SAR altimetry over ocean has attracted a lot of attention in the past years and huge progress has been made in a short timespan. CryoSat-2 was the first satellite to provide SAR, also known as delay/Doppler, altimetry data over the ocean, which permitted to demonstrate the significant benefits of SAR mode for ocean altimetry compared to conventional low resolution mode (LRM). Now Sentinel-3 will continue to provide SAR measurements over ocean continuous and globally.
The differences between the SAR mode and LRM in terms of shape of the waveforms and SNR can be appreciated in the figure below where two different echoes have been modelled.
fig 1. Delay/Doppler and conventional echoes for the same altimetric parameters (Pu = 1, τ = 31 gates, SWH = 2 m; credit: Abderrahim Halimi, ENSEEIHT)
fig 2. SAR waveforms modelled using SAMOSA3 for waves of 3 (red curve) and 10 meters (blue curve), credits NOC
Moreover, new retrackers are needed to process SAR waveforms.
There are different types of retrackers and different groups of people working on them:
- Numerical SAR waveform models, Phalippou & Enjolras, 2007, Phalippou & Demeestere, 2011 from TAS and F.Boy et al., 2012 from CNES.
- Semi-Analytical SAR waveform models, Wingham et al, 2004, Giles et al and Halimi et al., 2012 and different evolutions of the SAMOSA retracker from the ESA-funded project: Martin‐Puig et al., 2008, Gommenginger et al., 2012 and Ray et al., 2013.
- Fully-Analytical Physically-Based SAR waveform models
The convergence of results from different groups indicates a high level of confidence in the retrieval of geophysical information from SAR altimetry over ocean. Improvements offered by the SAR mode with respect to the conventional Low Resolution mode are the following:
- Improved precision in range (and derived parameters: SSH, SLA, etc.) with respect to LRM data. In terms of 1Hz noise, an improvement from 1.57 cm to 1.22 cm.
- Improved precision in SWH with respect to LRM data. In terms of 1Hz noise, an improvement from 11.09 cm to ~ 8.5 cm.
- Improved along track resolution, shown in SLA and SWH spectra, so that scales of less than 100km can be resolved.
- Improved SNR
- SAR products are consistent with LRM products or have known biases that can be corrected for.