Global Reference Systems

• Introduction
• International Earth Rotation and Reference System Service (IERS)
• International Laser Ranging Service (ILRS)
• International VLBI Service (IVS)

Introduction

Utilization of satellite methods for positioning and navigation purposes enables integration of the national reference systems into precise global reference systems.

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International Earth Rotation and Reference System Service (IERS)

The International Earth Rotation and Reference System Service (IERS) provides the astronomical and global terrestrial reference systems as well as data on the earth’s rotational behaviour. As a result of an international call for tenders BKG was entrusted with the direction of the Central Bureau of the IERS. The Central Bureau is the Coordination Centre between the Directing Board, the Analysis Coordinator, and the Technique, Product and Combination Research Centers of the IERS. It has to ensure the flow of information and data between the international organisations involved.

Extensive webpages were established that are continuously supplemented in order to provide information about the IERS with its manifold components. On these sites not only general information or important links to other WEB sites are given, but also the structure of the new IERS is represented and the pertaining functions are explained. By the publication of IERS messages, Annual Reports and Technical Notes information is supplied to the interested users. Besides maintaining the IERS Information Service, replying to inquiries concerning the IERS and providing general information on earth rotation constitute also a task of the IERS. Moreover, the Central Bureau organizes the meetings of the Directing Board. One of the essential tasks consists in setting up a database in which all relevant data and products of the IERS are saved and archived (IERS Website).

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International Laser Ranging Service (ILRS)

The ILRS and coordinates the laser rangings to artificial satellites (SLR) in order to support international geodetic programs and satellite missions, develops standards and specifications, which are a prerequisite for product consistency, and defines observation and analysis strategies in order to ensure the quality of the global coordinate network. Precise satellite ephemerides, coordinates and velocities of the observation stations, variations of the geocentre, statical and time-dependent coefficients of the earth’s gravity field, fundamental constants of physics, and moon parameters number among the fundamental products of the ILRS. The ILRS products are made available to IERS to serve as a basis for the creation and maintenance of a precise International Terrestrial Reference Frame (ITRF).

The principal emphasis of the work of the ILRS Analysis Centre (AAC) of BKG is on the determination of station coordinates, there motion rates and the earth rotation parameters within the frame of “single satellite, multi-satellite combined solutions”. For the corresponding computations the UTOPIA program package as well as the Centre’s own software developments are used. Further, procedures are established which allow to automate computation of the ILRS products.

The ILRS Group participates in the elaboration of an extended SINEX format for the ILRS. For this purpose, the series “SINEX solutions for single satellites” was integrated into an overall solution using a specifically developed software for network combinatorial computations.

The monthly SLR solutions are examined for geometric, numeric and temporal stability in order to assess the quality of the solutions as a basis for reference systems. To this end, methods serving an improved error analysis were developed. At the same time a-priori information referring to the observation network could be successfully counted back. These methods are very useful for the purpose of checking the stability of individual solutions before they are made use of for combinatorial computations.

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International VLBI Service (IVS)

BKG’s VLBI Group is one of the three "Primary Data Centers" worldwide of the IVS and is together with the VLBI Analysis Group of the Geodetic Institute of the University of BONN (GIUB) also one of the six IVS Analysis Centers worldwide (BKG VLBI Group Website). Within this framework the following activities are carried out:

• Generation of "VLBI Databases" for X – band and S – band from the raw data supplied to Leipzig by the correlator of the Max-Planck-Institute for Radio Astronomy. This includes calibration of the influences of the ionosphere, troposphere and cable transit times, the removal of ambiguities and outliers in the measuring data and their weighting. At the end of this process a set of files consisting of the databases and the calibration files is delivered to the IVS. 

• Compilation of a time series for UT1 – UTC from the “Intensive Experiment Series” as a IVS product (ca 200 experiments annually for the time being). The current time series can be retrieved from the IVS Data Centres under the name bkgint01.eopi. 

• Compilation of two differently parameterized time series for the earth orientation parameters (UTC1 – UTC, polar coordinates, nutation offsets) from the 24-hour VLBI experiments available since January 1984 (bkg00001.eops and bkg00002.eops). 

• Preparation of global solutions on the basis of all available 24-hour VLBI experiments for the derivation of reference systems (station and source coordinates, motion rates). 

• The analysis products provided within the frame of the IVS are retrievable from the IVS Analysis Center. Two of the three aforementioned EOP time series enter into Bulletin B published monthly.

In its function as a IVS Data Centre BKG mirrors several times a day the IVS datasets of the CDDIS, Washington, and of the Observatoire de Paris as well as the official IVS website. For the exchange of data an input area is available for all IVS components, through which VLBI databases, analysis products and other relevant data enter into the IVS system. After a successful syntax check these data are adopted into the Data Centre.

The "Operational Data Center" is maintained for the system’s own purposes of analysis. Presently, all databases of the VLBI experiments performed over the period 1976 to 2001 are stored in it. All data and the whole software are backed up in an automatic mode on a Tape Library.