Precise RTK Positioning Using the New RTCM-104 V2.1 Standard

J.P. Thomas Goguen and Timo Allison

Abstract:	Real-time Kinematic (RTK) surveying is now established as a reliable field technique, and has been commercially available since 1993. Combining both GPS and radio modem technologies, processing to yield baseline vectors can occur in real-time. This results in substantial increases in productivity when compared with traditional Kinematic and Static GPS techniques which require the downloading of stored GPS observables from two or more receivers and time consuming post-processing. Accuracies at the centimeter level are typical. Differential GPS (DGPS) achieves accuracies at the sub- meter level with high performance receivers. Although less accurate that RTK, there is no requirement to initialize the system (resolve integer wavelength ambiguities), allowing operation over longer baselines. In fact, both RTK and DGPS are differential systems with different levels of accuracy and operational constraints. However, DGPS has one big advantage from a customer viewpoint- the communication link provides differential corrections based on the RTCM-104 standard. This allows equipment from different manufacturers to be combined in a network comprising reference and rover receivers. The first RTK systems had to use proprietary data formats to allow doubledifference carrier phase processing. These proprietary formats can be very efficient from a data communications standpoint, but are inherently manufacturer dependent. A system is described which achieves similar positioning performance to an existing RTK product, the GPS Total StationTM, while using Version 2.1 of the RTCM-104 standard. While adhering to this industry standard, some new and interesting possibilities unfold. The system is capable of transmitting traditional RTCM DGPS corrections together with RTK-style messages on the same RTCM radio link. This provides a service for both RTCM DGPS and RTK users. Of course, if radio modem bandwidth (baud rate) restrictions are a problem, only those RTCM messages required for RTK operation should be selected for survey applications. As compared to DGPS, the effects of Selective Availability (SA) are reduced for RTK by time-synchronized double- differenced processing. The implementation of RTCM-104 Version 2.1 is discussed in an enhanced version of the GPS Total Stationm . Problems in Version 2.1 which affect RTK are highlighted, together with proven methods to alleviate these limitations. An application is described in which the system is being successfully used to provide both an RTCM DGPS and RTK service with a single communication link.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	1461 - 1466
Cite this article:	Goguen, J.P. Thomas, Allison, Timo, "Precise RTK Positioning Using the New RTCM-104 V2.1 Standard," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 1461-1466.
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