Radar level transmitter use microwaves to measure the level of fluid in a process tank. It beams microwaves downwards towards bottom of the tank. When microwaves hit the surface of the liquid, some of them get reflected back to the receiver built in the transmitter module. Time taken to travel to the liquid surface and back is measured by an electronic timing circuit.
Amount of microwaves reflected back depends on the die-electric constant of the fluid. If dielectric constant is low, less microwaves are reflected and measuring liquid level can be difficult. Water has high dielectric constant of 80, hence reflects microwaves excellently.
Horn type radar level instrument transmits microwaves through the air/vapors inside the tank. These microwaves can get reflected through the vessel internals, deposits on vessel internal parts etc. and complex fuzzy logic needs to be implemented to correctly measure the level.
Solution to above problem is Guided Wave Radar (GWR) transmitters. They incorporate a rigid or flexible cable antenna system to guide the microwaves down to the vessel. False echos from vessel walls and other parts are avoided. They are 20 times more efficient than through air radar systems due to focussed energy transmission, and can be used to measure level of liquids having dielectric constants down to 1.4 or lower.
Speed of radar waves are unaffected by vapor space gas composition, temperature or pressure. So they need no recalibration for vacuum or foams.
Check out these youtube videos to better understand it.