RFI Mapped by Spaceborne GNSS-R Data

Abstract

Global navigation satellite system (GNSS) signals, such as those transmitted by global positioning system (GPS) satellites, are used by both military and civilian users for navigation, positioning, and scientific purposes. Although signal transmission in GNSS bands by unauthorized persons is restricted, radio frequency interference (RFI) in the GNSS wavelengths can occur, either intentionally or unintentionally, which can limit or prevent the use of GNSS signals for their intended purpose. Often seen in conflict zones, RFI can be caused intentionally by transmitters sending out noise at the carrier frequency (jamming) or false or modified pseudorandom noise (PRN) codes to trick a receiver into calculating a false position (spoofing). However, not all cases of GNSS RFI are nefarious in origin, and there have been reports of RFI caused by faulty instrumentation or civilian communication systems (Berglund et al., 2011; National Academies of Sciences, 2023).One widely published source of GNSS RFI was first described by Murrian et al. (2021). Using an instrument on the International Space Station, the authors were able to pinpoint the location of a transmitter to a Syrian airbase, which has been a constant source of GNSS RFI since 2017. Murrian et al. (2021) indicated additional potential sources of RFI in both Libya and China, although they did not pinpoint

GNSS Signal Jamming as Observed From Radio Occultation

“…Different from broad L-band radio frequency interference (RFI) seen from space [11,12], the jamming on GNSS receivers is an intentional RFI, often concentrated in a narrow band targeted to a specific transmission frequency such as GPS [13]. These jamming or RFI signals must be flagged and removed in order to produce useful science data in GNSS-reflectometry (GNSS-R) observations [14][15][16][17]. Although GNSS-RO is largely based on phase measurements, the RO tracking can be adversely impacted by weak GNSS signals or signal-to-noise ratio (SNR).…”

“…For slowly varying high power sources, the algorithm might have some challenges to distinguish them from other natural variabilities. Furthermore, studying the GNSS-R delay doppler map (DDM) measurement noise, Chew et al [17] obtained GNSS RFI maps from 2017 to 2022 and found the approximate locations of regional hot spots in the subtropical latitudes over land. The technique can be used to track and evaluate jamming sources at a relatively high revisit rate from an 8-SmallSat constellation.…”