How the continuous wave radar works
A Continuous Wave (CW) radar does not transmit in pulses but transmits a single constant frequency without interruption, and receives at the same time. Because transmitting and receiving happen simultaneously, separate transmit and receive antennas (or very good isolation) are required — otherwise the receiver would be drowned out by its own transmitter.
What can a CW radar measure?
- Velocity: excellently. Every echo from a moving target returns with a Doppler shift fd = 2vr/λ. Because the transmitter works continuously and cleanly on one frequency, that shift can be measured very precisely and without ambiguity.
- Distance: not at all. There are no pulses, so there is no travel time to measure. A pure CW radar is completely blind to range.
Strengths: technically simple, no minimum range, no ambiguities, excellent velocity measurement from very close in to far out, and pulse characteristics that are hard to detect (there are no pulses). Applications:
- Speed meters (police "radar guns", artillery muzzle velocity radars, sports radars).
- CW illuminators for semi-active radar-guided missiles (Semi-Active Radar Homing): the ship or launcher continuously "illuminates" the target and the missile homes on the reflected CW energy.
- Proximity fuzes and Doppler motion sensors.
CW = continuous transmission = perfect velocity measurement, no range measurement. If you still want to measure range with CW, you have to vary something in the signal — which is exactly what FMCW does (Frequency Modulated Continuous Wave radar).
Peak power and average power of the CW radar
In a pulse radar, peak and average power differ enormously (Peak power and average power of the pulse radar). In a CW radar that distinction does not exist: the transmitter is always on, so:
Consequences of that 100% duty cycle:
- With a modest transmit power a CW radar still puts a lot of energy on the target — energy = power × time, and it has unlimited time. For target illumination that is ideal.
- The transmitter does not have to deliver extreme peaks: simpler, lighter and cheaper than a pulse transmitter with comparable detection performance.
- The downside for its own signature: a CW radar is continuously audible to ESM receivers — there is always a signal to take a bearing on. (The absence of a pulse rhythm does make classification harder.)
Pulse radar: Pp = 500 kW, duty cycle 0.1% → Pavg = 500 W. CW radar: P = 500 W continuously → also Pavg = 500 W, but without ever having to generate 500 kW. Both put the same average energy into the air.