Since the early days of the Global Positioning System (GPS), many researchers have investigated its application in aerial photogrammetry. Today, with the full constellation of 24 GPS (and up to 24 additional GLONASS) satellites operational, enabling excellent satellite geometry any time of the day, the need to apply the full potential of GPS for real-time aircraft navigation and photogrammetric mapping can be realized. The use of GPS to determine relative positional data for ground control points in a photogrammetric block adjustment is widely accepted and practiced. A number of simulations and tests have also shown that the determination of camera exposure positions using GPS drastically reduces, and could possibly eliminate, the number of horizontal and vertical control points needed in aerial triangulation. Given that control point determination is a major time and cost component of any photogrammetric project, it at first appears that commercial companies would be quick to implement GPS derived exposure stations. Furthermore, control points are often on or near roads making their field locating by ground based survey techniques dangerous. In practice, the number of companies using this method is extremely small, perhaps because of the natural uneasiness with the new technology. In this research, the application of GPS derived exposure stations for aerial triangulation is empirically evaluated in the commercial environment. Operational methodologies, practicality, and accuracy issues of airborne GPS are also discussed, as well as the capabilities and limitations of the system. Special considerations include multiple ground receiver comparisons, long base line processing using data from Continuously Operating Reference Stations, geoidal/ellipsoidal issues, and variance-covariance error propagation. Special emphasis has been placed on low altitude (higher precision desired) flights for collection of engineering design information.; The results of this research will help resolve any ambivalence about the use of the airborne GPS and foster its application in everyday photogrammetric operations. Airborne GPS is indeed practical and quite feasible with minimum or no ground control. The situation may be further enhanced when the Federal Aviation Administration (FAA) eventually implements the Wide Area Augmentation System. The high accuracy level of current GPS technology make it employable to meet the accuracy requirements for all photogrammetric applications including low-altitude engineering design projects. With the availability of Continuously Operating Reference System (CORS) data and “on the fly” ambiguity resolution techniques, Airborne GPS-Photogrammetry (ABGPS) may no longer be limited by the requirement for a base station on the project site. For some applications, data from nearby CORS stations (or “CORS equivalent” stations) may be used as the known station reference. This will bring airborne GPS-photogrammetry closer to conventional photogrammetric practice and enhance its widespread application.
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