Pulse Repetition Frequency

Pulse Repetition Frequency

Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It is a measure or specification ("pulses per second") mostly used within various technical disciplines (e.g. Radar technology) to avoid confusion with the unit of frequency hertz (Hz or "cycles per second"— mainly used for describing the continuous wave base frequency) of the transmitted electromagnetic signal. That electronic frequency may be thought of as switched on and off to form the pulse train of an active sonar or radar system, and if the radar has a characteristic (or known fixed) PRR, can be used in Electronic Warfare as a measurable attribute that can be used to identify the type or class of a particular platform such as a ship or aircraft—in some cases, a particular unit. Electromagnetic (radio or sound) waves are conceptually pure single frequency phenomena while pulses may be mathematically thought of as composed of a number of pure frequencies which sum and nullify in interactions creating a pulse train of the specific amplitudes, PRRs, base frequencies, phase characteristics, etcetera (See Fourier Analysis). The first term (PRF) is more common in device technical literature (Electrical Engineering and some sciences), and the latter (PRR) more commonly used in military-aerospace terminology (especially United States armed forces terminologies) and equipment specifications such as training and technical manuals for radar and sonar systems.

The reciprocal of PRF (or PRR) 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. Within radar technology 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. Conversely, a high PRR/PRF can enhance target discrimination of nearer objects such as a periscope or fast moving missile leading to practices of employing low PRRs for search radar, and very high PRFs for fire control radars, with many dual-purpose and navigation radars, especially naval designs having variable PRRs which might allow a skilled operator to also use a PRR adjustment to enhance and clarify an unclear radar picture, for example in bad sea states where wave action generates false returns, and in general for less clutter, or perhaps a better return signal off a prominent landscape feature (e.g. a cliff).