Unaided acquisition of weak GPS signals using circular correlation or double-block zero padding

Abstract

Acquisition of very weak GPS signals requires long coherent and incoherent integration. Increasing the coherent, or predetection, integration time (PIT) improves sensitivity, but is limited because of the unknown data and bit edges. Incoherent integration is less sensitive, because of squaring loss. Two algorithms are developed to use a PIT which is a multiple of one data bit interval, without requiring assisting information. Both algorithms estimate the most likely data bit combination and use this to subsequently reverse the data signs over each PIT and synchronize the start of each PIT. The algorithms differ in the method of coherent integration. One uses circular correlation (FFT/IFFT), the other uses double block zero padding (DBZP). Circular correlation requires larger numbers of Doppler bins as the PIT is increased. In contrast, DBZP does not use Doppler bins, but suffers from loss because it does not account for the Doppler effect on the code duration. Consequently, the varying delay between the received signal and the replica will cause subsequent incoherent integrations to be added at the wrong delay. Solutions have been developed for each of these problems. In the case of circular correlation, a small PIT is used initially and after few steps the Doppler bins which have the lowest likelihood are eliminated. Thus, the PIT can be increased without increasing the total number of Doppler bins. In the case of DBZP, an approach is developed to compensate for the change in the code duration and relative delay, with only a small increase in the processing requirement. Simulations indicate that both of these algorithms can acquire signals below 15 dB-Hz. The problem of acquiring weak signals in presence of strong interfering signals is also addressed.