Knowledge of the (sub)centimeter accuracy of the satellite position is a prerequisite for determining water levels from altimeter measurements. In addition to the accuracy on short time scales, the accuracy and stability of the orbit model on seasonal to decadal time scales is particularly important for the interpretation of water level data. As part of several projects (SEAVAR, REAPER, ESA_CCI), new orbit models were calculated at the GFZ for the Geosat, ERS-1, ERS-2, Topex, Envisat, Jason-1 and Jason-2 altimeter missions. The focus of the work was the provision of consistent orbits for long periods of time and various missions. In addition, the influence of various parameters and models (e.g. tracking systems, time-variable gravity fields, reference system) on the orbit quality was investigated. 

In the project "Influence of orbit modelling on sea level", cross-over analyses are used to investigate which orbit models produce the most consistent water levels. The analyzed orbit models originate from the original data sets (GDRs) and from different research centers (e.g. GFZ, ESOC, GSFC) and are integrated in the altimetry database of the GFZ (ADS). In addition, the contribution of orbit modelling to the regional sea level error budget on seasonal to decadal time scales is investigated. For this purpose, the differences of the radial orbit components of different current orbit models of the satellites Envisat, Jason-1, ERS-2 and Topex are calculated and analyzed on fixed grid points for each cycle.

It is shown that the choice of time-variable gravity fields for orbit modeling in particular has an impact on the estimation of seasonal variability and decadal trends. The estimates for missions prior to the launch of the GRACE mission (2002) differ particularly strongly. The orbit models vary on very large spatial scales, which in many cases can be explained by relative shifts in the geocenter of the individual models. For the Topex mission (1993-2005), the orbit modelling results in an error in the regional trend estimate of up to 2.5 mm/year. For the Topex mission, the uncertainty of the seasonal to decadal signal associated with orbit modeling reaches more than 10% of the actual signal in many regions (see Figure 1 for the trend).

Project partners:

• AWI
• CLS, DTU, TUD, University of Bonn und TU München 

Project duration:

• 2005-2019

Budget:

• BMBF, Geotechnologien: 2005-2008 (SEAVAR)
• ESA: 2009-2010 (REAPER)
• ESA: 2010-2013 (ESA-CCI sea level)

Project-related publications:

• Schreiner, P., Reinhold, A., Schöne, T., Esselborn, S., König, R., Three Decades of Altimetry Orbits: Consistent DORIS-based Orbit Series and Validation,  30 Years of Progress in Radar Altimetry Symposium, Montpellier, France, 2-7 September 2024.
• 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
• Rudenko, S., Esselborn, S., Schöne, T., Dettmering, D. (2019): Assessment of ITRF2014 for precise orbit determination of altimetry satellites. - Solid Earth, 10, 1, 293-305. https://doi.org/10.5194/se-10-293-2019
• Esselborn, S., Rudenko, S., Schöne, T. (2018): Orbit-related sea level errors for TOPEX altimetry at seasonal to decadal timescales. - Ocean Science, 14, 205-223. https://doi.org/10.5194/os-14-205-2018
• Rudenko, S., Dettmering, D., Esselborn, S., Fagiolini, E., Schöne, T. (2016): Impact of Atmospheric and Oceanic De-aliasing Level-1B (AOD1B) products on precise orbits of altimetry satellites and altimetry results. - Geophysical Journal International, 204, 3, 1695-1702. https://doi.org/10.1093/gji/ggv545
• Esselborn, S., Schöne, T., Rudenko, S. (2016): Impact of time variable gravity on annual sea level variability from altimetry. - In: Rizos, C., Willis, P. (Eds.), IAG 150 Years: Proceedings of the IAG Scientific Assembly in Postdam, Germany, 2013, (International Association of Geodesy Symposia; 143), Cham : Springer International Publishing, 55-62. https://doi.org/10.1007/1345_2015_103
• Rudenko, S., Dettmering, D., Esselborn, S., Schöne, T., Förste, C., Lemoine, J.-M., Ablain, M., Alexandre, D., Neumayer, K.-H. (2014): Influence of time variable geopotential models on precise orbits of altimetry satellites, global and regional mean sea level trends. Advances in Space Research 54:92–118, doi:10.1016/j.asr.2014.03.010
• Rudenko, S., Otten, M., Visser, P., Scharroo, R., Schoene, T., Esselborn, S. New improved orbit solutions for the ERS-1 and ERS-2 satellites. Advances in Space Research, 49, 8, 1229-1244, doi:10.1016/j.asr.2012.01.021, 2012.
• Schöne, T., Esselborn, S., Rudenko, S., Raimondo, J.-C. (2010): Radar Altimetry Derived Sea Level Anomalies - The Benefit of New Orbits and Harmonization. - In: Flechtner, F., Gruber, T., Güntner, A., Mandea, M., Rothacher, M., Schöne, T., Wickert, J. (Eds.), System Earth via Geodetic-Geophysical Space Techniques, Springer, 317-324. https://doi.org/10.1007/978-3-642-10228-8_25