Post-correlation semi-coherent integration for high-dynamic and weak GPS signal acquisition

Abstract

Acquisition and ultimate tracking of a weak GPS signal faces several technical challenges, notably, possible data bit sign reversal every 20 ms and tolerable frequency error inversely proportional to the integration interval. Brute force search over all possible combinations of the unknown values is computationally prohibitive. Assisted GPS relying on external infrastructure for timing, data bit, and frequency error information is costly. Coherent techniques such as the block accumulating coherent integration over extended interval (BACIX) have recently been proposed to increase coherent integration. Although efficient, such coherent methods may still be too expensive except for high-end receivers and may not maintain the SNR performance when there are large frequency changes over the extended integration interval. In this paper, we set forth a novel method that utilizes the semi-coherent scheme for post-correlation integration. It is named as block accumulating semi-coherent integration of correlations (BASIC) and can be viewed as an extension of the BACIX algorithm. Although less sensitive than coherent integration, semi-coherent integration based on inter-block conjugate products is computationally more efficient. In addition, it can handle large frequency changes. The BASIC algorithm is first formulated in the paper. Computer simulation results are then presented to illustrate the operation and performance of the BASIC algorithm for joint estimation of the initial frequency, chirping rate (rate of change in frequency), bit sync, and bit sign pattern.