The leading pioneer in GPS technology Copyright © 2007 NavCom Technology, Inc.Confidential A New Anti-Jamming Method for GNSS Receivers Jerry Knight, Charles.

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

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

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

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

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

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

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

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

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

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

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

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

The leading pioneer in GPS technology Copyright © 2007 NavCom Technology, Inc.Confidential A New Anti-Jamming Method for GNSS Receivers Jerry Knight, Charles.

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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.

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