# Calibrating the SAR SSH of Sentinel-3A and CryoSat-2 over the Corsica Facilities
## Data and Methodology  
### Data  
The data sets used for Sentinel-3 and CryoSat-2 are described below.  
Sentinel-3A:  
* Data sources: S3-PDGS (payload data ground segment) reprocessing 2017, Non-Time Critical (NTC) products: cycle 5-19, processing baseline 2.15 [10]. The processing baseline (PB) 2.15 consists of the following processors: SRAL L1 IPF V6.11, MWR L1 IPF V6.04, SRAL L2 IPF V6.07.
* Sentinel-3A SRAL (Ku/C Radar Altimeter) processing for SAR and Pseudo LRM (PLRM) and applied corrections
* Standard processing: dual-frequency ionosphere measurement and wet troposphere radiometry. In this study, we also compared the dual frequency ionosphere measurements with the global ionosphere maps model (GIM), together with a comparison of the wet troposphere radiometer measurements with the wet troposphere model from European Centre for Medium-Range Weather Forecasts (ECMWF)). The chosen wet correction for the model corresponds to the field that is computed at sea level (MOD_WET_TROPO_COR_ZERO_ALITITUDE)
* Dry troposphere
* Sea state bias (SSB)
* Solid, loading and pole tides
* In situ data
* Ajaccio: Service Hydrographique et Oceanographique de la Marine (SHOM) radar tide gauge data in real time
* Senetosa: pressure tide gauges (data retrieved end of July 2017)  
CryoSat-2:  
* Data sources: from SARvatore service at ESA G-POD ([[https://gprod.eo.esa.int/services/CRYOSAT_SAR/](https://gprod.eo.esa.int/services/CRYOSAT_SAR/)]([https://gprod.eo.esa.int/services/CRYOSAT_SAR/](https://gprod.eo.esa.int/services/CRYOSAT_SAR/))) using baseline B products with two different retrackers. It is important to note that this product does not correspond to the current baseline C products processed by the CryoSat-2 payload data ground segment (PDGS), the main difference being the correction of a known range bias of 673 mm:SAMOSA2 retracker (default): The SAMOSA (SAR Altimetry MOde Studies and Applications) model [11] has been implemented in its first-order formulation (SAMOSA2) in a retracker as in [12]. This retracker uses a look-up table to mitigate the effect of the model's approximation of the squared PTR (point target response) with a Gaussian curve, it implements a stack masking for the Doppler beams padded to zero and has in principle no enhancement in the coastal zone. SAMOSA+ retracker (the SAMOSA2 model tailored for inland water, sea ice and the coastal zone domain): in the case of SAMOSA+, the open ocean SAMOSA [11] model and retracker [13] has been implemented in the retracking scheme with two significant additions [12]: The first one concerns the selection of the first-guess epoch. This is not selected as the position of the waveform peak but as the position of the moving correlation peak in 20 consecutive waveforms (after aligning them for tracker shift). The rationale behind this choice is to attempt to mitigate the typical off-ranging effect in coastal data. The second concerns the treatment of land-contaminated waveforms. In case waveforms are not contaminated by land, the SAMOSA model was used with the mean square slope set to zero, i.e., as in [13], whereas in the case of land contaminated waveforms, we used a dual step retracking: in the first step, the SWH is still estimated as in [13], while in the second step, the SWH was set to zero and the third free parameter in the retracking becomes the mean square slope. The output of this second step is the range and the amplitude Pu (retracked waveform amplitude, see Equation (22) in [12]).
* Cryosat-2 SIRAL (synthetic aperture interferometric radar altimeter) data processing for SAR and applied corrections \(\circlearrowright\) Dry troposphere \(\circlearrowright\) Wet troposphere from ECMWF model \(\circlearrowright\) Ionosphere from GIM \(\circlearrowright\) Sea state bias = 3.5% of SWH \(\circlearrowright\) Solid, loading and pole tides
* In situ data \(\circlearrowright\) Ajaccio: SHOM radar tide gauge data in real time \(\circlearrowright\) Senetosa: pressure tide gauges (data retrieved end of July 2017)