Open-Loop Tracking of GNSS Signals at Audio Processing Rates

C.R. Benson, S.U. Qaisar

Abstract:	GNSS receivers typically use closed loop tracking to receive and measure incident signals. In situations with high-dynamics, tracking may be aided by inertial or other external sensors to minimize tracking errors that would result in phase jitter. In some non-navigation applications such as radio occultation and GNSS Reflectometry, open loop correlations may be performed – meaning that there is no direct feedback from the observations driving the local code and carrier generation functions. The output of the open-loop correlator can be downsampled to very modest audio sampling rates, making for much more efficient storage and processing than the intermediate frequency signal whose bandwidth is bounded by the spreading code rate. In this paper, we apply an open loop correlation technique to direct arrival GNSS signals to demonstrate the viability of open loop receivers for conventional GNSS functions, such as navigation and precise positioning. The key advantages of this scheme are the ability to defer hard decisions in the tracking process, and to preserve coherent carrier level data for later analysis at modest processing rates. Specifically, preserving audio avoids the capture effect inherent in phase detection. It does this by preserving the full signal characteristics in the region surrounding the correlator replica used. Those characteristics include both phase and amplitude, with rates of change bounded only by the audio sampling rate used. The open-loop correlation and down-sample operations can be performed by parts, and we demonstrate removal of the data signal after initial removal of the spreading code, the residual signal then represents unmodelled clock, orbit, receiver motion and propagation effects. Applications of this technique include navigation in urban canyons, GNSS-SAR, GNSS Array processing, track-before-detect systems, and as an affordable means to run multiple hypotheses – for applications such as vehicle navigation on diverging roads. Although this demonstration uses only the L1 C/A signal, the technique is also applicable to all other GNSS signals.
Published in:	Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016) September 12 - 16, 2016 Oregon Convention Center Portland, Oregon
Pages:	103 - 106
Cite this article:	Benson, C.R., Qaisar, S.U., "Open-Loop Tracking of GNSS Signals at Audio Processing Rates," Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016), Portland, Oregon, September 2016, pp. 103-106. https://doi.org/10.33012/2016.14582
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