GNSS Interference in L-Band SAR Missions � Assessment and Mitigation

Abstract

Synthetic Aperture Radar (SAR) satellites commonly make use of onboard Global Positioning System (GPS) receivers for precise orbit and baseline determination. In view of the extreme SAR transmit power levels, interference from SAR signals may inhibit proper GPS tracking and poses a particular challenge to space missions using L-band SAR signals with frequencies adjacent to or even overlapping GPS frequency bands. Within this study, the impact of simulated SAR signals on direct and semi-codeless GPS signal tracking is assessed in a signal simulator test bed using two commercial-off-the-shelf geodetic-grade receivers. A high robustness of GPS tracking to both adjacent-band and in-band SAR interference is obtained within the tests using representative chirp signals. For SAR signals next to or overlapping the GPS L2 band, proper tracking of the GPS L1 C/A code, GPS2 L2C, and semi-codeless L1/L2 P(Y)-code tracking is retained for interference powers up to 90 db above the natural GPS signal power. Apparently, a high level of immunity to high-power pulsed signals with repeat periods in the (sub-)ms regime is already provided by the automatic gain control of the receivers and/or a saturation of the analog-to-digital converters in the frontend that mimic an explicit pulse blanking. On the other hand, the addition of an external pulse blanking synchronized with the chirp pulses was found to be of marginal value. This unexpected results can presumably be understood by low power "noise" in the synthetic SAR signals that adds an additional signal outside the spectral and temporal limitations of the actual chirp signal and dominates the overall interference when simulating very high chirp signal powers.