Hight-Accuracy Airborne GPS/INS Positioning Augmented by Pseudolite

J. Wang, J. Wang, D.Sinclair, and L. Watts

Abstract:	GPS/INS integrated systems have been used in mapping applications for direct geo-referencing, which needs very accurate position and attitude solutions. However, due to the limited constellation of GPS satellites and their geometric distribution, the accuracy of GPS positioning cannot meet the requirements of large scale aerial photogrammetry. A GPS/INS integrated system with pseudolite (PL) augmentation for large scale airborne surveying is introduced in this paper. Comprehensive analysis is conducted for geometry design of the integrated system for airborne surveying. Only appropriately located PL can strengthen the GPS satellite geometry significantly, and minimize the impacts of PL location errors and the tropospheric delay modelling error at the same time. As PLs are comparatively close to receivers, the unit vectors from a PL to the reference and rover receivers can be significantly different. Tropospheric delay modelling error cannot be effectively mitigated in the differencing procedure. PL signals only propagate through the lower troposphere, where it is very difficult to model the signal delay due to spatial variations of atmospheric parameters and dense water vapour. Several models are introduced and evaluated in this paper for accurately estimating the PL tropospheric delay. Estimation of the PL troposphere modelling errors in a Kalman filter can improve position accuracy further. An extended Kalman filter is applied to process INS raw measurements and GPS/PL carrier phase data for system integration. Carrier phase ambiguities are estimated using the combination of GPS and PL code and phase observations. Double differenced GPS and PL measurements are processed to get initial positioning results. Computed positions based on fixed ambiguities, together with initial Kalman filter parameters are used to initialize the tightly coupled integrated system. Closeloop update in the system corrects INS errors, as the estimated errors are fed back to the inertial solutions. The inertial position errors are limited to the centimeter level. The INS measurement processed by the filter provides the relative movement during each GPS epoch and can be used to detect and repair GPS/PL cycle slips. The integer ambiguities are fixed by an OTF searching method before the filter updating.
Published in:	Proceedings of the 2006 National Technical Meeting of The Institute of Navigation January 18 - 20, 2006 Hyatt Regency Hotel Monterey, CA
Pages:	515 - 522
Cite this article:	Wang, J., Wang, J., D.Sinclair,, Watts, L., "Hight-Accuracy Airborne GPS/INS Positioning Augmented by Pseudolite," Proceedings of the 2006 National Technical Meeting of The Institute of Navigation, Monterey, CA, January 2006, pp. 515-522.
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