The space segment includes the 24 operational NAVSTAR satellites that orbit the earth every 12 hours at an altitude of approximately 20,200 kilometers. GPS encompasses three segments—space, control, and user. GPS is a satellite-based positioning system operated by the United States Department of Defense (DoD). The difference is applied to the GPS data recorded by the roving GPS receiver. The base station receiver calculates its position based on satellite signals and compares this location to the known location. The underlying premise of differential GPS (DGPS) requires that a GPS receiver, known as the base station, be set up on a precisely known location. Combining both methods provides flexibility during data collection and improves data integrity. Although both methods are based on the same underlying principles, each accesses different data sources and achieves different levels of accuracy. Differential correction can be applied in real-time directly in the field or when postprocessing data in the office. Differential correction techniques are used to enhance the quality of location data gathered using global positioning system (GPS) receivers.
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