the leading pioneer in gps technology copyright © 2007 navcom technology, inc.confidential a new...
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The leading pioneer in GPS technology
Copyright © 2007 NavCom Technology, Inc.Confidential
A New Anti-Jamming Method A New Anti-Jamming Method for GNSS Receiversfor GNSS Receivers
Jerry Knight, Charles Cahn and Sidharth Nair
Confidential - Copyright © 2007 NavCom Technology, Inc.2
GoalsGoals
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
Confidential - Copyright © 2007 NavCom Technology, Inc.3
Bandwidth RequirementsBandwidth 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
Confidential - Copyright © 2007 NavCom Technology, Inc.4
Receiver FilteringReceiver 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
Confidential - Copyright © 2007 NavCom Technology, Inc.5
Frequency PlanFrequency Plan
Diplexer
L1, L2, L5, .... plusStarFire Antenna
100 to 250 MHz Common IF Pseudo-basebandComplex Samples
L2, L5
BroadbandAmplifier
X X
Low LossFilter
30 MHzBandpass
A/D
L2 LOSynthesizer
X X A/D
L5 LOSynthesizer
X X A/D
L1 LOSynthesizer
L1, StarFire
BroadbandAmplifier
Low LossFilter
30 MHzBandpass
30 MHzBandpass
X X A/D
StarFireSynthesizer
200 kHzBandpass
Common2nd LO
StarFire2nd LO
21 Hz steps
Confidential - Copyright © 2007 NavCom Technology, Inc.6
Signal ProcessingSignal 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
Confidential - Copyright © 2007 NavCom Technology, Inc.7
Spread Spectrum Signal with CW Spread Spectrum Signal with CW Interference Interference
Confidential - Copyright © 2007 NavCom Technology, Inc.8
Noisy CW-Jammed Signal Noisy CW-Jammed Signal
Confidential - Copyright © 2007 NavCom Technology, Inc.9
Amaroso Sampling of Jammed Signal Amaroso Sampling of Jammed Signal
+1
0
-1
Confidential - Copyright © 2007 NavCom Technology, Inc.10
Theoretical Degradation from CW Jamming Theoretical Degradation from CW Jamming
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
ACTIVITY = PROBABILITY QUANTIZED MAGNITUDE = 1
DE
GR
AD
ATI
ON
OF
OU
TPU
T S
/N, D
B
J/N = -10 DB
-5 DB
0 DB
5 DB
10 DB
15 DB20 DB
25 DB
RANDOMLY PHASED JAMMER
Confidential - Copyright © 2007 NavCom Technology, Inc.11
FIG 3. OUTPUT S/N WITH 3-LEVEL QUANTIZATION, GAUSSIAN NOISE + CW JAMMING
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
-30 -20 -10 0 10 20 30
RANDOMLY PHASED JAMMER
Confidential - Copyright © 2007 NavCom Technology, Inc.12
Difficulties with AmorosoDifficulties 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
Confidential - Copyright © 2007 NavCom Technology, Inc.13
New MethodNew 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
Confidential - Copyright © 2007 NavCom Technology, Inc.14
Theory of 3-Level Quantized CorrelationTheory of 3-Level Quantized Correlation
V
V
n
dxxpdxxp
VpVpD
)()(
)]()([ 22
p(x) = probability density of jamming + noise= standard deviation of noiseV = magnitude quantizing thresholdDenominator = “Activity
Confidential - Copyright © 2007 NavCom Technology, Inc.15
Activity for a CW JammerActivity for a CW Jammer
16%
0.5
- 0.5
16%
16%
1.0
-1.0
Amplitude
InactiveInactive
Active Active
