SEASTAR consists of a single instrument on a single satellite flying in sun-synchronous Low-Earth Orbit. SEASTAR is the first single-platform SAR system dedicated to squinted along-track ocean interferometry from space. The payload features two squinted SAR beams pointing ±45° in azimuth fore and aft of the satellite plus a standard SAR beam broadside with multi-polarisation capability (VV/VH, HH/HV). This innovative approach makes it possible to retrieve both components of the ocean surface current vector and the wind vector simultaneously in a single pass, with fine resolution and high accuracy. SEASTAR produces a single-sided continuous swath 100-150km wide with a minimum incidence angle of 20 degrees. The inherent spatial resolution of the SAR images is a trade-off between observing swell waves and achieving high accuracy Level 2 products for ocean surface current vectors and wind vectors at 1km resolution over the full swath. Scientific objectives are traced to high accuracy requirements at 1 km resolution for TSCV (0.1 m/s) and OSVW (1 m/s). The instrument operates at Ku-band or Ka-band. The choice of microwave frequency and interferometric baseline must ensure high sensitivity across the wide swath in all sea state conditions, including sheltered coastal regions.