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Basics of RTCM 3.1 Transformation Messages Standard for GNSS Positioning Services

By Mohamed Eleiche


As concerns the use of RTCM observation correction data in GNSS positioning services, the coordinates are resulting within the ITRF or in a regional ITRF-realization (e.g. ETRF89 in Europe) as geocentric (x,y,z) or ellipsoidal (ϕ,λ ,h) coordinates. The users of GNSS services must however normally present their results in the coordinates of regional or local horizontal and vertical datum systems. Therefore, coordinate transformations are necessary. Currently, the pre-calculated transformation parameters and/or geoid-models have to be transferred in advance to GNSS controllers in particular ways. The aim of the transformation messages of the recent 2007 RTCM 3.1 was to define transformation algorithms and data structures by seven RTCM transformation messages, which allow the GNSS service to transmit respective RTCM transformation messages to the user of the GNSS service. In that way and by the transformation messages’ use in the GNSS-controllers, the above ITRF-based coordinates can automatically be transformed to the desired horizontal datum and height reference system, and the above preparation of a data transfer and further manual operations during the GNSS-measurement become obsolete. The software and communication architecture for the use the RTCM transformations messages in a GNSS service can be realized as a server-client concept. The desired reference transformations are implemented within the so-called transformation modules and as part of the RTCM transformation messages server. In that way, the NMEA-position of the GNSS-rover is used as the basic server request and it is passed through the administrating GNSS networking software to the RTCM transformation messages server. Depending on the configuration of the RTCM transformation messages server, different transformation modules are activated and different message design specifications are holding. Accordingly one or several binary RTCM transformation messages are generated and send via the GNSS network software back to the client of the GNSS-controller

Topics: Differential Global Navigation Satellite Systems, mathematical geodesy
Publisher: RTU
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