A New Anti-Jamming Method for GNSS Receivers
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The leading pioneer in GPS technology
A New Anti-Jamming Method for GNSS Receivers
Jerry Knight, Charles Cahn and Sidharth Nair
Confidential
Copyright © 2007 NavCom Technology, Inc.
Goals Provide protection from jamming of types commonly seen by commercial GNSS receivers such as specified in the DO-229 requirements for airborne equipment - Out of band signals - In band CW-interference - Pulse broadcast
Low cost, small size
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Bandwidth Requirements Semi-codeless P(Y) and L5 signals use 10 MHz codes - Minimum single-sided bandwidth of 10 MHz required - >12 MHz preferred for side-band power
GNSS bands are nominally ≥ 12 MHz Advance multipath mitigation and code tracking techniques prefer as wide a bandwidth as possible - Minimizes code edge distortion by receiver
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Receiver Filtering SAW filters provide nearly ideal filtering -
Nearly flat in-band gain pattern >60 dB of high-pole out-of-band protection Cell phone have driven down cost Small size
Use common IF for all GNSS bands - Use same 100 to 400 MHz SAW filter for all bands - Common IF and SAW make filtering biases nearly identical for all GNSS bands
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Frequency Plan L1, L2, L5, .... plus StarFire Antenna
100 to 250 MHz Common IF
Pseudo-baseband Complex Samples
X Diplexer
L2, L5 L2 LO Synthesizer
Low Loss Filter
Broadband Amplifier
L1 LO Synthesizer
X
A/D
X
A/D
X
A/D
30 MHz Bandpass
X L1, StarFire
A/D
30 MHz Bandpass
X L5 LO Synthesizer
X
30 MHz Bandpass Common 2nd LO
Low Loss Filter
Broadband Amplifier
X StarFire Synthesizer
5
200 kHz Bandpass
StarFire 2nd LO 21 Hz steps
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Signal Processing Amoroso (1983) recognized that if a spread spectrum signal is jammed by a random-phased CW signal, the SNR at the output of the receiver’s correlator is improved by using samples from the crest of the CW sine wave. AGC is set so that crest of the sine wave has a known magnitude. Use samples with magnitude > threshold (active) Inactive samples are not processed
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Spread Spectrum Signal with CW Interference
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Noisy CW-Jammed Signal
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Amaroso Sampling of Jammed Signal
+1
0
-1
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Theoretical Degradation from CW Jamming F IG 2 . O U T P U T S /N A F T E R 3 -L E V E L Q U A N T IZ A T IO N , G A U S S IA N N O IS E + C W J A M M IN G
0 R A N D O M LY P H A S E D JA M M E R J/N = -10 D B -2 -5 D B
D E GR A D A TIO N OF OU TP U T S /N , D B
-4
0 DB
-6
-8
-10 5 DB -12
-14
-16 25 D B
10 D B
-18 15 D B 20 D B -20 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
AC T IV IT Y = P R O B AB IL IT Y Q U AN T IZ E D M AG N IT U D E = 1
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FIG 3. OUTPUT S/N WITH 3-LEVEL QUANTIZATION, GAUSSIAN NOISE + CW JAMMING 0 RANDOMLY PHASED JAMMER
J/N = -10 DB
-2 -5 DB
DEGRADATION OF OUTPUT S/N, DB
-4
-6
0 DB 15 DB
-8 20 DB -10
25 DB 5 DB
-12
-14
-16 10 DB -18
-20 -30
-20
-10
0
10
20
30
INPUT GAIN, DB (FIXED QUANTIZING THRESHOLD =1.0)
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Difficulties with Amoroso Difficult to determine J/S The ideal AGC level and threshold are functions of J/S The ideal threshold for weak jamming gives poor results for strong jamming and vice versa - Activity = 0.54 is ideal if no jamming 0.3 to 0.7 provide near-optimal results
- Activity < 0.10 for strong jamming
Amoroso used 4-level sampling - It is well known that 3-level sampling provides additional anti CW-jamming capability - 3-level sampling greatly simplifies digital signal processing
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New Method 2-bit, 3-bit or 4-bit A/D samples of IF signal - 4-bit best for pulse jamming
