Acquisition to Tracking and Coasting for Software GPS Receiver

John J. Schamus, James B. Y. Tsui

Abstract:	The goal the software GPS receiver is to do acquisition and real-time tracking of GPS C/A codes in software using a receiver front-end, an Analog-to-Digital converter (ADC) and a PC to do the processing. To start tracking two parameters need to be obtained from the acquisition process: the Doppler frequency of the signal, and the sample on which the C/A code sequence for the satellite begins. Software based acquisition normally uses 1 msec of data (5 msec for fine frequency resolution), but the process can take 1 sec or more for weak signals. The C/A code position shifts over time, and for stationary receivers there is a linear relationship between the rate of this shift and the Doppler frequency of the signal. One way to deal with this shift in the C/A code is to store all samples collected during the acquisition process so as to start tracking at a point immediately after acquisition, but this would be prohibitive in both processing time and storage requirements. A better way is to develop a method to estimate on which sample the C/A code sequence begins after a period of time. The linear relationship between the Doppler frequency and the C/A code shift can be used to predict the code position, and this method is affective for a period of up to 30 seconds after acquisition. This linear relationship can also be used to allow C/A code tracking to coast and immediately resume after a drop out due to interference or jamming. Since A/D converters clocks often have small variances from their specified sampling frequencies and this variance leads to a bias in the amount of C/A code shifting relative to the Doppler frequency. Because of this, one must test each A/D converter to find this bias.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	325 - 328
Cite this article:	Schamus, John J., Tsui, James B. Y., "Acquisition to Tracking and Coasting for Software GPS Receiver," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 325-328.
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