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

Chapter: 2 GPS Applications and Requirements

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Suggested Citation:"2 GPS Applications and Requirements." 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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2
GPS Applications and Requirements

Introduction

GPS specifications were originally developed by the DOD (Department of Defense) in the late 1960s with the primary objective of satisfying military navigation requirements. A secondary objective was to provide a separate, less accurate signal for both military and civilian use. This signal, described in Appendix C, and known as the Standard Positioning Service (SPS), was intentionally degraded in accuracy (100 meters, 2 drms) to avoid its exploitation by potentially unfriendly users.

As the GPS satellite constellation expanded and was eventually completed in 1993, the use of the freely available SPS signal for civil applications also continuously expanded. GPS is now used for positioning, navigation, and timing applications in a number of civil and commercial activities related to aviation; maritime commerce and recreation; land transportation; mapping, surveying, and geodesy; scientific research; timing and telecommunications; and spacecraft. Each of these broadly defined civilian user categories, along with military applications, is discussed in this chapter.

Many of the innovative civilian applications that this chapter will address were not foreseen by the original designers and developers of GPS and cannot be accomplished without augmenting and/or enhancing the stand-alone capabilities of the system as currently configured. As a result, differential correction methods and user equipment integrated with other positioning technologies, as described in Appendix C, have been utilized to meet the requirements of many of these applications. Within this context, there have been no requirements imposed on the basic GPS by civilian users to date beyond the assurance that the basic SPS signal-in-space will remain freely available at its currently defined accuracy level.1 Users have taken this signal and adapted it to their applications. The basic GPS has therefore become a ''dual-use" system, which is still designed to meet the requirements of only a single user, the Department of Defense.2

1  

This official U.S. government policy is currently reiterated every 2 years in the Federal Radionavigation Plan.

2  

The term "dual-use" usually refers to use by both the military and civilians.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Although the continued existence of GPS as a dual-use system clearly requires some trade-offs between civilian utility and national security, the NRC committee has concluded from its deliberations that because GPS provides tremendous benefits to both civilian and military users, as the remainder of this chapter will clearly illustrate, it should firmly remain a dual-use system. From the committee's perspective, recognition that GPS is truly a dual-use system brings with it the responsibility of meeting the requirements of all users to the highest degree possible. This implies that the system must be designed to the specifications of both civilian and military requirements. Many nonmilitary users of GPS have requirements that have been validated by standard-setting bodies and federal agencies that can now only be achieved through the additional cost of differential GPS (DGPS). Because human safety is an important consideration for many of these applications, a specified level of accuracy is not the only requirement. Integrity, availability, and resistance to RF (radio frequency) interference (both intentional and unintentional), as defined in Appendix C, are of significant importance as well. The sections that immediately follow discuss these requirements for each user category.

The task given to the NRC committee by Congress also recognized the dual-use nature of GPS and the trade-offs that exist between civil and military utility when it asked the following questions: "What augmentations and technical improvements to the GPS itself are feasible and could enhance military, civilian, and commercial use of the system?"; and, "What are the implications of security-related safeguards and countermeasures for the various classes of civilian GPS users?" These questions are examined in the remainder of the chapter by determining the challenges that currently exist for full utilization of GPS in each user community, including challenges that are related to Selective Availability (SA) and Anti-Spoofing (A-S). Although some of these challenges relate to the limitations of associated technologies and technology policies, findings in this chapter reveal that the biggest challenge for most users is meeting the requirements of a given application through augmentation of the GPS SPS. It stands to reason, therefore, that improving the basic capabilities of GPS and the freely available SPS signal will enhance the ability of civilian users to meet their requirements more easily, more cost-effectively, and in some cases, without augmentation or enhancement from DGPS or other positioning technologies. Improvements to the basic GPS can be made that will improve the military's ability to meet its requirements as well. Specific technical recommendations that would achieve this goal and address the tasks assigned to the NRC committee are discussed in detail in the next chapter.

GPS Military Applications

Although the overall use of GPS in the civilian sector has grown much faster than military usage, the system was designed with military requirements in mind, and the importance of the system to national security has not diminished. GPS is more accurate than any other radionavigation or positioning technology developed by the DOD, and is beginning

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

to replace all other operational systems.3 The coalition military forces demonstrated the effective use of GPS for many of these proposed applications during the Persian Gulf War, despite the fact that the GPS constellation consisted of only 16 satellites at the time. This limited three-dimensional coverage of the Persian Gulf region to 18 hours per day. Another limiting factor was the small number of P-code military receivers in the DOD inventory at the time of the conflict. This prompted a National Command Authority decision to turn SA to zero during the war and led to the DOD's purchase of thousands of civilian GPS receivers, which became known as "sluggers".4 In addition to this official procurement, many units and individuals deployed to the Persian Gulf ordered their own GPS receivers directly from vendors and manufacturers.5

Current and Future Applications and Requirements

The most common use of GPS during the Persian Gulf War, and perhaps the most critical, was for land navigation. U.S. Army tanks and infantry relied heavily on GPS to avoid getting lost during movements to various destinations in the featureless desert. GPS also was used by coalition forces for en route navigation by aircraft, helicopter search and rescue, marine navigation, and even munitions guidance in the case of the U.S. Navy's Standoff Land Attack Missile (SLAM).6

The use of GPS for precision-guided munitions such as the SLAM will increase in the future. The U.S. military currently has, or is developing, eight additional types of precision land attack weapons that utilize GPS integrated with inertial navigation systems for mid-course guidance.7 Another important GPS application under consideration is the

3  

The DOD plans to phase out use of Loran-C and Omega in 1994, Transit in 1996, and land-based navigation aids by 2000, depending on the progress of GPS installation and integration. Civilian use of these systems, however, may continue. Source: Radionavigation System Users Conference held in Washington D.C. on November 9-10, 1993, (unpublished).

4  

The "slugger" or Small Lightweight GPS Receiver is a Trimble Navigation TRIMPACK, three-channel receiver that utilizes the L1, C/A-code to provide three-dimensional navigation capability. More than 10,000 receivers were purchased by the DOD from Trimble Navigation and other receiver manufacturers during the Persian Gulf War.

5  

Bruce D. Nordwall, "Imagination Only Limit to Military, Commercial Applications for GPS," Aviation Week & Space Technology, 14 October 1991, p. 60.

6  

Joseph Wysocki, "GPS and Selective Availability—The Military Perspective," GPS World, July/August 1991, pp. 38-43.

7  

These eight weapons include: the Tomahawk Block III and IV cruise missile; the Tri-Service Stand-Off Attack Missile; the Joint Direct Attack Munition; the Joint Stand-Off Weapon; the GBU-15 precision glide bomb; the AGM-130, a powered version of the GBU-15; and finally, the ATACMS ballistic missile. Source: J.G. Roos, "A Pair of Achilles Heels: How Vulnerable to Jamming are U.S. Precision-Strike Weapons?" Armed Forces Journal International, November 1994, p. 22.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

use of GPS for the precision delivery of cargo by parachute or paraglider. For this application, GPS must be capable of providing steering commands to a reefing system to steer the parachute or paraglider to the desired landing point.

Combat search and rescue is another important function for which the use of GPS is increasing. Although GPS is already used for navigation by helicopters and other aircraft involved in combat search and rescue, it also will be used in the future to determine the exact location of downed aircrew members. By combining GPS with space-based communications capabilities, individuals can be found quickly, saving lives, time, and money. Communications capabilities would allow the location of aircraft, helicopters, and tanks to be monitored in real time, reducing casualties by friendly fire. Further, if GPS and communications capabilities are combined with guidance systems, unmanned aerial vehicles could be used for surveillance of target areas.

Tables 2-1 through 2-3 represent an extensive list of the military's positioning and navigation applications and their requirements.

Challenges to Full GPS Utilization

Accuracy and Integrity

The shaded cells in Tables 2-1 through 2-3 point out positioning and navigation requirements that cannot be met with the current 16 meter (SEP) specified accuracy8, or 8-meter (CEP) derived accuracy9, of the GPS PPS (Precise Positioning Service). Presumably, many of these requirements are currently being met by other guidance systems, such as highly accurate inertial navigation systems and terminal seekers, and other radionavigation systems, such as the microwave landing system. If these applications were to rely on GPS alone in the future, their accuracy requirements could only be met with some form of DGPS or a significantly improved PPS.

Some of the aviation applications listed in Table 2-1 also have specified integrity requirements. These requirements cannot be met with the PPS as currently configured.

8  

SEP, or spherical error probable, represents an accuracy that is achievable 50 percent of the time in all three dimensions (latitude, longitude, and altitude). PPS accuracy is normally represented in this manner. The 2 drms PPS specified accuracy value is 21 meters SEP, as shown in Figure C-7 in Appendix C.

9  

CEP, or circular error probable, represents an accuracy that is achievable 50 percent of the time in two dimensions (latitude and longitude). Most military accuracy requirements are defined in this manner. CEP, and other positioning accuracy definitions are discussed in greater detail in Appendix D.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-1 Military Aviation and Precision-Guided Munitions Applications and Requirementsa

 

Application

Accuracy

Integrity

 

Resistance to RF Interference

 

 

 

1 minus PHE times PMDb

Time to Alarm

 

Aviationb

Low-level Navigation and Air Drop

50.0 m (2 drms)

0.999

10 sec

High

 

Non-precision Sea App/Landings

12.0m (2 drms)

0.999

10 sec

High

 

Precision App/Landings Unprepared Surface

12.5m (2 drms)

0.999

6 sec

High

 

Precision Sea App/Landings

0.6m (2 drms)

0.999

6 sec

High

 

Amphibious and Anti-submarine Warfare

50.0 m CEP

Not specified

Not specified

High

 

Anti-air Warfare

18.1 m CEP

Not specified

Not specified

High

 

Conventional Bombing

37.5 m CEP

Not specified

Not specified

High

 

Nuclear Bombing

75.0 m CEP

Not specified

Not specified

High

 

Close Air Support/Interdiction

9.0 m CEP

Not specified

Not specified

High

 

Electronic Warfare

22.5 m CEP

Not specified

Not specified

High

 

Command, Control & Communications

37.5 m CEP

Not specified

Not specified

High

 

Air Refueling

370.0 m CEP

Not specified

Not specified

High

 

Mine Warfare

16.0 m CEP

Not specified

Not specified

High

 

Reconnaissance

18.1 m CEP

Not specified

Not specified

High

 

Magnetic and Gravity Survey

20.0 m CEP

Not specified

Not specified

High

 

Search & Rescue and Medical Evacuation

125.0 m CEP

Not specified

Not specified

High

 

Mapping

50.0 m CEP

Not specified

Not specified

High

Precision-guided Munitions

Precision-guided Munitions

3.0 m CEP

Not specified

Not specified

High

a. Availability and continuity of service requirements are not specified for military aviation and precision-guided munitions applications.

b. This measure relates the probability that a hazardously misleading error will occur (PHE) and the probability that this error will go undetected (PMD).

c. Peacetime requirements for the en route through Category I approach and landing phases of flight are identical to FAA requirements.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-2 Naval Applications and Requirementsa

 

Application

Accuracy

Resistance to RF Interference

En route Navigation

Pilotage & Coastal Waters

72.0 m CEP

High

 

Inland Waters

25.0 m CEP

High

 

Open Waters

2400.0 m CEP

High

 

Rendezvous

380.0 m CEP

High

 

Harbor

8.0 m CEP

High

Mine Warfare

Swept Channel Navigation & Defensive Mining

16.0 m CEP

High

 

Offensive Mining

50.0 m CEP

High

 

Anti-mine Countermeasures

<5.0 m CEP

High

 

Geodetic Reference Guide (WGS-84)

128.0 m CEP

High

Special Warfare

Airdrop

20.0 m CEP

High

 

Small Craft

50.0 m CEP

High

 

Combat Swimming

1.0 m CEP

High

 

Land Warfare & Insertion/Extraction

1.0 m CEP

High

 

Task Group Operations

72.0 m CEP

High

Amphibious

Beach Surveys

185.0 m CEP

High

Warfare

Landing Craft

50.0 m CEP

High

 

Artillery & Reconnaissance

<6.0 m CEP

High

Surveying

Hydrographic

<5.0 m (2 drms)b

High

 

Ocean & Geophysical Deep Ocean

90.0 m (2 drms)

High

 

Oceanographic

100.0 m (2 drms)

High

a. Availability, integrity, and continuity of service requirements are not specified for naval applications.

b. This requirement can currently be met with data post-processing.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-3 Military Land Applications and Requirementsa

 

Application

Accuracy

Resistance to RF Interference

 

Chemical Warfare

100.0 m CEP

High

Engineer

Mine Neutralization

100.0 m CEP

High

 

Mine Dispensing & Gap Crossing

50.0m CEP

High

Field Artillery

MLRS

20.0 m CEP

High

 

Howitzer

175 m CEP

High

 

Mortars

50.0 m CEP

High

 

Fist-V & Forward Observer

30.0 m CEP

High

 

Artillery and Mortar Radar

10.0 m CEP

High

 

Infantry & Armorb

100.0 m CEP

High

 

Missile Munitions

93.0 m CEP

High

 

Signal

15.0 m CEP

High

 

Special Operations Forces

30.0 m CEP

High

 

Intelligence Electronic Warfare

20.0 m CEP

High

 

Ordnance

84.0 m CEP

High

Air Defense Artillery

Patriot

10.0 m CEP

High

 

Hawk

40.0 m CEP

High

a. Availability, integrity, and continuity of service requirements are not specified for military land transportation applications.

b. The Infantry & Armor category also includes transportation, soldier support, military police, and quartermaster.

