A range detection apparatus comprising a transmitter (4) adapted to transmit a microwave signal and a receiver (4) adapted to receive an echo signal reflected from a target which corresponds to a portion of the transmitted signal; a signal generating means (2) adapted to generate a drive signal to be applied to the transmitter (4) to produce the transmitted signal, the signal generator (2) producing a drive signal which includes a first signal frame (300a) comprising at least two frequencies and a second signal frame (301a) comprising at least two frequencies, the second signal frame (301a) differing from the first (300a), and a processor (11) adapted to process the echo signal together with the transmitted signal so as to determine the distance to the target that produced the echo signal. The apparatus is particularly suited for Frequency Shift Key (FSK) radar in automotive, typically adaptive cruise control functions. It may reduce ambiguities in range measurements. An equivalent method of reducing ambiguity is also disclosed.