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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.
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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.
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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.
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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
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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.
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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.
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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
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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
--
--
Representative terms from entire chapter:
standard antenna
