Acquisition to Tracking and Coasting for Software GPS Receiver

摘要

The goal the software GPS receiver is to do acquisitionrnand real-time tracking of GPS C/A codes in softwarernusing a receiver front-end, an Analog-to-Digital converterrn(ADC) and a PC to do the processing. To start trackingrntwo parameters need to be obtained from the acquisitionrnprocess: the Doppler frequency of the signal, and thernsample on which the C/A code sequence for the satelliternbegins. Software based acquisition normally uses 1 msecrnof data (5 msec for fine frequency resolution), but thernprocess can take 1 sec or more for weak signals. The C/Arncode position shifts over time, and for stationary receivers there is a linear relationship between the rate of this shiftrnand the Doppler frequency of the signal. One way to dealrnwith this shift in the C/A code is to store all samplesrncollected during the acquisition process so as to startrntracking at a point immediately after acquisition, but thisrnwould be prohibitive in both processing time and storagernrequirements. A better way is to develop a method tornestimate on which sample the C/A code sequence beginsrnafter a period of time. The linear relationship between thernDoppler frequency and the C/A code shift can be used tornpredict the code position, and this method is affective forrna period of up to 30 seconds after acquisition. This linearrnrelationship can also be used to allow C/A code trackingrnto coast and immediately resume after a drop out due torninterference or jamming. Since A/D converters clocksrnoften have small variances from their specified samplingrnfrequencies and this variance leads to a bias in the amountrnof C/A code shifting relative to the Doppler frequency.rnBecause of this, one must test each A/D converter to findrnthis bias.