Differential Operation of NAVSTAR GPS

Abstract

Under the Selective Availability program the Department of Defense plans to provide the civil community with Standard Positioning Service (SPS), having an accuracy of 500 meters (2drms) when the NAVSTAR GPS becomes operational. Subsequent improvements in accuracy are expected to be instituted as national security considerations permit. However, provisions are also made to degrade accuracy to worse than 500 meters, if necessary. Depending on the type and level of Selective Availability imposed, different civil capabilities can be realized. These capabilities are described in this report. Differential operation, wherein a high-quality, surveyed-in receiver installation determines satellite pseudorange errors and communicates them to nearby users, offers a promising technique of improving SPS on a local scale. It can improve the accuracy of the GPS even when SPS is provided at full accuracy, i.e., when Selective Availability is removed. The type of corrections, the associated accuracy, and the update rate are discussed here. Differential system design alternatives and the advantages and disadvantages of each are also described.