Pulse repetition frequency

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Pulse Repetition Frequency (PRF) is the number of pulses transmitted per second by a radar. The reciprocal of this is called the Pulse Repetition Time (PRT), Pulse Repetition Interval (PRI), or Inter-Pulse Period (IPP), which is the elapsed time from the beginning of one pulse to the beginning of the next pulse. PRF is important since it determines the maximum target range (R_max) and maximum Doppler velocity (V_max) that can be accurately determined by the radar.

[edit] Range ambiguity

A radar system determines range through the time delay between pulse transmission and reception by the relation:

$Range = \frac{c\tau}{2}$

For accurate range determination a pulse must be transmitted and reflected before the next pulse is transmitted. This gives rise to the maximum range limit:

$Max Range = \frac{c\tau_{PRT}}{2} = \frac{c}{2PRF} \qquad \begin{cases} \tau_{PRT} = \frac{1}{PRF} \end{cases}$

The maximum range also defines a range ambiguity for all detected targets. Because of the periodic nature of pulsed radar systems, it will be impossible for a radar system to determine the difference between targets separated by integer multiples of the maximum range using a single PRF. More sophisticated radar systems avoid this problem through the use of multiple PRFs either simultaneously on different frequencies or on a single frequency with a changing PRT.