A Combined DGPS/INS and Synthetic Aperture Radar System for Geoid Referenced Elevation Models and Ortho-Rectified Image Maps

The objective of this paper is to introduce and describe the concept of using airborne gravimetry by strapdown INS/DGPS to reference mapping products derived from Interferometric Synthetic Aperture Radar (IFSAR) to the geoid. Calgary-based Intermap Technologies Ltd. is a digital mapping company that focuses on providing digital elevation models and ortho-rectified images using an interferometric radar system called the STAR-3i. Intermap Technologies Ltd., in partnership with the University of Calgary, has developed a product capable of providing geoid referenced digital elevation models and ortho-rectified images. Using the IFSAR technology, the STAR-3i provides a new generation of airborne radar digital elevation models and ortho-rectified image maps. The STAR-3i consists of two X-band radar antennas mounted on a LearJet 36. Data is collected simultaneously for the two antennae, and is combined by a digital correlation process to extract terrain information. STAR-3i uses integrated DGPS/INS navigation results to obtain highly accurate position and orientation control. Because DGPS is used as the position reference all terrain information is referred to the ellipsoid. The University of Calgary has developed software that extracts gravity disturbance information from the DGPS/INS data collected by the STAR-3i system. Using the gravity information, a precise relative geoid is determined for the flight area. By combining this geoid with the EGM 96 geopotential model, the mapping products derived from the STAR-3i system can be directly referenced to the geoid. The STAR-3i system, and the method of extracting the geoid from the integrated DGPS/INS data are presented. An analysis of geoid determination accuracy for system is given. The results are based on a flight test in California on August 23, 1997. Based on a preliminary analysis, for a flight elevation of 6000 metres and flight speed of 700 km/hr, the following results were obtained: • 1.5 mGal RMS difference at 12 km half wavelength spatial resolution • 2.0 mGal RMS difference at 9 km half wavelength spatial resolution. These RMS values are calculated by comparison to upward continued ground gravity disturbances. The accuracy of the STAR-3i determined geoid is 5 centimetres (1σ) when compared with an independent geoid reference. For the given flight conditions the accuracy of the STAR3i elevation map is 0.5 to 1.5 metres (1σ).