GEROS-ISS: Innovative GNSS reflectometry/occultation payload onboard the International Space Station for the Global Geodetic Observing System

In response to a European Space Agency (ESA) announcement of opportunity, the GEROS-ISS proposal was submitted and accepted by ESA to proceed to Phase A. GEROS-ISS is an innovative ISS experiment primarily focused on exploiting reflected signals of opportunity from Global Navigation Satellite Systems (GNSS) at L-band to measure key parameters of ocean and land/ice surfaces. Secondary mission goals are global atmosphere and ionosphere observations using the GNSS radio occultation technique complementing other current satellite missions. GEROS-ISS will pioneer the exploitation of signals from Galileo and possibly other GNSS systems, for reflectometry and occultation, thereby improving the accuracy as well as the spatio-temporal resolution of the derived geophysical properties. GEROS-ISS will contribute to the long-term S.I. traceable observation of the variations of major climate components of the Earth System: Oceans/Hydrosphere, Cryosphere/Snow, Atmosphere/Ionosphere and solid Earth/landcover changes with innovative and complementary aspects compared to current Earth Observation satellite missions. Therefore, the data from GEROS-ISS allow for climate change related scientific studies addressing the challenges of ESA's Earth Observation strategy (SP 1304 The Changing Earth: New scientific challenges for ESA's living planet). GEROS-ISS will mainly provide mid- and low-latitude observations on submesoscale or longer oceanic variability with a focus on the coastal region, surface ocean currents, surface winds, wave heights and the vertical atmospheric temperature, water vapour and electron density structure for a period of at least ten years. These observations will lead to a better understanding of the climate system, including Rossby wave large-scale structures, ocean roughness and wind, eddy-current systems, fronts and coastal upwelling. GEROS-ISS takes advantage of the capacious infrastructure onboard the ISS, to potentially derive additional Earth's surface parameters critical to understand anthropogenic climate change and, e.g., snow, soil moisture and mountain glacier elevation change. GEROS-ISS also provides a sensor calibration/validation option for other upcoming satellite missions including ISS-RapidScatt, CYGNSS and ROCSat/COSMIC-II. The GEROS-ISS mission requirements prepared by the Scientific Advisory Group are introduced and an overview of the current status of development and research is provided with focus on the science mission.