The new RTCM 3.1 transformation messages - declaration, generation from reference transformations and implementation as a Server-Client concept for GNSS-Services

摘要

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) as geocentric (x, y, z) or ellipsoidal (B, L, 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. Such are the transfer of transformation parameter databases with a direct access on the GNSS-controllers, or the mapping of a desired reference transformation and/or geoid into a discrete so-called grid and its transfer to the GNSS-controller by the user. The aim of the transformation messages of the recent 2007 RTCM 3.1, and so that of the RTCM working group (composed of representatives of GNSS industry, GNSS-services and science) behind, respectively, was to define transformation algorithms and data structures, 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. Further, the GNSS service providers are able to ensure, that unique and current information for the computation of the different transformations is used. 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. The networking software of the GNSS service administrates the distribution of the transformation messages. 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. The paper describes at first the so-called reference transformations by the mathematical models, algorithms and data structures. The various reference transformations (continuous transformation parameter databases, grids, plate transformations, geopotential models, DFHBF , DFLBF are divided into three different categories. The concept of gridding using virtual fitting points generated from the reference transformations for a set up of RTCM transformation messages is treated. The difference between static grids and the general and powerful dynamic RTCM grids is discussed. Finally the results of the use of the RTCM server GZTraS with the active configuration for DFHBF/DFLBF databases as reference transformations of the transformation modules in country-wide tests in the area of Bavaria, Germany, are presented.