

Aiding can be defined as the usage of any non-GPS-derived user dynamics and clock information in GPS receiver signal-tracking and navigation functions. The availability of such data can have a profound impact on GPS signal acquisition time, code and carrier tracking thresholds, interference and jamming resistance, anti-spoofing capability, and receiver integrity. Aiding works by providing auxiliary observations, which sense a vehicle's motion parameters. Inertial aiding is especially effective because of its invulnerability to electromagnetic interference and because its error characteristics are complementary to those of radionavigation systems, that is, inertial noise errors are low frequency and GPS signal tracking errors are high frequency.

Since the earliest days of GPS, the military has exploited synergism, at first with loosely-integrated inertial navigation systems (INS)/GPS built around existing aircraft INS mechanization, and more recently with "embedded" INS in which the inertial sensor and

55  

These concepts have been privately developed and patented by the Magnavox Electronic Systems Company (MESC), Patent 4734701. MESC has been continuing to enhance these concepts since inception.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

GPS components reside in the same box. Embedded architectures combine GPS tracking-loop estimates with INS accelerometer and gyroscope outputs to correct INS biases. This provides fast GPS loop-aiding commands for a 10 dB-15 dB increase in tracking threshold and jamming margins and also supports rapid pull-in after signal blockage. This is referred to as a "tightly-coupled" INS/GPS structure. In less sophisticated aiding systems, often referred to as "loosely-coupled" structures, inertial positions and GPS positions or pseudorange data are merged in a cascaded filter structure, missing the benefits of improved GPS signal tracking margins. However, these loosely-coupled INS/GPS structures do extend the length of time that inertial operations can provide useful accuracy and a GPS integrity check, and also speed GPS signal acquisition.

Historically, inertial aiding had been too expensive for many tactical military applications. It was not until the 1980s that less-costly strapdown ring-laser gyroscope technology became common aboard military aircraft. However, in the last 5 years there have been other encouraging developments that could lead to wider implementation of aided GPS in tactical military applications. Fiber-optic gyroscopes and solid-state accelerometer configurations have come into use, and more recently, batch-fabricated quartz rate sensors and quartz and silicon accelerometers have been developed. These technologies should have a major impact on the cost of aided receiver systems.

The development of low-cost, solid-state, tightly-coupled integrated inertial navigation system/GPS receivers to improve immunity to jamming and spoofing should be accelerated.

Signal Processing Improvements

The estimation of path delay and Doppler for all satellites in view is the most fundamental task of any navigation receiver.56 Conventionally, delay and Doppler parameter estimates are extracted in delay-lock and phase-lock tracking loops consisting of dedicated loop software and correlator ASIC channels for each satellite. The resulting pseudorange and carrier phase quantities are then fed to the navigation filter routines, wherein these estimates are combined to produce updated position and velocity. In this traditional setup, predating the availability of fast and inexpensive digital signal processing and reduced instruction set computing (RISC) capable of hundreds of millions of double-precision floating point instructions per second, raw correlator data from a given satellite are processed without reference to state and error from other tracking loops, or from the receiver as a whole.

Fast computing permits statistically optimal validation and weighting of correlator data from all satellites early in the processing chain, based upon the full receiver state model. By taking advantage of inter-satellite path correlations, and by rapidly adapting filter gains to encountered signal amplitude and noise fluctuations, tracking thresholds can be improved on the order of 10 dB, and tracking can be made resistant to spoofing, multipath

56  

Doppler refers to the relative shift of frequency due to satellite-to-user motion.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

capture, and cycle slip. This signal processing approach also can be combined with the inertial aiding techniques above, whereby correlator data, as well as accelerometer and gyroscope data are combined in an optimal fashion.

The development and operational use of GPS receivers with improved integration of signal processing and navigation functions for enhanced performance in jamming and spoofing should be accelerated.

Improved L2 Ionospheric Correction

In a hostile environment where the L1 signal is jammed, PPS receivers will have access only to L2 signals, thus eliminating their ability to perform dual-frequency ionospheric corrections. In such situations, military users must rely on an ionospheric model such as that contained in the broadcast navigation message or on a single-frequency correction process such as DRVID (Difference Range Versus Integrated Doppler). 57

As the GPS signal travels through the ionosphere, the modulating codes (Y and C/A) are delayed by an amount proportional to the inverse square of the frequency. To first order, the carrier phase is advanced by the same amount, producing an effect termed "code-carrier divergence." DRVID is a technique that exploits this effect to compute the change in ionospheric delay over time. In order to determine the total ionospheric delay, an initial delay value must be known. This would work well in a scenario in which a receiver is initialized in a clear environment, that is, outside the region of L1 jamming, and then tracks signals into the jammed region using DRVID to make ionospheric corrections. The primary disadvantage of DRVID is that it relies on continuous carrier tracking, which is not likely to be possible in a high-jamming and possibly high-blockage environment.

