- Schematic overview of the GEROS experiment to be installed aboard the International Space Station. Red lines indicate the reflectometry measurements for water, ice and land surface monitoring. Blue lines indicate GNSS RO and coherent reflectometry observations and the green lines symbolize the GNSS signals, received from zenith for Precise Orbit Determination (POD) of the GEROS payload and 3D upside ionosphere monitoring.
- © J. Wickert (TUB/GFZ)
GEROS-ISS  is an innovative experiment for
climate research, which was proposed within a call of the European
Space Agency ESA 2011 for installation at the International Space
Station ISS. The proposal of the international team from Germany,
Sweden, Spain, U.S., Denmark, and Switzerland was the only selected
from ESA out of about 25 submitted proposals for further studies.
Within GEROS-ISS it is planned to use navigation satellite signals for remote sensing of ocean, ice and land surfaces and also for atmosphere sounding to derive information on climate change. Main mission goal is the precise determination of the sea surface height using reflected GNSS signals.
A group of scientists (Science Advisory Group), led by Prof. Jens Wickert (GFZ), works on the scientific preparation of the mission and contributes to the technical preparation. ESA initiated two industrial feasibility studies under lead of ADS (Airbus Defense and Space, Madrid, Spain) and TAS (Thales Alenia Space, Rome, Italy) to investigate the technical realization of GEROS-ISS in detail. These Phase A studies were finished after two years duration in 2016. GARCA, a scientific study, also initiated by ESA and conducted under lead of GFZ, contributed to the scientific preparation of the mission and also supports the industrial studies. GARCA was also performed within two years and finished in November 2016 .
Beside the scientific coordination GFZ is mainly involved in GEROS with investigations related to the phase altimetry  and to the usage of the new GNSS-R data to improve ocean models .
Recently ESA performs a cost reduction study for the GEROS mission, which will be finalized late 2017. Within the recent Earth Explorer 9 call from ESA, the GEROS/GARCA science team initiated the submission of the G-TERN proposal  in June 2017, which is based on the GEROS technical concept. G-TERN is jointly coordinated by Jens Wickert (GFZ) and Estel Cardellach (IEEC, Spain).
GARCA project 
Webstory by European Space Agency 
 Wickert, J., et al., GEROS-ISS: GNSS REflectometry, Radio Occultation, and Scatterometry Onboard the International Space Station, IEEE Journal of selected topics in applied Earth observations and Remote Sensing, Vol. 9, Issue: 10, p. 1_30, 10.1109/JSTARS.2016.2614428, 2016.
 GARCA team, GNSS-R assessment of requirements and consolidation of retrieval algorithms, Final Project Report, ESA-AO1-7850/14-GARCA-FR, pp 463, 2016.
 Semmling, M., Leister, V., Saynisch, J., Zus, F., Heise, S., Wickert, J. (2016): A Phase-Altimetric Simulator: Studying the Sensitivity of Earth-Reflected GNSS Signals to Ocean Topography. - IEEE Transactions on Geoscience and Remote Sensing, 54, 11, p. 6791-6802.
 Saynisch, J., Semmling, M., Wickert, J., Thomas, M. (2015): Potential of space-borne GNSS reflectometry to constrain simulations of the ocean circulation. - Ocean Dynamics, 65, 11, p. 1441-1460.
 Wickert, J./Cardellach, E. et al., G-TERN: GNSS Transpolar Earth Reflectometry moNitoring system, Proposal for an Earth Explorer 9 Mission, European Space Agency, 2017.