ION GNSS 2012
Session C6: Interference and Spectrum Management 2

Title: Time-Frequency Based Intentional Interference Suppression Techniques in GNSS Receivers
Author(s): A. Sicramaz Ayaz, R. Bauernfeind, University FAF Munich, Germany; T. Pany, IFEN GmbH, Germany; B. Eissfeller, University FAF Munich, Germany
Date/Time: Friday, September 21, 2012, 3:20 p.m.
Room: 205 (NCC)

Interference suppression is a challenging issue in GNSS receivers. Interference can be incured by Tv/Radar emitters in nearby frequency bands, Lightsquared interference, chirp jammers etc. The interference immunity can be increased in the receiver via numerious suppression methods depending on the interference signal type such as conventional Fourier transform methods, statistical detection methods or time-frequency transform based methods e.g. Wigner-Ville distribution (WVD), Karhunen-Loeve, Short-time Fourier Transform (STFT) or Fractional Fourier transforms (FrFT). The interest in time-frequency transforms is the efficient analysis accomplished in non-stationary signals in order to improve the time-frequency resolution with minimum cross-term interferences. Among time-frequency transforms, WVD provides the best time-frequency resolution, however it causes cross-term interferences that narrows its applications.
Chirp signals present a change in frequency with time and are well localized in FrFT transform domain. Chirp jammer signals are recorded via open-field tests in the Galileo Test range (GATE) Berchtesgaden. Measurements were taken with GNSS software receiver (ipexSR)[2] and the triple-frequency USB Front-end. This receiver enables recording of intermediate frequency (IF) samples and detailed analysis of the interference effects on the receiver.

In this paper, we propose to use discrete approximation of Short time FrFT (STFrFT) to detect chirp jammer signals. We show that the proposed method can increase the sensitivity of the receiver in the presence of mono and multi chirp jammers and is superior to other transform based methods investigated. STFrFT uses chirp signals as the basis functions and provides a measure about the angular distribution of signals energy in time-frequency plane [1], therefore chirp signals can be better localized with STFrFT. The optimal angle estimation is the key point in the localization via STFrFT.

Another time-frequency method is the well-known STFT. It is the ´windowed Fourier transform´ that splits the non-stationary chirp signal into small segments which are then assumed as stationary. This is achieved by multiplying the signal with a window function and then the FFT operation is applied to the windowed signal. By using the method, the interference and the useful signal components are recognized easily due to the concentration of their distributions in the frequency domain and the excision is realized in frequency domain. However there is uncertainty on the window size.

The purpose of this paper is twofold: First, to introduce STFrFT based jammer suppression method with chirp parameter estimation and second to present sensitivity of the GNSS receiver applied with chirp localization methods including STFrFT in the presence of chirp jammers on L1 band. The analysis will be provided by acquisition correlation magnitude, code/carrier tracking sensitivity and Front-end settings.

References:

[1] Ozaktas et al, "Digital computation of the Fractional Fourier Transform", IEEE Signal Processing Transactions, Vol. 44, No 9, 1996 [2] C. Stober et al, "ipexSR: A Real-Time Multi-Frequency Software GNSS Receiver", IEEE ELMAR,Zadar, Croatia, 2010.

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