Modeling and Geometry Design for Pseudolite Augmented Airborne DGPS

Carrier phase DGPS systems have been increasingly used in airborne surveying. However, the accuracy and availability of GPS positioning cannot meet the stringent requirements of large-scale photogrammetry. Groundbased pseudolites can strengthen the measurement geometry for airborne GPS systems, and improve ambiguity resolution. As a result, positioning accuracy and reliability can be improved, especially in the vertical component. In order to effective ly augment DGPS aided airborne surveying with pseudolites, carrier phase measurements of pseudolites need to be employed in the positioning process. However, due to the comparatively small separations between pseudolites and receivers, some challenging issues have to be investigated, such as geometry design, nonlinearity, tropospheric delay and pseudolite location errors. Geometry design is extremely important for pseudolite augmented GPS positioning system. Optimally located pseudolite and reference receiver can significantly improve the geometric strength of positioning solutions and thus reduce the effects of nonlinearity and pseudolite location error. The optimal locations of pseudolite and reference receiver are proposed based on analyzing these issues and the simulation results in airborne surveying scenarios. Nonlinearity is a challenging issue in pseudolite augmented DGPS. The nonlinear geometry bias in processing pseudolite measurements can be effectively eliminated by Projected Single Difference strategy, which can also be used in processing GPS measurements to improve the positioning accuracy. Several pseudolite tropospheric delay models are introduced and evaluated, and optimal models corresponding to different applications are proposed. Simulation and real data test results have shown the effectiveness of the proposed methods.