Currently, single-frequency receivers employ the 8-parameter Klobuchar model that is contained in the broadcast navigation message. This model is considered to be effective in eliminating approximately 50 percent of the total ionospheric delay with a day-to-day variability of 20 percent to 30 percent. It is suggested that enhancements to this model could improve the performance to the 70 percent to 80 percent level.58 Furthermore, based on the current performance of local-area DGPS, the NRC committee believes that local-area estimates of ionospheric conditions made just outside the jamming region could provide even greater improvements.

57  

P.F. MacDoran, "A First-Principles Derivation of the Difference Range Versus Integrated Doppler (DRVID) Charged-Particle Calibration Method," JPL Space Programs Summary 37-62 II, 31 March 1970.

58  

JA. Klobuchar, "Potential Improvements to the GPS Ionospheric Algorithm." Presentation at the GPS/PAWG Meeting, 14 July 1993, Peterson Air Force Base, Colorado.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Military receivers should be developed that compensate for ionospheric errors when L1 is jammed, by improved software modeling and use of local-area ionospheric corrections.

Possible Interim Operational Procedures

The NRC committee believes that the most significant shortcoming of a GPS denial strategy is the current inability to operate in high levels of enemy jamming, while at the same time denying GPS to an adversary. The implementation of technical enhancements to military user equipment, such as direct Y-code acquisition capability, improved nulling antennas, better inertial aiding capabilities, enhanced signal processors, and improved L2 ionospheric corrections would assist in the optimal solution to this problem. Although the NRC committee believes that these technical capabilities are now available, unfortunately, such capabilities are not currently fielded by the military.

GPS receivers are especially vulnerable during their signal acquisition phase. This weakness is magnified by the inability of most military GPS receivers to acquire the Y-code during periods when the C/A-code is being jammed. Future receivers capable of direct Y-code acquisition will go a long way toward solving this problem. In any event, tactics must be developed and put in place to facilitate acquisition during jamming. Some possible disciplines that can be implemented in the near-term are presented below.

(1)  

Develop military procedures to remove jammers and DGPS stations. As with existing plans to destroy radars in a hostile area, plans and procedures should be developed to remove jammers and DGPS stations.

(2)  

Acquire the Y-code outside the jamming area. Prior to entering the jamming area, the C/A-code can be used to acquire the Y-code. Once the Y-code is obtained, and while still within the active jamming area, PPS receivers should be operated continuously or be re-powered every few hours in order to maintain accurate time. Accurate time will aid in faster, direct reacquisition of the Y-code. This technique can be extended to aircraft-based GPS-guided munitions using low-powered C/A-code retransmissions aboard, or by hardwiring of time-transfer circuits.

(3)  

Review training exercises, procedures, and policy manuals. The current training procedures and policy manuals should be examined to make sure U.S. troops are properly instructed to operate in both hostile jamming and denial jamming environments. For example, ground forces can make use of natural terrain and man-made obstructions to obtain some shielding from ground-based jammers.

Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

(4)  

Schedule denial jamming/spoofing. Tactically, the U.S. military can interrupt denial jamming/spoofing for short time periods, typically 2 to 3 minutes hourly, to assist those friendly forces in need of C/A-code to reacquire the Y-code. These scheduled times would be short and random to prevent hostile troops from taking advantage of interrupted jamming. Dependence upon this technique will diminish as improved training procedures and time discipline techniques are disseminated into the force structure.

(5)  

Develop and utilize C/A-code selective denial techniques that minimize impact upon friendly L1-only military receivers, such as the Plugger. The L4 selective denial analysis of Appendix J suggests a four-part approach to selective denial of C/A-code on the L1 band:

    • apply shaped denial jamming combined with spoofing;
    • use a switchable retrofit bandstop RF front-end filter; and,
    • improve clock discipline, through operator training.
  • Improvement Implementation Strategy

    Because of the relatively long life time of GPS satellites (5 to 10 years) and the length of time required to replace the total constellation of 24 satellites, opportunities for introducing enhancements and technology improvements to the system are limited.

    Figure 3-6 shows the current plan for satellite replacements. According to the GPS Joint Program Office, current plans for the Block IIF contract include 6 short-term, and 45 long-term "sustainment" satellites. As currently planned, the Block IIF satellites will be designed to essentially the same specifications as the Block IIR satellites. The current program and schedule make it possible for another country to put up a technically superior system that uses currently available technology before the United States can do so. Under current planning, the earliest opportunity for an infusion of new technology in the GPS space segment would be after Block IIF, probably sometime after the year 2020.

