Reference frames are the basis for the quantification of processes in the Earth's system and thus essential for referencing monitored global change parameters such as sea level change. The future requirements on the reference frames in terms of accuracy, long-term stability and reliability will necessitate an entirely physically based datum definition and realisation. Dynamic satellite reference frames bear the potential for novel concepts for datum realisation. Access to the centre of mass can only be provided by dynamic reference frames realised by satellite orbits. Global navigation satellite systems as well as low Earth orbiting satellites are not yet fully exploited for retrieval of datum parameters (that are origin, scale and orientation of the reference frame). It is expected that the datum parameters will strongly benefit from a combination of the different satellite missions with respect to accuracy, precision and stability. The improvement of orbit models is, however, a prerequisite as deficiencies in radiation pressure modelling today affect datum realisation and geodetic parameter time series.
The main goal of the project is the maximum exploitation of available SLR and GNSS satellites to realise a dynamic reference frame with high accuracy, long-term stability and reliability. The main tasks are (1) Improvement of orbit modelling for SLR and GNSS satellites, (2) sensitivity analysis of satellites / satellite constellations w.r.t. the datum parameters, (3) development of novel concepts for geodetic datum definition that have a more physical basis than definitions applied today for the realisation of the International Terrestrial Reference System.
The questions to be answered are the following:
- How well can datum parameters be determined using satellite techniques today?
- What improvements can be expected by combining satellites in different orbit heights, with different inclinations and observed with different tracking techniques?
- To what extent will new and physically based concepts for the definition and realisation of the datum improve the terrestrial reference frame?
The accuracy of today's satellite o the potential for improvements by including satellites with various orbit characteristics and tracked using different observing techniques. Terrestrial and space-borne GNSS receiver data are analysed together, in order to study the sensitivity to the dynamic origin, orientation and scale resulting from consistently combined satellites at different orbit heights.
In the context of the project the solutions of space geodetic techniques GNSS and SLR are compared and analysed in order to identify systematic effects. Especially, the scale is directly related to many of the reduction models (e.g. geophysical loading, troposphere refraction, thermal deformation of the instruments). The influence of existing biases on the datum parameters are studied and model refinements are performed. Based on the results of the GNSS and SLR analysis the data are combined to a consistent GNSS-SLR system and the obtained datum parameters are analysed in terms of reliability, accuracy and long-term stability.
Novel concepts for defining the time evolution of the terrestrial reference system orientation are developed and investigated in the final phase of the project. The results for the second degree spherical harmonics of the gravity field and up-to-date geophysical models of the deformation of the Earth's crust are the basis for this work.
Project PN6 contributes to the general goals of the Research Unit. In particular, it contributes to the consistency of reference systems, the definition of high-precision reference systems, and the realization of next generation reference systems, but also to the improvement of physical background models through improved orbit modelling and to the integration of all space geodetic and astronomic observations through direct linking of dynamic satellite reference frames with VLBI observations.
