GARCA was a scientific study, funded by the European Space Agency ESA. It was contributing to the preparation of the GEROS-ISS  Mission for the application of the innovative GNSS-Reflectometry (GNSS-R) aboard the International Space Station. Within GEROS-ISS signals from the Global Navigation Satellite Systems (GNSS) are foreseen to be exploited for Remote sensing of Ocean, Ice and Land surfaces but also for atmosphere/ionosphere sounding on a near-global scale to derive information on Climate Change.
The GARCA study was conducted by an international consortium under lead of GFZ and finished November 8, 2016 after a two-year project period . Seven institutions from six different countries belong to the GARCA consortium:
The objective of GARCA was to support the assessment and consolidation of scientific requirements and the consolidation of retrieval algorithms for a spaceborne GNSS-R experiment, focusing on the GEROS-ISS concept and its primary and secondary data products (sea surface height and ocean surface roughness). The main work was the development of an end2end-Simulator for the GEROS-ISS measurements (GEROS-SIM), and the evaluation of the expected Level2-data products (L2 performance). A GEROS-SIM version was made available through a web-server, freely accessible to registered scientists. Validation and performance tests benefit from the large amount of GNSS-R data acquired by the consortium members in the last decade, and new dat, which was acquired during a flight campaign in 2015.
Three Observing-System Simulation Experiments (OSSE) by JPL, GFZ and NERSC assessed the oceanographic significance of the expected GEROS-ISS measurements and demonstrated the usefulness of the GEROS-ISS concept. Twelve additional external scientific experts supported the GARCA project and were involved in the work as beta-testers of the developments and also to initiate the sustainable formation of an interdisciplinary GEROS-ISS user community, which also initiated new GNSS-R mission proposal, as, e.g., G-TERN, within the Earth Explorer 9 call from ESA .
Beside the scientific and administrative coordination GFZ was mainly active in GARCA with investigations related to phase based altimetry  and OSSE .
 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.
 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.
 Semmling, M., Leister, V., Saynisch, J., Zus, F., Heise, S., Wickert, J., 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, 2016.