JSG T.24: Integration and co-location of space geodetic observations and parameters
Chair:Krzysztof Sośnica (Poland)
Affiliation:Commissions 1, 2, 3 and 4,GGOS
Many geodetic parameters can be retrieved using various techniques of space geodesy. For instance, all satellite techniques are sensitive to a geocenter motion and gravity field variations. However, some techniques are affected more by systematic observation errors than other techniques. Earth’s rotation parameters from sub-daily to daily temporal scales can be determined using all techniques of space geodesy with higher or lower accuracy, and with better or worse temporal resolutions. Precise orbits of satellites may be based on a single technique or multiple techniques that shall mitigate system-specific orbital systematic errors.
Recently, a series of satellite missions co-locating different space geodetic techniques has been launched:
- SLR and GNSS: Galileo (all satellites), GLONASS (all satellites), QZSS (all satellites), IRNSS (all satellites), GPS (2 satellites), BeiDou/COMPASS (selected satellites), CHAMP, GRACE-A/B, GOCE, SWARM-A/B/C, ICESat-2, COSMIC-2, Terra-SAR, TanDEM-X, GRACE Follow-On A/B, etc.
- DORIS and SLR: TOPEX/Poseidon, ENVISAT, CRYOSAT-2, SARAL and Jason-1 (after 2009),
- VLBI and SLR: RadioAstron,
- VLBI, SLR, and GNSS: APOD,
- DORIS, GNSS and SLR: Jason-2/3, HY-2A and Sentinel-3A/B,
- SLR, VLBI, GNSS and DORIS: GRASP and E-GRASP (planned missions).
SLR retroreflectors are passive and relatively cheap devices; thus, they are installed on-board many low- and high-orbiting satellites. Many low-orbiting satellites for ocean monitoring are equipped with DORIS and GNSS receivers for precise orbit determination, and with SLR retroreflectors for the orbit validation. DORIS receivers are installed on many satellites which require precise ephemeris and orbit below 2000 km. Missions dedicated to Earth’s gravity field recovery are typically equipped with GNSS receivers and SLR retroreflectors. Most of the GNSS satellites are equipped with SLR retroreflectors (except for GPS). VLBI telescopes are typically slow as they are dedicated to tracking extra-galactic quasars. Hence, many VLBI telescopes have problems with tracing fast-moving low-orbiting targets that are planned for co-location on-board satellites. However, first experiments using the APOD satellite with a VLBI transmitter and SLR retroreflector was successfully performed in Australia. Unfortunately, the APOD GPS receiver failed soon after the satellite launch which caused some issues with the accuracy of the determined orbit when the number of SLR observations was insufficient.
Despite many LEO satellites equipped with two or three techniques of space geodesy, the full potential of the co-location on-board LEO has not yet been entirely explored in terms of deriving combined geodetic parameters. SLR observations to LEO are typically used only for validation of GPS-based or DORIS-based orbits. SLR observations to LEO and GNSS do not contribute at all to realization of the International Terrestrial Reference System despite GNSS and LEO satellites contributing to GNSS and DORIS solutions, respectively.
Recently, the International Laser Ranging Service initiated a series of special tracking campaigns dedicated to tracking new LEO and GNSS spacecraft which increased the amount of collected data with a perspective of their full co-location in space. The combination of solutions based on GNSS, SLR, LLR, DORIS and VLBI requires a profound investigation of biases and systematic effects affecting all individual techniques. Neglecting systematic effects may lead to degradation of solutions and to absorption of various systematic effects by global geodetic parameters.
The main goal of this JSG is to investigate methods to combine global geodetic parameters derived from multiple techniques of space geodesy with the major focus on those missions capable of co-locating and integrating different observation techniques. We aim at improving quality and reliability of global geodetic parameters and realization of the terrestrial reference system through integration of different microwave techniques with laser techniques. We will also explore benefits emerging from co-locating geodetic techniques on-board low and high-orbiting satellites. We aim at detailed analyses related to system-specific issues and systematic effects emerging from combining different techniques of space geodesy, and at the assessment of their contributions to combined global geodetic parameters.
- Determination of global geodetic parameters using combined space geodetic observations.
- Determination of geocentre motion from: SLR observations to passive and active satellites, DORIS, Galileo, GPS, GLONASS, and BeiDou or possibly also VLBI (using inverse methods).
- Separation of geophysical signals in the geocentre motion from the technique-specific and system-specific errors, employing the co-location in space between SLR and GNSS using Galileo, BeiDou, and GLONASS satellites for deriving common parameters.
- Analysis of potential usability of SLR observations to active LEO satellites together with GNSS and DORIS data for deriving global geodetic parameters.
- Analysis of daily pole coordinates and of length-of-day variations using combined SLR and microwave observations to different GNSS and LEO satellites with DORIS receivers, and the comparison with respect to LLR and VLBI results.
- Determination of sub-daily Earth’s rotation parameters from VLBI, GPS, GLONASS, Galileo and SLR observations to LEO and geodetic satellites.
- Precise orbit determination of LEO and GNSS satellites using combined SLR and microwave observations – GNSS and DORIS.
- Estimation of geodetic parameters using GNSS employing time-variable gravity field models derived from SLR, active LEOs and GRACE. Assessment of the vulnerability of satellite orbits to low-degree Earth’s gravity field depending on the satellite heights.
- Homogenization of tropospheric delay models for co-located space geodetic stations. Separation of the wet and hydrostatic tropospheric delay; analysis of the horizontal gradients for optical and microwave techniques.
- Combination of SLR observations to various LEO missions: Sentinel-3A/3B, GRACE, GRACE-FO, GOCE, SWARM-A/B/C, Jason-2/3 to realize the terrestrial reference frames.
- Determination of time-variable low-degree gravity field using SLR observation to passive geodetic satellites and GNSS-based orbits of LEO satellites to fill a gap between GRACE and GRACE-FO missions,
- Estimation of Earth’s rotation parameters by combining LLR, SLR and GNSS, and their comparison with VLBI results.
Program of Activities
- To launch a questionnaire for the current integration of techniques and finalized co-location in space experiments.
- To open a web page with information concerning the co-location in space, the combination of global geodetic parameters and the exchange of ideas, provision and updating the bibliographic list of references of research results and relevant publications from different combination centers.
- To launch a proposal for two state-of-the-art review papers on co-location on-board LEO and GNSS and combination of global geodetic parameters co-authored by JSG members.
- To organize a session at the Hotine-Marussi Symposium 2022.
- To promote sessions and presentation of the research results at international symposia both related to Earth science, e.g., IAG/IUGG, EGU, AGU, EUREF, IGS and ILRS.
Krzysztof Sośnica (Poland), chair
Mathis Blossfeld (Germany)
Janina Boisits (Austria)
Grzegorz Bury (Poland)
Florian Dilssner (Germany)
Susanne Glaser (Germany)
Toshimichi Otsubo (Japan)
Erik Schnoemann (Germany)
Dariusz Strugarek (Poland)
Daniela Thaller (Germany)
Tzu-Pang Tseng (Australia)
Radosław Zajdel (Poland)
Julian Zeitlhöfler (Germany)