Pulse Repetition Frequency
Understanding PRF
PRF is one of the most important radar design parameters because it directly creates a trade-off between unambiguous range and unambiguous Doppler measurement. This fundamental trade-off drives radar waveform design.
PRF Trade-offs
| Parameter | Low PRF | Medium PRF | High PRF |
|---|---|---|---|
| PRF | 100-3000 Hz | 3-30 kHz | 30-500 kHz |
| Max unamb. range | 50-1500 km | 5-50 km | 0.3-5 km |
| Max unamb. Doppler | Limited | Moderate | Good |
| Range ambiguity | No | Yes | Severe |
| Doppler ambiguity | Severe | Some | No |
R_max = c / (2 x PRF)
Maximum unambiguous Doppler:
f_d_max = PRF / 2
Example at PRF = 10 kHz:
R_max = 3e8 / (2 x 10000) = 15 km
f_d_max = 5 kHz = 750 m/s at X-band
Frequently Asked Questions
What is PRF?
PRF is the pulse repetition rate of a radar. It determines maximum unambiguous range (c/2PRF) and Doppler (PRF/2). Low PRF: good range, poor Doppler. High PRF: good Doppler, poor range. A fundamental radar trade-off.
Can you resolve both range and Doppler ambiguity?
Yes, using multiple PRFs (PRF diversity). By transmitting at two or more PRFs and comparing the apparent range/Doppler, the true values can be determined. This is standard in medium-PRF airborne radar.
What is duty cycle?
Duty cycle = pulse width x PRF = tau x PRF. It is the fraction of time the radar is transmitting. Average power = peak power x duty cycle. Typical duty cycles: 0.001 (pulsed) to 1.0 (CW). Higher duty cycle = more energy on target.