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Ground Surveys
The USGS uses global positioning system (GPS) differential correction techniques to improve the quality of aircraft trajectories. The underlying premise of differential GPS (DGPS) is that any two receivers, a base station or reference receiver and a rover receiver, are relatively close together and experience similar atmospheric errors. Atmospheric errors that are not accounted for may reduce the quality of the aircraft trajectories. DGPS requires that the reference GPS receiver be set up on a precisely known location (Figure 1). For a particular survey, the USGS either set up their own reference station or use National Oceanographic and Atmospheric Administration/National Geodetic Survey Continuous Operating Reference Stations (NOAA/NGS CORS) stations. The base station receiver calculates its position based on satellite signals and compares this location to the precise known location. The difference is applied to the GPS data recorded by the aircraft's GPS receiver, thus improving the overall quality or the trajectory.
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