Block Correlator for Tracking GPS/GNSS Signals
Phani K. Sagiraju, Pradeep Kashyap, David Akopian · 2008
GPS receiver’s primary tasks are measurements of range, range-rate, and demodulation of the navigation data which are used to compute position, velocity, and time information. A GPS signal is a direct sequence spread spectrum (DSSS) signal and measurements are extracted by synchronizing a locally generated replica of GPS signal code with the received signal. The Doppler frequency estimate and code offset parameters of a satellite signal are given by an initial synchronization called acquisition, followed by a fine synchronization called tracking. The main purpose of tracking is to continuously refine these values using feedback loops. For example, the code tracking loop in the GPS receiver is called delay lock loop (DLL) and it is based on correlations with multiple replica codes delayed and advanced in code phase. The outputs of correlators are fed into a discriminator which estimates the tracking code phase error and DLL adjusts phases to approximate received code. Advanced tracking loops use multiple correlators for more accurate tracking and multipath mitigation. This is a challenge for so-called software GPS receiver implementations. In this paper, a new block correlator concept is introduced to significantly reduce computational complexity for software receivers. Block tracking correlator implements multiple correlators in a joint mode by sharing computations. Overall system is implemented at the cost of a single correlator plus insignificant overhead.