Dynamics Performance of Carrier and Code Tracking Loops in Ultra-Tight GPS/INS/PL Integration

Abstract

Integrating the correlator measurements, I (in-phase) and Q (quadrature), with position, velocity and attitude from INS in a Kalman filter characterizes the ultra-tight GPS/INS integrated system. The Doppler feedback derived from the corrected INS is fed back to the carrier-tracking loop to remove the dynamics from the GPS/PL ranging signals, thereby reducing the carrier tracking bandwidth. This reduction in bandwidth results in better code and carrier measurements accuracy. Therefore accurate estimation of the Doppler frequency from the INS becomes paramount in leveraging the benefits of ultra-tightly coupled systems. Although the code loops are not directly aided by the INS derived Doppler in ultra-tightly coupled system, nevertheless, an accurate knowledge of Doppler information from INS results in better code thresholds. This improvement in threshold further increases the accuracy of raw measurements and also jamming immunity. In unaided mode, the carrier and code tracking loop bandwidths are about 15 Hz and 3 Hz respectively, whereas in ultra-tight integration mode the bandwidths can be reduced to 3 Hz and 1 Hz respectively. In this paper, the performance of the carrier and code tracking loops are analyzed. A Software GPS receiver is used to perform the analysis. A Costas Loop is used for carrier tracking with an arctan discriminator function, while a narrowband DLL with a E-L/E+L discriminator is used for the code tracking loops.