GNSS RFI Detection: Finding the Needle in the Haystack

M. Scaramuzza, H. Wipf, M. Troller, H. Leibundgut, S. Rämi, R. Wittwer

Abstract:	What does a needle in the haystack have in common with GNSS radio frequency interference (RFI)? According to the Cambridge Dictionary this expression means that it is impossible or extremely difficult to find something, especially because an area to search is simply too large. This is exactly the situation for GNSS RFI detection in space and time. GNSS RFI is increasingly becoming important. Safety critical applications using GNSS, which are exposed to RFI, might lead to unacceptable performance degradations. Aviation, with GNSS based flight procedures conducted under Instrument Meteorological Conditions (IMC), are particularly concerned about this threat. Therefore it is crucial to develop the capability to assess the GNSS RFI situation over a large region and during a sufficiently long period. A method based on [1] is presented on how the RFI situation can be assessed over the whole area of Switzerland. It consists in installing mini quick access recorders (mQAR) on board of two dozen helicopters operated by Rega, the main Swiss Helicopter Emergency and Medical Service (HEMS), and by the Swiss Air Force, and collecting data during a period of three years. Daily missions of the two operators are used to record data. In this way, large parts of Switzerland are randomly covered. The low flight altitude is common to all helicopter missions. Therefore it is expected, that the probability having the aerial vehicles exposed to ground based RFI is higher compared to commercial fixed wing operations. Based on recorded C/No (Carrier to Noise Ratio), GPS satellite azimuth and elevation angles as well as roll, pitch and yaw angles of the helicopters, the GPS signals are normalized with use of an empirical derived GPS antenna pattern. The normalized GPS signals are finally statistically assessed for the determination of potential RFI. Only GPS L1 C/A data is taken into account within this paper, but with few modifications this method can be adapted to any GNSS providing pseudo range services. Over 6000 hours of recorded flight data were assessed so far. The empirical derived antenna pattern for GPS signal normalization is shown. Horizontal and vertical distribution of the flights are depicted. First results on potential GPS RFI are presented and discussed.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	1617 - 1624
Cite this article:	Scaramuzza, M., Wipf, H., Troller, M., Leibundgut, H., Rämi, S., Wittwer, R., "GNSS RFI Detection: Finding the Needle in the Haystack," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 1617-1624.
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