A radar water level sensor measures water level without touching the water by timing microwave pulses sent down to the surface and reflected back.
A radar water level sensor measures the distance from a fixed point down to a water surface by sending microwave signals and timing their echo. Water level is found by subtracting that distance from the known height of the sensor. Most units used in hydrology are pulse radar or frequency modulated continuous wave (FMCW) radar. A pulse radar times the round trip, and distance is d = c·t/2, where c is the speed of light and t the travel time. An FMCW radar sends a sweeping frequency and works out distance from the frequency difference between sent and returned signals. Unlike ultrasonic sensors, radar is barely affected by air temperature, wind or humidity, and no part of it sits in the water. Radar sensors are mounted on bridges, gantries or brackets over rivers, canals, reservoirs, tanks and wet wells. They are used at river gauging stations, in flood warning, for reservoir levels and to watch sewer overflows. With a rating curve, the level gives discharge. Some units also measure surface velocity with Doppler radar. Placement matters. The beam spreads in a cone, so it needs a clear view of the water with no pier, bank or debris in its footprint, and the sensor must sit above the highest expected flood. The sensor is set to a survey datum so readings match the staff gauge. Foam, floating debris, ice and very rough water can scatter the echo. Bridges vibrate, and a mounting that moves adds noise. Check readings against the staff gauge on each site visit. For example, a sensor fixed 8.00 m above gauge datum that reports a distance of 5.35 m gives a stage of 2.65 m. Strengths are no contact with the water, low upkeep, and no fouling or silt problems. Limits are the need for a stable structure above the water, a clear beam path and a short blind zone close to the sensor.