Which phenomenon occurs when the Doppler frequency shift exceeds half the PRF?

Which phenomenon occurs when the Doppler frequency shift exceeds half the PRF?

• A. Nyquist limit
• B. Aliasing
• C. Doppler smear
• D. Range resolution

Answer: (B)

Doppler measurements in pulsed radar are limited by Nyquist: you can only unambiguously measure Doppler shifts up to half the PRF. When the true Doppler shift is larger than PRF/2, the sampled Doppler values wrap around and appear as a different, smaller frequency within the -PRF/2 to +PRF/2 range. This folding is Doppler aliasing, creating velocity ambiguity where fast motion can look slower or even reverse direction. Nyquist defines the unambiguous boundary; aliasing is what you see once you exceed it. Doppler smear and range resolution involve different aspects (pulse timing and range discrimination, respectively) and do not describe this folding behavior.

Doppler measurements in pulsed radar are limited by Nyquist: you can only unambiguously measure Doppler shifts up to half the PRF. When the true Doppler shift is larger than PRF/2, the sampled Doppler values wrap around and appear as a different, smaller frequency within the -PRF/2 to +PRF/2 range. This folding is Doppler aliasing, creating velocity ambiguity where fast motion can look slower or even reverse direction. Nyquist defines the unambiguous boundary; aliasing is what you see once you exceed it. Doppler smear and range resolution involve different aspects (pulse timing and range discrimination, respectively) and do not describe this folding behavior.