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The leading pioneer in GPS technology Copyright © 2007 NavCom Technology, Inc.Confidential A New Anti-Jamming Method for GNSS Receivers Jerry Knight, Charles Cahn and Sidharth Nair
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Confidential - Copyright © 2007 NavCom Technology, Inc. 2Goals 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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Confidential - Copyright © 2007 NavCom Technology, Inc. 3 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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Confidential - Copyright © 2007 NavCom Technology, Inc. 4 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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Confidential - Copyright © 2007 NavCom Technology, Inc. 5 Frequency Plan
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Confidential - Copyright © 2007 NavCom Technology, Inc. 6 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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Confidential - Copyright © 2007 NavCom Technology, Inc. 7 Spread Spectrum Signal with CW Interference
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Confidential - Copyright © 2007 NavCom Technology, Inc. 8 Noisy CW-Jammed Signal
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Confidential - Copyright © 2007 NavCom Technology, Inc. 9 Amaroso Sampling of Jammed Signal
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Confidential - Copyright © 2007 NavCom Technology, Inc. 10 Theoretical Degradation from CW Jamming
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Confidential - Copyright © 2007 NavCom Technology, Inc. 11
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Confidential - Copyright © 2007 NavCom Technology, Inc. 12 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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Confidential - Copyright © 2007 NavCom Technology, Inc. 13 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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Confidential - Copyright © 2007 NavCom Technology, Inc. 14 Theory of 3-Level Quantized Correlation p(x) = probability density of jamming + noise = standard deviation of noise V = magnitude quantizing threshold Denominator = “Activity
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Confidential - Copyright © 2007 NavCom Technology, Inc. 15 Activity for a CW Jammer Sin(30ْ) = 0.5 Threshold = 0.5 Activity = 0.67 30ْ
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Confidential - Copyright © 2007 NavCom Technology, Inc. 16 Population Distribution for AGC
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Confidential - Copyright © 2007 NavCom Technology, Inc. 17 Population Distribution for 3-Level Samples
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Confidential - Copyright © 2007 NavCom Technology, Inc. 18 A/D to AGC and 3-Level Sample Conversion A/D (Binary) Sign - MagnitudeAGC3-Level 1111+7Active+1 1110+6Active+1 1101+5Active+1 1100+4Active+1 1011+3Active+1 1010+2Active+1 1001+1Active0 1000+0Inactive0 0111-0Inactive0 0110Active0 0101-2Active 0100-3Active 0011-4Active 0010-5Active 0001-6Active 0000-7Active
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Confidential - Copyright © 2007 NavCom Technology, Inc. 19AGC
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Confidential - Copyright © 2007 NavCom Technology, Inc. 20 Proposed and Optimum CW Jamming Performance
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Confidential - Copyright © 2007 NavCom Technology, Inc. 21 CW Jamming Test
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Confidential - Copyright © 2007 NavCom Technology, Inc. 22 C/N0 vs. CW Jamming
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Confidential - Copyright © 2007 NavCom Technology, Inc. 23 I/Q vs. J/S - Varying GPS Signal Strength
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Confidential - Copyright © 2007 NavCom Technology, Inc. 24 AGC vs. CW Jamming
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Confidential - Copyright © 2007 NavCom Technology, Inc. 25 C/N0 vs. J/S – In Band CW Jamming
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Confidential - Copyright © 2007 NavCom Technology, Inc. 26 AGC vs. J/S – Out of Band CW Jammer
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Confidential - Copyright © 2007 NavCom Technology, Inc. 27 Sweep Test Setup
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Confidential - Copyright © 2007 NavCom Technology, Inc. 28 Frequency Sweep Test Results Jamming Strength (dBm) J/S in dBStatus -70 + (-30) = -100-100-(-121) = 21LOCK -65 + (-30) = -95-95-(-121) = 26LOCK -64 + (-30) = -94-94-(-121) = 27LOCK -63 + (-30) = -93-93-(-121) = 28LOCK 62 + (-30) = -92-92-(-121) = 29LOCK -61 + (-30) = -91-91-(-121) = 30LOCK -60 + (-30) = -90-90-(-121) = 31Loss of LOCK
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Confidential - Copyright © 2007 NavCom Technology, Inc. 29 Frequency Sweep J/S 30 dB
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Confidential - Copyright © 2007 NavCom Technology, Inc. 30 Frequency Sweep J/S 31 dB
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Confidential - Copyright © 2007 NavCom Technology, Inc. 31 Broadband Jamming Test
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Confidential - Copyright © 2007 NavCom Technology, Inc. 32 30 MHz Broadband Jamming
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Confidential - Copyright © 2007 NavCom Technology, Inc. 33 10 MHz Broadband Jamming
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Confidential - Copyright © 2007 NavCom Technology, Inc. 34 1 MHz Broadband Jamming
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Confidential - Copyright © 2007 NavCom Technology, Inc. 35 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 > Threshold 1 -Blanking off when Threshold 2 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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Confidential - Copyright © 2007 NavCom Technology, Inc. 36 Probability of Sample of Give Magnitude Magnitude# Standard Deviations Probability 10.430.666 20.860.390 31.290.197 41.720.085 52.150.032 62.580.0099 73.010.0026
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Confidential - Copyright © 2007 NavCom Technology, Inc. 37 Pulse Jamming
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Confidential - Copyright © 2007 NavCom Technology, Inc. 38Conclusions 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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