The dynamics of satellite orbits are determined by conservative (gravitational field) and non-conservative (e.g. due to atmospheric drag or solar radiation pressure) forces. All forces are modelled and parameterised in our central “Earth Parameter and Orbit System” software package EPOS. By evaluating observations such as “Doppler Orbitography and Radiopositioning Integrated by Satellite” (DORIS), “Global Navigation Satellite System” (GNSS), “Satellite Laser Ranging” (SLR) and other techniques such as inter-satellite distance measurements on GRACE or GRACE-FO, EPOS allows us to calculate all our parameters of interest with highest accuracy in the sense of a least squares adjustment and at the same time obtain highly precise satellite orbits.
We provide our results to the IAG services “International Earth Rotation and Reference Systems Service” (IERS), “International Laser Ranging Service” (ILRS) and “International DORIS Service” (IDS) and make the calculated satellite orbits available to the scientific community via the GFZ ISDC. At the same time, we also offer our precise satellite orbits or inter-satellite baselines to various project partners such as GMV, ESA or DLR as part of third-party funded projects. Since EPOS also has a simulation mode, we can investigate the effects of future instrumentation on geodetic target parameters on various satellite missions planned by ESA (e.g. Next Generation Galileo or GENESIS).
Of particular interest to us is the method of distance measurement using satellite laser radar (SLR). We regularly provide various products as an official SLR Analysis Center for the International Laser Ranging Service (ILRS) and thus also contribute to the development of new international coordinate systems such as the “International Terrestrial Reference Frame” (ITRF). Additionally, we operate a SLR station on the Telegraph Hill as part of the worldwide station network of the ILRS.
Similar to the ILRS Analysis Centre, we have also been operating an Associated Analysis Centre for the International Doris Service (IDS) since several years. To analyse GNSS radio occultation data for the improvement of weather forecasts for various European weather services, we have been operating an operational 24/7 Rapid Science (RSO) and Near Realtime Orbit (NRT) orbit determination for Low Earth Orbiters (LEO) such as CHAMP, GRACE, GRACE-FO, TerraSAR-X or TanDEM-X since 2000.
For these and various other LEOs, we also calculate orbit predictions, which contribute significantly to the success of these missions, as they are the basis for planning the download of satellite data (e.g. with the help of our satellite receiving station in Ny-Ålesund) or for controlling instruments on satellites. The most demanding application is the control of the SLR ground stations of the ILRS. This requires an accuracy of approximately 70 metres in the orbit direction, which corresponds to a deviation of 10 ms from the time at which the satellite becomes visible above a station (i.e. the satellite is too early or too late). The accuracy of the predicted orbits is constantly monitored and compared, for example, with the RSOs mentioned above, which serve as highly accurate (a few centimetres) reference orbits.
The Lense-Thirring effect, which describes the precession of the orbital node of a particle flying around a rotating mass and which is a manifestation of the phenomenon of general relativity, was measured by us from the node drifts of the LAGEOS and LARES satellites using the GRACE gravity fields with an accuracy of approx. 10%.
In addition to these topics, we are working on a number of third-party funded projects, e.g. by
- using our SLR station to participate in long-distance time transfer experiments,
- investigating the benefits of future global navigation satellite systems (NextGNSS), which will be a further development of the current GPS, GLONASS, Galileo or Beidou constellations, in order to better achieve some important goals of the Global Geodetic Observing System (GGOS),
- routinely calculating high-precision orbits for the Sentinel satellites of the Copernicus Earth observation programme of the European Commission and the European Space Agency (ESA), or
- routinely performing high-precision baseline determination between the two radar satellites TanDEM-X and TerraSAR-X on behalf of DLR.
Literature
Below you find some recent publications of the working group 3. The section's complete list of publications can be found here.
Neumayer, K. H., Schreiner, P. A., König, R., Dahle, C., Glaser, S., Mammadaliyev, N., Flechtner, F. (2024 online): EPOS-OC, a Universal Software Tool for Satellite Geodesy at GFZ. - In: (International Association of Geodesy Symposia), Berlin, Heidelberg: Springer.
https://doi.org/10.1007/1345_2024_260
Schreiner, P. A., König, R., Neumayer, K., Reinhold, A. (2023): On precise orbit determination based on DORIS, GPS and SLR using Sentinel-3A/B and -6A and subsequent reference frame determination based on DORIS-only. - Advances in Space Research, 72, 1, 47-64.
https://doi.org/10.1016/j.asr.2023.04.002
Testa, A., Michalak, G., Dassie, M., Neumayer, K., Giorgi, G. (2023): Estimating Satellite Navigation Broadcast Ephemeris via Inter-Satellite and Ground-to-Satellite Ranging. - Engineering proceedings, 54, 1, 15.
https://doi.org/10.3390/ENC2023-15463
König, R., Reinhold, A., Dobslaw, H., Esselborn, S., Neumayer, K., Dill, R., Michalak, A. (2021): On the effect of non-tidal atmospheric and oceanic loading on the orbits of the altimetry satellites ENVISAT, Jason-1 and Jason-2. - Advances in Space Research, 68, 2, 1048-1058.