Anti-Jam and Anti-Spoof Capability

Although the "Resistance to RF Interference" column in Tables 2-1 through 2-3 does not include quantitative values, a high level of resistance to RF interference is a critical requirement for most military applications.10 For the military, the primary interference concerns are deliberate jamming and spoofing by an adversary or by our own forces. In future conflicts, a potential enemy also will be utilizing the capabilities of GPS and DGPS against U.S. and allied military forces. In order to deny this use, friendly forces must have the ability to eliminate an adversary's use of GPS signals without impacting the effectiveness of their own user equipment. This dictates that military GPS receivers also must be capable of continued operation in an environment populated with both U.S. and enemy jammers. Therefore, GPS-based navigation systems used on aircraft, ships, land vehicles, and precision-guided munitions must possess one or more of the following capabilities:

10  

Quantifiable values for resistance to RF interference are given in decibels (dB), and relate to the ratio of jammer power to signal power (J/S). These values are very specific to a given mission and operational environment, making a generic J/S requirement for a given application difficult to determine.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
  • have sufficient jamming-to-signal ratio strength to navigate through the jamming environment successfully;
  • be able to null out the jamming signal; and/or
  • have an alternative to GPS for navigating through the jamming environment.

The military currently relies on its SA and A-S (Anti-spoofing) security procedures to deny full GPS accuracy to the enemy while maintaining the use of a highly accurate spoof resistant signal. Anti-jam antennas and antenna electronics also are deployed on many weapons systems to provide increased jam resistance, and integrated GPS/inertial navigation systems provide a means of navigating to a target in spite of successful jamming. None of these procedures and technical measures, however, can be considered the final solution to the military's requirement to simultaneously use GPS and deny its use to the enemy. A number of candidate improvements in this regard are presented in the next chapter.

Findings

The GPS PPS meets most of the military's positioning and navigation accuracy requirements, although some applications require accuracy and integrity that is beyond the capability of the PPS as currently configured.

The anti-jamming and anti-spoofing capabilities of military GPS user equipment are critical to successful mission completion in a battlefield environment characterized by both U.S. and enemy spoofers and jammers.

GPS Aviation Applications

Despite the fact that investigations into the use of satellites for civil aviation applications have been conducted for over 20 years, the concept was not considered financially or technically feasible until the development of GPS.11 Instead, a large number of ground-based radionavigation systems have been relied upon around the world for air navigation services, and ground-based air traffic controllers have utilized radar, voice position reporting, and visual sightings for aircraft surveillance.12 It now appears that a

11  

Federal Aviation Administration (FAA), FAA Satellite Navigation Program Master Plan. FAA Research and Development Service, Satellite Program Office (ARD-70), 15 February 1993, p. 2.

12  

Existing ground-based radionavigation systems include NDBs (Non-Directional Beacon), VORs (VHF Omni-directional Range), VOR/DMEs (VORs with Distance Measuring Equipment), TACANs (Tactical Air Navigation), and VORTACs (combined VORs and TACANs). Other systems include the Instrument Landing System (ILS), used for precision approach and landing, and Loran-C and Omega, both of which are used for en route navigation. Each of these systems is described in detail in the 1992 Federal Radionavigation Plan.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Global Navigation Satellite System (GNSS), based on GPS and additional satellite augmentations, could eventually replace most of these ground-based systems.

Current and Future Applications and Requirements

Civilian pilots have been utilizing GPS in uncontrolled airspace for applications such as crop dusting, aerial photography and surveying, search and rescue, and basic point-to-point navigation for some time.13 On June 9, 1993, the Federal Aviation Administration (FAA) approved GPS for supplemental use in the domestic, oceanic, terminal, and non-precision approach phases of flight in controlled airspace as well. This supplemental use required that another navigation source, such as a ground-based radio aid, must still be monitored while using GPS as the primary system. Once initial operating capability was declared for GPS by the DOD and DOT (Department of Transportation) on December 8, 1993, the monitoring of another navigation system for integrity purposes became unnecessary, provided that the GPS receiver utilized meets the FAA's TSO C-129 criteria for Receiver Autonomous Integrity Monitoring (RAIM).14 In addition, traditional navigation sources such as VORs and TACANs must still be operational, and their associated receiver equipment must be on board the aircraft as a backup. Several GPS receivers have already been certified under the FAA's TSO C-129 criteria.

The use of GPS as the primary means of navigation for the domestic en route through non-precision approach phases of flight will require better availability and continuity of service (reliability) than is currently available from the stand-alone system. Phase I of the FAA's Wide Area Augmentation System (WAAS), which is scheduled to be in place by 1997, will make this possible. Table 2-4 contains the quantitative performance requirements that the WAAS is being designed to meet.15

In the near future, the FAA hopes that GPS also will be used for Category I precision approaches. Precision approaches are required when the weather conditions at a given airport reduce the ceiling, or height of the base of a cloud layer, and the visibility, or the distance a pilot can see visually, to levels that are below non-precision approach criteria.16 Phase II of the FAA's WAAS implementation, scheduled for completion in 2001, will improve GPS-derived accuracy enough to allow the system to be used for these types of approaches. This increased accuracy requirement, which was also derived from the WAAS request for proposal (RFP), is included in Table 2-4.

13  

In uncontrolled airspace, pilots are not in direct communications with air traffic controllers, are responsible for their own navigation, and must be able to avoid terrain and collisions with other aircraft visually.

14  

RAIM is discussed in the next chapter, and is further explained in Appendix C.

15  

Federal Aviation Administration. Wide Area Augmentation System (WAAS), Request For Proposal, DTFA01-94-R-21474.

16  

Category I approaches can be flown when the visibility is no less than 0.81 kilometers (0.5 miles), and the ceiling is no lower than 61 meters (200 feet).

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Testing by the FAA and several contractors is currently underway to determine the feasibility of also using GPS to conduct Category II and III approaches and landings, and the results to date have been very promising. These approaches are flown when the weather conditions at an airport are even worse than those described previously for Category I.17 As can be expected, the accuracy, integrity, and continuity of service requirements are stricter than those for Category I landing systems, and therefore, the concepts currently under development utilize local-area differential GPS augmentations, rather than the WAAS. The requirements for Category II and III, which were derived from the Federal Radionavigation Plan and existing International Civil Aviation Organization (ICAO) requirements for instrument landing systems (ILS), are listed in Table 2-4.18

GPS also shows promise for use in Traffic Alert/Collision Avoidance Systems (TCAS) and Automatic Dependent Surveillance (ADS) systems. TCAS is already used by U.S. airlines and by many airlines in Europe.19 Testing of an updated TCAS, which broadcasts an aircraft's position and velocity derived from GPS on the existing Mode-S datalink, has proven to be more accurate than the existing system.20 The requirements for this application are listed in Table 2-4.

ADS systems, which are still under study and development, would automatically broadcast an aircraft's GPS-derived position to the air traffic management (ATM) system via geostationary communications satellites in oceanic airspaces, and via terrestrial-based communications links in domestic airspace.21 This would allow for more efficient ocean crossings than are currently possible using the existing ATM reporting system. ADS would also be useful in the domestic en route and terminal phases of flight, where current aircraft separation is primarily the responsibility of air traffic controllers who utilize secondary surveillance radars. ADS systems are also being considered for monitoring the land-based operations of an airport, such as aircraft taxiing, and service-vehicle collision avoidance. The requirements listed for ADS in Table 2-4, which are based on current radar-based surveillance requirements, should be considered preliminary because the FAA is in the early phases of studying how to use GPS in performing the surveillance function.

17  

For example, a properly equipped aircraft can fly a Category IIIB approach when the ceiling is below 15 meters (50 feet) and the visibility is between 50 and 200 meters. Source: Federal Aviation Administration, FAA Advisory Circular No. 120-28C: Criteria for Approval of Category III Landing Weather Minima, 9 March 1984.

18  

These requirements are currently under review and may be revised due to an emerging concept known as required navigation performance (RNP). See: R. J. Kelley and J. M. Davis, "Required Navigation Performance (RNP) for Precision Approach and Landing with GNSS Application," Navigation: Journal of the Institute of Navigation 41, no. 1 (1994): pp. 1-30.

19  

The current TCAS configuration uses a data link known as Mode-S to measure the vertical separation between two aircraft in close proximity to one another. Measurements that are determined to be too close by the TCAS software set off an alarm that warns the flight crew and allows them to take action.

20  

"FAA Redirects TCAS-3 Effort," Aviation Week and Space Technology, 27 September 1993, p. 37.

21  

The exact method of transmission in U.S. domestic airspace has not yet been determined.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-4 GPS Performance Requirements for Aviation Applicationsa

 

Application

Accuracy (2 drms)

Integrity

 

Availability

Continuity of Service

Resistance to RF Interference

 

 

 

1 minus PHE times PMDb

Time to Alarm

 

 

 

Navigation

En route Oceanicc

23.0 km

Not Specified

30.0 s

99.977%

Not Available

High

 

En route to Non-Prec. App/Landing

100.0 m

1-1 x10-7 per hourd

8.0 s

99.999%

1-1x 10-8 per hour

High

 

CAT I App/Landing

7.6 m

1-4 x 10-8 per app.

5.2 s

99.9%

1-5.5x10-5 per app.

Very High

 

CAT II App/Landing

1.7 m (vertical)

1-0.5 x 10-9 per app.

2.0 s

Not specified

1-2x10-6 per 15 sec.

Very High

 

CAT III App/Landing

0.6-1.2 m(vertical)

1-0.5 x 10-9 per app.

2.0 s

Not specified

1-2x10-6 per 15 sec.

Very High

Survei- llance

TCAS

14.4 me

Not Specified

Not Spec.

Several daysf

Essential equip.g

Installed equipmenth

 

Oceanic ADS

Not specified

Not specified

Not spec.

Not specified

Not specified

Not specified

 

Domestic ADS

200.0 mi

Not specified

Not Spec.

99.999%i

Not specified

Very High

 

Surface Surveillance

12.0 m (resol.)j

Not specified

Not spec.

99.87%j

Not specified

Very High

a. Unless otherwise annotated, GPS aviation requirements were provided by the MITRE Corporation.

b. This measure relates the probability that a hazardously misleading error will occur (PHE) and the probability that this error will go undetected (PMD).

c. Source of en route oceanic requirements: U.S. Department of Commerce, National Telecommunications and Information Administration, A Technical Report to the Secretary of Transportation on a National Approach to Augmented GPS Services, NTIA Special Publication 94-30, November 1994, p. 12. It is likely that the accuracy requirement will become significantly more stringent in the future to allow tighter spacing between aircraft.

d. This number is equivalent to 0.9999999 or 99.99999 percent.

e. Based on current TCAS specifications.

f. According to airline minimum equipment list (MEL) practice approved by FAA certification.

g. Based on reliability certification for essential equipment.

h. Must meet installed equipment test. Otherwise unspecified.

i. Based on current radar surveillance.

j. Based on Airport Surface Detection Equipment-3 specifications, which require the resolution of two targets separated by 12 meters.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Challenges to Full Utilization of GPS

Selective Availability and Anti-Spoofing

When SA dithering of the GPS signals is employed, the DGPS corrections required to circumvent the resulting accuracy degradation must keep up with the dithering rate. This is not a problem for local-area DGPS, since the local correction broadcast usually has a sufficient data rate to provide timely corrections. The space-based WAAS, however, broadcasts its differential corrections as part of the navigation message data carried by a GPS-like L1 signal. SA has a negative effect on this signal format; the high correction data rate necessary to keep up with the SA dither rate constrains the flexibility of providing additional information on this navigation message.

