How does the PRF of a pulsed radar affect its maximum unambiguous range and velocity?
PRF and Ambiguity
Medium PRF ambiguity resolution: transmit at several different PRFs and identify the true range and velocity by finding the values consistent across all PRFs (Chinese Remainder Theorem or coincidence algorithm). Selecting the PRF set: choose 3-8 PRFs that are mutually prime and provide a combined unambiguous range and velocity that covers the required operational envelope. The PRFs must also avoid blind ranges (eclipsing, where the receiver is blanked during transmission of the next pulse) and blind velocities (where the Doppler shift equals a multiple of the PRF).
Frequently Asked Questions
What is eclipsing?
Eclipsing occurs when a target's return arrives during the transmit pulse of a subsequent pulse. The receiver is blanked during transmission, so the return is lost. Eclipsed ranges: R_eclipse = n × c / (2 × PRF) ± c × τ / 2, where τ is the pulse width and n is any positive integer. Mitigation: use pulse-to-pulse PRF jitter or staggered PRF to ensure that eclipsed ranges move between dwells.
How do I select the PRF?
Consider: required unambiguous range, required unambiguous velocity, clutter Doppler spread (PRF must place the target Doppler outside the clutter region), eclipsing at ranges of interest, and blind speeds. For ground-based surveillance: PRF = 300-1000 Hz (range priority). For airborne intercept: PRF = 10-30 kHz (velocity priority).