Comparison of Assisted and Stand-Alone Methods for Increasing Coherent Integration Time for Weak GPS Signal Tracking

In the GPS C/A signal tracking process, the longest coherent integration time possible in real-time is limited by the 50 Hz navigation data. However, in order to track weak GPS signals longer coherent integration is required. Even if the effect of data modulation can be removed, traditional GPS loop filters are based on digital approximations to analog filters and these approximations break down as integration time increases and/or as the tracking loop bandwidth decreases. This paper addresses both of these points. For the cases when external data aiding is not available, a decision feedback principle is used herein, in which the data bits are estimated through the tracking process itself. By using a DPLL (Digital Phase Lock Loop) and assuming low residual phase error, a sign detector is used as an optimum solution for the data message. This method also has the lowest computational burden. The reliable operational range of this technique is limited by the Bit Error Rate (BER) performance of the carrier tracking loop. In order to use navigation data as external aiding in weak signal environments and to compare the results with the decision feedback stand-alone software receiver, a NovAtel OEM4 receiver operating under line-of-sight conditions is used to assist the software receiver with the navigation data. The results for live attenuated GPS signal are compared with the commercial U-blox high sensitivity receiver. It is shown that by utilizing the optimum loop filters, which ensure stability in long integration times, it is possible to track the GPS signal as low as 10 dB-Hz with a phase locked loop.

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