What is 'blind speed' in radar and why does it matter?

• A. The speed of the radar dish.
• B. The speed at which a target disappears from screen due to range.
• C. The speed at which weather returns saturate.
• D. Specific target speeds where Doppler processing cannot detect motion due to PRF limitations.

Answer: (D)

Blind speed happens in pulsed Doppler radar when the target’s true Doppler shift lines up with the sampling rate of the radar. The radar samples echoes every PRF interval, so Doppler information is wrapped into an unambiguous range. If the phase change from one pulse to the next equals an integer multiple of 2π, the motion looks like no motion at all. In other words, the Doppler frequency shift lands on a place that maps back to zero due to the sampling, so the processor can’t detect movement. Those specific velocities depend on wavelength and PRF and are called blind speeds.

Why this matters: at these speeds, motion signals are aliased or indistinguishable from stationary, so you can miss or misestimate wind or precipitation motion in Doppler velocity data. Operators mitigate it by using multiple PRFs or staggered PRFs to shift the blind speeds and by applying velocity-dealiasing techniques to recover the true motion.