Weak Signal GPS Synchronization for Locating in-Building Cellular Telephones

This paper investigates the application of a parallel noncoherent matched filter that can allow GPS signal acquisition even for the very low SNR signals found within large buildings. Recent government regulations require technology that can supply emergency "9-1-1" dispatchers with cellular telephone location to within 50 meters and this is very difficult with current technology. With surroundings at room temperature, our simulation results show codephase acquisition at GPS signal strengths as low as -175 dBW (29 dBHz). Segmentation of the matched filter and non-coherent summation allows Doppler and carrier frequency offsets as large a 5 kHz. The matched filter operates at half-chip intervals and provides a correlation output for each of the 2046 codephases in the 1023 chip code sequence. These outputs are individually accumulated (or averaged) over many code cycles to improve the probability of correct codephase detection. At -175 dBW signal strength, simulations indicate a mean acquisition time of 1.2 seconds. After accumulating for 10,000 cycles (10 seconds), the probability of correct detection exceeds 99%