Radar altimeters

"It is commonly understood that an altimeter is a down-looking radar device designed to measure relative altitude, thus to establish the height of the radar above the surface below. Although similar in form and function, the purpose of a remote-sensing altimeter is different, since usually it is mounted on a spacecraft whose orbital height is known. Remote-sensing altimeters are designed to retrieve certain geophysical parameters about the surface from the timing and shape of the radar reflections (waveforms)." (Raney, Keith. 2014)

Sources

Raney, Keith. 2014. “Radar, Altimeters.” In: Njoku E.G. (eds) Encyclopedia of Remote Sensing. Encyclopedia of Earth Sciences Series, 525–32. https://doi.org/10.1007/978-0-387-36699-9_134.