After Selective Availability (SA) was turned off, the rate of change with time of the DGPS common errors (atmospheric delay, satellite orbit and clock error) became quite slow. This inevitably leads to a requirement to modify various configurations of DGPS correction message broadcasting, and reference station (RS) managers need to examine the characteristics of GPS measurement errors with SA-off. GPS error sources are temporally and spatially decorrelated, so the DGPS user position accuracy is varied by the baud-rate of the RS, the distance between the user and the station, and the noise statistics of the receiver. We identify the minimum and maximum size of correction data, interval time, the coverage range and the baud-rate that are required to maintain the existing DGPS service. Moreover, the compatibility and accuracy can be assessed to meet the users' requirements without measurements being needed. The results in this paper are used in the study and testing for the redesign of United States Coast Guard (USCG) RS. We hope that our study will be a great help in determining the flexible factors of both the RS and the user.
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