A French radar altimeter on the Indo-French SARAL satellite. It was the first in space to work in the Ka band, and measures the height of the sea surface.
An altimeter fires radar pulses straight down and times the echo. Knowing the satellite's exact orbit, scientists turn that time into the height of the sea surface. Small bumps and dips in sea height reveal ocean currents and eddies.
AltiKa works at 35.75 GHz in the Ka band, a higher frequency than earlier altimeters. That gives it a smaller footprint on the sea. It also carries a two-frequency radiometer to correct for water vapour in the air. France's space agency CNES built it.
PSLV-C20 launched SARAL in February 2013. Reaction wheel problems in 2015 meant the satellite moved into a drifting orbit from July 2016, but AltiKa kept working. Beyond the open ocean, its data have been used for coastal seas, river and lake levels, ice sheets and wave forecasting.