Differentially coherent acquisition algorithm for indoor GPS

The acquisition of weak signal is a time consuming process for a GPS receiver. If the acquisition is slow, the cold-start and warm-start time is long and the information passed to the tracking program obtained from old data may be out-of-date. Differentially coherent combining of correlation results is proposed for the acquisition of weak GPS signals. It is shown that GPS signals down to −177dBW can be acquired in 200msec without any assistance data and the need for a complex FFT post-processing of correlation results is avoided. Based on the scheme, a novel adaptive threshold algorithm is presented, which can automatically adjust the threshold according to different satellite signal power.