Performance Analysis of a RTK Vector Phase Locked Loop Architecture in Degraded Environments

This paper investigates the application of the vector tracking approach to carrier phase tracking to improve phase tracking in degraded signal environments. Carrier phase tracking is more sensitive to environmental factors than code phase or carrier frequency tracking. In contrast to many vector based carrier phase tracking approaches, the algorithms described in this paper do not rely on local loop filters to maintain carrier phase lock. Rather than estimating the global position, the position of a rover receiver relative to the position of a static base station is estimated in the navigation processor. The relative position vector (RPV) estimates, along with integer fixed carrier ambiguities, and measurements from the base station are used to drive the replica carrier. To test the carrier phase tracking approaches described in this paper, nonlinear Monte Carlo simulations were performed to compare the RTK-VPLL performance to traditional scalar tracking loops. Simulations were performed at the correlator level to analyze the carrier phase tracking performance of scalar and vector tracking approaches at various carrier to noise ratios. In addition to the simulation study, the proposed algorithms are tested using live sky data collected in static and dynamic tests in clear sky and in foliage environments. During the dynamic tests, the baseline ranged for 10s of meters to 1000s of meters. The test locations are classified as either moderate or heavy foliage environments. The RTK-VPLL performance is compared to a commercial GPS receiver. A Novatel Propak V3 receiver is used to assess accuracy when an accurate reference solution is available, or to assess improved tracking performance relative to a conventional scalar tracking receiver when the reference solution is degraded. A local GPS base station was used to calculate the RTK solution for the vector receiver and for the reference receiver. The RTK-VPLL was shown to significantly reduce carrier phase cycles slips and loss of lock in the moderate foliage environments.