Radar methods of level measurement are sometimes referred to as microwave types.
Both use electromagnetic waves, typically in the microwave X-band (10 GHz) range.
Basically, all types operate on the principle of beaming microwaves downward from a sensor located on top of the vessel. The sensor receives back a portion of the energy that is reflected off the surface of the measured medium. Travel time for the signal (called the time of flight) is used to determine level.