Active Active
Sin(30ْ) = 0.5
Threshold = 0.5
Activity = 0.6730ْ
Confidential - Copyright © 2007 NavCom Technology, Inc.16
Population Distribution for AGC Population Distribution for AGC
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
-3 -2 -1 0 1 2 3
Standard Deviations
Pro
ba
bil
ty
33%
0.43
33% 33%
Confidential - Copyright © 2007 NavCom Technology, Inc.17
Population Distribution for 3-Level Samples Population Distribution for 3-Level Samples
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
-3 -2 -1 0 1 2 3
Standard Deviations
Pro
ba
bil
ty
0.86
60%20% 20%
Confidential - Copyright © 2007 NavCom Technology, Inc.18
A/D to AGC and 3-Level Sample Conversion A/D to AGC and 3-Level Sample Conversion
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.19
AGCAGC
AGC_M
AGC_P
> ThresholdImag[2:0]
> ThresholdQmag[2:0]
T = 1F = 0 +
Sample Enable
+ IQ Sum[8:0]
EN CLR
Div NTCEN
IQ Sum > TargetEN T
F2 9
Confidential - Copyright © 2007 NavCom Technology, Inc.20
Proposed and Optimum CW Jamming Proposed and Optimum CW Jamming Performance Performance
-12
-10
-8
-6
-4
-2
0
-10 -5 0 5 10 15 20 25
J/N, DB
DE
GR
AD
ATI
ON
OF
OU
TPU
T S
/N, D
B
ASYMPTOTES
PROPOSED
OPTIMUM
Confidential - Copyright © 2007 NavCom Technology, Inc.21
CW Jamming Test CW Jamming Test
Noise Com Generator(-30dBm)
Spirent GNSS Simulator
(-121 dBm)
CombinerSapphire GNSS
Receiver
AGC Voltage110 dBm – 0 dBm 11 dBm – 0 dBm
110 dBm – 0 dBm 11 dBm – 0 dBm
Jamming signal strength is varied by
varying the attenuators
LNANoise Figure 2 dBm
Confidential - Copyright © 2007 NavCom Technology, Inc.22
C/N0 vs. CW Jamming C/N0 vs. CW Jamming
-140 -130 -120 -110 -100 -90 -80 -70 -6020
25
30
35
40
45
50
55
Jamming in dBm
I/Q
in d
B-H
z
I/Q vs CW Jamming - Varying GPS Signal Attenuation
-121dbm GPS Signal
-123dbm GPS Signal
-126dbm GPS Signal
-128dbm GPS Signal-131dbm GPS Signal
-133dbm GPS Signal
-136dbm GPS Signal
Confidential - Copyright © 2007 NavCom Technology, Inc.23
I/Q vs. J/S - Varying GPS Signal Strength I/Q vs. J/S - Varying GPS Signal Strength
-20 -10 0 10 20 30 40 50 60 7020
25
30
35
40
45
50
55
J/S in dB
I/Q
in d
B-H
z
I/Q vs J/S - Varying GPS Signal Attenuation
-121dbm GPS Signal
-123dbm GPS Signal
-126dbm GPS Signal
-128dbm GPS Signal-131dbm GPS Signal
-133dbm GPS Signal
-136dbm GPS Signal
Confidential - Copyright © 2007 NavCom Technology, Inc.24
AGC vs. CW Jamming AGC vs. CW Jamming
-140 -130 -120 -110 -100 -90 -80 -70 -600.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
Jamming in dBm
AG
C V
AGC Voltage vs Jamming for CW Jamming - Varying GPS Signal Attenuation
Confidential - Copyright © 2007 NavCom Technology, Inc.25
C/N0 vs. J/S – In Band CW Jamming C/N0 vs. J/S – In Band CW Jamming
-20 0 20 40 60 80 100 12015
20
25
30
35
40
45
50
55
J/S in dB
I/Q
in d
B-H
z
I/Q v/s J/S - Varying Center frequency of CW jammer from 1575Mhz to 1558 Mhz
Confidential - Copyright © 2007 NavCom Technology, Inc.26
AGC vs. J/S – Out of Band CW JammerAGC vs. J/S – Out of Band CW Jammer
-20 0 20 40 60 80 100 120 14015
20
25
30
35
40
45
50
55
J/S in dB
I/Q
in d
B-H
z
I/Q v/s J/S - Varying Center frequency of CW jammer from 1525Mhz to 1625 Mhz
