National Academies Press: OpenBook

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

Chapter: Appendix L: Enhanced Signal Structures for the Military

« Previous: Appendix K: Direct Y-Code Acquisition
Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Appendix L
Enhanced Signal Structures for the Military

A significant increase (approximately 10 dB) in anti-jam capability could possibly be achieved on the Block IIF satellites by employing another wide-band signal, occupying perhaps 100 MHz to 200 MHz. Such a broad signal would require that the carrier be at Sband (approximately 3 GHz) or higher frequency. The move to a higher frequency also would reduce nulling antenna size and increase its performance. Such a high frequency would also provide increased immunity to the effects of ionospheric scintillation, which can degrade receiver performance when it is present.1

To demonstrate the anti-jam effectiveness of a wide-band, fine ranging signal, calculations for seven possible signal scenarios (with various bandwidths, antennas, and inertial aiding) have been performed for jammers operating at power levels of 100 watts and 10 kilowatts. In each case, the jammers were assumed to be co-located with the target. At these two power levels, code- and carrier-tracking thresholds were estimated as a function of range from the jammer. For many applications, the key parameter is not the minimum range for signal lock, but the minimum range for acceptable range error. Therefore, the minimum range-to-jammer for a 1-meter range error was also determined. It is important to distinguish two quite different operating scenarios: direct attack and loiter. In direct attack, the range-to-target is closed as rapidly as possible. Once GPS is lost, guidance to the target is by inertial guidance alone. Mission success then depends upon the remaining distance to target as well as the inertial drift rate. By contrast, in loitering scenarios such as remotely piloted vehicle reconnaissance and other scenarios involving sustained area-wide high accuracy, loss of GPS means loss of high accuracy positioning, as inertial drifts can quickly exceed mission error bounds.

Table L-1 summaries the seven signal scenarios. Scenario 1, 2, and 3 with Y-code signaling (20-MHz bandwidth) were considered as baseline for comparison with the other scenarios, each with a 100-MHz chipping rate (200-MHz bandwidth). A high chipping rate direct-sequence modulation was chosen to improve both the jamming margin and pseudorange accuracy. Under the assumption that a wide region of the L-band would be hard to come by and that beam-forming antenna structures are large at L-band, a fourfold

1  

Ionospheric scintillation is a phenomenon in which the Earth's ionosphere introduces rapid phase and amplitude fluctuations in the received signals.

Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

frequency increase was predicted. In each scenario, attention was given to the thermal noise limited region and the interference limited region. For military users in a combat environment, receiver and thermal noise is negligible compared with jamming power.

Table L-1 Summary of Seven Signal Scenarios with Different Bandwidths, Antennas, and Inertial Aiding

Scenario

Bandwidth

Antenna Used

Inertial

Aiding

Code Loop

Tracking

Bandwidth

Carrier Loop

Tracking

Bandwidth

1

(Baseline)

Y-code

Bandwidth

20 MHz

Standard

Antenna

No

1.0 Hz

(20 MHz)

2

(Baseline)

Y-code

Bandwidth

(20 Hz)

Standard

Antenna

Yes

1.0 Hz

(aided)

1.0 Hz

(aided)

3

(Baseline)

Y-code

Bandwidth

(20 MHz)

Nulling Antenna

(25 dB nulls)

Yes

0.1 Hz

(aided)

1.0 Hz

(aided)

4

Wide

Bandwidth

(200 MHz)

Standard

Antenna

No

1.0 Hz

20 Hz

5

Wide

Bandwidth

(200 MHz)

Standard

Antenna

Yes

1.0 Hz

(aided)

1.0 Hz

(aided)

6

Wide

Bandwidth

(200 MHz)

Miniature

Antenna

(25 dB nulls)

Yes

0.1 Hz

(aided)

1.0 Hz

(aided)

7

Wide

Bandwidth

(200 MHz)

Null/

Beamforming

Antenna

(31 dB nulls and 6 dB beam gain)

Yes

0.1 Hz

(aided)

1.0 Hz

(aided)

Scenario 1: Unaided Y-Code Bandwidth Signal With A Standard Antenna

For comparison purposes, a baseline of an unaided Y-code bandwidth GPS receiver operating with a standard antenna will be used.

Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Scenario 2: Aided Y-Code Bandwidth With A Standard Antenna

For comparison purposes, a baseline of an aided Y-code bandwidth GPS receiver operating with a standard antenna will be used.

Scenario 3: Aided Y-Code Bandwidth With Nulling Antenna

For comparison purposes, a baseline of an aided Y-code bandwidth GPS receiver operating with a nulling antenna will be used.

Scenario 4: Unaided Wide Bandwidth With Standard Antenna

This scenario is compared with the baseline described in Scenario 1.

Receiver Thermal Noise Limited Case

In this condition, the four times higher radio carrier frequency will give a free-space carrier-to-noise ratio disadvantage of 12 dB. Above the code-tracking loop threshold, the 12 dB loss is more than offset by increased signal bandwidth. Multipath susceptibility is reduced by factors of 10 and 100, respectively, over Y-code and C/A-code.

Noise Jammer Limited Case

Importantly, any increase in free-space loss with frequency is equal for both the interference source and the GPS satellite. With the narrower code chip of the wide-band signal structure, better calibration of the constellation will be needed.

Scenario 5: Aided Wide Bandwidth Standard Antenna

The comparative baseline is the aided Y-code receiver operating with a standard antenna, Scenario 2.

Receiver Thermal Noise Limited Case

In this condition, the four times higher radio carrier frequency will give a free-space carrier-to-noise ratio disadvantage of 12 dB. Above the code-tracking loop threshold, the 12 dB loss is more than offset by increased signal bandwidth. Multipath susceptibility is reduced by factors of 10 and 100, respectively, over Y-code and C/A-code.

Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Noise Jammer Limited Case

As for Scenario 4, any increase in free-space loss with frequency is equal for both the interference source and the GPS satellite. Therefore the wider bandwidth yields a 10-dB advantage in break-lock margin and a further operational advantage at a specified pseudorange accuracy level.

Scenario 6: Aided Wide Bandwidth With Miniaturized Nulling Antenna

The comparative baseline is the aided Y-code receiver operating its nulling antenna, Scenario 3. For equal nulling performance, a fourfold increase in radio frequency would reduce the overall antenna footprint to one-sixteenth the original area, making for a much more practical design in many applications. With aiding, the code- and carrier-tracking loop bandwidths are conservatively reduced to 0.1 Hz and 1 Hz, respectively.

Receiver Thermal Noise Limited Case

Same comments as for Scenario 4.

Noise Jammer Limited Case

As in Scenario 5, the widened signal bandwidth gives an immediate improvement in effective carrier-to-noise ratio of 10 dB against the reference system and a consequent 10-dB increase in jamming-to-signal ratio code and carrier tracking margin. As shown in Table L-1, this factor, together with narrowed tracking loop bandwidths yields a factor of three improvement in minimum jammer distance before loss of lock. More importantly, a factor of six reduction in jammer distance to the 1-meter error threshold is obtained. These results are achieved with a much smaller antenna than at L1.

Scenario 7: Aided Wide Bandwidth With Nulling And Beam-Forming Antenna

The baseline is Scenario 3, an aided Y-code receiver operating with a null-steering antenna. The size advantages of Scenario 4 are now given up in favor of a wide-band antenna possessing four times as many elements. This translates into more (and deeper) nulls and the capacity to form beams in the direction of GPS satellites. It is assumed that nulls are improved by 6 dB over the reference antenna and that a 6-dB gain may be

Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

obtained in the direction of each satellite. Obviously these parameters need future study and verification.

Receiver Thermal Noise Limited Case

Because of antenna beam-forming, there is just a 6-dB loss in carrier-to-noise ratio as compared with the reference Y-code system. Above tracking threshold this loss is more than offset by increased signal bandwidth, with an order of magnitude ranging error improvement.

Noise Jammer Limited Case

This is the most important case. Over the reference system, the widened signal bandwidth gives an immediate improvement in effective carrier-to-noise ratio of 10 dB. To this add 12 dB from improved antenna nulling and beamforming, for a total of 22 dB increase in the jamming-to-signal ratio code- and carrier-tracking margin. As shown in Tables L-2 and L-3, and Figures L-1 and L-2, there is an order of magnitude improvement in minimum jamming distance before loss of lock and a factor of 20 improvement in minimum jamming distance at 1-meter error threshold.

