OSCAR: validation of 2D total surface current vector fields during the SEASTARex airborne campaign in Iroise Sea, May 2022.

OSCAR (Ocean Surface Current Airborne Radar) is a new airborne instrument which provides unique 2D synoptic views of ocean and atmosphere dynamics (currents, waves, winds) below km-scale. OSCAR is a Ku-band (13.5 GHz) SAR system with Doppler and scatterometry capabilities in three azimuth look directions. The OSCAR instrument features an along-track interferometric (ATI) baseline in two lines-of-sight squinted 45° fore and aft from the broadside direction. The fore and aft antenna pairs provide interferometric Doppler measurements in two views angularly separated by 90 degrees. This ensures two orthogonal measurements of the ocean surface motion velocity that enable the retrieval of the total ocean surface current vector. In addition, backscatter measurements from the broadside antenna in the zero-Doppler direction serve to retrieve wind direction and wind speed, which are critical to correctly measure total ocean surface currents.

In each line-of-sight, the ocean surface motion sensed by the microwave radar (after correcting for navigation and geometry) has two constituents: the total ocean surface current – consisting of all currents contributing to actual horizontal transport of water – and a measurement bias associated with the Doppler signature of the surface scatterers responsible for the backscatter, a term known as Doppler wave bias or Wind-wave induced Artefact Surface Velocity — WASV (Martin et al., 2016). The WASV is caused by the phase velocity of the surface scatterers responsible for the microwave backscatter (e.g. Bragg waves) and the effect of the orbital motion of longer ocean waves. The magnitude of the WASV can reach 0.5-1 m/s and is, at first order, a function of the wind direction. A number of geophysical model functions (GMFs) have been published in recent years to correct this effect.

In May 2022, OSCAR was flown during the SEASTARex campaign over Iroise Sea (French Brittany). The campaign consisted of three flights on three different days, including acquisitions over a well-instrumented site with ground truth measurements of total ocean surface current fields from a WERA HF radar, supported by data from an X-band marine radar, stereo-video and a down-looking ADCP. Here, we present the first results of the validation of the OSCAR retrieved current fields against data from independent ground truth sensors and models.