Stability Analysis of Tracking Weak GPS Signals through Non-coherent Ultra-tight GPS/INS Integration

In comparison to the coherent ultra-tight GPS/INS integration, the non-coherent ultra-tight integration is more preferable for tracking weak signals because the code discriminator used in the non-coherent integration is independent of the carrier phase, which makes it to be computed regardless of whether the SNR being strong enough or not to track the carrier phase. This paper proposes a specific noncoherent ultra-tight integration approach to the weak signal tracking problems. Stability of this integration system is investigated under weak signal conditions using hardware-in-theloop simulation. The experimental data including GPS IF(intermediate frequency) signal and INS true data are collected in the same simulation scenario with different levels of SNR by using Spirent GPS signal simulator and Nordnav Rxx-2 receiver. The performance of the system under weak signal condition is analysed in comparison to a stand-alone GPS receiver. The analysis results lead to the following conclusions. First, in comparison to the classical scalar tracking loops used in the stand-alone receiver, the non-coherent ultra-tight integration system can enhance continuous positioning when the GPS signal is severely attenuated. Second, outliers in one loop may severely corrupt the results of integration Kalman filter, which shows importance of quality control to the non-coherent ultra-tight GPS/INS integration system. Keywords-ultra-tight integration, GPS/INS, weak signal tracking, hardware-in-the-loop