IMM Methods for Carrier Phase Tracking and Navigation Data Bits Estimation Through Interference

In this paper, we develop a phase lock loop (PLL) tracking algorithm that allows continuous phase tracking through interference events. This concept is directly applicable to GPS receivers subject to wideband radio frequency interference. Kalman filters have been previously proposed as more flexible alternatives for carrier phase tracking than traditional PLLs using phase discriminators. However, the characteristics of the GPS signal lead to a hybrid estimation problem, requiring simultaneous estimation of the discrete navigation data bits and the continuous carrier phase. Interacting multiple model (IMM) algorithms are often used in such problems when systems are constrained to a finite set of dynamic or measurement models. In the case of GPS phase tracking, there are only two measurement models corresponding to the two choices of navigation data bits (+1 and -1). The estimated phase is obtained at each measurement timestep by combining the two modes’ estimation results using their respective likelihood functions. In this way, the IMM avoids generation of exponentially-growing candidate data bit sequences, which cannot be handled in real time GPS receivers. Batch simulation results are provided as a benchmark best case comparison for IMM phase tracking performance. Two choices of state variables are investigated for their feasibility and performance.