Frequency Modulated Continuous Wave radar
An FMCW radar transmits continuously, with a tone that keeps rising. By the time the echo comes back, the radar is already transmitting a higher tone; the difference between the two reveals how long the echo was under way, and therefore the range.
In practiceThe radar altimeter in helicopters and aircraft measures height above the ground this way, even when flying low in the dark. Small FMCW radars are also used to detect drones around a camp: they are small, economical and, because of their low power, hard to intercept.
The FMCW radar (Frequency Modulated Continuous Wave) solves the CW radar's big shortcoming: range measurement. The trick: the transmit frequency is varied continuously, usually as a triangular or sawtooth sweep. The frequency itself thus becomes a "clock".
The principle: by the time an echo returns, the transmitter has already swept on to a different frequency. The difference between the instantaneous transmit frequency and the received echo frequency — the beat frequency (fb) — is directly proportional to the travel time, and therefore to the range:
Properties of FMCW:
- Measures range and velocity with low, continuous power.
- Excellent resolution at short range; no minimum range worth mentioning.
- Small, light and cheap to build (a single chip) — which is why it is the radar of radio altimeters, automotive sensors, level gauges and short-range surveillance radars.
- The low, spread-out power makes FMCW radars relatively hard to detect for classic ESM receivers: an LPI radar (Low Probability of Intercept). Modern navigation and surveillance radars (and, for example, missile altimeters) use FMCW partly for that reason.