Flight test of a GPS/INS/Pseudolite integrated system for airborne mapping
Carrier phase based DGPS systems have been used for airborne mapping over many years. Integrated GPS/INS systems are also becoming popular in order to improve positioning accuracy and system reliability. 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 research has shown that ground-based pseudolites (PLs) can strengthen the constellation of GPS satellites and their geometric distribution, and improve the measurement geometry. GPS/INS systems augmented by PLs can improve positioning reliability and accuracy, especially in the vertical component.
A tightly integrated GPS/INS/PL system is described in this paper. In this system, GPS and PL measurements are processed with a projected single difference (PSD) method to eliminated nonlinear geometry bias. Different troposphere and ionosphere delay models are applied to GPS and PL 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 double-differenced carrier phase data are processed in the filter time and measurement updates. The positioning results of GPS/PL/INS, GPS/INS integrated systems, and GPS only with different data rates are compared with aerial triangulation results to evaluate their performances. The geo-referencing performance of the integrated system is also evaluated.