Radar altimetry is an essential component for observing the complex ocean circulation. 95% of the ocean socioeconomic activity is developed in coastal areas. For this reason (and many others), understanding the physical processes driving the coastal ocean dynamics should be a priority (Vignudelli et al., 2011). The coastal altimetry community has the responsibility of providing the most accurate product to study the coastal processes.
One of the most novel concepts in coastal altimetry is the Fully Focused SAR (FF SAR) processing. FF SAR introduces improvements in terms of along-track resolution in high Pulse Repetition Frequency (PRF) radar altimeters. The processing is similar to the SAR altimetry, but with an unprecedented high along-track resolution up to the theoretical limit equal to half the antenna length (~0.5 m). This is in contrast to the ~300 m unfocused SAR along-track resolution given in coastal altimeter standard products (Egido and Smith 2017). The FF SAR footprint is SAR focused along-track and pulse-limited across-track. The main drawback of FF SAR is its high computational effort, but some works have been done to reduce the computational time without losing accuracy (Guccione et al., 2018).
In this work, the performance of FF SAR products in the Gulf of Cadiz (Spain) was analysed in terms of accuracy and precision. The analysis was done in the first 5 km from the coast. Two FF SAR algorithms still in development were used: (i) FF SAR Back Projection (BP) (S3 prototype version of Kleinherenbrink et al., 2020); and (ii) FF SAR Omega-Kappa (WK) (Guccione et al., 2018). Four retracking algorithms were used to estimate the retracked range: Threshold retracker (Davis 1993), SAMOSA (Ray et al., 2015), SAMOSA+ (Dinardo et al., 2018) and ALES+ SAR (Passaro et al., 2020). Also, the results were compared to unfocused SAR using data provided by the ESA Grid Processing On Demand (GPOD). Table 1 shows a description of the datasets used in this work.
Four tracks were processed in the study area according to the availability of in-situ measurements (Figure 1): Two tracks from Sentinel-3A and two from Sentinel-3B. The posting rate used was 80 Hz. A total of 45 cycles for S3A tracks and 15 cycles for S3B were analysed in the period 2016-2019. The products accuracy was obtained by comparing time series of Sea Level Anomaly (SLA) with those obtained from a radar tide gauge. The statistics used were the standard deviation of the difference (sdd). Also, an analysis of the Percentage of Cycles for High Correlation (PCHC) was done (Passaro et al., 2015). To evaluate the precision, the SLA differences between consecutive measurements along-track were calculated for each cycle. These differences were considered a good estimation of noise, since SLA is not expected to change significantly in 85 m along track, which is the distance between consecutive measurements at 80 Hz. Subsequently, using the average of these differences, the noise over a single cycle was obtained. Finally, the precision over each track segment was determined by averaging the noise of all the cycles.
Our preliminary results show percentages of PCHC higher in FF SAR than unfocused SAR. In terms of sdd, the result obtained comparing the in-situ data and the different S3 datasets were similar, ranging between 7 and 10 cm. Although it should be noted that lower values of sdd were obtained at 1-2 km of the coast in FF SAR. In addition, preliminary results indicate that the along-track noise is lower in FF SAR than in unfocused SAR. Moreover, some improvements in the results were observed when the ALES+ SAR retracker was applied.
Besides, in order to assess the potential of the different algorithms and retrackers for their use in coastal oceanographic applications, across-track surface current velocities, derived from the different SLA retrievals, will be compared with high-frequency radar (HFR) data.
The innovation of this study lies in the use of new products from FF SAR to improve our knowledge of coastal processes such as the coastal countercurrent observed in the Gulf of Cadiz (its origin is still on debate), taking advantage of the good quality of data closer to the coast (1-2 km) compared to unfocused SAR (up to 3 km) (Mulero-Martinez et al., 2021; Aldarias et al., 2020).
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