What term describes an aircraft becoming invisible to radar due to Doppler shift equal to the radar's PRF?

• A. Doppler Null
• B. Blind Speed
• C. Cloaked Velocity
• D. Shadow Speed

Answer: (B)

In pulsed-Doppler radar, motion is detected by the Doppler shift of the returned signal, but the shift is measured modulo the radar’s pulse repetition frequency (PRF) because the signal is sampled at that rate. The Doppler frequency shift is fd = 2 v cosθ / λ. If the true Doppler shift equals an integer multiple of the PRF (fd = n × PRF), the measured Doppler wraps around to zero. That makes the target’s motion indistinguishable from stationary clutter in the Doppler processing, effectively rendering the target invisible to the Doppler filter at that speed. This situation is known as a blind speed. It depends on wavelength, PRF, and the aspect angle, and it explains why certain speeds hide in the Doppler spectrum. Other terms aren’t the standard name for this effect.