Flight Test of a Gps/ins/pseudolite Integrated System for Airborne Mapping

Carrier phase based DGPS systems have been used for airborne mapping for many years. Integrated GPS/INS systems are also becoming popular in order to improve the positioning accuracy, system reliability and achieve direct geo-referencing. However, this approach is limited by the current GPS constellation. Further improvement is required to achieve centimetre level positioning accuracy, which becomes increasingly important for large scale airborne mapping. Previous researches have shown that ground-based pseudolites can strengthen the constellation of GPS satellites and their geometric distribution, and improve the measurement geometry. GPS/INS systems augmented by pseudolites demonstrate improved positioning reliability and accuracy, especially in the vertical component. This paper describes a flight test of a tightly integrated airborne GPS/INS system augmented by pseudolites. In this system, GPS and pseudolite measurements are collected from two airborne antennas mounted up-facing and down-facing respectively. Different troposphere and ionosphere delay models are applied to GPS and pseudolite data before computing the double-differences. Widelane and geometry-free ambiguities are estimated using combinations of independent GPS code and phase observations. Positioning results based on the fixed ambiguities are used to initialize an extended Kalman filter in the integrated system. INS measurement and GPS/PL doubledifferenced carrier phase data are processed in the filter time and measurement updates. The positioning results of GPS/INS integrated systems with and without pseudolites augmentation, and GPS only with different data rates are analyzed. The geo-referencing performance of the integrated system is also evaluated.