RFI Situational Awareness in GNSS Receivers: Design Techniques and Advantages

Phillip W. Ward

Abstract:	This paper describes a simple but effective radio frequency interference (RFI) monitoring design for GNSS receivers that does not require the receiver to be tracking for it to reliably provide RFI (jamming) situational awareness. There are numerous advantages to RFI situational awareness in a GNSS receiver design. One unique advantage is that it provides the noise floor information required to perform an adaptive initial search and acquisition process. The design continues to operate even when the RFI level is so high that the GNSS receiver is incapable of acquiring and tracking satellites, an RFI situation that cannot be determined by most of today’s GNSS receivers, even most military receivers. This robustness in RFI situational awareness measurement permits the GNSS receiver to inform the user that it will most likely be unable to acquire the signals and why. The jamming measurement is obtained by the measurement of the GNSS receiver RF front end gain using measurements obtained from the automatic gain control (AGC). The preferred embodiment of the jamming meter is digital gain control of the AGC. This gain control is derived from the analog-to-digital converter (ADC) process that normally occurs at the intermediate frequency (IF) of the GNSS receiver. In the absence of RFI, the AGC gain is set by the thermal noise power level (N). The digital value of N becomes the jamming reference level; not the GPS satellite signal power (S), which is well below N at the IF. When RFI occurs, the digital gain control acts to attenuate the gain of the AGC amplifier in order to keep the rms amplitude of the AGC amplifier output at a constant level. That AGC operating point is normally selected to provide the least amount of quantization noise in the ADC that follows the AGC. In the presence of RFI, the new digital value is now a measure of the jamming power (J) plus N. This can be converted to a useful measure of the RFI level called the jamming-to-noise power ratio (J/N). Therefore, the AGC digital gain control reading can be a J/N meter as a natural by-product of the AGC gain control. The J/N meter technique works because the AGC control voltage does not change from the measure of N unless RFI is present at the IF. The gain of a modern AGC amplifier is linear in decibels (dB), so reading the AGC digital gain control value provides a precise linear measure in dB of the RFI power level, thus providing RFI situational awareness. An estimate of J/S can be made from this measurement assuming a uniform level of S for all satellites plus certain GNSS receiver front end parameters. The estimation of J/S from the measure of J/N cannot account for differences in S for each satellite, but if the initial search fails and the J/N meter says that RFI is not the problem, then the receiver can reasonably conclude that the GNSS signals must all be critically attenuated. When and if the receiver is tracking a subset of the GNSS constellation, it can accurately measure the carrier-to-noise power ratio in a 1 Hz noise bandwidth, C/N0 = S/N0, for each satellite being tracked. It can then estimate J/S more accurately for each satellite based on S/N0 and certain GNSS receiver design parameters. There are numerous enhancements that will improve the accuracy of the J/N meter and the estimate of J/S such as calibration features and RFI characterization. The design of the calibration features is described. In addition, both the advantages and the limitations of the design are presented including a new concept of an adaptive initial search technique based on RFI situational awareness.
Published in:	Proceedings of the 63rd Annual Meeting of The Institute of Navigation (2007) April 23 - 25, 2007 Royal Sonesta Hotel Cambridge, MA
Pages:	189 - 197
Cite this article:	Ward, Phillip W., "RFI Situational Awareness in GNSS Receivers: Design Techniques and Advantages," Proceedings of the 63rd Annual Meeting of The Institute of Navigation (2007), Cambridge, MA, April 2007, pp. 189-197.
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