the leading pioneer in gps technology copyright © 2007 navcom technology, inc.confidential a new...

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


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


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


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


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


Spread Spectrum Signal with CW Spread Spectrum Signal with CW Interference Interference


Noisy CW-Jammed Signal Noisy CW-Jammed Signal


Amaroso Sampling of Jammed Signal Amaroso Sampling of Jammed Signal

+1

0

-1


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


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


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


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


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


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


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%


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%


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


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


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


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


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


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


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


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


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


Sweep Test Setup Sweep Test Setup

HP Signal Generator(-30 dBm)


(-121 dBm)


Receiver




Sweep 1575.32213 MHz to 1575.32233 MHz at 1 Hz

steps


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


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


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


Broadband Jamming Test Broadband Jamming Test

Noise Com Generator(-30dBm)


(-121 dBm)


Receiver



Jamming signal strength is varied by

varying the attenuators



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



0 20 40 60 80 100 120 14020

25

30

35

40

45

50

55


J/S in dB

I/Q

in d

B-H

z


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


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


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


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

the leading pioneer in gps technology copyright © 2007 navcom technology, inc.confidential a new...

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