Ultra-tight GPS/INS/PL Integration: A System Concept and Performance Analysis

The architecture of the ultra-tight GPS/INS/PL integration is the key to its successful performance; the crux of this architecture is the Doppler feedback to the GPS receiver tracking loops. This Doppler derived from INS, when integrated with the carrier tracking loops, removes the Doppler due to vehicle dynamics from the GPS/PL signal thereby warranting a significant reduction in the carrier tracking loop bandwidth. The bandwidth reduction provides several advantages such as: improvement in anti-jamming performance, increase in post correlated signal strength which increases the dynamic range, accuracy of measurements etc. Therefore, any degradation in the derived Doppler estimates will directly affect the tracking loop bandwidth and hence its performance. The quadrature signals from the receiver correlator, I (in-phase) and Q (quadrature), form the measurements, whereas the inertial sensor errors, position, velocity and attitude errors form the states of the complementary Kalman filter. To understand the measurement model of the filter for this type of integrated system, a good understanding of GPS/PL signal characteristics is essential. It is shown in this paper that phase and frequency errors are the variables that relate the measurements and the states in the Kalman filter. The main focus of this paper is to establish the fundamental mathematical relationships that form the measurement model, and to show explicitly how the system error states are related to the GPS/PL signals. Furthermore, to test the validity of the derived mathematical relationships, simulation experiments were conducted to test the performance of the Kalman filter, and our results shown in this paper confirm the validity of the theoretical arguments.