Validation of PRS Tracking Algorithms Using Real Life Signals

J. Wendel, F.M. Schubert, C. Kurzhals, D. Fernandez-Prim, M. Soellner

Abstract: The Galileo PRS signals and the GPS M code signals use a binary offset carrier (BOC) modulation. A BOC signal is obtained by modulating the carrier with the pseudorandom noise (PRN) code chips and a rectangular square wave subcarrier. The autocorrelation function (ACF) of a BOC signal shows multiple peaks, while the envelope of the ACF is given by the triangle shaped correlation function of the PRN code. Therefore, the main peak of the ACF is sharper than the peak of the corresponding binary phase shift keying (BPSK) signal, which allows for a more accurate tracking, i.e. less tracking jitter, and an increased multipath robustness. On the other hand, there is a risk that a tracking loop locks to a secondary peak, which results in a systematic error in the pseudorange measurements if this is not detected and countered. Algorithms for tracking BOC signals are widely researched, but a performance assessment of these algorithms is for most researchers restricted to software simulations or simulations with space segment simulators only, both employing dummy signals. The reason for this is that the PRN code and the navigation message of the signals in space (SIS) are encrypted: without the secret keys, the required security infrastructure and/or hardware, it is not possible to generate replica PRN codes that correlate with the SIS and can be used to for tracking and acquisition of these signals. This paper presents an approach that allows to validate tracking algorithms using real life encrypted signals like Galileo PRS, without needing cryptographic keys and knowledge about the crypto mechanisms employed. A static directive antenna is used to collect samples containing the encrypted signal from a satellite, while at the same time a rover is collecting samples using a conventional GNSS antenna and front-end. From the samples of the directive antenna, a blind demodulation technique is used to extract a signal that is given by the multiplication of the ranging code chips with the navigation message symbols. This information allows constructing appropriate replicas, which can be used by the tracking algorithms under test to post-process the samples of the SIS collected by the rover. The feasibility of the proposed approach is shown in an experiment.
Published in: Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014)
September 8 - 12, 2014
Tampa Convention Center
Tampa, Florida
Pages: 476 - 485
Cite this article: Wendel, J., Schubert, F.M., Kurzhals, C., Fernandez-Prim, D., Soellner, M., "Validation of PRS Tracking Algorithms Using Real Life Signals," Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 476-485.
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