Comparing the cygnss simulator forward scattering model with TDS-1 and cygnss on-orbit DDMS

The CYGNSS (Cyclone GNSS) Mission is a constellation of 8 micro-satellites successfully launched in December 2016. Each satellite carries a GNSS-R (Global Navigation Satellite System Reflectometry) receiver which forms a Level-1 delay-Doppler map (DDM) measurement of reflected GPS L1 C/A-coded signals off the surface of the Earth. A very similar instrument is also operating onboard the TechDemoSat-1 (TDS-1) satellite. Prior to launch, the CYGNSS End-to-End Simulator (E2ES) was developed in order to simulate CYGNSS measurements and develop Level-2 ocean surface mean-square-slope (MSS) and wind speed retrieval algorithms. As part of initial validation process, this paper will present a detailed comparison between the downlinked Level-1 DDM data (from both TDS-1 and CYGNSS) and those predicted by high-fidelity CYGNSS forward model. This comparison includes a statistical analysis of DDM data pixels and their residuals with respect to simulated waveforms as a means of analyzing the on-orbit properties of the instrument, identifying outliers, as well as validating predicted scattering models, particularly the model's predicted dependence on incidence angle and wind speed.