SA also decreases navigation availability and integrity monitoring availability for SPS users because the ranging errors it introduces require better satellite geometry for the specified 100-meter level of navigation accuracy. This sometimes rules out the operational use of GPS, especially when there are failed satellites present, and significantly reduces the effectiveness of RAIM.22

The employment of A-S, which overlays the Y-code on L2 rather than the P-code, denies the second frequency needed for real-time ionospheric correction to all but authorized PPS users. Without dual-frequency receivers on board aircraft, the WAAS needs to employ a large network of ground sites to collect ionospheric data, that will be interpolated by the user to estimate the ionospheric delay in the pseudorange measurements. The disadvantages of this constraint are a decrease in the vertical positioning accuracy of wide-area DGPS, and an increase in the size, complexity, and cost of the WAAS ground network.

Resistance to Radio Frequency Interference

A-S also limits an SPS receiver's ability to deal with RF interference from known sources such as the third harmonic of some UHF (ultra-high frequency) television channels and airborne VHF (very-high frequency) transmitters. Solutions to the potential problem of RF interference must be found if GPS is to become the primary navigation and surveillance system for aviation, and organizations such as the RTCA are actively studying the issue. Resistance to interference can be greatly improved through the use of dual-frequency receivers that can track the code on both L1 and L2 because it is unlikely that interference from a single source will simultaneously affect both frequencies. As discussed in Appendix G, access to the wider bandwidth of the P-code, which is approximately 20 MHz (versus 2 MHz for the C/A-code), also would increase resistance to interference and reduce vulnerability to multipath.

22  

An analysis of the effects of SA on RAIM was conducted for this study by the MITRE Corporation. The results are presented in the next chapter, and the full analysis can be found in Appendix F.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Operational Procedures

Currently GPS is being approved by the FAA to operate under the same procedures used for existing navigation aids. If GPS is to provide more efficiency to present operations, however, there have to be accompanying changes in technical infrastructure and institutional culture. The major benefits of GPS navigation and surveillance will only be achieved when its coverage and accuracy are exploited to enable aircraft to fly user-preferred flight paths with minimal command and control from air traffic controllers. The benefits and enabling factors for these new operational procedures are discussed below.23

Most instrument flights are constrained to specified paths that facilitate the air traffic management (ATM) system's human-controlled separation of aircraft. Since GPS-equipped aircraft will be able to fly any desired flight path with high accuracy, users (especially air carriers) can potentially gain significant fuel and time efficiencies by having the ability to fly the most advantageous routing from one destination to another while independently amending their flight path as necessary to avoid congestion and potential conflicts with other aircraft. In order to make this change in procedure possible, as a minimum, the following enabling factors will have to be in place:

  • automation that can cope with numerous aircraft flight path crossings, unlike the present essentially linear flow of traffic;
  • a changed ATM culture that accepts a high level of automation for conflict prediction and resolution, and allows more autonomy in the cockpit for route selection and aircraft separation;
  • highly reliable flight management systems aboard all aircraft to ensure that the same airport and route information is available to each aircraft flying in the national airspace system;
  • two-way data links that provide an interface between ATM and aircraft flight management systems for such purposes as automatic negotiation of flight clearances (with pilot approval) and updates to airport and air route databases; and,
  • cockpit display of traffic information to allow all aircraft to provide self-separation and enhanced collision avoidance.

23  

More information on this concept, known as ''free flight", can be found in the following document: RTCA, Inc., Report of the RTCA Board of Directors' Select Committee on Free Flight (Washington, D.C., 18 January 1995).

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Findings

The implementation of the FAA's WAAS should enable all navigation requirements through Category I precision approach to be met with wide-area DGPS. Category II/III approaches and landings will still require local DGPS augmentations.

The presence of SA and A-S increases the cost and complexity of WAAS and limits the effectiveness of RAIM.

The full navigation and surveillance capabilities of GPS will not be realized until air traffic management procedures and related technical systems are revised and modernized. In addition, GPS requirements based on the simultaneous use of the system for both navigation and surveillance must be established.

Radio frequency interference with GPS signals could prove to be a significant problem for aviation applications. Techniques to mitigate its effects, such as the use of a second GPS frequency, must be explored.

Maritime Use Of GPS

In general, mariners use GPS for either navigation or positioning, although GPS has recently been applied to surveillance applications as well. It is important to define these broad categories of use before discussing more specific marine GPS applications and their requirements.

Marine navigation can be defined as the process of planning, recording, and controlling the movement of a craft or vessel from one place to another. During this process, there are generally concerns regarding commerce, expediency in transport, human safety, and environmental protection. When a vessel is navigating, it is often in situations where it is committed to a course of action based on these concerns. This has led to specific requirements for accuracy, integrity, availability, and area of coverage.

Marine positioning usually refers to activities such as hydrographic surveying, locating underwater objects, or other activities on the water where a vessel is not traversing a path to a destination. As with marine navigation, marine positioning generally has well-defined accuracy requirements, but because of the amount of time on station, integrity requirements can often be relaxed. Because of the cost of the resources used in conducting some positioning operations, however, lack of availability can have a severe economic impact.

In an effort to avoid the economic and environmental costs of vessel collisions and groundings, many of the nations ports and harbors are being equipped with surveillance systems known as vessel traffic services (VTS), which monitor the course and speed of ships, just as the air traffic control system tracks the flight paths of aircraft. Some of these systems operate with personnel similar to air traffic controllers who monitor and advise ships and

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

pilots. Others are more automated and rely on the ability of ships to monitor themselves.24 In both cases, GPS and DGPS are used to provide accurate positioning information that is integrated with other positioning, communications, and computing technologies.

Current and Future Applications and Requirements

The navigational use of GPS has evolved slowly in the maritime community, due in part to the lack of continuous service available from GPS until initial operational capability was declared in December 1993. Commercial shippers are now beginning to equip their vessels with GPS for navigation, however, and in April 1994 the Coast Guard declared that a GPS receiver meets the requirements for carriage of electronic position fixing devices as prescribed under US CFR Title 33, Part 164, section 164.41. The U.S. Coast Guard also has established a DGPS network that will eventually provide coverage to all U.S. coastal areas, ports, harbors, and inland waterways. Some commercial shippers have begun to experiment with DGPS capability as well.

Use of GPS among recreational boaters is becoming widespread. The low cost ($400-$2,000) of marine GPS equipment has made it an attractive alternative to other systems such as Loran-C and Transit. Recreational use of DGPS, however, has been limited to applications, such as yacht racing, that require improved position and velocity information. The additional cost of the beacon receiver used to receive the Coast Guard DGPS corrections has limited the recreational use of DGPS.

Maritime navigational requirements are well documented in the Federal Radionavigation Plan.25 It breaks the marine navigation problem down into several distinct phases that relate to different geographical considerations. These are oceanic, coastal, harbor/harbor approach, and inland waterway. The oceanic and coastal requirements have been derived from the limitations of systems that have been used for these phases of navigation for some time, such as celestial plotting techniques, Loran-C, and Transit, whereas the harbor/harbor approach requirements were developed through research on ship maneuvering and the human factors involved in piloting large commercial vessels. Table 2-5 lists the current GPS requirements for the oceanic, coastal, and harbor/harbor approach phases of navigation.

Because official inland waterway requirements have not yet been adopted, the values shown in Table 2-5 for this phase of navigation should be considered as tentative estimates. Recent field trials of the Coast Guard's DGPS service, however, have demonstrated sufficient accuracy to satisfy the Army Corps of Engineers inland waterway construction requirement of 6 meters (2 drms). It is likely that this same system also could satisfy inland waterway navigation requirements. The Coast Guard's goal for their DGPS service is to

24  

An example of the former type system would be the U.S. Coast Guard's ADS (automatic dependent surveillance) system now in use in Prince William Sound, Valdez, Alaska. The private-sector VTS being developed for Tampa Bay by Tampa Bay VIPS, INC., is a good example of the latter type.

25  

Section 2.4 Civil Marine Radionavigation Requirements, pages 2-24 through 2-34.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

achieve 3-meter (2 drms) accuracy for these operations, and provide the needed integrity and availability for navigation as well.

In contrast to navigation, GPS became practical for many positioning applications as soon as there were a few hours of satellite coverage each day. The Coast Guard, for example, began positioning navigation buoys with DGPS in 1990, when there were only 12 hours of GPS coverage per day. Other applications include the positioning of offshore oil platforms by petroleum companies and hydrographic surveying conducted by the National Oceanic and Atmospheric Administration (NOAA) to develop nautical charts. These users often augment the GPS standard positioning service with DGPS services provided by the Coast Guard or private sector companies.

Requirements for positioning applications are not well documented. Generally, positioning applications strive to achieve the best accuracy possible within a user's practical limitations, which are often related to time and cost. A system that satisfies marine navigation requirements for accuracy often satisfies some marine positioning requirements as well. Many high frequency, very-high frequency, ultra-high frequency and microwave systems have been developed and successfully used over the years to provide high accuracy positioning information in specific geographic areas. With DGPS coming on line and meeting the harbor/harbor approach requirement of 8 meters (2 drms), however, the need for these other systems has waned.

Marine surveillance systems, such as Coast Guard and commercial VTS, require accurate velocity data in addition to accurate positioning information. The continuous broadcast of velocity from each ship in a given VTS coverage area will allow pilots and VTS operators to take evasive action when two or more ships are approaching the same location at a fast closure rate. DGPS currently yields velocity accuracy on the order of 0.1 nautical miles per hour, which is sufficient for this application.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-5 Requirements for Maritime Applicationsa

 

Application

Accuracy (2 drms)

Integrity

Availability

Coverage

Resistance to RF Interference

 

 

 

Time to Alarmb

 

 

 

Navigation

Oceanic

1800-3700 m (1-2 naut. mi.)

Not specified

99.0%

Global

Moderate

 

Coastal

460 m (0.25 naut. mi.)

Not specified

99.7%

U.S. Coasts

Moderate

 

Harbor/ Harbor Approach

8.0-20.0 m

6-10 s

99.7%

Harbors and Approaches

High

 

Inland Waterwaysc

3.0 m

6-10 s

Not yet defined

Inland Waterwayc

High

 

Recreational Boatingc

10.0 m

Not specified

99.9%

Coasts and Inland Waterways Nationwide

Moderate

Surveillance

Vessel Traffic Servicesd

10.0 m

Not specified

99.9%

Local

Very High

Positioning

Resource Exploration

1.0-3.0 m

Not applicable

99.0%

Global

Moderate

a. Integrity (1 minus PHE times PMD) and continuity of service requirements are not defined for maritime applications. Other maritime GPS requirements originate from the Federal Radionavigation Plan, pp. 2-26 through 2-28 unless annotated otherwise.

b. Source of time-to-alarm requirements: U.S. Department of Commerce, National Telecommunications and Information Administration, A Technical Report to the Secretary of Transportation on a National Approach to Augmented GPS Services, p. 11.

c. These values are not firmly established requirements. They are estimated useful values determined by the committee.

d. Source of Vessel Traffic Services Requirements: D. H. Alsip, J. M. Butler, and J. T. Radice, Implementation of the U.S. Coast Guard's Differential GPS Navigation Service (Washington, D.C.: USCG Headquarters, Office of Navigation Safety and Waterway Services, Radionavigation Division, 28 June 1993).

Challenges to Full Utilization of GPS

Associated Technologies

The positioning and navigation capabilities of GPS and DGPS do not solve the user's problems by themselves. For coastal and oceanic navigation, a GPS position (latitude and

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

longitude) can be directly plotted on a paper nautical chart in the traditional fashion. This procedure often limits the accuracy of a position solution, not because of GPS errors but because of chart errors and plotting errors. More precise charts and plotting methods are therefore required in order to take advantage of the accuracy of GPS and DGPS.

The current state of the art in marine navigation is the Electronic Chart Display Information System (ECDIS), which is capable of displaying information from nautical publications, electronic navigational charts, and navigation sensors simultaneously. The International Maritime Organization (IMO) has recently drafted an assembly resolution on performance standards for ECDIS. Final approval of this draft document at the next meeting of the IMO assembly in 1995 would represent the first step in replacing paper nautical charts with computer-generated electronic charts for commercial vessel navigation. Under current IMO regulations, all merchant vessels are required to carry and use up-to-date paper charts. Once ECDIS's built to IMO standards are in use, vessels will not be burdened by a requirement to maintain paper charts and will have superior navigation capabilities in coastal and harbor areas.

