Using Sentinel-3 SAR altimeter data for detection of coastal currents along the Northwest Atlantic shelf

Hui Feng (University of New Hampshire, United States)


Doug Vandemark (University of New Hampshire, United States of America ); Alejandro Egido (NOAA – Laboratory for Satellite Altimetry, United States of America ); John Wilkin (Rutgers University, United States of America )

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Coastal Altimetry

Presentation type: Type Forum

Delay Doppler (DD) or so-called Synthetic Aperture Radar (SAR) altimetry is a relatively new technique aiming to coherently receive the responses from a surface target during its entire illumination time, containing phase information between subsequent echoes. DD/SAR altimetry thus provides the potential for much finer along-track resolution. It is expected to be highly valuable to coastal ocean applications. By now, both CryoSat-2 and Sentinel-3 satellite altimeters have been operated in SAR Mode and in Low Resolution Mode (LRM), equivalent to conventional altimetry. For the upcoming Sentinel-6, full-time high resolution SAR and LRM mode measurements will be available. New processing approaches have been proposed to analyze SAR mode waveforms (i.e. re-track processing), including Un-Focused SAR (UFSAR) and Fully-Focused SAR (FFSAR) processing. FFSAR processing utilizes both the returned power and phase for optimally focusing on the complex returned echoes along the ground track, attempting to reach maximum DD theoretical limits.
The present study documents the potential for SAR-mode measurements to better resolve near-shore and short-scale currents on the Northwest Atlantic (NWA) shelf, a connected system that includes the Nova Scotian Shelf, marginal sea Gulf of Maine, and Mid-Atlantic Bight. Along this extended coastline there are several persistent and narrow coastally-trapped currents with cross shore scales less than 10-30 km that conventional altimetry has failed to adequately capture.
Our overall goal is to improve regional monitoring and prediction by using most up-to-date altimeter SSH data. Specific objectives are to evaluate Sentinel 3A PLRM, UFSAR, FFSAR data to quantify: 1) data recovery rates near to the coast, 2) noise reduction in sea surface height (SSH) measurements across the shelf and (3) SSH-derived geostrophic velocity estimations at scales of 10-40 km.
This study demonstrates the advantages of SAR mode altimetry in this region: 1) SSHA noise in SAR is significantly reduced, compared to in PLRM, 2) SSHA noise reduction in FFSAR is noticeable, compared to UFSAR, particularly close to the coastline with little degradation (inside of 5 km), 3) FFSAR SAR derived geostrophic current estimates are less biased and exhibit the lowest noise, quite equivalent to UFSAR performance in the bulk sense, and 4) fine-scale (~20 km) analysis across nearshore currents and small-scale gyres reveals SAR data provide sharper and more realistic altimetry based circulation observations in this coast/shelf region.
In the next year, we intend to extend this ongoing assessment to the HR SAR data from the early phase upcoming Sentinel-6 along with Jason-3 LRM data as part of S6 cal/val work in this region.
Hui Feng
University of New Hampshire
United States