Recent developments in space VLBI geodesy demonstration experiment (GEDEX)

The unique radio astronomical technique of Space Very Long Baseline Interferometry (SVLBI) is an extension of the ground-based VLBI into the space. It has some important potential applications in geodesy and geodynamics, including the definition, practical realization, and interconnection of different reference frames, determining the geocentric positions of VLBI stations, estimation of the gravity field of the Earth, and satellite orbit determination using the delay and delay rate observables. With the launching of the first SVLBI satellite of the VLBI Space Observatory Programme (VSOP) of Japan, in February 1997, this technique has become a reality. An international team of scientists, working under the auspices of the FOMI Satellite Geodetic Observatory, Hungary, has designed the GEDEX, for the purpose of exploring the feasibility of the geodetic applications of SVLBI. A brief description of the SVLBI technique and references to the significant theoretical developments in the field of geodetic SVLBI are provided, followed by an over-view of salient features of the Highly Advanced Laboratory for Communications and Astronomy (HALCA), the orbital component of the VSOP mission. The state of the art and the projected developments in this field are summarised. The various aspects of GEDEX, including the objectives, the tasks involved, the scope of the experiment, the software development, the data formats and recent results of data pre-processing and test data analysis are discussed. A test VSOP data set has been pre-processed and analysed. Preliminary results show that the SVLBI delay residuals are at the level of a few meters. Conclusions drawn, based upon the experience gained in this project, and recommendations for future work are presented. The major practical problems with HALCA data for geodetic use include the single observing frequency, the limited bandwidth and sensitivity, the unreliable instrumental phase-cal data, the poor sky coverage and the sparse co-observing array of ground-based VLBI telescopes