Figures L-1 and L-2 show the pseudorange errors as a function of distance for various receiver alternatives described in Table L-1 and the two jammer power levels.2 The difference between the Y-code and wide-band options is rather dramatic, even on the log-log plots. The most capable system operates below the 1-meter level to within about 45 meters of the 100-watt source. At 1,000 meters, the code-tracking error is below the centimeter level. As shown in Table L-2, carrier-phase tracking and code-loop aiding are available within several hundred meters of the jammer. The miniaturized nulling antenna with aiding is good down to about 175 meters. Both aided wide-band options are substantially more capable than the best performing existing Y-code system.

Tables L-2 and L-3 summarize the results of this exercise. The most significant finding, perhaps, is that with the wide-band signal using unaided tracking and a simple antenna a vehicle can approach a 100-watt jammer to within 6 kilometers before a 1-meter range error has accumulated. With aided tracking, this range is reduced to about 3 kilometers. For many airborne weapons systems this is sufficiently close to permit a successful mission when employing inertial navigation for the balance of the flight (i.e., assuming the worst case scenario in which the jammer and target are co-located). Considering that the size and cost of nulling antennas may prohibit their use on certain weapon systems, this is a significant finding and supports the notion that consideration should be given to the eventual inclusion of a new, very wide-band waveform. Note also that a move to higher frequency makes the nulling antenna more feasible for many vehicles. As a means of defeating enemy jamming, the Air Force should explore the feasibility of adding

2  

Data generated by J. W. Sennott, Bradley University, Peoria, Illinois.

Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

a new wide-band ranging signal on Block IIF satellites operating at S-band or higher frequency.

Figure L-1 Wide-band GPS with 100-watt jammer.

Figure L-2 Wide-band GPS with 10-kilowatt jammer.

Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table L-2 GPS Wide-Band Signal Augmentation Performance 100-Watt Jammer

System Option

Code Status

Carrier Telemetry

Status

Jammer

distance at

loss of lock

(meters)

Jammer

distance for 1-

meter range

error (meters)

Jammer

distance at

loss of lock

(meters)

Range

error at

loss of lock

(meters)

1. Y-code

unaided

standard antenna

18,000

90,000

90,000

1.0

2. Y-code

aided

standard antenna

10,000

35,000

21,000

--

3. Y-code

aided

nulling antenna

550

1,000

1,400

1.9

4. Wide-band

unaided

standard antenna

6,000

6,000

35,000

0.1

5. Wide-band

aided

standard antenna

3,100

3,100

6,500

0.27

6. Wide-band

aided

miniature antenna

175

175

450

0.19

7. Wide-band

aided

null/beamforming

antenna

45

45

215

0.19

Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×

Table L-3 GPS Wide-Band Signal Augmentation Performance 10-Kilowatt Jammer

System Scenario

Code Status

Carrier Telemetry Status

Jammer

distance at

loss of lock

(meters)

Jammer

distance for 1-

meter range

error (meters)

Jammer

distance at

loss of lock

(meters)

Range

error at

loss of lock

(meters)

1. Y-code

unaided

standard antenna

--

--

--

--

2. Y-code

aided

standard antenna

--

--

--

--

3. Y-code

aided

nulling antenna

--

20,000

--

--

4. Wide-band

unaided

standard antenna

--

60,000

--

--

5. Wide-band

aided

standard antenna

--

31,000

--

--

6. Wide-band

aided

miniature antenna

--

1,800

--

--

7. Wide-band

aided

null/beamforming

antenna

--

450

--

--

Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 255
Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 256
Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 257
Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 258
Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 259
Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 260
Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 261
Suggested Citation:"Appendix L: Enhanced Signal Structures for the Military." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
×
Page 262
Next: Appendix M: Accuracy of a 14-Satellite Ensemble Versu a 24-Satellite Ensemble »
The Global Positioning System: A Shared National Asset Get This Book
×
Buy Paperback | $61.00 Buy Ebook | $48.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!