https://doi.org/10.1016/j.asr.2020.05.047
Glaser, S., Michalak, G., Männel, B., König, R., Neumayer, K., Schuh, H. (2020): Reference system origin and scale realization within the future GNSS constellation "Kepler". - Journal of Geodesy, 94, 117.
https://doi.org/10.1007/s00190-020-01441-0
Glaser, S., König, R., Neumayer, K., Balidakis, K., Schuh, H. (2019): Future SLR station networks in the framework of simulated multi-technique terrestrial reference frames. - Journal of Geodesy, 93, 11, 2275-2291.
https://doi.org/10.1007/s00190-019-01256-8
Glaser, S., König, R., Neumayer, K., Nilsson, T., Heinkelmann, R., Flechtner, F., Schuh, H. (2019): On the impact of local ties on the datum realization of global terrestrial reference frames. - Journal of Geodesy, 93, 5, 655-667.
https://doi.org/10.1007/s00190-018-1189-0
Operational RSO and NRT orbit determination
Processing of the GNSS radio occultation data within the GNSS Atmosphere Sounding Project (ATMO) requires availability of the precise orbits of the GPS and Low Earth Orbiting (LEO) satellites.
The application of the radio occultation products for the operational Weather Prediction Services requires additionally generation of the satellites orbits with very low latency.
To fulfill the above requirements GFZ has developed and operates two orbit determination systems.
The first one, the so called Rapid Science Orbit (RSO) determination system produces precise orbits with 1 day delay which are used for off-line validation and gap-less generation of the radio occultation products (vertical profiles of bending angles, temperature and humidity) .
The second one, the Near-Real Time (NRT) orbit determination system generates the orbits with a frequency of one satellite revolution (approximately every 1.5 hours) and latency of 15-30 min which are prerequisite for operational generation of the radio occultation products in Near-Real Time.
Currently the NRT and the RSO processing systems deliver orbits for all GPS satellites and for the LEO satellites CHAMP, GRACE A/B, SAC-C, COSMIC 1-6, TerraSAR-X and TanDEM-X. The orbits are stored in the GFZ Information System and Data Center (ISDC) and are freely available for the scientific community world-wide.
Rapid Science Orbits
The Rapid Science Orbit system was developed first in 2001 for the CHAMP mission to generate orbits on a daily basis and to enable processing of the radio occultation data collected by the CHAMP on-board receiver (see GPS radio occultation with CHAMP).
The RSO system generates the satellite orbits from GPS data using the GFZ's “Earth Parameter and Orbit System – Orbit Computation” (EPOS-OC) software in the so-called two-step approach.
In the first step the GPS orbits and clock biases are estimated using a globally distributed network of ground stations. In the second step, the estimated GPS orbits and clocks are used as fixed in the subsequent estimation of the orbits of the Low Earth Orbiting (LEO) satellites.
The LEO orbits generated by the RSO system are validated by independent Satellite Laser Ranging (SLR) observations and obey an accuracy of approximately 5 cm.
The RSO system was extended by the SAC-C satellite in 2003, the GRACE A/B satellites in 2004 to enable processing of the occultation data from these satellites (GRACE RO), the COSMIC 1-6 satellites (2006) (COSMIC-RO), TerraSAR-X in 2007 (TerraSAR-X RO) and TanDEM-X in 2010 (TanDEM-X RO).
The RSO system can easily be extended by other satellite missions. The continuous availability of the orbits is accured by human interaction/repair has to performed if the orbits can not be computed automatically due to data or quality problems.
Near-Real Time orbits
To enable assimilation of the satellite radio occultation products in the Weather Prediction Systems, a Near-Real Time orbit determination system for CHAMP and GRACE was developed and became operational in June 2006.
The NRT processing system, similarly to RSO, is also using the two-step approach for the LEO orbit determination.
The frequency of the NRT orbits, in contrast to RSO, is determined by the frequency of the LEO data 'dumps' (downloads) at S-band receiving staions. F.i. by employing our station in Ny Alesund dumps arrive approximately once per satellite revolution. Once a new portion of data is available, the NRT system generates a new orbit. The latency of these orbits is very low and amounts to 15-30 minutes after the last epoch of the data used in the processing. The GPS NRT orbits, needed for the two step approach, are generated every 15 minutes with a latency of 10 minutes to be available at any time of the LEO dump.
The NRT system contains three subsystems with different sources of the GPS orbits (estimated in-house from ground data or retrieved from the International GNSS Service) what increases redundancy and reliability of the orbit products.
The system was used to generate CHAMP orbits till the very end of the mission in October 2010, for SAC-C satellite it was activated for validation purpose in a period November 2010 - August 2011.
For the GRACE-A satelite the NRT orbits are continuously generated since August 2006.
The NRT system for TerraSAR-X was activated in August 2007 (see also TerraSAR-X RO), for TanDEM-X (TanDEM-X RO) on 24 June 2010, 3 days after the launch of the satellite, what demonstrates the ability of the system to rapidly include new satellites in to the processing chain.
The accuracy of the NRT orbits depends on the NRT subsystem and is in the range of 5-10 cm obtained from the SLR validation. The system is fully automatic, needs offline and postfact human monitoring and maintenance.