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National Reference Systems Position

Introduction

For the setup and maintenance of the geodetic reference system in Germany BKG takes care of the analysis of GPS and GLONASS observations. Evaluation work is subdivided into five areas whose core element consists of the observations of the permanent stations of the "Geodetic Reference Network". The coordinates derived for this network from daily analysis and issued weekly as mean values constitute Germany’s national contribution to an overall European solution, which is integrated into a EUREF combination solution through the inclusion of further national partial solutions of other European countries. In parallel with the determination of coordinates troposphere-related signal transit delays are estimated from the relevant GPS observations.

The Europe-wide combination of these troposphere parameters constitutes a new task performed since June 2001. BKG is involved in the provision of the precise GPS satellite orbits required for these activities, through the "International GNSS Service" (IGS), within the frame of an award of contracts to the "Center for Orbit Determination in Europe" (CODE). Precise orbits of the Russian GLONASS satellites are determined independently within the framework of the “International GLONASS Service” (IGLOS). A GNSS database is connected with the analysis processes, which receives observations from the projects GREF, EUREF, IGS and IGLOS (functions as a Regional IGS Datacenter for Europe).

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Integrated Geodetic Reference Network
of Germany - GREF

The Integrated Geodetic Reference Network of Germany GREF serves the high-precision coordinate determination for the integration into international networks, the generation of real-time products for coordinate determination and the connection with the reference systems for height and gravity.

GNSS analysis of the Integrated Geodetic Reference Network of Germany GREF (click to enlarge)

Some GREF stations are at the same time part of superior networks, of the EUREF Permanent Network (EPN) and of the network of the International GNSS Service (IGS). The network analysis of the GREF is performed with the data of the GREF stations, the EPN or IGS stations, respectively, and of some SAPOS stations of the German national survey. Thus, the GREF network is on the one hand connected to the IGS stations, on the other hand a connection is created to the superior network EPN (Central Bureau of the EPN). This post-processing results in precise coordinates the time series of which allow to precisely determine station movements.

Detailed information for the BKG stations of the GREF reference network are available in a special Information System GREF (only in German).

The establishment of Internet connections in most GREF stations allows the generation of real-time products, especially the generation of RCTM data and RCTM correction data for DGPS and RTK applications. A networking software is applied that computes observation data for Virtual Reference Stations (VRS) from the connected GNSS permanent stations. Under the name GREF-IP a concept was worked out that provides the delivery of these VRS data, observation data in RCTM format or receiver raw data via Internet.

The propagation delays appearing because of the inclusion of the Internet showed themselves far predominantly in a range of few seconds which are harmless for most DGPS applications.

Further information on the real-time technology of the GREF network please find in the GNSS Data Centre of the BKG (GDC of the BKG) under the project NTRIP.

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Geodetic Reference Network for Europe - EUREF

Europe-wide use of a uniform reference system constitutes an important geodetic task in the field of georeferencing. On the recommendation of EUREF the European Commission has proposed to use for this task the system ETRS89. BKG is involved in the maintenance of this system through the analysis of a national subnet of its own as well as the weekly combination of the coordinates of further subnets into a continental overall solution. The relevant stations covered amount to ca 160. Coordinates and station velocities are estimated, which are integrated with sub-cm accuracy into the global reference system ITRF. Accumulated multi-year solutions enter into the determination of the global reference system ITRF.

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Refraction

The estimation of troposphere-related signal runtime delays constitutes part of the Europe-wide analysis of permanent GPS observations. These parameters allow to determine the water vapour content of the troposphere and are of importance for climate research purposes as well as for an improved determination of heights. BKG has the function of a EUREF analysis centre for the weekly combination of troposphere parameters from national European partial solutions. This combination solution is a weighted average carried out epochwise of single solutions. Systematic differences between the partial solutions are taken into account. Today, nearly 20 Local Analysis Centres (LAC) supply troposphere files. For altogether 160 permanent stations in Europe troposphere parameters are estimated in each case by three LACs in parallel, the homogeneity of results being secured through a Europe-wide adjustment of the relevant analysis strategies.

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GNSS Satellite Orbits

BKG makes a German contribution to the utilization of GPS through its support given to the computation of precise orbits of the GPS system within the frame of CODE. The analysis performed there in cooperation with the IGS yield the most precise satellite orbits worldwide, the continuous knowledge of which is a primary prerequisite with regard to the geodetic utilization of GPS. Computation of precise satellite orbits of the Russian GLONASS system on the basis of microwave observations is nowadays performed exclusively by the Institutions ESA/ESOC and BKG. The prospect held out of the availability of new satellites to the system’s users documents Russia’s interest in continuing to operate the system. The accuracies for orbit determination reach, as a rule ±15cm. The combined use of GPS and GLONASS observations is in the field of geodesy of particular importance for RTK applications. Further, from the combined use of GPS and GLONASS improved possibilities are obtained for the estimation of troposphere parameters.

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GNSS Data Center

Nowadays the GNSS Data Center records the data of nearly 200 permanent stations distributed worldwide. More than 100 000 files are exchanged on a weekly basis. Data management comprises the projects SAPOS / GREF, EUREF, IGS and IGLOS. About half of the stations connected provides RINEX data at hourly intervals, the remaining half provides RINEX daily files. Besides observation data the most relevant derived results are additionally stored, as e.g. coordinates, troposphere parameters and satellite orbits.

A web server is an integral part of the Data Center, which as a graphical surface serves to access the archived data material. Since in this context mainly European projects are concerned the web server language is English (GNSS Data Center)

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