at 1575 MHz
at 1555 MHz
at 1550 MHz
at 1525 MHzat 1545 MHz
at 1595 MHz
at 1625 MHz
Confidential - Copyright © 2007 NavCom Technology, Inc.27
Sweep Test Setup Sweep Test Setup
HP Signal Generator(-30 dBm)
Spirent GNSS Simulator
(-121 dBm)
CombinerSapphire GNSS
Receiver
AGC Voltage70 dBm – 50 dBm 5 dBm – 0 dBm
110 dBm – 0 dBm 11 dBm – 0 dBm
LNANoise Figure 2 dBm
Sweep 1575.32213 MHz to 1575.32233 MHz at 1 Hz
steps
Confidential - Copyright © 2007 NavCom Technology, Inc.28
Frequency Sweep Test Results Frequency Sweep Test Results
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
Confidential - Copyright © 2007 NavCom Technology, Inc.29
Frequency Sweep J/S 30 dB Frequency Sweep J/S 30 dB
100 200 300 400 500 600 700 800 900 1000 11000
10
20
30
40
50
SV
1 C
/No
Run Time in Seconds
C/No and Costas Ratio v/s time - J/S = 30dB
100 200 300 400 500 600 700 800 900 1000 1100-1.5
-1
-0.5
0
0.5
1
1.5
SV
1 C
R
Run Time in Seconds
Confidential - Copyright © 2007 NavCom Technology, Inc.30
Frequency Sweep J/S 31 dB Frequency Sweep J/S 31 dB
100 200 300 400 500 600 700 800 900 10000
10
20
30
40
50
SV
1 C
/No
Run Time in Seconds
C/No and Costas Ratio v/s time - J/S = 31dB
100 200 300 400 500 600 700 800 900 1000-1.5
-1
-0.5
0
0.5
1
1.5
SV
1 C
R
Run Time in Seconds
Confidential - Copyright © 2007 NavCom Technology, Inc.31
Broadband Jamming Test Broadband Jamming Test
Noise Com Generator(-30dBm)
Spirent GNSS Simulator
(-121 dBm)
CombinerSapphire GNSS
Receiver
AGC Voltage110 dBm – 0 dBm 11 dBm – 0 dBm
110 dBm – 0 dBm 11 dBm – 0 dBm
Jamming signal strength is varied by
varying the attenuators
LNANoise Figure 2 dBm
Confidential - Copyright © 2007 NavCom Technology, Inc.32
30 MHz Broadband Jamming30 MHz Broadband Jamming
0 10 20 30 40 50 6020
25
30
35
40
45
50
55
60I/Q v/s J/S - Broadband Jamming BW:30MHz at 1575.42MHz
J/S in dB
I/Q
in d
B-H
z
Confidential - Copyright © 2007 NavCom Technology, Inc.33
10 MHz Broadband Jamming10 MHz Broadband Jamming
0 20 40 60 80 100 120 14020
25
30
35
40
45
50
55
60I/Q v/s J/S - Broadband Jamming BW:10MHz at 1575.42MHz
J/S in dB
I/Q
in d
B-H
z
Confidential - Copyright © 2007 NavCom Technology, Inc.34
1 MHz Broadband Jamming1 MHz Broadband Jamming
0 20 40 60 80 100 120 14020
25
30
35
40
45
50
55
60I/Q v/s J/S - Broadband Jamming BW:1MHz at 1575.42MHz
J/S in dB
I/Q
in d
B-H
z
Confidential - Copyright © 2007 NavCom Technology, Inc.35
Pulse JammingPulse 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
Confidential - Copyright © 2007 NavCom Technology, Inc.36
Probability of Sample of Give Magnitude Probability of Sample of Give Magnitude
Magnitude # StandardDeviations
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.37
Pulse JammingPulse Jamming
-20 0 20 40 60 80 100 120 140 1600
10
20
30
40
50
60
J/S in dB
I/Q
in d
B-H
z
Pulse Jamming Tests - C/No v/s J/S
10% duty cycle
20% duty cycle
30% duty cycle40% duty cycle
50% duty cycle
Confidential - Copyright © 2007 NavCom Technology, Inc.38
ConclusionsConclusions
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
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