Poseidon-4 is the primary payload of the Copernicus Sentinel-6 mission. This nadir-pointing dual-frequency synthetic aperture radar (SAR) altimeter is designed to provide high accuracy and high precision altimetry measurements, including Sea Surface Height (SSH) from the radar range measurement, Significant Wave Height (Hs), and wind speed derived from normalized radar cross section.

Over the open ocean, the European Space Agency's (ESA) CryoSat mission and the Copernicus Sentinel-3 mission have demonstrated that SAR mode altimetry offers a significant improvement (a reduction of the random noise by a factor of ~3 compared to the LRM mode) to both range noise and σ0 measurements.