PRC Generation in Time-Latency: IS RRC Still Required Even If S/A Has Been Turned Off?

Changdon Kee, Byungwoon Park, Jeonghan Kim, and Seongjoon Cho

Abstract:	The SA termination improved the positioning accuracy of standalone-GPS dramatically and the robustness of DGPS corrections to latency. Before the discontinuance of SA, there was such a large distortion due to SA that DGPS users had to receive the PRC (pseudo range correction) as frequently as possible (every several seconds) to use the DGPS solution continuously. The rate of signal distortion was also large, and so the reference station had to send RRC (range rate correction) with PRC to compensate for the time latency. Though the SA is off now, the DGPS reference station is still sending RRC with PRC, but it is wondered whether the RRC is still indispensable to DGPS positioning. The RRC has been used to model the time varying characteristic of the PRC over the period in which the PRC is generated at the reference station and applied at the mobile station (the age of correction). Now, the SA has been off, the time rate of the DGPS common errors (atmospheric delay, satellite orbit and clock error). Therefore, the update-rate of DGPS correction messages can be much reduced without serious accuracy degradation. Although the PRC without SA is far robuster than that with SA, many people still believe that the compensation for time-latency by RRC is useful, and many standard message formats such as RTCM SC-104 use it. The variance and the offset of the RRC, however, are added to the error of the PRC, when the PRC is predicted by extrapolation using the RRC in time-latency. If the time-rate of the common errors sum is small, the error of the PRC predicted by the RRC can be larger than that without prediction in time-latency, i.e. the PRC´s compensation by the RRC can be rather harmful to DGPS positioning than helpful. In this paper, we generate the PRC and the RRC of 24hr GPS data in Seoul using various techniques: 1) Without RRC (RRC=0) 2) Time difference of the CPC (carrier phase correction) 3) Time difference of the Hatch-filtered PRC for various averaging constant value 4) Time difference of the divergence-free Hatch-filtered PRC for various averaging constant value. And we examine how the amount of the time-latency effects on DGPS positioning accuracy for each technique. From the result we propose to use the PRC without RRC in some time latency. We also adopt this proposal to real-time internet-based DGPS process, and we compare the result without RRC and that with RRC from a dynamic test in intentional time-latency. Now many GPS researchers move foci from the accuracy to the popularization of GPS. The GPS researchers in RTCM committee and U.S Coast Guard have tried to reduce the correction message size. They also have interest in our research topic, and we presented our result at the RTCM meeting. From our results, we can examine the duration time for which PRC without RRC is as accurate in positioning as PRC with RRC, and it can be used to reduce the data size by deduction RRC from the message. And then we can implement DGPS service in low rate communication sending PRC message as rarely as possible.
Published in:	Proceedings of the 2004 National Technical Meeting of The Institute of Navigation January 26 - 28, 2004 The Catamaran Resort Hotel San Diego, CA
Pages:	869 - 874
Cite this article:	Kee, Changdon, Park, Byungwoon, Kim, Jeonghan, Cho, Seongjoon, "PRC Generation in Time-Latency: IS RRC Still Required Even If S/A Has Been Turned Off?," Proceedings of the 2004 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2004, pp. 869-874.
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