Accessing and Processing Real-Time GPS Corrections for Precise Point Positioning - Some User Considerations

摘要

Natural Resources Canada makes its GPS Corrections (GPS·C) available in real-time through the Canada-wide Differential GPS Service. These corrections have also recently become available over the Internet using prototype UDP and TCP servers. The latter uses the NTRIP model of data transfer to allow, in part, for the creation of Virtual Reference Stations and the production of RTCM-SC104 local-area corrections for Differential GPS. In addition, both servers provide high-precision wide-area corrections in a modified RTCA-SC159 format to provide the necessary precision to eventually realize real-time Precise Point Positioning (PPP) to the sub-decimetre level. Obtaining the highest possible accuracy from GPS corrections requires an awareness of the clock reference, or datum, to which they refer. The pseudorange measurements selected for the correction computation determine the clock reference for those corrections. The GPS·C corrections are currently referenced to the C/A (C1) and P2' (C1+P2-P1) pseudorange observables and users must match these to achieve the best possible accuracy. Failure to do so can degrade positioning precision by up to a factor of three or more.