Use two thresholds - First threshold sets activity level - Second threshold controls conversion from A/D samples to 3-level
Near optimal Amoroso thresholds and AGC are obtained when the AGC threshold is 0.5 times the 3-level conversion threshold
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Theory of 3-Level Quantized Correlation
D
[ p (V ) p ( V )] 2 n
V
p ( x ) dx
2
p ( x ) dx
V
p(x) = probability density of jamming + noise = standard deviation of noise V = magnitude quantizing threshold Denominator = “Activity 14
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Activity for a CW Jammer Amplitude 1.0
Active
0.5
Active
Sin(30ْ) = 0.5 Threshold = 0.5
Inactive 16
%
30ْ
Inactive
Activity = 0.67
16% 16%
- 0.5
Active
Active
-1.0
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Population Distribution for AGC
0.5
0.45
0.4
Probabilty
0.35
0.3
0.25
0.2
33%
33%
33%
0.15
0.1
0.43 0.05
0 -3
-2
-1
0
1
2
3
Standard Deviations
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Population Distribution for 3-Level Samples
0.5
0.45
0.4
Probabilty
0.35
0.3
0.25
0.2
0.15
20%
60%
20%
0.1
0.86 0.05
0 -3
-2
-1
0
1
2
3
Standard Deviations
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A/D to AGC and 3-Level Sample Conversion
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A/D (Binary)
Sign - Magnitude
AGC
3-Level
1111
+7
Active
+1
1110
+6
Active
+1
1101
+5
Active
+1
1100
+4
Active
+1
1011
+3
Active
+1
1010
+2
Active
+1
1001
+1
Active
0
1000
+0
Inactive
0
0111
-0
Inactive
0
0110
-1
Active
0
0101
-2
Active
-1
0100
-3
Active
-1
0011
-4
Active
-1
0010
-5
Active
-1
0001
-6
Active
-1
0000
-7
Active
-1 Confidential - Copyright © 2007 NavCom Technology, Inc.
AGC
Sample Enable EN
TC Div N
Imag[2:0] > Threshold
T=1 F=0
+
2
+
EN
IQ Sum CLR [8:0]
9
T EN IQ Sum > Target F
AGC_M AGC_P
Qmag[2:0] > Threshold
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Proposed and Optimum CW Jamming Performance F IG 4 , O U T P U T S /N W IT H 3 -L E V E L Q U A N T IZ A T IO N W IT H G A U S S IA N N O IS E + C W J A M M IN G
0
ASYMPTOTES
D E GR A D A TIO N OF OU TP U T S /N , D B
-2
-4
O P T IM U M -6
PROPOSED -8
-10
-12 -10
-5
0
10
5
15
20
25
J/N , D B
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CW Jamming Test
110 dBm – 0 dBm
11 dBm – 0 dBm
Noise Com Generator (-30dBm)
Jamming signal strength is varied by varying the attenuators
Sapphire GNSS Receiver
Combiner
Spirent GNSS Simulator (-121 dBm)
LNA Noise Figure 2 dBm
110 dBm – 0 dBm
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AGC Voltage
11 dBm – 0 dBm
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C/N0 vs. CW Jamming I/Q vs CW Jamming - Varying GPS Signal Attenuation 55
50
I/Q in dB-Hz
45
40
35
30
25
20 -140
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-121dbm -123dbm -126dbm -128dbm -131dbm -133dbm -136dbm -130
GPS GPS GPS GPS GPS GPS GPS
-120
Signal Signal Signal Signal Signal Signal Signal -110 -100 -90 Jamming in dBm
-80
-70
-60
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I/Q vs. J/S - Varying GPS Signal Strength
I/Q vs J/S - Varying GPS Signal Attenuation 55
50
I/Q in dB-Hz
45
40
35 -121dbm -123dbm -126dbm -128dbm -131dbm -133dbm -136dbm
30
25
20 -20
23
-10
0
GPS GPS GPS GPS GPS GPS GPS
Signal Signal Signal Signal Signal Signal Signal
10
20 30 J/S in dB
40
50
60
70
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AGC vs. CW Jamming AGC Voltage vs Jamming for CW Jamming - Varying GPS Signal Attenuation 1.3 1.2
1.1 1
AGC V
0.9
0.8 0.7 0.6 0.5 0.4 0.3 -140
24
-130
-120
-110
-100 Jamming in dBm
-90
-80
-70
-60
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C/N0 vs. J/S – In Band CW Jamming
I/Q v/s J/S - Varying Center frequency of CW jammer from 1575Mhz to 1558 Mhz 55 50 45
I/Q in dB-Hz
40 35 30 25 20 15 -20
25
0
20
40 60 J/S in dB
80
100
120
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AGC vs. J/S – Out of Band CW Jammer
I/Q v/s J/S - Varying Center frequency of CW jammer from 1525Mhz to 1625 Mhz 55 50 45
I/Q in dB-Hz
40 35 at at at at at at at
30 25 20 15 -20
26
0
1575 MHz 1555 MHz 1550 MHz 1525 MHz 1545 MHz 1595 MHz 1625 MHz 20
40
60 J/S in dB
80
