On the Performance of Cryosat-2 Sar Mode over Water Surfaces

This paper presents analyses of L1B waveforms in SAR mode and Low-Rate Mode (LRM) over water surfaces from numerical simulations from the CryoSat Mission Performance Simulator (CRYMPS) and from CryoSat-2 satellite measurements. This work was performed as part of the ESA project "Development of SAR Altimetry Studies and Applications over Ocean, Coastal zones and Inland waters (SAMOSA)". The project aims to determine the performance of SAR altimeters over the ocean and the coastal zone, compared to those of conventional pulse-limited altimeters. The performance for range and wave height retrieval in SAR mode is assessed with the help of the SAMOSA1 theoretical waveform retracker developed in the SAMOSA contract to retrack peaky SAR mode waveforms (Gommenginger et al., 2010). The SAMOSA1 SAR ocean retracker has been delivered to the Sentinel-3 team to form the basis of the operational SAR ocean retracker for the Sentinel3 Surface Topography Mission. In this paper, we present an analysis of range retrieval accuracy as a function of significant wave height from CRYMPS L1B LRM and SAR waveforms, and for pseudo-LRM waveforms obtained from the SAR mode data. First results of the SAMOSA1 SAR ocean retracker applied to CryoSat-2 waveforms over ocean are presented. 1. MOTIVATION & CONTEXT The CryoSat-2 SIRAL altimeter operates in three modes: a) Low-Rate Mode (LRM) over the open ocean, similar to conventional pulse-limited altimetry; b) SAR Interferometric mode (SARIN) over continental ice; and c) SAR mode (SAR) over sea ice. The CryoSat-2 instrument is also switched in SAR mode over some ocean test sites as a precursor for the SRAL altimeter on the GMES Sentinel-3 Surface Topography Mission (STM), which will also feature SAR capability. At present, it is expected that S3-STM will operate in SAR mode in ocean regions of high mesoscale variability and in the coastal zone. The work presented in this paper was performed in the context of the ESA project SAMOSA for the "Development of SAR Altimetry Studies and Applications over Ocean, Coastal zones and Inland waters". Key aims of the project included a) to develop a theoretical retracker for SAR altimeter ocean waveforms; b) to compare range retrieval accuracy in LRM and SAR mode as a function of significant wave height (SWH); c) to investigate how to reduce SAR mode data to pseudo-LRM to establish their statistical equivalence with LRM. Here, we first focus on a) and b), before presenting first results of retracking CryoSat2 L1B SAR waveforms over the ocean. 2. THE SAMOSA1 SARALT OCEAN RETRACKER SAR Altimeter waveforms are much peakier than conventional Brown-type waveforms and therefore need a different approach to extract geophysical information. Within SAMOSA, a new theoretical model was developed (Martin-Puig et al., 2009) to fit SAR altimeter waveforms over water surfaces. The model gives analytical and numerical solutions to compute the Delay-Doppler Map for a single SAR burst. The model depends on the following geophysical parameters: range, significant wave height, along-track mispointing and Sigma0. Fig. 1 shows the typical peaky shape of SAR waveforms at zero Doppler in single-look DDM, and the broadening of the echo with increasing SWH. Figure 1. SAMOSA1 modelled SAR Altimeter waveforms for different values of significant wave height. The waveforms correspond to zero Doppler frequency in the single-look delay-Doppler maps. Successive bursts are subsequently subjected to Doppler-beam selection to build the Stack, which is incoherently integrated to obtain multi-looked waveforms comparable to the SAR L1B products. The SAMOSA1 SARAlt ocean retracker has now been documented in the form of a Detailed Processing Model (Gommenginger et al., 2010) and delivered to the Sentinel-3 team responsible for the implementation of the operational processor for S3-STM. 3. SIMULATED SAR AND LRM WAVEFORMS FROM CRYMPS OVER WATER The CryoSat Mission Performance Simulator (CRYMPS) was designed by UCL/MSSL to assess the performance of CryoSat over ice. It provides simulated products in LRM, SAR and SARIN mode for specified surfaces. Within SAMOSA, CRYMPS L1B LRM and SAR products were obtained for ocean surfaces with a number of scenarios with variable significant wave height. Explicit 3D ocean surface Digital Elevation Maps were generated for input into CRYMPS, featuring realistic ocean surface waves generated by inverting wave spectra based on the Elfouhaily et al. (1997) theoretical model for given SWH values. Figure 2. (a) CRYMPS LRM waveform over ocean and fitted Brown model; (b) CRYMPS SAR waveform over ocean and fitted SAMOSA1 SAR model. Significant wave height is around 3 meters in both cases. Fig. 2 shows two examples of CRYMPS waveforms in LRM (top) and SAR (bottom) mode extracted from one of the ocean scenarios. The waveforms were fitted respectively with the NOC implementation of the Brown ocean model and the SAMOSA1 SARAlt ocean retracker. Based on the Full-Bit-Rate (FBR) products, the SAR mode data were sampled and incoherently averaged to generate pseudo-LRM waveforms, according to an averaging strategy developed by Starlab (Martin-Puig et al., 2011). The pseudo-LRM waveforms are Brown-like and were also retracked with the NOC Brown retracker. 4. RANGE RETRIEVAL ACCURACY AS A FUNCTION OF SWH CRYMPS L1B LRM and SAR data were obtained for three ocean scenarios, which spanned values of SWH from 0.1 m to 5 m. The top row in Fig. 3 shows the profiles of significant wave height from the DEM of the three ocean scenarios. The subsequent rows in Fig. 3 show the retrieved SWH obtained by retracking the CRYMPS LRM, pseudo-LRM and SAR waveforms with the relevant retrackers. The colours identify those segments of the scenarios that correspond to a fixed value of SWH. A similar figure for the retracked range (not shown) provides the data used to assess the range retrieval accuracy as a function of significant wave height shown in Fig. 4. Figure 3. (top) CRYMPS DEM SWH and retracked SWH for (middle-top to bottom) LRM, pseudo-LRM and SAR mode, for three CRYMPS scenarios over ocean. Colours indicate data segments corresponding to 2 seconds of data at a fixed value of SWH. Fig. 4 shows the retrieval accuracy at 1Hz of the relative range (position of the leading edge in the window) and a)