High accuracy GPS phase tracking under signal distortion

Array signal processing is a viable method to provide protection against interference to the desired GPS (Global Positioning System) signal. Adaptive array processing can also be used to provide GPS multipath mitigation. As reported in the literature (Hatke, 1998), adaptive nulling to suppress interference can cause errors in the carrier phase and code phase measurements. The output of multi-element controlled reception pattern antennae (CRPA) may be subject to sizeable variations in phase patterns that arise from the application of non-trivial time varying adaptive array weights (Moelker, 1998). Part of this phase distortion can be compensated in the carrier phase tracking loop of the baseband GPS receiver processing. Classical approaches implement constrained beam steering mechanisms to control the phase of the array in the look direction. An alternative approach, using unconstrained adaptive array processing with compensation for the subsequent phase distortion of the GPS signal is demonstrated by the use of a software radio with results to support the theory. This unconstrained adaptive array implementation does not require attitude information and obviates the need for an attitude sensor such as an inertial reference unit. In addition to addressing carrier phase distortions due to adaptive array processing, this body of work evaluates the carrier phase distortion resulting from GPS signal multipath. Phase multipath characterization is performed for GPS receiver architectures that use typical baseband receiver processing designs (using coherent and non-coherent code tracking loops). Prior to this effort, scant attention had been given to validating carrier-phase multipath theory against GPS data (Brodin, 1996). A comprehensive phase multipath equation (Braasch & Van Dierendonck, 1999) which captures the true nature of carrier multipath is presented. Validation of this equation is carried out for wide and narrow correlator spacing within coherent and non-coherent code tracking architectures by comparing bench test data to theoretical predictions. In addition, data collected for non-zero multipath phase rates are presented. The impact of GPS receiver architecture on the mitigation of phase-rate multipath will be discussed.