The Jason-Class Altimeter, also known as the Poseidon-3C altimeter, is a Ku and C band nadir altimeter similar to that of the Jason series. It complements the Ka-band Radar Interferometer (KaRIn) with nadir measurements of ocean surface topography. As one of seven instruments aboard the Surface Water and Ocean Topography (SWOT) platform, the Jason-class altimeter measures altimeter range, radar backscatter (sigma0), significant wave height, and range delay due to the ionosphere electron content in the nadir gap between the left and right KaRIn swaths. By providing a nadir altimeter dataset, it calibrates and complements the wide-swath measurements from the KaRIn instrument.