What is Pulse Repetition Interval?

Pulse Repetition Interval (PRI) is the time difference between two transmitted pulses. This term is used in a number of technical fields but most notably importantly associated with radars. As radar systems usually consist of a powerful transmitter and sensitive receiver connected to the same antenna, PRI is one of the defining characteristics of radar technology. It is counted from the start of transmission of a pulse to the start of transmission of the next pulse.

After producing a brief pulse of radio signal, the transmitter has to be turned off to receive the reflections from distant targets. This time taken to receive the reflected signals is called receiving time. If the receiving time ends before the next transmitting pulse, the leftover time before the next transmission is called dead time. Hence, PRI is the combination of receiving time and dead time.

The receiving time is always smaller than the difference between the pulse repetition period and the length of the transmitter's pulse. It is sometimes also limited by the dead time, in which the receiver is already switched off just before the next transmitting pulse. Modern radars generally carry out system test loops during this dead time. Radars that use phased-array antennas especially need dead time because the phase shifters of the antenna are reprogrammed during the dead time to prepare the antenna for the next direction of the antenna's beam.

In some radars, there is a short recovery time for the duplexer (switch that controls the timing of the transmitter and receiver) between the transmitting pulse and the receiving time. This recovery time is mostly required when the duplexer must switch off the receiver response to transmit at high powers. However, when transmitting at low power, reflected signals can be received even during transmission. In this case, the receiving time includes transmission time too.

Since the radio signal has to travel out to the target and back again, the radar’s desired range of operation is dependent on this quiet-period (transmitter ‘off’ state) between 2 transmission pulses. Longer range signals require longer PRIs and similarly shorter PRIs produce shorter maximum ranges but broadcast more pulses and radio energy in a given time. Shorter PRIs hence create stronger reflections that make detection easier. Radar systems balance the two competing requirements of range and pulse energy for optimum usage of the technology for various purposes.