A key issue for the timely implementation of ECDIS is the availability of digital data for the production of Electronic Navigational Charts. NOAA has begun the process of developing digital databases for electronic nautical charts, but faces serious resource limitations in this endeavor.26 Existing hydrographic surveys are often very old, and must be updated before accurate digital data can be developed from them. At today's rate of progress, NOAA expects that it will take 5 to 10 years to digitize paper charts of U.S. waters.27 Until this task is completed, charts will continue to be a source of marine navigation error that cannot be overcome by the widespread use of GPS.

Selective Availability

Despite the fact that users who desire accuracy better than 100 meters (2 drms) can now get it from DGPS services such as the U.S. Coast Guard's, SA still has a negative impact on the marine use of GPS. For recreational boaters, who prefer not to spend additional money on DGPS-capable receivers, this is especially true. Loran-C, which is still the most popular marine navigation system, is frequently used by fishermen to return to previously known fishing grounds with an accuracy of 20 to 30 meters. If GPS cannot meet or better this capability, recreational boaters, who could represent a large market for GPS, will be reluctant to embrace it in their operations.

SA also has a negative impact on the ability of commercial ocean-going vessels to use GPS as the navigation sensor for automatic piloting equipment. Controlling a ship by

26  

NOAA's electronic chartmaking efforts are the focus of an NRC report titled: Charting a Course Into the Digital Era: Guidance for NOAA's Nautical Charting Mission, Marine Board, National Research Council (Washington, D.C.: National Academy Press, 1994).

27  

NRC, Minding the Helm, Marine Board, National Research Council (Washington, D.C.: National Academy Press, 1994), pp. 227-228.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

autopilot is preferred to manual control during long voyages because it saves fuel and reduces crew workload. This equipment requires stable velocity inputs, which are unavailable from the SPS with SA present. Methods exist to smooth or limit SA errors, such as the integration of inertial navigation systems with GPS, but vessel operators may be unwilling to bear this additional cost burden.

The Coast Guard's DGPS service itself is also affected by the presence of SA. In order to keep up with the high rate of clock dither present with SA, the system's radiobeacons must broadcast differential corrections at a high update rate. These corrections then require most of the bandwidth available on the 283 KHz to 325 KHz signal. A slower correction rate would allow the broadcast of other safety-related information that may be critical to mariners in the coastal and harbor regions.

Integrity

Under current operational procedures, the GPS master control station (MCS) does not monitor the integrity of the SPS. An improperly operating satellite can be detected by observing errors in the broadcast of the Y-code, but it is possible for errors to exist in the C/A-code regardless of the status of the Y-code. Because of this situation, the Coast Guard has stated that DGPS radiobeacons would still be required even in the absence of SA. Other integrity issues for maritime DGPS users result from the potential lack of accurate electronic nautical charts used in ECDIS's as was discussed above.

Availability and Radio Frequency Interference

RF interference to both GPS and DGPS radiobeacon's are significant issues for the commercial maritime user because interference has a direct impact on signal availability. In the marine radiobeacon band (283 KHz to 325 KHz), atmospheric interference from electrical storms will occasionally interfere with operations. Vessels operating with additional sources of navigation information can cope with lapses in availability, but users of only GPS and DGPS cannot.

Findings

GPS and DGPS are now in use in the maritime community for a number of navigation, positioning, and surveillance applications.

The full benefit of GPS and DGPS will not be realized by maritime users until systems such as ECDIS's eliminate errors produced by inaccurate charts and incorrect plotting. Up-to-date digital hydrographic data is required for the electronic charts utilized by these systems.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

The presence of SA affects the acceptance of GPS by recreational boaters and some commercial users, and limits the ability of the Coast Guard's DGPS service to provide important safety-related information to its users.

Land Transportation Applications

The civil land transportation sector of the nation's economy has generally been slow to adopt high technologies from other sectors such as aerospace or electronics. Recently, however, this trend is beginning to change due to an increased focus on initiatives such as the Intelligent Transportation System (ITS), which adapt defense-related technologies for uses in the civilian community.28 More specifically, land transportation applications of GPS are growing rapidly, triggered by ever cheaper and more sophisticated equipment, an accelerated maturation of technology, widely available differential augmentations, and competition for economic and environmental responsiveness. All modes of land transportation, including trains, trucks, automobiles, all-terrain vehicles, bicycles, backcountry skiers, hikers, and even pedestrians, have applications in which safety, position-location, and navigation are important, and have users who are therefore willing to use low-cost GPS, DGPS augmentations, or other comparable systems.

Current and Future Applications and Requirements

The trucking and railroad industries are currently the dominant land users of GPS for vehicle location and navigation, in part for reasons of competitive advantage in meeting the needs of just-in-time manufacturers and goods distributors. As on-time delivery becomes increasingly important to U.S. manufacturers and distributors, the trucking and rail industries and the international freight industry will require the ability to locate not only their vehicles or shipping containers, but also the components of their cargo when it consists of divisible elements, such as the packages handled by United Parcel Service or Federal Express. This must be accomplished with ever-greater accuracy and in near real-time. The tentative quantitative requirements for these GPS applications are listed in Table 2-6.

One of the largest near-term markets for GPS will probably be for automobile and light truck navigation and position-location. This market can evolve in a number of ways, since the automobile is used for a variety of purposes. On-board GPS and CD-ROM map systems are already being utilized by several rental car agencies, and at least one major U.S. automobile manufacturer already offers a GPS-based navigation system to its customers as

28  

ITS was formerly known as the Intelligent Vehicle/Highway System (IVHS). The name was changed to recognize the multi-modal nature of transportation.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

an option.29 It also is estimated that over half a million automobiles owned and operated in Japan already carry a GPS-based automobile navigation system.30

Although the final systems architecture and standards for the nation's ITS program have yet to be determined, the FHWA (Federal Highway Administration) anticipates that GPS will be an important component.31 Potential ITS applications for GPS, in addition to vehicle navigation and position-location, include collision avoidance and control, vehicle command and control, automated bus stops, automated toll collection, accident data collection, a number of commercial vehicle regulatory activities, and ITS infrastructure management. Tentative requirements for these applications are included in Table 2-6.

GPS can also be used for the automatic guidance of farm vehicles engaged in precision farming. Also known as prescription farming, or site-specific crop management, precision farming gives the farmer the ability to apply precise amounts of fertilizer and pesticide to exact field locations based on the type of crop planted and the soil composition, potentially improving both the efficiency and cost-effectiveness of these operations. The positioning and navigation accuracy required for precision farming, as shown in Table 2-6, can only be met with local-area DGPS.

Much of the growth in the so called low-end, personal GPS receiver market can be attributed to transportation-related recreation activities involving both vehicles and pedestrians. Examples include "off-roading" with four-wheel drive vehicles, back-country skiing, mountain climbing, bicycling, hiking, and even golfing.32 For those activities in which the potential for "getting lost" is high, and search and rescue services are often required as a result, GPS is much more than a useful gadget; it is a potentially life-saving device.

29  

This system, known as Guidestar, is offered as an option in General Motor's Oldsmobile 88 model. It uses GPS as an accuracy monitor for a dead-reckoning and map-matching navigation system.

30  

Source of information: Personal conversation with Michael Swiek, Executive Secretary of the U.S. GPS Industry Council.

31  

The leaders of the two teams that have been awarded Phase II ITS contracts for continuation of architecture design are Rockwell International and Loral Federal Systems. It is too early in the design process to determine exactly what role GPS will play in either team's final architectures. Source of Information: personal conversation with Mr. Lee Simmons, National Architecture Team Leader for ITS, FHWA, 22 February 1995.

32  

Several golf courses in the United States have experimented with DGPS systems mounted on golf carts to provide golfers with exact distances to the pin based on their location on the course.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-6 Land Transportation Requirementsa

 

Application

Accuracy (2 drms)

Integrity

Availability

Coverage

Resistance to RF Inter-ference

 

 

 

Time to Alarm

 

 

 

Railroad

Train Control

1.0 m

5 s

99.7%

Nation

High

ITS and Vehicle Navigation/ Position- Alert Location

Highway Navigation and Guidance

5.020.0 m

1-15 s

99.7%

Nation

High

 

Mayday/Incident

5.0-30.0 m

1-15 s

99.7%

Nation

High

 

Fleet Management (AVL/AVI)

25.0-1500 m

1-15 s

99.7%

Nation

High

 

Emergency Response

75.0-100.0 m

1-15 s

99.7%

Nation

High

 

Automated Bus/RailStop Annunciation

5.0 -30.0 m

1-15 s

99.7%

Nation

High

 

Vehicle Command and Control

30.0 -50.0 m

1-15 s

99.7%

Nation

Very High

 

Collision Avoidance, Control

1.0 m

1-15 s

99.7%

Local

Very High

 

Collision Avoidance, Hazardous Situation

5.0 m

1-15 s

99.7%

Local

Very High

 

Accident Data Collection

30.0 m

1-15 s

99.7%

Nation

Moderate

 

Infrastructure Management

10.0 m

1-15 s

99.7%

Nation

Moderate

Hazmat

Vehicle or Cargo Locations

5.0 m

1 s

99.7%

Nation

High

Precision Farming

Automatic Vehicle Guidanceb

0.05 m

5 s

99.7%

Local

High

Search & Rescue

Location Determinationc

10.0 m

minutes

99.0%

Nation

High

Recreation

Off-road Vehicles, Hikers, Back-country Skiers, etc.c

5.0 m

minutes

99.0%

Nation

Moderate

a. Integrity (1 minus PHE times PMD) and continuity of service requirements are not defined for land transportation applications. Source of other requirements, unless otherwise annotated: U.S. Department of Commerce, National Telecommunications and Information Administration, A Technical Report to the Secretary of Transportation on a National Approach to Augmented GPS Services, p. 9.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

b. Precision farming requirements were derived from information provided by the U.S. Department of Agriculture and the Deere & Company, Precision Farming Group.

c. The values listed for these applications are not firmly established requirements. They are estimated useful values determined by the committee.

Challenges to Full GPS Utilization

The major challenges for most uses of GPS in the land transportation sector include the need to meet the desires and requirements of an ever-increasing number of creative applications and the need to do so with technologically integrated equipment that is affordable, reliable, and reasonably durable. Each of these challenges deserves further explanation.

Accuracy Versus Other Requirements

For many civil land transportation purposes, the issue of accuracy of location will be dominated by right-of-way and construction surveying needs, not by vehicle, cargo, or personnel position-location or navigation issues. Where positioning and navigation accuracy is important, it is often related to requirements such as availability and integrity. For example, it is important in the cities for both people and freight movements, and dispatch control, to have good accuracy resolution not compromised by loss of signal lock due to tall buildings or interference from other radio sources. Similarly, the problem of resolution to a few meters, essential in mountainous terrain for numerous applications such as avalanche search and rescue, and forest fire control, is made more difficult by terrain and foliage, which can mask GPS signals.

For trucking and shipping, where vehicle and cargo location, and fleet dispatch and management are important, it seems clear that availability and coverage may be a greater challenge than greater accuracy. For example, a truck traveling from Boston, Massachussets to Seattle, Washington will not need to be located to within 25 meters of its actual position throughout the entire trip; however, it will need to be located. Satellite or terrestrial-based DGPS augmentation techniques must be adopted that provide better availability and coverage to the entire nation, not just the densely populated areas.

Users who do perceive a need for higher navigation and position-location accuracy than is available from the GPS SPS can generally meet this requirement by utilizing one of several commercially provided DGPS services. Systems that provide the differential correction via FM subcarrier seem especially suited to land transportation users, although their networks do not yet cover all of the nation. Additional enhancements to GPS receivers, such as the integration of small solid-state inertial gyroscopes and accelerometers, can also improve accuracy and other performance characteristics. Those users with accuracy requirements in the 20-meter range, however, which includes the rapidly growing automobile navigation market, would not need to augment or enhance GPS with DGPS or inertial gyroscopes if SA were eliminated.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

For future ITS applications of GPS, such as automatic vehicle control and collision avoidance, integrated systems that use inertial navigation units and differential corrections will be required to meet the stringent accuracy, integrity, and availability requirements placed on any system that is fundamental to public safety. Current legal limitations, which restrict the ability for private sector interests to provide ''navigation" services as opposed to "positioning" services, however, may negate the ability to use private sector DGPS providers to help meet these requirements. Although the removal of SA would not allow the standard positioning service to meet these requirements either, it would improve the performance of both wide-area differential systems such as the WAAS, and local-area systems.33

The Cost of Integrated Systems

Land transportation seems to offer unlimited opportunities for integrating GPS and DGPS with other complementary technologies related to communications, scanning, and digital imaging. Example technical systems include cellular phones, on-board fax and computer resources, driver performance and alertness equipment, and vehicle operations sensors. It seems reasonable to suggest that private sector creativity will be able to devise these integrated systems that will likely form the core of the nation's future transportation systems such as ITS. A few words of caution, however, should be considered.

