In general, pulsed radar transmits and receives a train of pulses, as illustrated by Figure given below. The Inter Pulse Period (IPP) is T , and the pulse width is Δt . The IPP is often referred to as the Pulse Repetition Interval (PRI). The inverse of the PRI is the PRF, which is denoted by fr,

$$fr=\dfrac{1}{PRI}=\dfrac{1}{T}$$

PRF is often governed by the maximum unambiguous range, Run, beyond which targets are not expected. The maximum range Run is given by

$fr=\dfrac{2C}{R_{un}}$ where c is speed of propagation

So to keep Run larger fr must be kept smaller i.e. pulses must be transmitted at lower rate, so that the echo from target gets sufficient time to reach to RADAR and the range of the target detected remain unambiguous.

Other factors affecting the fr are the echoes from beyond the maximum unambiguous range, such as large target or clutter source ( like mountain). Such echoes arrive at a time later than the pulse-repetition interval and are called second-time-around echoes. Such echoes reflect the false position of the actual target. In order to avoid such ambiguity in range by changing the fr.