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
Confidential - Copyright © 2007 NavCom Technology, Inc.
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
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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
18
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
21
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
22
-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
Confidential - Copyright © 2007 NavCom Technology, Inc.
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
Confidential - Copyright © 2007 NavCom Technology, Inc.
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
28
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
Confidential - Copyright © 2007 NavCom Technology, Inc.
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
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Probability of Sample of Give Magnitude
36
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
Confidential - Copyright © 2007 NavCom Technology, Inc.
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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