From the pulse out to the blip.

The basic idea

Shout, then listen for the echo.

The radar transmits a short, powerful burst of radio energy — a pulse — then goes quiet and listens. If the pulse hits something, a tiny fraction reflects back. Radio waves travel about 300 metres every microsecond, and the pulse travels out and back, so every microsecond of echo delay means roughly 150 metres of range. Everything else in radar exists to answer three questions about that echo: how far (timing), which direction (where the antenna was pointing), and how fast (the Doppler shift in the echo's frequency).

The surprise is how faint the echo is. The pulse spreads as it travels, and the reflection spreads again on the way back — so received power falls with the fourth power of range. Double the distance and the echo returns sixteen times weaker. That's why transmitters are measured in kilowatts while receivers strain for millionths of a millionth of a watt, and it's what the range equation is for.