Airborne pseudolites in a Global Positioning System degraded environment

The Global Positioning System (GPS) is a continuous, all-weather navigation service that supports precise navigation for authorized military users. As with all satellite navigation systems, GPS is requires the user to maintain direct line-of-sight access to four or more satellites, and therefore performance is degraded or denied when the sky view is obstructed. To minimize this problem, various types of augmentations have been developed and tested. Pseudo-satellites, also called pseudolites, are one such augmentation system in which GPS-like ranging signals from additional transmitters supplement the satellite signals. This paper examines the ability of airborne pseudolites to support precise positioning in an urban environment. The impact of restricted satellite availability due to obstructions is examined for various urban terrain zones. Then the ability of the pseudolite to improve both availability and accuracy is investigated. A comparison of the system performance when relying on GPS only and when using an airborne pseudolite is presented for various positioning scenarios. Simulation results show that required accuracy and availability can be obtained by using an appropriately equipped airborne pseudolite.