In order for these systems to be widely accepted by potential users, their cost must be considered modest; they must be easy to use; and the equipment itself must be durable, reliable, and essentially maintenance free. "Gadgets" that fail to meet the above criteria or compromise the operational safety of a vehicle will never be accepted voluntarily by users in the surface-transportation community, especially the competitive commercial vehicle market. In addition, integrated positioning and communications systems utilizing GPS and other technologies will be most widely accepted if they help to fulfill bonafide publicand private-sector customer needs in a cost-effective manner. Systems introduced to the marketplace because of technology push are not likely to achieve widespread success.

Findings

There is a tremendous market for land navigation and positioning systems that integrate GPS with other technologies such as digital communications systems, driver performance and alertness equipment, vehicle operations sensors, and CD-ROM-based digital mapping and applications software. These systems, however, will only become widely accepted if costs continue to drop, high levels of reliability can be maintained, and reasonable durability can be assured.

33  

Potential improvements to DGPS techniques as a result of the elimination of SA are discussed in the next chapter.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Improved integrity, availability, and resistance to RF interference are as important to many land transportation GPS users as defeating the accuracy degradation caused by SA.

GPS-based automobile navigation systems, which require accuracies in the 5 to 20-meter range, would no longer require DGPS if SA were eliminated and further improvements were made to the basic GPS as suggested in Chapter 3. The elimination of SA would also improve the performance of those DGPS systems required for higher-accuracy applications, such as collision avoidance, that are important to the future Intelligent Transportation System.

Mapping, Geodesy, and Surveying Applications

Currently, the fields of mapping, surveying, and geodesy are being transformed by a number of new and innovative technologies, including Geographic Information Systems (GIS), high-resolution remote sensing, and GPS. Of these, GPS has had the most important and immediate impact because of both cost savings and accuracy improvements over previous positioning technologies and techniques. The single most powerful feature related to GPS, which is not true of traditional mapping and surveying techniques, is that its use does not require a line of sight between adjacent surveyed points. This factor is paramount in understanding the impact that GPS has had on the surveying and mapping communities.

Current and Future Applications and Requirements

GPS has been used by the surveying and mapping community since the late 1970s when only a few hours of satellite coverage were available. It was immediately clear that centimeter-level accuracy was obtainable over very long baselines (hundreds of kilometers). In the early 1980s, users of GPS faced several problems: the cost of GPS receivers; poor satellite coverage, which resulted in long lengths of time at each survey location; and poor user-equipment interfaces. Today, instantaneous measurements with centimeter accuracy over tens of kilometers and with one part in 108 accuracy over nearly any distance greater than 10 kilometers can be made. The cost of "surveying-level" receivers in 1994 ranged from $10,000 to $25,000, and these costs are falling rapidly. Practitioners are developing numerous new applications in surveying, such as the use of GPS in a kinematic mode to determine the elevation of terrain prior to grading it for a storm water basin.34

Traditional land surveying is increasingly being accomplished using GPS because of a continuous reduction in receiver costs, combined with an increase in user friendliness. This

34  

"Kinematic" GPS surveying is accomplished using a reference receiver and one or more moving remote receivers. The carrier-phase measurements observed by the remote receivers and the static receiver are used in an interferometric mode to allow the positions of the remote receivers to be determined to the centimeter level in real time. More information on carrier-phase (interferometric) GPS techniques can be found in Appendix C.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

trend towards the use of GPS has enhanced the volume of survey receiver sales because land surveyors outnumber geodesists (control surveyors) by at least one order of magnitude.35 This usage has also increased the accuracy and accuracy requirements of surveying in general.

GPS is also increasingly being used as the core technology for integrated mapping systems. These systems are usually mobile (e.g., a van, train, airplane or any other vehicle) and contain a combination of sensors. These sensors include vision or imaging systems, laser ranging and profiling systems, ground penetrating radars, and other navigation sensors such as inertial navigation units. GPS provides positioning data when satellites are visible, and other sensors provide the spatial location data required for map making. The inertial systems, and sometimes the vision systems, are used to interpolate between GPS positions through periods when GPS satellites are lost from the vehicle's field of view. These mapping systems provide the surveying and mapping community with powerful new ways of acquiring accurate and current digital data.

In general, the availability of higher GPS accuracy has influenced various mapping and surveying requirements for three reasons: (1) people want the latest and the best; (2) past requirements were in some cases dictated by the cost of acquisition; and (3) if higher accuracy can be obtained, multiple purposes can be satisfied. As an example of requirements changing as a function of new capability, consider a problem of facilities management which deals with the inventory of transportation features such as the location and attributes (type, condition, and so forth) of a guardrail along a highway. Previously, the location was "required" by transportation departments to be accurate to ± 6 meters (20 feet), which is generally the best that is possible from scaling or plotting on a 1/24,000 USGS quadrangle. Using differential techniques a GPS position can easily be obtained in a real-time, dynamic environment to within ± 1.5 meters (5 feet). Users now realize that if accuracies of ± 0.3 meters (1 foot) can be obtained (and they can), the length of the guardrail, in addition to its location, can be obtained so that if the guardrail needs to be upgraded or replaced, an accurate estimate of the cost is available. This kind of analysis is growing rapidly as GPS becomes understood and applied to various problems. Clearly, concepts of this kind are widespread in GIS applications in natural resource planning, environmental problems, civil infrastructure enhancements, an so on. Analogous examples can be given for surveying and geodesy.

Accuracy requirements for surveying applications are generally satisfied at this time. The quest for better and better accuracy will continue, but any reasonable distance can currently be measured, with significant care, to one part in 108. In each of the categories in Table 2-7, the most stringent accuracy requirements are adopted because of the potential for multipurpose applications.

35  

Land surveying usually ignores the curvature of the Earth (except in leveling) and assumes that the Earth's surface is a plane. Control surveying does not make this assumption and is generally performed with an accuracy an order of magnitude better than land surveying.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-7 Current and Future GPS Requirements for GIS, Mapping, Surveying, and Geodesya

 

Application

Accuracy (2 drms)

Integrity

Availability

Coverage

 

 

 

Time to Alarmb

 

 

 

Geographic Information Systems (GIS)

1.0-10.0 m

Minutes

98%

Worldwide

 

Photogrammetry

0.02-0.05 m

Minutes

98%

Worldwide

 

Remote Sensing

0.1-20.0 m

Not specified

98%

Worldwide

 

Geodesy

0.01-0.05 m

Hoursc

98%

Sites Worldwide

 

Mapping

0.1-10 m

Hoursc

98%

Sites Worldwide

Surveying

Hydrographic

0.05-10.0 m

Hoursc

98%

Sites Worldwide

 

Topographic

0.01-0.5 m

Hoursc

98%

Sites Worldwide

 

Boundary

0.01-0.05 m

Hoursc

98%

Sites Worldwide

a. Integrity (1 minus PHE times PMD), continuity of service, and resistance to RF interference requirements are not defined for mapping, survey, and geodetic applications. Source of other requirements, unless otherwise annotated: The Ohio State University, Center for Mapping.

b. Source of time-to-alarm requirements: A Technical Report to the Secretary of Transportation on a National Approach to Augmented GPS Services, p. 13.

c. The integrity of the positioning data for each of these applications is validated in post-processing.

Challenges to Full GPS Utilization

It is important to understand that nearly all accuracy requirements presently can be met using DGPS. However, the cost of meeting these requirements would decrease if various enhancements to the basic GPS itself were implemented. In particular, eliminating SA and/or A-S would drive down the costs of new applications. GIS applications would benefit most from the elimination of SA because many GIS requirements could potentially be satisfied by the accuracies obtained from stand-alone GPS with SA set to zero and with other potential accuracy improvements. All real-time, dynamic surveying and mapping applications would benefit from improved signal acquisition. Faster integer ambiguity resolution, important to real-time kinematic survey and mapping applications, would be achievable with a second frequency unencrypted by A-S. As discussed in Appendix G, access to the wider bandwidth of the P-code, which is approximately 20 MHz (versus 2 MHz for the C/A-code), also would increase resistance to RF interference and reduce vulnerability

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

to multipath. Improving the GPS orbit information (ephemeris) available to SPS users also would have a significant impact on the surveying and mapping community as longer and longer baselines could be measured in real-time with centimeter accuracy.

Additional challenges to the use of GPS in the mapping, surveying, and GIS community deal with receiver cost, service and maintenance, user friendliness, and interfaces with other software and hardware. For example, at the Center for Mapping at the Ohio State University, researchers have developed a real-time positioning capability accurate to 1.5 centimeters with as few as five satellites in view. The system is interfaced with software developed by the construction service industry that displays "cuts and fills" on a screen so that an operator of earth-moving equipment can grade earth in a prescribed fashion. One problem with the overall system is the necessity to use two GPS receivers that currently cost $25,000 each. The competitive technology costs about $40,000. If the price of the GPS receivers falls to $10,000 each, however, the GPS technology will dominate the market. This is especially true because GPS offers coordinates in three dimensions without line of sight requirements. The rival technology (laser plane) is one dimensional and requires line of sight. Hence, the challenge to using GPS for earth moving is focused on software integration and the costs of receivers. This same scenario is applicable to many other potential GPS applications envisioned at the moment.

Findings

Greater geodetic accuracy for mapping and surveying will be pursued in part because of the challenge of obtaining it. A few applications, such as determining the position of the blades of earth-moving equipment in real time will demand increased accuracy. Most applications, however, will be enhanced by cost savings from quicker acquisition of the same data. The elimination of SA and A-S, and the use of dual-frequency user equipment can improve data acquisition time.

For surveying, the weakest link in the utilization of GPS, aside from SA and A-S, is the precision of the GPS satellite orbits. While improving ephemerides will not significantly enhance positioning over short baselines, they will have a noticeable impact over baselines greater than 50 kilometers.

GPS Earth Science Applications

One's ability to measure the Earth, including its atmosphere and its ocean surfaces, has been greatly enhanced by GPS. New departures in scientific endeavor and commercial enterprise have begun, and initial results are very promising.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Current and Future Applications and Requirements

Meteorology

In meteorology, GPS can be used to measure atmospheric water vapor. Water vapor is the principal mechanism by which moisture and latent heat are transported in the atmosphere and is therefore closely linked to weather and climate. As discussed in Appendix C, GPS signals are delayed by the ionosphere and neutral atmosphere as they travel towards the surface of the Earth. This delay can be estimated by a receiver observing the two principal GPS transmission frequencies. When combined with surface pressure data, the estimated signal delay can provide a measurement of wet delay, which in turn, can be converted into precipitable water vapor. GPS sensing of precipitable water vapor with millimeter accuracy has been demonstrated successfully. The use of this technique for weather forecasting is being explored, and has been proposed for climate research.

Another innovative use of GPS for meteorology is the new field of Earth-atmospheric occulation measurements. This technique uses a GPS receiver on a satellite in low-Earth orbit to track a GPS satellite as it sets behind the Earth. As the GPS signal passes through the edge of the atmosphere it is refracted, causing delay and Doppler shift, which is measured with millimeter accuracy by the spaceborne receiver. The index of refraction of the atmosphere can then be determined as a function of height. This index can then be analyzed to produce atmospheric temperature profiles and a measure of water vapor content. The first demonstration of this promising GPS application, which is also important to global change research, is scheduled to take place in 1995.

Oceanography

One importance of GPS to the field of oceanography is its potential ability to determine precise orbital parameters for the Topex/Poseidon satellite, which in turn, provides accurate radar altimetry of the ocean's surface. In general terms, Topex/Poseidon data improve in several ways as more precise orbital information becomes available. The issue is to separate orbital error from tides, general circulation, and gravity-field error. General circulation needs to be determined at the 1-centimeter level, a reasonably easy task with the GPS precise positioning service (PPS), but difficult, or perhaps even impossible, with other methods of orbit determination. Orbital error would no longer be a significant factor for all Topex/Poseidon data if orbits could be determined with an accuracy of ± 1 millimeter. Using the GPS PPS, this is a distant, although not unobtainable goal.

