Radar Tool
PRF & ambiguity explorer
How often should a radar transmit? That single choice — the pulse repetition frequency — sets how far the radar can see without confusion, which target speeds it goes blind to, and how close a target can get before it disappears. This tool puts numbers on all of it.
The trade-off, in plain language
After each pulse, the radar listens. It must assume any echo it hears belongs to the most recent pulse. So if an echo from a very distant target arrives after the next pulse has already gone out, the radar mistakes it for a close echo of the new pulse — a range ambiguity, or "second-time-around" echo. The listening time between pulses sets a ceiling:
Pulse repetition also creates blind speeds. The radar measures target motion by comparing the echo's phase from pulse to pulse. If a target moves exactly half a wavelength (or any whole multiple of it) between pulses, its echo phase looks identical each time — to the radar, it appears stationary and gets filtered out along with the ground clutter:
And the pulse's own width matters too. While transmitting, the receiver is switched off (the pulse would destroy it), so anything closer than Rmin = c·τ/2 is invisible. The same c·τ/2 sets the range resolution — two targets closer together than this merge into one blip.
High PRF gives clean speed measurement but short unambiguous range; low PRF the opposite. There's no free lunch — modern radars dodge the problem by switching between several PRFs and cross-checking, but each individual PRF still obeys everything on this page.
Inputs
Two transmit pulses (cyan) with the listening window between them. Drag the slider to move a target: its echo (amber) slides through the window. Push it beyond the unambiguous range and the echo lands after the second pulse — the radar would wrongly read it as a short-range echo.
Blind speeds at this PRF and frequency
| n | m/s | km/h | knots |
|---|
Real systems complicate this picture with pulse compression (long coded pulses that behave like short ones), staggered PRFs and multiple PRF sets — but they all start from the timing shown above.