Implementation of Galileo Geodesy Service Provider prototype
Background & Objectives
For many Earth science applications as well as for satellite navigation, geodesy has the potential to provide a TRF, as a realisation of a TRS, to which are related valuable observations of parameters that govern the Earth system. Since the 1980s, continuous improvement of space geodesy techniques, in terms of technology and modelling of their observations, has drastically improved our ability of TRF determination and its maintenance over time to reach the one-millimetre accuracy level.
While any individual space geodesy technique (VLBI, SLR, DORIS, GNSSs including Galileo in the future) is able to provide observations allowing a TRF determination, a combination of station positions and velocities from independent techniques has long been the standard method to realise global terrestrial reference frames. In principle, the particular strengths of one observing method can compensate for weaknesses in others if the combination is properly constructed, suitable weights are found, and accurate local ties in co-location sites are available. None of the space geodesy techniques is able to provide all the necessary parameters for the TRF datum definition (origin, scale and orientation).
The utility of multi-technique combinations is therefore recognised for the reference frame implementation, and in particular for accurate datum definition. Since the creation of the International Earth Rotation and Reference Systems Service (IERS), the current implementation of the International Terrestrial Reference Frame (ITRF) is based on a multi-technique combination, incorporating individual TRF solutions derived from space geodesy techniques as well as local ties of co-location sites. The IERS has recently initiated a new effort to try to improve the quality of ties at existing co-location sites, without which the ITRF could not exist.
The main function of GGSP is the realisation of a precise and stable GTRF, fully compatible with the ITRF and a realisation of the ITRS. The GTRF will be the basis for all Galileo products and services, serving both the Galileo core system (GCS) and the Galileo user segment. This implies that the GGSP should enable all users of the Galileo system to rapidly access the GTRF with the precision required for their specific application.
The main Work Packages of the GGSP prototype are:
- Consolidation of GGSP prototype (specification, definition and planning phase);
- Realisation, validation and maintenance of the GTRF;
- Validation of satellite orbits, clocks and earth rotation parameters;
- Outreach and applications in geodesy;
- Recommendations for the implementation and operation of the permanent GGSP.
The prime task of the GGSP is to provide station coordinates and velocities for the Galileo sensor stations for the In-Orbit Validation-phase of Galileo. Furthermore, the GGSP responsibilities will include the generation of other precise products that are needed by the advanced Galileo geodetic user community to get full access to the GTRF. These products, generated simultaneously with the GTRF activities performed within the GGSP, will comprise of at least:
- precise Galileo satellite orbit and clock estimates;
- Earth rotation parameters (ERPs);
- GTRF data and information for the users.
The GGSP contract covers the specification, design, implementation, testing and operation of the prototype Galileo Reference Service Provider (GRSP) to support the Galileo IOV phase. The prototype of the Galileo Reference Service Provider is responsible for establishing the Galileo Terrestrial Reference Frame (GTRF), and its relation with the International Terrestrial Reference Frame (ITRF) within tight specification.
The GTRF has to fulfil general requirements as there are accurate TRFs for all relevant Galileo operations, long-term stability with high accuracy and reliability, connection to ITRF at the same accuracy level and, to the maximum extent, independent maintenance of GTRF from other TRF realisations. There are a wide range of users, user groups and user communities of Global Navigation Satellite Systems such as Galileo. These users need knowledge about the inherent reference frame as the basis of all Galileo services and applications. Among the potential users of the GGSP are geodetic users, scientific users and application users.
Work performed & results
The GGSP results will comprise specification for the GTRF implementation, datum definition and analysis strategy for GTRF derivation and maintenance over time. The GTRF is designed to be compatible with the International Terrestrial Reference Frame (ITRF) and will therefore be a realisation of the International Terrestrial Reference System (ITRS). The GTRF station coordinates shall be identical to the ITRF station coordinates within a tolerance of 3 cm (at 95% confidence level) for all station markers that appear in both frame realisations.