In the wider context of oceanography, one can assert that every time there has been a real improvement in navigation whole new fields of study have opened. GPS with SA set to zero provides a real improvement in navigation. Ocean-surface height measured by ships at sea, and the positioning of a tomographic lagrangian drifter also can be accomplished

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

with useful accuracy.36 Other oceanographic positioning applications, such as the location of objects on the ocean floor, which is essential for drilling and sampling activities, require real-time accuracy of about 10 meters.

Geodynamics

In geodynamics, GPS is used to study relative motions on the surface of the Earth. The tectonic plates of the Earth's outer layers move relative to each other at rates within the range of 1 centimeter per year to 20 centimeters per year. Many earthquakes occur along the plate boundaries, a recent example being the earthquake in Northridge, California, which occurred on January 17, 1994. A few permanent GPS reference stations provided important data for the early determination of the Northridge earthquake mechanism, which had a displacement on the order of 1 meter.

Arrays of permanent GPS stations, coupled with a few interferometric strain meters, can be used to study crustal deformation in the time intervals between earthquakes. This information could be used to estimate the varying amounts of seismic risk in, for example, different parts of the Los Angeles area. The risk assessment could be used to determine appropriate local variations in building codes, freeway and subway construction, and other public projects.

In a very speculative vein, if GPS arrays and associated strain meters reveal premonitory or precursory signals for earthquakes, and if the signals are detected early enough to provide meaningful warnings to a region's population and public authorities, then it would become important to measure these signals in as near real time as possible, that is, with minimal post-processing. Only time will tell whether GPS arrays will become useful in this very speculative vein. If not, the improved study and understanding of the deformation of the Earth's crust and of the rupture process of earthquakes will still provide ample reason to establish and operate permanent geodetic GPS arrays.

Airborne Geophysics

Many of the measurement tools used historically by Earth scientists for regional studies are not sufficiently accurate to model physical processes and improve the understanding of natural hazards and the distribution of nonrenewable resources. Physical barriers, such as inaccessibility by land due to hazardous terrain, and limited resources which prevent the surveying of large areas by conventional means, pose other difficulties.

Collecting data remotely from satellites or aircraft can overcome some of the sampling problems. Satellite missions, however, require long lead times between concept and realization, making airborne platforms an attractive alternative for regional Earth studies.

36  

Tomography is the use of acoustic travel time to infer changes in acoustic wave speed due to changes in sea temperature and composition. A tomographic lagrangian drifter is a neutrally buoyant buoy equipped to record the arrival of acoustic pulses for use in tomography studies.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Aeromagnetic surveys have been used for half a century with great effect, but airborne gravity and topographic mapping depend on the ability to determine the aircraft's motion and position.37

Positioning to 100 meters horizontally and 3 meters vertically is required to provide useful measurements of gravity and terrain. GPS and DGPS are ideal for such positioning and a combination of GPS and an inertial navigation system to provide the acceleration of the aircraft, could enable studies of dynamic changes in topography and gravity, such as the expansion of a volcanic dome caused by the injection of magma. Using GPS and radar altimetry to obtain precise gravity anomaly maps, the regional prospecting for ore bodies, salt domes (petroleum reservoirs), or large anticlines (big domes that contain petroleum) can also be accomplished quickly and economically.

Accuracy Requirements

In general, the Earth science applications described above require much better positioning accuracy than was ever anticipated or intended from GPS, as Table 2-8 clearly illustrates. However, any static GPS reference station equipped with a dual-frequency geodetic receiver can currently be positioned "absolutely" at the centimeter level with respect to the international terrestrial reference frame with less than 24-hours of data. Relative positions between stations at regional scales can be determined at the few-millimeter level with very short observation times. This capability is due to major improvements in GPS software packages and the availability of very precise satellite ephemerides (10-centimeter accuracy) determined by the International GPS Service for Geodynamics (IGS).

The ephemeris information available from the IGS can also be used for post-processed dynamic positioning applications. Moving platforms up to several hundred kilometers away from a fixed DGPS base station can achieve 10-centimeter to 20-centimeter positioning accuracy using corrections based on IGS ephemeris data. Satellite clock information distributed by the IGS also is helpful for mitigating SA effects in post-processing of position, particularly in airborne and oceanographic applications.

37  

Airborne Geophysics is the subject of a recent NRC report titled: Airborne Geophysics and Precise Positioning: Scientific Issues and Future Directions, Board on Earth Sciences and Resources, National Research Council (Washington, D.C.: National Academy Press, 1995).

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-8 GPS Earth Science Requirementsa

 

Application

Accuracy (2 drms)

Integrity (time to alarm)

Static

Meteorology

0.001 m

Hours

 

Oceanography- General Ocean Circulation Determination

0.01 m

Hours

 

Geodynamics

0.001 m + 109 x baseline length

Hours

Dynamic

Oceanography - real-time Positioning and Navigation

10.0-30.0 m

Not specified

 

Airborne Geophysics

3.0 m vertical

Minutes

a. Integrity (1 minus PHE times PMD), availability, continuity of service, and resistance to RF interference requirements are not available for the GPS Earth Science applications covered by this table. Other requirements were derived from input received from the appropriate scientific community.

Challenges to Full GPS Utilization

Meteorology

A third GPS radio frequency would be very helpful in atmospheric studies. Also, the presence of A-S greatly increases costs and limits the performance of many techniques due to loss of low-elevation angle data and signal-to-noise ratio, even when using dual-frequency codeless receivers.

Oceanography

In general, spacecraft orbits determined from GPS data with A-S off are superior to those determined by other means, with A-S on this is not the case. A successor mission to Topex/Poseidon could be designed with receivers that would work well in the presence of A-S and, essentially, overcome this obstacle. However, it has been estimated that the additional cost of adding a space-qualified PPS receiver to a satellite would be about $500,000.38 Much of this cost stems from the security measures that are required for the proper handling of classified equipment.

For other types of oceanographic research, SA is the central challenge to the usefulness of GPS. The 10-meter to 30-meter accuracies required to navigate research vessels, position buoys, and locate objects on the ocean floor cannot be achieved using GPS

38  

W. G. Melbourne et al., "GPS Flight Receiver Program for NASA Science Missions - A Unified Development Plan," (JPL D-10489). Jet Propulsion Laboratory, 10 February 1993.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

alone, unless SA is eliminated and other improvements are made to increase the accuracy of the SPS.

Geodynamics

Despite improved post-processing software and the use of differential GPS, the effects of A-S and SA degrade the results by 50 percent or more, primarily through the loss of the signal-to-noise ratio using dual-frequency codeless receivers. The loss can be partially recovered by replacing existing receivers that are a few years old with newer equipment. Significant savings in time and costs would occur, however, if this was not necessary.

Airborne Geophysics

SA has little effect on airborne geophysical applications when differential GPS and post-processing are utilized. As with geodynamic applications, however, the presence of A-S greatly reduces the signal-to-noise ratio available to dual-frequency receivers. The dynamic, high-multipath environment that exists for GPS receivers on aircraft makes codeless receivers especially vulnerable to losing lock on the L2 signal and requires a lengthy reacquisition time. In lieu of code-tracking capability on L2 or an alternative L-band signal, improvements to the tracking loops in codeless receivers could improve this situation.39

Findings

Using post-processed GPS orbits provided by the IGS network of differential reference stations, the effects of SA can be eliminated for most Earth science applications, and with the use of dual-frequency "codeless" receivers, centimeter-level positioning accuracies can be achieved.

The availability of a second GPS frequency for civil use with unencrypted code would greatly enhance many Earth science applications that require high-precision accuracy. Dynamic, high-multipath applications, such as airborne geophysics, would benefit from faster acquisition and more robust tracking. Applications such as remote atmospheric sensing require submillimeter precision in the carrier-phase observables, which may be achievable using a second unencrypted signal.

39  

The effects of SA and A-S on the use of GPS in airborne geophysics are discussed in more detail in the NRC report Airborne Geophysics and Precise Positioning: Scientific Issues and Future Directions, Appendix A: Effects of Selective Availability and Anti-spoofing.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

GPS Timing And Telecommunications Applications

Because the pseudoranging method used by GPS to establish three-dimensional position locations requires a highly accurate time standard, the system is ideally suited for applications that require precision timing and precise time transfer. GPS pseudorange measurements are based on the transit time of a signal from the GPS satellite to the user. Thus, if the locations of both the satellite and the observer are known, the difference in the user-clock offset from that of the satellite can be readily determined. Furthermore, if the satellite clock is referenced to a standard such as Universal Coordinated Time (UTC), as is the case with GPS, the observer can then determine user-clock offset from UTC.40

Current and Future Applications and Requirements

The time-transfer community was one of the first to realize benefits from GPS, since a full satellite constellation is not required for most time-transfer methods. In fact, the most accurate method of time transfer to date, known as GPS common-view, relies on the ability of two users on the globe to observe the same GPS satellite simultaneously, despite a large geographic separation. GPS common-view is currently used by the 55 international timing centers that are charged with the task of maintaining International Atomic Time (TAI) and UTC throughout the world.41 A chain of common-view observations also is used to link the widely separated sites that are part of the National Aeronautics and Space Administation's (NASA) Deep Space Network.42 Other time-transfer methods that utilize a single GPS satellite, as well as methods that require observations from multiple satellites, are used for a number of scientific research activities that require precise time synchronization of equipment located in different laboratories.43

40  

UTC is often referred to as Greenwich Mean Time because it refers to the time of day in Greenwich, England (U.K.).

41  

The official international timing center in the United States is the National Institute of Standards and Technology (NIST) Metrology Laboratory in Boulder, Colorado. This facility, along with 53 others, keep time relative to the master facility at the Bureau International des Poids et Measures (BIPM) in France.

42  

The Deep Space Network (DSN) consists of three tracking stations located near Barstow, California; Canberra, Australia; and Madrid, Spain. These stations receive telemetry data from deep space missions such as Galileo, and send commands that control spacecraft navigation and operation. The three tracking stations are monitored by the DSN's control center at the NASA Jet Propulsion Laboratory in Pasadena, California.

43  

Single satellite time-transfer methods in addition to common-view include GPS direct and clock flyover. Methods using multiple satellites include Enhanced GPS, GPS used as Very Long Baseline Interferometry (VLBI), and Geodetic Positioning Time Transfer. For more information on these methods see: David Allen, Jack Kusters, and Robin Giffard, ''Civil GPS Timing Applications," in Proceedings of ION GPS-94:7th International Technical Meeting of the Satellite Division of the Institute of Navigation (Salt Lake City, Utah, September 1994), pp. 25-32.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

GPS is also increasingly utilized by many telecommunications companies to synchronize their land-based digital telecommunications networks.44 Most often, these users compare a reference clock directly to GPS time by viewing one or more satellites, rather than transferring time from one reference clock to another. AT&T, in particular, now uses GPS to maintain time synchronization throughout its long distance telephone system,45 and an international digital telecommunications system that uses a GPS-based timing system began operating in Moscow in 1991.46 As synchronous fiber optic networks such as SONETs increase in size and complexity, GPS time synchronization may replace the more common practice of using land lines to disseminate timing information from a small number of land-based clocks.47

The "Stratum n" performance level hierarchy, developed by the American National Standards Institute (ANSI) T1 Committee on Network Synchronization Methods and Interfaces, specifies the requirement for synchronization. At the present, the one to four Stratum performance levels (with one being the most stringent) could be satisfied by the long-term frequency stability available from the GPS standard positioning service.48 The ANSI T1 requirements are listed in Table 2-9.

Precise GPS timing also has the potential to significantly improve mobile cellular communications. Currently most cellular telephone networks are subject to transmission degradation as a call is transferred from one cell's channel to another, but if all of a network's cells used the same channel, this problem would be eliminated. This can be accomplished by providing each cell with a unique code rather than a unique frequency using a technique known as Code Division Multiple Access (CDMA).49 Major CDMA manufacturers have recognized GPS as an effective way to provide the precise time synchronization required by their systems.50 Timing accuracies similar to those required for digital networks are sufficient for this application.

44  

Information presented in this section on the use of GPS by the telecommunications industry, unless annotated otherwise, is based on the following report: Eric A. Bobinsky, GPS and Global Telecommunications: A Summary Briefing Prepared for the National Research Council Committee on the Future of the Global Positioning System (Washington, D.C., 29 July 1994).

45  

E. Krochmalny, "GPS Synchronizes the Lines," GPS World, May 1992, p. 39.

46  

M. J. Toolin, "GPS in a Russian Telecommunications Network," GPS World, June 1992, pp. 28-34.

47  

SONETs, or Synchronized Optical NETworks, were originally proposed by Bellcore, and are now becoming the worldwide standard format for optical transmissions. The term "synchronous" highlights the fact that a SONET is aligned in time with respect to a common timing source.

