The Doppler effect in 30 seconds
When a wave source moves toward you, the waves arrive more often — higher frequency. When it moves away, lower frequency. You hear it with sirens. Radar sees it with reflected pulses. If the transmitter emits at 10.000 GHz and the echo returns at 10.0001 GHz, the target is closing at roughly 3 metres per second.
Why Doppler is hard to fake
Chaff, decoys and most stealth tricks can hide a radar return's size but not its velocity. A moving target produces a Doppler shift; a stationary metal strip does not. This is why pulse-Doppler radar dominates modern fighter intercept — it filters out ground clutter that has zero Doppler shift and shows only the things actually moving.
Weather radar in colour
A NEXRAD weather radar fires 1000+ pulses a second in a slowly rotating beam. Each raindrop reflects a tiny return. The system computes the Doppler shift per range cell and colours the map by it: green for raindrops moving toward the antenna, red for raindrops moving away. The boundary between green and red, with strong shifts on both sides, is the signature of a rotating storm — a mesocyclone that may produce a tornado.
Police radar guns
A handheld police radar is a continuous-wave Doppler device. It fires a microwave beam at your car, listens for the reflection, and measures the frequency shift. At 24 GHz, every 1 m/s of closing speed produces about 160 Hz of shift. A small DSP chip converts the shift directly to km/h.