Changes of the global, regional and coastal sea level are, among others, the major indicators of global climate change in the Earth system. Satellite radar altimetry is the unique method for regional and global sea level measurements. This method, however, directly depends on the accuracy of the positions of altimeters onboard satellites, from which the distance to the sea surface is measured, and, therefore, on the accuracy of the satellite positions, especially in the radial direction. Even though significant progress in the precise orbit determination (POD) of altimetry satellites has been reached in the recent 30 years, the user requirements for the accuracy of regional sea level trends have not been yet achieved. Orbit-related errors remain a major contribution to the sea level uncertainty budget. Therefore, investigation and mitigation of these errors are still necessary and very important for a reliable determination of global and regional sea levels, their trends and their extrapolation. This information is very important for scientists and also for decision makers in climate policy to support them with reliable quantification of global and regional sea level variations.

The objective of this project is to investigate and mitigate the current remaining errors in POD of altimetry satellites. The key tasks of the project are the following:

• to perform a detailed review of various state-of-the-art models used in the POD of altimetry satellites,
• to investigate the impact of these models on the orbit quality for these satellites and altimetry products based on these orbits,
• to find possible sources of the current remaining errors in POD of these satellites,
• to find possibilities for further improvement of orbit quality for these satellites,
• to mitigate the current remaining errors in POD of altimetry satellites and thus improve the orbit quality for these satellites.

Through the use of new models, recent reference frame realizations, improved corrections and algorithms, the errors coming from the deficiencies in the following components will be investigated and mitigated:

1. measurement corrections and data processing algorithms,
2. gravitational and non-gravitational forces acting on altimetry satellites, and
3. terrestrial reference frame realizations and corrections to station positions.

Some other error sources will be also investigated, if found relevant to the project during its execution. With these investigations, the project aims to contribute to the improved quality of altimetry satellite orbits as a fundamental requirement for estimating reliable sea level variations.