100
120
140
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Sweep Test Setup
Sweep 1575.32213 MHz to 1575.32233 MHz at 1 Hz steps 70 dBm – 50 dBm
AGC Voltage
5 dBm – 0 dBm
HP Signal Generator (-30 dBm) Sapphire GNSS Receiver
Combiner
Spirent GNSS Simulator (-121 dBm)
LNA Noise Figure 2 dBm
110 dBm – 0 dBm
27
11 dBm – 0 dBm
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Frequency Sweep Test Results
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Jamming Strength (dBm)
J/S in dB
Status
-70 + (-30) = -100
-100-(-121) = 21
LOCK
-65 + (-30) = -95
-95-(-121) = 26
LOCK
-64 + (-30) = -94
-94-(-121) = 27
LOCK
-63 + (-30) = -93
-93-(-121) = 28
LOCK
62 + (-30) = -92
-92-(-121) = 29
LOCK
-61 + (-30) = -91
-91-(-121) = 30
LOCK
-60 + (-30) = -90
-90-(-121) = 31
Loss of LOCK
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Frequency Sweep J/S 30 dB C/No and Costas Ratio v/s time - J/S = 30dB 50
SV 1 C/No
40 30 20 10 0
100
200
300
400
500 600 700 Run Time in Seconds
800
900
1000
1100
100
200
300
400
500 600 700 Run Time in Seconds
800
900
1000
1100
1.5
SV 1 CR
1 0.5 0 -0.5 -1 -1.5
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Frequency Sweep J/S 31 dB C/No and Costas Ratio v/s time - J/S = 31dB 50
SV 1 C/No
40 30 20 10 0
100
200
300
400 500 600 Run Time in Seconds
700
800
900
1000
100
200
300
400 500 600 Run Time in Seconds
700
800
900
1000
1.5
SV 1 CR
1 0.5 0 -0.5 -1 -1.5
30
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Broadband Jamming Test
110 dBm – 0 dBm
11 dBm – 0 dBm
Noise Com Generator (-30dBm)
AGC Voltage
Jamming signal strength is varied by varying the attenuators
Sapphire GNSS Receiver
Combiner
Spirent GNSS Simulator (-121 dBm)
LNA Noise Figure 2 dBm
110 dBm – 0 dBm
31
11 dBm – 0 dBm
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30 MHz Broadband Jamming
I/Q v/s J/S - Broadband Jamming BW:30MHz at 1575.42MHz 60 55 50
I/Q in dB-Hz
45 40 35 30 25 20
32
0
10
20
30 J/S in dB
40
50
60
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10 MHz Broadband Jamming
I/Q v/s J/S - Broadband Jamming BW:10MHz at 1575.42MHz 60 55 50
I/Q in dB-Hz
45 40 35 30 25 20
33
0
20
40
60 80 J/S in dB
100
120
140
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1 MHz Broadband Jamming
I/Q v/s J/S - Broadband Jamming BW:1MHz at 1575.42MHz 60 55 50
I/Q in dB-Hz
45 40 35 30 25 20
34
0
20
40
60 80 J/S in dB
100
120
140
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Pulse Jamming Near by radios or pseudolites sometimes create brief interference with very great power 4-bit A/D samples allow automatic detection of a pulsed jammer - Blanking on when > X of 16 samples > Threshold1 - Blanking off when < Y of 128 samples > Threshold2
During the pulse, AGC feedback and digital signal processing must be disabled (samples are blanked by setting them all inactive) - The strength of the un-blanked signal is inversely proportional to the pulse duty cycle The receiver’s front end must quickly recover from the pulse 35
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Probability of Sample of Give Magnitude
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Magnitude
# Standard Deviations
Probability
1
0.43
0.666
2
0.86
0.390
3
1.29
0.197
4
1.72
0.085
5
2.15
0.032
6
2.58
0.0099
7
3.01
0.0026 Confidential - Copyright © 2007 NavCom Technology, Inc.
Pulse Jamming Pulse Jamming Tests - C/No v/s J/S 60
50
I/Q in dB-Hz
40
30
20 10% 20% 30% 40% 50%
10
0 -20
37
0
duty duty duty duty duty 20
cycle cycle cycle cycle cycle 40
60 80 J/S in dB
100
120
140
160
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Conclusions We have demonstrated a simple and effective method of implementing 3-level sampling that maintains Carrier phase tracking in the presence of CW jamming with J/S as large as 60 dB - The method does not overcome spectral line densities weaknesses of the C/A codes
Use of 4-bit A/D samples allows automatic detection and mitigation of very strong pulse jamming signals - Post-correlation C/N0 is reduced in proportion to the duty cycle of the jammer 38
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