GEROS-ISS, a demonstration mission of GNSS remote sensing capabilities to derive geophysical parameters of the earth surfaces: Altimetry performance evaluation

The GNSS rEflectometry, Radio Occultation and Scatterometry onboard International Space Station (GEROS-ISS) is an innovative experiment for climate research, proposed in 2011 within a call of the European Space Agency (ESA) for installation at the ISS. This international proposal was the only one selected for further studies by ESA out of ~25 submitted ones. In this work, the assessment of the instrument performance for the near-nadir altimetry (GNSS-R) mode is assessed, including the effects of multi-path in the ISS structure, the electromagnetic-bias, and the orbital height decay. In the absence of ionospheric scintillations, the altimetry rms error is <; 50 cm for a swath <; ~250 km and for U10 <; 10 m/s. If the transmitted power is 3 dB higher (likely to happen at beginning of life of the spacecraft), mission requirements (rms error is <; 50 cm) are met for all ISS heights and U10 up to 15 m/s. However, around 1.5 GHz, the ionosphere can induce significant fading, from 2 to > 20 dB at equatorial regions, mainly after sunset, which will seriously degrade the altimetry and the scatterometry performances of the instrument.