If something is coming towards you, the waves it reflects are squeezed together slightly: the frequency goes up. If it is moving away, the frequency goes down. That small shift gives the speed towards or away from the radar.
In practiceA pulse-Doppler radar — such as the one in an F-16 or an AWACS aircraft — measures the radial velocity of every target this way. That lets it filter out echoes from the terrain, which moves differently from an aircraft, and still see a low-flying aircraft over land (Pulse-Doppler radar and Moving Target Indicator). The same technique is in a police speed gun.
You know the Doppler effect from an ambulance: the pitch is higher as it approaches and lower as it drives away. The same happens with radio waves. When a radar signal reflects off a moving target, the frequency of the echo shifts:
- Target approaching the radar → echo has a higher frequency (positive Doppler shift).
- Target moving away from the radar → echo has a lower frequency (negative Doppler shift).
- Target moving tangentially (across the line of sight) → no Doppler shift.
An X-band radar (10 GHz, λ = 3 cm) sees a target coming straight at the radar at 300 m/s. The Doppler shift is fd = (2 × 300) / 0.03 = 20,000 Hz = 20 kHz. On a carrier of 10,000,000,000 Hz that is a minuscule shift — yet coherent radars can measure it.
The Doppler effect gives radar two enormous capabilities:
- Direct velocity measurement of any target, without having to track it over time.
- Separation of moving targets from stationary clutter: a mountain has zero Doppler shift, an aircraft does not. This is the foundation of MTI and pulse-Doppler radar (Pulse-Doppler radar and Moving Target Indicator) — and therefore of almost every modern military radar.
Doppler measures only the radial velocity component. A target flying exactly tangentially has zero Doppler shift and can disappear into the radar's clutter filter ("notching").