The 2022 SEASTARex campaign over the Iroise Sea (offshore Brittany in France), is the first scientific demonstration and validation campaign for the SEASTAR satellite mission. SEASTAR was one of the candidates for ESA's 11th earth explorer, which was intended to address the scientific need to advance our understanding of fast-evolving small-scale ocean surface dynamics in coastal seas, shelf seas, and marginal ice zones. SEASTAR was not selected to proceed after assessment.

The SEASTAR mission concept is developed to observe these dynamics with daily revisits and 1 km resolution, focusing on exchanges of heat, carbon, and nutrients at boundaries between land, the cryosphere, the atmosphere, and the deep ocean. To achieve this, SEASTAR proposes to measure 2D Total Surface Current Vectors (TSCV) and Ocean Surface Vectors Wind (OSVW) fields, and Directional Ocean Wave Spectra (DOWS) using a highly innovative technology on a single independent satellite. The satellite consists of two orthogonal squinted Along-Track Interferometry (ATI) Synthetic Aperture Radar (SAR) beams plus a third beam for scatterometry a la ASCAT.

The Ku-band airborne demonstrator OSCAR (Ocean Surface Current Airborne Radar) reproduce at best the acquisition geometry of the satellite. The objectives of the SEASTARex Iroise Sea campaign were:

• Simultaneous acquisitions of OSCAR airborne data and ground truth data during a dedicated campaign over an instrumented and well-characterised ocean site;
• Data processing, calibration, analysis and dissemination of all airborne and ground truth data acquired during the campaign;
• Development of Level-2 processing chain to retrieve TSCV and OSWV from OSCAR L1 Doppler (Radial Surface Velocity) and NRCS data;
• Evaluation of OSCAR retrieved TSCV against ground truth measurements;
• Assessment of OSCAR retrieved OSVW against spaceborne scatterometers.

What was the outcome of SEASTARex?

For the SEASTARex Iroise Sea campaign, OSCAR data were acquired over four sites:

1. The calibration site along the airport base in Morlaix where corner reflectors were deployed;
2. The Ouessant island site characterised by strong gradients of macrotidal currents. In this area, ground truth surface currents were monitored with an X-band marine radar and a stereo-video system deployed on La Jument lighthouse, and an HF radar sited at two land stations on the continent;
3. The open-ocean site over the Trefle mooring characterised by a homogeneous area with uniform bathymetry and environmental conditions. The mooring was deployed during SEASTARex to measure in situ wind, waves and current in addition to the HF radar monitoring the whole area. Over this area, the aircraft acquired data using a star pattern flight and a circular flight to sample the environmental conditions in different azimuth directions relative to OSCAR beams;
4. The offshore site, in the Bay of Biscay, to fly OSCAR simultaneously with overpasses of the ASCAT space-borne scatterometer, aiming to calibrate OSCAR NRCS and validate OSVW retrievals. OSCAR flight tracks over the four sites can be seen on the Figure.

Over the Ouessant island site, OSCAR TSCV showed excellent agreement with X-band marine radar TSCV data and the position of NRCS gradients in satellite NOVASAR-1 data (see figure enclosed). Overall comparisons against HF radar and ADCP measurements over the open-ocean site over the Trefle mooring show performances better than 10cm/s. Performance for OSVW are better than 1 m/s.

The successful outcomes of this first campaign underscore the unparalleled potential of a SEASTAR-like instrument to collect critical data on ocean currents and winds in the challenging coastal regions where alternative measurement methods are both expensive and difficult to implement. The SEASTARex project paves the way for future advancements in the field of Doppler Oceanography. The insights gained from the SEASTARex campaigns will undoubtedly prove invaluable for future satellite missions aiming to measure ocean surface currents, winds, and waves, ultimately leading to improved monitoring and management of our oceans and coastal environments.

Download the 2022 SEASTARex Final Report