48  

There are currently no ANSI T1 "Stratum n" requirements for absolute timing accuracy. The absolute timing accuracy specification for the GPS SPS is 340 nanoseconds relative to UTC.

49  

Code Division Multiple Access is the same technique that allows a GPS receiver to distinguish one satellite from another despite the fact that they all use the same frequency.

50  

U. H. Werner, "Improving Mobile Communications with GPS," GPS World, May 1993, pp. 40-43.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Cellular signals are also subject to the local conditions in each cell that may vary from cell to cell, such as weather or landform geometry. By putting GPS positioning capability in the mobile receiver and by transmitting the position information to the mobile control and operations center of the mobile system, the network control operations could determine user location and travel direction. With this information available, the network controller can provide optimal hand over as well as real-time dynamic performance optimization for each location. A typical communications cell ranges from a few tens of meters to over a hundred square kilometers, so a positioning accuracy of a few hundred meters will suffice. When dealing with small, oddly shaped cells, however, or when trying to map signal and propagation characteristics within a complex area such as an "urban canyon," accuracy on the order of a few meters in three dimensions may be required. These general values for positioning accuracy have not yet been defined as requirements, and therefore are not included in Table 2-9.

In the future, many information services may require "time-of-day" information to a much higher degree of accuracy than is typical of today's services. Examples include universal personal communications services and broadband integrated services digital networks which may require a high degree of time-of-day precision in order to interface with several different types of communications systems to transmit tremendous amounts of digitally packeted information.51 Timing accuracies of 100 to 300 nanoseconds relative to UTC will likely be required for these services.

Table 2-9 Timing and Telecommunications Requirementsa

 

Application

Accuracyb

 

Reliabilityc

 

 

Time

Frequency

 

Common-View Time Transfer

NASA Deep Space Network

1 ns

1 x 10-15

Not specified

 

BIPM for TAI and UTC

1 ns

1 x 10-14

Not specified

 

International Timing Centers

0.1-1 ns

1 x 10-14

Not specified

 

NIST Global Time Service

10 ns

1 x 10-14

Not specified

Time

Power Industry

10 ns

Not available

High

Synchronization

 

 

 

 

 

ANSI T1 Stratum 1

Not specified

1 x 10-11

High

 

Time-of-Day Services

100-300 ms

Not specified

High

a. Source of requirements for common-view time transfer and power industry time synchronization: David Allen, Jack Kusters, and Robin Giffard, "Civil GPS Timing Applications," p. 28. Source of time-of-day requirement: Eric A. Bobinsky, "GPS and Global Telecommunications." ANSI Stratum 1 requirements provided by Mr. Bruce M. Penrod of True Time, Santa Rosa, CA.

51  

Iridium, Orbcomm, Globalstar and other proposed low-Earth orbit (LEO) satellite communications systems are all examples of UPC services. Broadband integrated services digital networks, are digital telephone lines capable of transmitting data, voice, graphics, and video information at a rate much faster than modems.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

b. The timing accuracies listed include both time relative to UTC in nanoseconds (ns) or milliseconds (ms), and long term frequency stability measured over one day, except for the ANSI Stratum 1 long term frequency stability, which is measured over any time interval greater than 1000 seconds.

c. For commercial time synchronization applications, "reliability" corresponds to overall system reliability as explained in this section, not the continuity of service requirement applicable to GPS aviation applications.

Challenges to Full GPS Utilization

Time Transfer

For GPS time-transfer applications, the challenge of mitigating the effects of SA's clock dithering in order to improve accuracy appears to have been met. Methods to filter SA-induced noise have allowed time transfers to occur using C/A-code receivers, which achieve accuracies of better than 1 nanosecond relative to UTC, and long-term frequency stabilities of better than 1 x 10-14.52 Laboratories responsible for the world's primary time standards, such as NISTs metrology laboratory in Boulder, Colorado, are hoping to conduct time transfers with this type of accuracy on a routine basis. These accuracies are required in order to maintain standards that are two orders of magnitude better than the timing accuracies required by industry.

If errors from SA are removed and ionospheric errors are minimized by using dual-frequency receivers, clock and ephemeris errors become dominant. Improvements to the GPS space and ground control segments will be required in order to reduce these errors.53

Time Synchronization

For the telecommunications industry, requirements such as integrity and availability fall under the general category of overall system reliability. Communications and information service providers will not rely on any technical system that does not guarantee them the ability to satisfy the needs of their customers on a continuous 24 hour-a-day basis. Many potential users of GPS in the telecommunications industry feel that GPS, as currently configured, cannot provide this level of reliability. As with many other GPS applications, the absence of SPS integrity monitoring is unacceptable to many in the telecommunications industry. These potential users have expressed a desire to have GPS performance monitoring data available to them in real time in order to feel comfortable with its reliability.

52  

David Allan, Jack Kusters, and Robin Giffard, "Civil GPS Timing Applications," pp. 26-27.

53  

Candidate improvements are discussed in Chapter 4.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

The presence of SA, despite the fact that it does not degrade timing accuracy to less than currently acceptable levels, is considered to be another limitation on overall system reliability. The telecommunications industry believes that GPS, being "subject to failures or deliberate denial of signal", cannot and should not be used without being backed up by other technologies able to provide the same information.54 In the future, it is also likely that accuracies in the range of 50 to 100 nanoseconds will be required for some telecommunications applications. It will be difficult for direct GPS timing to meet this requirement, even without the presence of SA.

Findings

GPS currently meets all accuracy requirements for both GPS time transfer and time synchronization using direct GPS time.

Many telecommunications companies are still hesitant to utilize GPS because of concerns about system reliability and the presence of SA.

Future accuracy requirements for both time synchronization and time transfer will be difficult to achieve with the current capabilities of GPS.

Spacecraft Uses of GPS

The application of GPS to spacecraft navigation and control has the potential to provide significant savings in spacecraft costs and mission operations and is being introduced into spacecraft systems today in both government and commercial programs. The feasibility of using GPS for satellite navigation was first demonstrated in 1982 by a receiver placed aboard Landsat 4.55 Since then a number of additional missions and satellites have utilized GPS, including the Topex/Poseidon satellite launched in 1992. Other spacecraft programs have flown GPS, but it has been used primarily in an experimental mode.

GPS receivers also have been used experimentally for launch vehicle applications. The experimental Ballistic Missile Defense Organization/McDonnell Douglas Delta Clipper (DC-X) utilized a GPS receiver integrated with an inertial navigation unit and flight control avionics during its flight testing. The system has reportedly cut the rocket's development

54  

GPS and Global Telecommunications: A Summary Briefing Prepared for the National Research Council Committee on the Future of the Global Positioning System, p. 8.

55  

H. Heuberger and L. Church, "Landsat-4 Global Positioning System Navigation Results," (Presentation to the American Astronautical Society/American Association of Aeronautics and Astronautics (AAS/AIAA) Astrodynamics Conference, AAS 83-363, August 1983).

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

time and contributed to its success.56 An integrated GPS/inertial navigation unit is also being test flown on the Orbital Science Corporation's Pegasus launch vehicle. The company hopes that an operational version of the unit will one day improve the vehicle's en route navigation and orbital injection accuracy.

Current and Future Applications and Requirements

GPS is currently being tested or used for several spacecraft applications, including orbit determination, attitude determination, launch and reentry vehicle positioning and trajectory determination, and time synchronization. Precise time synchronization, which is required by many spacecraft, such as telecommunications satellites, to an accuracy of 100 nanoseconds was discussed in some detail in the previous section, but the remaining applications are discussed below.

Orbit Determination

The use of GPS for real-time determination of orbital parameters provides an economical means of determining a spacecraft's orbit very accurately. A properly designed, space-qualified GPS receiver can replace several conventional orbital positioning spacecraft sensors, reducing both weight and cost, and in some cases relieving the requirement for worldwide, ground-based stations to track orbital positions. In addition, the orbital parameters determined with GPS can in some cases be input to an on-board control computer and propulsion system to provide autonomous station keeping. This would alleviate or reduce the need for mission operations personnel to control a spacecraft's orbital position from the ground.

In general the requirements for real-time orbit determination are not very stringent, ranging from about 50 meters to several kilometers. Although these requirements are quite lax, the same is not true for post-flight or post-processed solution accuracies. Many spacecraft, in particular those used for scientific missions, require very precise knowledge of where the satellite was when scientific data were being collected. The desire to achieve ± 1 centimeter orbit determination accuracy for the Topex/Poseidon spacecraft, as discussed in the Earth Science section of this chapter, provides an excellent example. In order to achieve this level of accuracy, GPS measurements from the spacecraft are processed together with GPS data from a worldwide network of ground stations and an extensive set of dynamic models. Future science missions are likely to push this requirement even further towards the millimeter level.

56  

"Delta Clipper Contractors Tout Components' Success," Space News, 27 September - 3 October 1993, p. 17. The DC-X is a one-third-scale sub-orbital, single-stage-to-orbit (SSTO) technology demonstrator developed with funding from the Ballistic Missile Defense Organization (BMDO).

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Attitude Determination

In the last several years, several manufacturers of GPS receivers have started collaborating with spacecraft developers to design GPS receivers for use as attitude sensors on board spacecraft. On-board attitude determination is a requirement for virtually every modern spacecraft, and most also require an automatic attitude control system. The traditional suite of sensors used for attitude determination range from relatively low-cost magnetometers and horizon sensors to precise gyroscopes, sun sensors, and star trackers. GPS may provide a cost-effective complement or even alternative to many of these existing systems.

GPS attitude determination is accomplished by observing the carrier phase of an incoming GPS signal at two or more antennas on board the spacecraft. The difference in phase between the antennas can be related to the vehicle orientation and the rate of change of these phase observations is an indication of the attitude rate of change. The accuracy of GPS for this application is limited by multipath, the phase noise in the receiver, the separation of the antennas, and the stability of the structure supporting the antennas. With the best current technology, accuracies as good as 0.1 degrees (2s) can be expected.

The accuracy requirements for satellite attitude determination range from 5 degrees for some simple spacecraft to well below 3 x 10-6 degrees (0.01 arc seconds) for a spacecraft like the Hubble Telescope. At this stage GPS cannot replace the high performance of star trackers for this ultimate precision, but may provide a cost-effective alternative for many mission requirements.

Launch and Re-entry Vehicle Guidance

GPS also has applications to space launch vehicles as a sensor in the vehicle's navigation system and for providing positioning information to ground controllers for range safety purposes. As previously mentioned, an integrated GPS/inertial navigation system has been tested on the experimental BMDO/McDonnell Douglas Delta Clipper (DC-X), and on Orbital Science Corporation's Pegasus launch vehicle. In addition, an experimental space re-entry vehicle called the Spacewedge, designed for re-entry rather than launch, is demonstrating the ability to make an automatic precision landing using a parafoil and a commercial GPS receiver. A full-scale space vehicle, either piloted or unpiloted, may one day use GPS-based technology for emergency crew return or cargo return from Earth's orbit.57 Accuracy requirements have not been provided for these experimental applications.

Most range safety tracking for launch vehicles currently is conducted using a rather elaborate and expensive system consisting of ground tracking radars and associated equipment. According to a previously published NRC study, it is conceivable that pending

57  

Spacewedge, known formally as the "spacecraft autoland gliding parachute experiment," has been developed by NASA's Dryden Flight Research Facility, Edwards AFB, California for under $100,000 annually. J. R. Asker, "Space Autoland System Shows GPS' Wide Uses," Aviation Week & Space Technology, 18 October 1993, pp. 54-55.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

further study by range safety experts, GPS-derived trajectory data could be used as a more cost-effective alternative.58 The DOD has been considering the use of GPS as the primary time and space position information source for the national ranges ever since the Range Applications Joint Program Office was established approximately 5 years ago, and the Navy has been utilizing GPS trajectory data for Trident missile testing since the early 1980s.59 Accuracy requirements for GPS range safety applications are very mission specific, and have not been generalized.

GPS also could be used to improve range safety by sending flight termination commands to missiles and launch vehicles carrying GPS receivers. This could be accomplished using a DGPS datalink or a pseudolite located at the range or, as suggested by one expert in range safety, by using some spare data bits available in the GPS navigation message itself.60 Current flight termination telecommands, which are used to initiate self-destruction, are broadcast in the UHF frequency band. This band is very susceptible to spoofing, jamming, and interference. Integrating a telecommand with other GPS and DGPS equipment and datalinks already under development for time and position range applications could provide a more secure and cost effective means of initiating a flight termination when it is necessary.

