Performance of code tracking loops in ultra- tight GPS/INS/PL integration

With its several inherent advantages, an ultra-tightly coupled GPS/INS/PL system will have a dramatic increase in both commercial and defence applications. Integrating the correlator measurements, I (in-phase) and Q (quadrature), with position, velocity and attitude from INS in a Kalman filter characterises this type of system. The Doppler feedback derived from the corrected INS is then fed back to the carrier-tracking loop to remove the dynamics from the GPS/PL ranging signal, thereby reducing the carrier tracking bandwidth. This reduction in bandwidth is the principal reason behind its popularity. Some of the applications where this system can be used are: dynamic scenarios, weak signal conditions, interference & jamming environments, etc. Although the code loops are not directly aided by the INSderived Doppler in an ultra-tightly coupled system, 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 the jamming immunity. Normally the code loop bandwidth is about 1 to 3 Hz in an unaided case, but in this type of integrated system the bandwidth is typically less than 1 Hz as the code dynamics are mitigated as well. In this paper, the mathematical analysis pertaining to code dynamics is presented. An E-L/E+L discriminator is utilised as a software-based receiver is used for the experiments. Various trajectories with different dynamics are generated and the code-tracking loops are tested, and the results show substantial improvement in the code thresholds.