A New Anti-Jamming Method for GNSS Receivers

February 7, 2018 | Author: Anonymous | Category: Science, Physics, Electronics
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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

8

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Amaroso Sampling of Jammed Signal

+1

0

-1

9

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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

10

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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

15

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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

16

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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

17

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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

19

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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

20

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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

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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

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

29

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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

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

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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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