Ultra-Tight Integration of Pseudolites with INS

Abstract

Ultra-tight integration of GPS with INS has been proposed to address various problems associated with these individual systems. However, a low cost INS may not sustain prolonged GPS signal losses which frequently occur in indoors, foliages, urban environments etc. Moreover, the upcoming applications such as LBS require positioning systems to work in all these environments and this stimulates the interests in developing alternative positioning systems. A 'Pseudolites-only' constellation which artificially simulates the GPS-constellation is an ideal choice to provide positioning solutions in these non- benign environments. The main advantage in using the pseudolites is its flexibility - the geometry and the signal characteristics can be optimized for a particular application. To combat signal degradation due to reflecting/refracting environments and receiver dynamics, the pseudolites can be integrated with INS to improve the overall system robustness. Two problems that affect the receiver performance in using 'Pseudolite-only' constellation are: higher rate changes in Doppler due to large geometry changes, and signal fluctuations. This paper proposes the ultra-tight integration of pseudolites with INS to address these potential problems. Firstly, the tracking loop performance in dynamic scenarios is enhanced by integrating the INS measured dynamics with the carrier tracking loops. Secondly, as the tracking loops are closed by the integration Kalman filter, any larger fluctuations in the Pseudolite I and Q signals will not severely affect the receiver tracking. In this paper, a series of numerical experiments are performed to study the performance of the pseudolite/INS ultra-tight integration. The results show a significant improvement in measurements accuracy, acquisition times over conventional ultra-tight integration which uses only GPS signals. The propagation effects of the pseudolite signals, its characteristics and their effect on tracking loops are also discussed in detail.