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The first successful integration of the optical and microwave observations onboard Galileo and GLONASS

22-10-2021

Scientists from the Institute of Geodesy and Geoinformatics of the Wroclaw University of Environmental and Life Sciences (UPWr) and Technical University of Munich (TUM) for the first time successfully conducted an integration of two space geodetic techniques, i.e., microwave – GNSS and optical – SLR, aiming at the geodetic datum realization in space, independently on the ground measurements.

The role of space geodetic techniques in the realization of the terrestrial reference frames

The description of the Earth system demands identification and positioning in time and space all the phenomena occurring with it. One of the main goals of global geodesy is to provide an accurate and stable in time global geodetic reference system. Currently, the global reference frame is resolved by a combination of observations provided solely by space techniques, that is, Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). All the techniques are treated separately in the current realization of the International Terrestrial Reference Frame (ITRF2014) and the linkage between the observing stations of each technique is resolved by ground measurements using the so-called “local ties”. The local ties are calculated based on the ground observations conducted using precise instruments. The local ties cannot be obtained operationally and are contain information about the linkage between different techniques in certain localization.

Integration of the space geodetic techniques onboard navigation satellites

Modern navigation satellites of the emerging the global navigation satellites systems such as Galileo, GLONASS, and BeiDou are equipped with laser retroreflector arrays for SLR providing an opportunity for the ground laser stations to be tracked. As a result, the navigation satellites provide an excellent platform for the co-location of the two independent space techniques in space onboard the GNSS satellites. The linkage is resolved using the so-called “space-tie” which constitutes a vector between the GNSS antenna phase center and the centroid of the SLR retroreflector. So far, the SLR-to-GNSS observations were mainly used for the validation of the microwave-based GNSS orbit products, but its great potential was neglected by the realization of the international terrestrial reference frames due to the complexity of the data processing of two completely different space geodetic techniques: microwave and optical.



Pioneer achievement of the UPWr and TUM scientists

Scientists from the Institute of Geodesy and Geoinformatics of the Wroclaw University of Environmental and Life Sciences (UPWr) together with scientists from the Technical University of Munich (TUM) conducted a combination of the microwave and optical techniques onboard the GNSS satellites aiming at the realization of the Geodetic datum in space independently of the ground measurements.

For this study both were used, the microwave observations broadcasted by the GPS, Galileo, and GLONASS satellites, and optical observations to navigations satellites which are conducted by the laser stations. A series of different strategies were tested to define the best network constraining scenario for two independent networks of GNSS and SLR stations.

The best results were obtained using a consistent network constraining to both GNSS and SLR networks, i.e., no-net-translation and no-net-rotation. Moreover, the accuracy of the integration was assessed to be at the level of 40-50 mm for a single solution and at the level of a few millimeters for long-term measurements. Based on the space ties, the quality of the local ties recovery was evaluated to be at the level of 3 mm.

Realization of the geodetic datum onboard the GNSS satellites comprises an alternative for the classical solution in which all the techniques are connected using ground measurements. The pioneer methodology developed by the scientists from UPWr and TUM can be adopted for the future realization of the terrestrial reference frames. Moreover, the integration of microwave and optical observations allows for the better quality determination of the length-of-day excess as well as allows for inter-continental station connection using space ties. More details can be found in the article published in Journal of Geophysical Research: Solid Earth under the link: https://doi.org/10.1029/2021JB022211

The research was funded by the grant UMO-2019/35/B/ST10/00515 awarded by the National Science Centre (NCN), Poland