A consolidated list of available GPS requirements for spacecraft applications is provided in Table 2-10.

Table 2-10 Requirements for GPS Spacecraft Applicationsa

 

Application

Accuracy

Satellites

Orbit Determination (Real Time)

50 m (2 drms)

 

Orbit Determination (Post-Process)

± 0.001 m (2 drms)

 

Attitude Determination

5 degrees to 3 x 10-6 degrees

 

 

(2σ)b

Launch Vehicles

Launch Trajectory and Position

Mission Specific

 

Determination

 

a. Accuracy is currently the only specified requirement for GPS spacecraft applications. The values in this table were derived by the committee from input received during the study.

b. Accuracy as good as 3 x 10-6 degrees (2σ) is currently available only from star trackers. GPS is currently capable of 0.1 degree (2σ) attitude determination accuracy, which is suitable for most spacecraft missions.

58  

NRC, Technology For Small Spacecraft, Aeronautics and Space Engineering Board, National Research Council (Washington, D.C.: National Academy Press, 1994), p. 16.

59  

Source of Information: Personal conversation with Daniel F. Alves, Jr. of Alpha Instrumentation/Information Management, AI2M, Santa Maria, California, 20 February 1994.

60  

Daniel F. Alves, Jr., Global Positioning System Telecommand Link, U.S. Patent number 5,153,598, 6 October 1992.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Challenges to Full GPS Utilization

Orbit Determination and Orbital Positioning

The GPS SPS currently delivers sufficient accuracy for real-time orbit determination requirements. The well-known orbital dynamics of most spacecraft allow filtering of the data, which helps to mitigate the effects of SA. A-S does not have a significant effect on real-time orbit determination because most satellites are above the densest part of the ionosphere and can probably ignore the ionospheric delay contribution.

For precise post-flight requirements SA does not pose a problem, but the presence of A-S reduces the orbit determination accuracy of spacecraft missions which rely on codeless L2 measurements. Topex/Poseidon, for example, relies on the use of dual-frequency data, but does not carry a receiver capable of tracking the Y-code. The processing of raw data from a large network of stations will still be required, however, even if the basic GPS accuracy is improved. Nevertheless, all improvements to the basic system will aid in the search for the last centimeter or millimeter of precision.

Attitude Determination

Because GPS attitude determination techniques use differential carrier-phase measurements, SA has little or no effect on the accuracy achievable. A-S may have some effect in that it prevents the use of differential P-Code measurements for coarse attitude determination and makes the use of dual frequency differential carrier-phase measurements more difficult. As mentioned previously, however, the accuracy of GPS for this application is limited primarily by design parameters related to receiver electronics and antenna structure.

Signal Visibility

Satellites in orbit near or above the GPS constellation are only able to track GPS signals that pass beyond the limb of the Earth. On the current Block II/IIA satellites there is sufficient antenna beamwidth to allow orbit determination to be performed at geosynchronous altitudes using GPS and a significant amount of dynamic modeling.61 The Block IIR and IIF satellites, however, may not have the same antenna beamwidth, and the L-band signals broadcast from these antennas may no longer pass beyond the limb of the Earth. This could eliminate the ability of a geosynchronous satellite to receive GPS signals, precluding a potentially important GPS application.

61  

S. C. Wu et al., "GPS-Based Precise Tracking of Earth Satellites from Very Low to Geosynchronous Orbits," in Proceedings of the National Telesystems Conference (Ashburn, Virginia, May 1992), pp. 4-1 to 4-8.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Findings

The presence of SA has little or no effect on the ability to use GPS for orbit determination, but A-S limits the performance of orbit determination for spacecraft that rely on dual-frequency measurements, such as Topex/Poseidon.

SA has no effect on the accuracy of GPS attitude determination methods for spacecraft. A-S may place some limitations on achievable accuracy, but so do design parameters related to receiver electronics and antenna structure.

The ability to use GPS for orbit determination on board geosynchronous satellites will be lost if the Block IIR and IIF spacecraft are built with narrower beamwidth antennas than the Block II/IIA satellites.

Summary

Although this chapter does not represent a complete list of all GPS applications and their requirements, it should be clear from its content that the Global Positioning System has become an integral part of our nation's technical infrastructure, which contributes to our security, economy, and overall quality of life. Indeed, a fully exhaustive list of GPS applications may be impossible to compile, for as soon as it was completed, dozens of new and innovative applications, such as navigation systems for the visually impaired, would be developed that exploit GPS to the limits of its technological capability. Although requirements for currently undiscovered applications such as this one cannot be quantified, a strong case can be made for not only maintaining the basic system's operational capability but also for continuously improving it in order to meet the increasingly demanding requirements of a multitude of military and civilian users who rely on GPS on a routine basis.

The tables included in this summary represent a compilation of the GPS applications that have been discussed in this chapter. Military applications with accuracy requirements currently unmet by the PPS are included in Table 2-11, and civil applications are grouped according to their accuracy requirements in tables 2-12 though 2-16. As these tables and the preceding discussions in this chapter clearly illustrate, the civilian applications that currently require augmentation or enhancement of the GPS SPS far outweigh those that do not. Most integrity and availability requirements for civilian applications are also unmet by the GPS SPS and are highlighted in the tables through the use of grey shading. Candidate technical improvements and modifications to the basic GPS that would enhance its functionality and make it more capable of meeting the requirements of both civilian and military users are discussed in the next two chapters.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-11 Summary of Military Applications with Accuracy Requirements Unmet by the GPS PPS as Currently Specifieda

 

Application

Accuracy

Integrity

 

 

 

 

1 minus PHE times PMD

Time to Alarm

Aviation

Non-precision Sea Approach/Landings

12.0 m (2 drms)

0.999

10s

 

Precision Approach/Landings Unprepared Surface

125 m (2 drms)

0.999

6 s

 

Precision Sea Approach/Landings

0.6 m (2 drms)

0.999

6

Mine Warfare

Anti-mine Countermeasures

< 5.0 m CEP

Not specified

Not specified

Special Warfare

Combat Swimming

1.0 m CEP

Not specified

Not specified

 

Land Warfare & Insertion/Extraction

1.0 m CEP

Not specified

Not specified

Amphibious Warfare

Artillery & Reconnaissance

< 6.0 m CEP

Not specified

Not specified

 

Precision-guided Munitions

3.0 m CEP

Not specified

Not specified

a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

Table 2-12 Summary of Civilian Applications with Accuracy Requirements of 100 Meters or Greater (currently achievable with the basic GPS SPS)a

 

Application

Accuracy (2 drms)

Integrity

 

Availability

 

 

 

1 minus PHE times PMD

Time to Alarm

 

Aviation

En route Oceanic

23 km

Not specified

30 s

99.977%

 

En route through Non-precision Approach/Landings

100 m

1-1x10-7 per hour

8 s

99.999%

 

Domestic Automatic Dependent Surveillance (ADS)

200 m

Not specified

Not specified

99.999%

Maritime

Oceanic Navigation

1800 to 3700 m

Not specified

Not specified

99.0%

 

Coastal Navigation

460 m

Not specified

Not specified

99.7%

a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-13 Summary of Civilian Accuracy Requirements Between 25 and 100 Metersa

 

Application

Accuracy (2 drms)

Integrity

 

Availability

 

 

 

1 minus PHE times PMD

Time to Alarm

 

ITS and Vehicle Navigation/Position Location

Fleet Management (AVL/AVI)

25 to 1500 m

Not specified

1 to 15 s

99.7%

 

Emergency Response

75 to 100 m

Not specified

1 to 15 s

99.7%

 

Vehicle Command and Control

30 to 50 m

Not specified

1 to 15 s

99.7%

 

Accident Data Collection

30 m

Not specified

1 to 15 s

99.7%

Spacecraft (Satellites)

Orbit Determination (real time)

50 m

Not specified

Not specified

Not specified

a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

Table 2-14 Summary of Civilian Accuracy Requirements Between 10 and 25 Metersa

 

Application

Accuracy

(2 drms)

Integrity

 

Availability

 

 

 

1 minus PHE times PMD

Time to Alarm

 

Aviation

TCAS

14.4 m

Not specified

Not specified

Several Days

 

Surface Surveillance

24.0 m

Not specified

Not specified

99.87%

Maritime

Recreational Boating

10.0 m

Not specified

Not specified

99.9%

 

Vessel Traffic Services

10.0 m

Not specified

Not specified

99.9%

ITS

Infrastructure Management

10.0 m

Not specified

1 to 15 s

99.7%

Search & rescue

Location Determination

10.0 m

Not specified

minutes

99.0%

Oceanography

Real-time Navigation and Positioning

10.0 to 30.0 m

Not specified

Not

Not specified

a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2-15 Summary of Civilian Accuracy Requirements Between 1 and 10 Metersa

 

Application

Accuracy (2 drms)

Integrity

 

Availability

 

 

 

1 minus PHE times PMD

Time to Alarm

 

Aviation

CAT I Approach/Landing

7.6 m

1-4 x 10-8 per approach

5.2s

99.9%

 

CAT II Approach/Landing

1.7 m (vertical)

1 -0.5 x 10-9 per approach

2.0s

Not specified

Maritime

Harbor/Harbor Approach

Navigation

8.0 to 20.0 m

Not specified

6 to 10 s

99.7%

 

Inland Waterway Navigation

3.0 m

Not specified

6 to 10 s

Not specified

Railroad

Train Control

1.0 m

Not specified

5 s

99.7%

ITS and Vehicle Navigation/Position-Location

Highway Navigation and Guidance

5.0 to 20.0 m

Not specified

1 to 15 s

99.7%

 

Mayday/Incident Alert

5.0 to 30.0 m

Not specified

1 to 15 s

99.7%

 

Automated Bus/Rail-Stop

Annunciation

5.0 to 30.0 m

Not specified

1 to 15 s

99.7%

 

Collision Avoidance, Control

1.0 m

Not specified

1 to 15 s

99.7%

 

Collision Avoidance,

Hazardous Situation

5.0 m

Not specified

1 to 15 s

99.7%

Hazmat

Transport

Vehicle or Cargo Location

5.0 m

Not specified

1 s

99.7%

Land

Recreation

Off-road Vehicles, Hikers,

Back-country Skiers, etc.

5.0 m

Not specified

Minutes

99.0%

Earth Science

Airborne Geophysics

3.0 m (vertical)

Not specified

Minutes

Not specified

Mapping/

Surveying

Geographic Information

Systems (GIS)

1.0 to 10.0 m

Moderate

Not specified

98%

a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

Suggested Citation:"2 GPS Applications and Requirements." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table 2—16 Summary of Submeter Civilian Accuracy Requirementsa

 

Application

Accuracy (2 drms)

Integrity

 

Availability

 

 

 

1 minus PHE times PMD

Time to Alarm

 

Aviation

CAT III Approach/Landing

0.6 to 1.2 m (vertical)

1 -0.5 X 10-9 per approach

2.0 s

Not specified

Precision Farming

Automatic Vehicle Guidance

0.05 m

Not specified

5.0 s

99.7%

Mapping/ Surveying/ Geodesy

Photogrammetry

0.02 to 0.05 m

Not specified

Minutes

98.0%

 

Remote Sensing

0.1 to 20.0 m

Not specified

Not specified

98.0%

 

Geodesy

0.01 to 0.05 m

Not specified

Hours

98.0%

 

Mapping

0.1 to 10.0 m

Not specified

Hours

98.0%

 

Surveying

0.01 to 10.0 m

Not specified

Hours

98.0%

Earth Science

Oceanography (ocean circulation determination)

0.01 m

Not specified

Hours

Not specified

 

Geodynamics

0.001 m + 109 x baseline length

Not specified

Hours

Not specified

Spacecraft (satellites)

Orbit Determination (post-process)

± 0.001 m

Not specified

Not specified

Not specified

 

Attitude Determination

3 x 10-6 degrees (0.01 arc second), 2σ

Not specified

Not specified

Not specified

a. References and/or additional notes for each of the requirements listed in this table can be found by referring to previous tables (2-1 through 2-10) included in this chapter.

Suggested Citation:"2 GPS Applications and Requirements." 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:"2 GPS Applications and Requirements." 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:"2 GPS Applications and Requirements." 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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Page 64
Suggested Citation:"2 GPS Applications and Requirements." 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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Page 65
Suggested Citation:"2 GPS Applications and Requirements." 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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Page 66
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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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