"A rose by any other name would smell as sweet"
I remember the Lockheed-Martin Hercules being fitted with such named "Radar Altimeters" (although they had a longer usable range than that used on typical airliners*).
Radio altimeters operate on frequencies around 4.3GHz. C-band radar is 3.7 to 4GHz. X-band radar is 9~15.5GHz. They are all in the Super High Frequency Band (3Ghz to 30GHz)
As Gary Scott says, the highest land in the "cone" is displayed.
(EDIT) *Lesser known fact is that the Aux Fuel Tank Transfer system logic on the 747-400ER is tied into the Radio Altimeter system (well outside the normal range as seen by pilots on their displays).