Radar level instruments measure the distance from the transmitter/sensor (located at some high point)to the surface of a process material located further below in much the same way as ultrasonic level sensor, by measuring the time-of -flight of a traveling wave and then determine the level of the process material. They are regarded as continuous level measurement devices because they continue to measure level even as the level of the liquid in the vessel changes.

The fundamental difference between a radar level instrument and an ultrasonic level instruments is the type of wave used. Radar level instruments use radio waves instead of sound waves used in ultrasonic instruments. Radio waves are electromagnetic in nature (comprised of alternating electric and magnetic fields), with very high frequency in the microwave frequency range - GHz.

There are two basic types of level radar instruments: guided-wave radar and non-contact wave radar. Guided-wave radar instruments use wave guide "probes" to guide the radio waves into the process liquid while non-contact radar instruments send radio waves out through open space to reflect off process material. Note that guide-wave radar instruments are used in applications where the dielectric of the process liquid is quiet low. All radar level instruments use an antenna to broadcast or send radio signals to the process liquid whose level is to be determined.