RADAR At Any Frequency
Although most practical radars operate in the higher frequencies, there are radars at many areas in the electromagnetic spectrum ranging from the HF through light (lasers). Engineers use laser range finding to accurately measure distances allowing them to start a bridge on both sides of a river and have them meet squarely in the middle. Navy ships use frequencies in the VHF part of the spectrum for Air Search. Short range fire control systems on ships and aircraft are in the gigahertz range.
Back in the 1950s, amateur radio operators operating in the HF bands detected echos of their own signals. It turned out that they were hearing their own transmissions bouncing off the moon. As I remember, the time delay was around a half of a second, something easily detected by the human ear/brain system.