The Need for Range Rate Corrections in DGPS Correction Messages

Byungwoon Park, Jeonghan Kim, Sanghoon Jun, Changdon Kee and Rudolph Kalafus

Abstract:	The Range Rate Correction (RRC) has been used in differential GPS reference station broadcast messages to account for variations that occur between successive Pseudo-Range Corrections (PRC´s). While Selective Availability (SA) was in effect on the C/A-Code, RRC´s were useful in reducing the rate at which differential corrections were broadcast, because the errors introduced by SA could change by several meters in a ten-second period. Now that SA has been removed, the issue has been raised as to whether the RRC term is still necessary. With SA removed, the temporal variations in the corrections are much smaller, and are determined primarily by atmospheric variations, reference station clock drift, and reference station receiver noise. This presents the possibility of broadcasting corrections at a greatly reduced rate, and thus reducing the data requirements for differential GPS service. The RRC term has typically been computed based on carrier phase variations. Because the SA technique caused the carrier and code to vary in lock step, variations in the carrier phase were excellent predictors of the variations in the corrections. However, with SA removed, this is no longer true, because ionospheric variations play a major role. Because of the dispersive nature of the ionosphere, code and carrier diverge. In a previous analysis it was demonstrated that this effect could actually cause RRC terms computed in this manner to introduce errors into the position solution if the correction update is reduced to once every few tens of seconds. This previous analysis also addressed a number of different methods of computing RRC´s, and showed that in all cases it would be better to set the RRC term to zero. The implication would be that future differential correction messages should omit the RRC term. Such a conclusion might apply to satellite systems other than GPS. However, the work performed up to this time has not fully addressed temporal variations of ionospheric and tropospheric delays for very low-lying satellites. This paper provides an analysis of these effects based on the WAAS tropospheric model and on established ionospheric data that cover the 11-year solar cycle. The paper will provide results on code and carrier delays that account for seasonal, diurnal, and regional differences in the atmosphere. These results will be used by the RTCM SC-104 committee to determine what recommendations to provide to service providers and reference station vendors using the Version 2 standard on how to handle the RRC terms, i.e., whether to set them to zero, establish a minimum update rate, or use a new algorithm. Similarly, the results will be used for defining differential correction messages in the new Version 3 standard. The U.S. Coast Guard is conducting tests to address these issues based on results to date because of the potential benefits of reducing the data requirements for differential corrections and thus making available new services over the existing radio-beacon broadcast links. The results of this paper are expected to provide a thorough understanding of the factors affecting the temporal variations of the corrections, and will support the development of standards worldwide.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	1266 - 1276
Cite this article:	Park, Byungwoon, Kim, Jeonghan, Jun, Sanghoon, Kee, Changdon, Kalafus, Rudolph, "The Need for Range Rate Corrections in DGPS Correction Messages," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 1266-1276.
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