Doppler Search as Pre-acquisition Step

I. Suberviola, S. Köhler, J. Mendizabal, G. Rohmer

Abstract:	Every GNSS receiver has an acquisition module/process, which detects the presence the satellite signals, synchronizing the receiver in time (code delay) and frequency domain (Doppler Effect) with each detected signal. This synchronization is done simultaneously, that means, all combinations (Fig. 1) of Doppler frequency bins and code offsets bins are tested to find the correlation peak. There are many different algorithms that perform these combinations and they have been widely explained in different publications ([1]-[3]). However, all of them are based on three basic architectures. These are explained in detail in [4]. A brief summary is done hereafter along with their respective illustrating figures. The simplest one (Fig. 2) searches serially for the peak in the two-dimensional search space. The second one (Fig. 3) parallelizes the frequency search thanks to the FFT operation. The most complex one (Fig. 4), but also the fastest one, parallelizes the code offset bin search. This is done performing the correlation in the frequency domain and it is the most complex method, because the number of code bins to parallelize is much greater than the number of frequency bins (parallelized in the second method). However, in this last method, all frequency bins must still be serially verified. The number of frequency bins depends on the coherent integration time ???which defines the Doppler bin size. For a frequency search interval of and an integration of a whole L1/E1 epoch (1 ms for GPS and 4 ms for Galileo, respectively), the Doppler bin size is approximately 500 Hz and 150 Hz respectively. This means that 28 and 93 Doppler bins (for GPS and Galileo respectively) will have to be searched every time a satellite is to be acquired. The method proposed here, "Doppler Search as preacquisition step", reduces the number of Doppler bins. Thanks to it, the number of the new frequency bins, will be exactly the number of visible satellites (between 7 and 10). Moreover, these frequency bins will be much more precise (2 Hz precision) than the ones used in the conventional methods and therefore, the power loss due to Doppler frequency mismatch will be negligible. In addition to this, due to the precision on the Doppler frequency determination, a much faster stabilization of the tracking loops will be achieved. The “Doppler Search as pre-acquisition step” can also be used at a cold start to quickly determine (0.5 sec) if there is some satellite visible at all, with an outdoor signal power (around -130 dBm). If no satellite is detected, the receiver can consider itself to be in an indoor scenario and start its high sensitive algorithms.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	2646 - 2652
Cite this article:	Suberviola, I., Köhler, S., Mendizabal, J., Rohmer, G., "Doppler Search as Pre-acquisition Step," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 2646-2652.
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