ALBERT

Description: Orbital products describe positions and velocities of satellites, be it the Global Navigation Satellite System (GNSS) satellites or Low Earth Orbiter (LEO) satellites. These orbital products can be divided into the fastest available ones, the Near Realtime Orbits (NRT), which are mostly available within 15 to 60 minutes delay, followed by Rapid Science Orbit (RSO) products with a latency of two days and finally the Precise Science Orbit (PSO) which, with a latency of up to a few weeks, are the most delayed. The absolute positional accuracy increases with the time delay. This dataset compiles the RSO products for various LEO missions and the appropriate GNSS constellation in sp3 format. The individual solutions for each satellite mission are published with individual DOI as part of this compilation. GNSS Constellation: • GNSS 24h (v01) • GNSS 30h (v02) LEO Satellites: • CHAMP • GRACE • GRACE-FO • SAC-C • TanDEM-X/ TerraSAR-X Each solution is given in the Conventional Terrestrial Reference System (CTS). • The GNSS RSOs are 30-hour long arcs starting at 21:00 the day before the actual day and ending at 03:00 the day after. The accuracy of the GPS RSO sizes at the 3-cm level in terms of RMS values of residuals after Helmert transformation onto IGS combined orbit solutions (Version 1 GNSS RSOs are 24-hour long arcs starting at 00:00 and ending at 24:00 the actual day). • The LEO RSOs are generated based on these 30-hour GNSS RSOs in two pieces for the actual day with arc lengths of 14 hours and overlaps of 2 hours. One starting at 22:00 and ending at 12:00, one starting at 10:00 and ending at 24:00. The accuracy of the LEO RSOs is at the level of 1-2 cm in terms of SLR validation. The exact time covered by an arc is defined in the header of the files and indicated as well as in the filename. This dataset compiles RSO products for various LEO missions and the corresponding GNSS constellation in sp3 format in a revised processing version 2. The switch from previous version 1 to 2 was performed on 18-Feb-2019. Major changes from version 1 to 2 are the change from IERS 2003 to IERS 2010 conventions and ITRF 2008 to ITRF-2014, as well as the temporal extension of the GNSS constellation from previous 24 hours (version 1) to 30 hours (version 2) arcs. This temporal expansion eliminates the chaining of two consecutive 24-hour GNSS constellation solutions previously used to process day-overlapping LEO arcs in Version 1. This 24h GNSS constellation (Version 1) will continue to operate and be stored on the ISDC ftp server, as discussed in more detail in Section 8.1. All RSO LEO arcs will no longer be continued in version 1 after the changeover date and will only be available in version 2 since then.

Description: There is an ongoing global effort to improve the Space geodetic techniques contributing to the Global terrestrial reference frames, which do not yet fulfill the Global Geodetic Observing System (GGOS) scientific requirements. Next-generation Global Navigation Satellite Systems (NextGNSS) satellites are planned to be equipped with optical inter-satellite links and ultra-stable clocks. The motivation of the study is to assess the improvement in the reference frames and Earth orientation parameters (EOP)achieved by the NextGNSS. In addition, transmitters on NextGNSS satellites for Very Long Baseline Interferometry observations (VLBI) are envisaged that will result in co-location in Space in addition to co-location on the ground between the Space geodetic techniques. The VLBI will observe the satellites along with the radio sources realizing the ICRF (International Celestial Reference Frame). This will empower the NextGNSS to directly determine the Earth's Rotation angle, which is otherwise impossible. Furthermore, it would allow for independent validation of satellite orbits. For this study, we will investigate multiple scenarios, such as having a NextGNSS satellite constellation with and without VLBI transmitters, and determine the improvement in the station positions and EOP.

Description: Improving the accuracy of global reference frames has increasingly become a vital task for Earth system monitoring over the last decade. In this sense, co-location in space using a single-satellite space tie is a frequently recurring concept, which has nevertheless not yet been realized. In this study, we perform simulations for the techniques DORIS, GNSS, SLR and VLBI towards the co-location in space at a single satellite striving to reach the goals of the Global Geodetic Observing System (GGOS). Therefore, Precise Orbit Determination (POD) to multiple existing missions starting from TOPEX for DORIS, LAGEOS for SLR and GRACE for GNSS up to state-of-the-art missions, such as Sentinel-6, is performed to obtain individual station and receiver accuracy, availability, and further technique-specific effects. Realistic simulations of the existing missions are extended by simulations to six fictional orbit scenarios over a time span of seven years. VLBI observations to a satellite and ensuing POD are a novelty at this point. For the fictional space-tie satellite scenarios, we included the formerly proposed NASA and ESA missions GRASP and E-GRASP, a modified version of E-GRASP with lower eccentricity, and three circular orbits with different inclinations reaching from near polar to 30 degrees. Single-technique and combined terrestrial reference frame (TRF) solutions were generated based on existing infrastructure plus the space-tie satellite and compared to real TRFs. The effect on the TRF is quantified in terms of changes in the origin and scale, in terms of formal errors of adjusted ground station coordinates, and in terms of solved Earth rotation parameters. Based on all the scenarios, we aspire to answer the question if and how the important GGOS goals can be fulfilled.