How is aliasing in Doppler processing avoided?

• A. By increasing the display brightness.
• B. By using a larger antenna.
• C. By ignoring high-speed targets.
• D. By selecting an appropriate PRF or multiple PRFs.

Answer: (D)

Aliasing in Doppler processing comes from how often the radar samples the returning signal. The Doppler frequency you can measure unambiguously is limited to half the pulse repetition frequency (the Nyquist limit). If a target’s Doppler shift tries to exceed that limit, its true velocity wraps around and you see an incorrect, aliased value. The way to prevent this is to pick a PRF that keeps the expected Doppler shifts within the unambiguous range, or use multiple PRFs (staggered PRFs) to cover a wider Doppler span and then combine the data to recover the true speed. Other options don’t address the sampling issue: increasing display brightness doesn’t affect Doppler sampling, a larger antenna changes spatial aspects but not aliasing, and ignoring fast targets simply loses those targets rather than solving the aliasing problem.