    As noted throughout this chapter, the NRC committee believes that there are significant improvements that could be made to the system that would not only enhance its performance for civilian and military use, but also make it more acceptable and competitive internationally. One method to incorporate technology in an efficient and timely manner is through a preplanned product improvement (P3I) process. With this type of approach, planned changes and improvements could intentionally be designed into the production of the satellites at specific time intervals.

    Assuming that the first improvements suggested in this report are incorporated in the later half of the Block IIR satellites, additional funding might be required to incorporate changes for the already completed Block IIR satellites. However, the NRC committee believes that the timely improvement in system performance is adequate justification for the additional cost. Recommended improvements to the space segment and the operational control segment are summarized in Tables 3-13 and 3-14.

    Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
    ×

    In addition to the specific recommendations given in this report, the NRC committee also discussed several enhancements that it believes have particular merit and should be seriously considered for future incorporation. These items are discussed in Chapter 4. Although a few enhancements could be included on the Block IIR spacecraft, especially if a P3I program were implemented, most of the enhancements would have to be incorporated in the Block IIF spacecraft design.

    Figure 3-6

    Current Plan for Satellite Replacement. (Courtesy of the GPS Joint Program Office)

    Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
    ×

    Table 3-13 Space Segment Enhancements

    Proposed Enhancement

    Satellite Block

    Implementation Date

    Benefit of Enhancement

    Turn SA to zero.

    Block II/IIA, IIR, IIF

    Immediately

    Approximately 30- meter (2 drms) stand-alone accuracy for civil users.

    Add a new L-band signal (would be usable before the Block IIR constellation is complete).

    Block IIR, IIF

    As soon as possible

    Approximately 12- meter (2 drms) stand-alone accuracy for civil users. Enhanced integrity monitoring.

    Use inter-satellite crosslinks to relay satellite health information and commands.

    Block IIR, IIF

    As soon as possible

    Improve overall system reliability and availability

    Use inter-satellite crosslinks to relay ground-based integrity monitoring information and commands.

    Block IIR, IIF

    As soon as possible

    Improve GPS signal integrity for all users.

    Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
    ×

    Table 3-14 Operational Control Segment Enhancements

    Proposed Enhancement

    Implementation Date:

    Benefit of Enhancement

    Provide more frequent correction updates.

    As soon as possible

    Improve stand-alone GPS accuracy for PPS and SPS users (if SA is off and 48-hour embargo is lifted) by reducing combined clock and ephemeris errors by half.

    Add more monitoring stations.

    As soon as possible

    Improve stand-alone GPS accuracy Improve overall system reliability by allowing prompt detection of satellite anomalies. Allow for uninterrupted tracking of all satellites.

    Improve Kalman Filter and dynamic models.

    Added to 1995 Operational Control Station Request for Proposal

    Improve accuracy by reducing combined clock and ephemeris errors with non-partitioned Kalman Filter (15%) and with improved dynamic model (5%).

    Establish procurement coordination of improved monitor station receivers, computers, and software contracts.

    As soon as possible and in conjunction with the 1995 contract award

    Improve accuracy. Allow for integrity monitoring of C/A-code.

    Reexamine planned Block IIR operation and compare to the accuracy advantages gained by incorporating inter-satellite ranging data in the ground-based Kalman Filter and uploading data at some optimal time.

    Immediately

    Possibly improve accuracy over planned Block IIR operation.

    Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
    ×

    Proposed Enhancement

    Implementation Date:

    Benefit of Enhancement

    Use Block IIR satellite communication crosslinks to the extent possible with the existing crosslink data rate to support on-board satellite health monitoring.

    Block IIR satellites

    Improved reliability and availability Permits more rapid response time by the ground station.

    Use Block IIR inter-satellite communication crosslinks to relay integrity information determined through ground-based monitoring.

    Block IIR satellites

    Permits more rapid response time for integrity monitoring.

    Permanent backup master control station.

    Immediately

    Reduce risk and improve reliability of overall system.

    Provide simulator to test software and train personnel.

    Immediately

    Reduce risk and improve reliability of overall system, improve efficiency of operations.

    Update the operational control segment software using modern software engineering methods in order to permit easy and cost-effective updating of the system and to enhance system integrity. This should be specified in the 1995 OCS upgrade request for proposal.

    Should be specified in 1995 Operational Control Station Request for Proposal.

    Easier to make modifications to software. Reduces cost and complexity

    Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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    Suggested Citation:"3 Performance Improvements to the Existing GPS Configuration." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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    The Global Positioning System (GPS) is a satellite-based navigation system that was originally designed for the U.S. military. However, the number of civilian GPS users now exceeds the military users, and many commercial markets have emerged. This book identifies technical improvements that would enhance military, civilian, and commercial use of the GPS. Several technical improvements are recommended that could be made to enhance the overall system performance.

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