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| '''Pulse repetition frequency (PRF)''' or '''pulse repetition rate''' ('''PRR''')<!-- common term in radar/sonar ---> 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 to describe the [[continuous wave]] base frequency) of the transmitted electromagnetic signal.
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| 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.
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| 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 that sum and nullify in interactions that create 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.
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| The [[Multiplicative inverse|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. The IPP term is normally used when referring to the quantity of PRT periods to be processed digitally. Each PRT producing a variable number of Range Gates depending on the PRF. For example, the APY-1 radar used 128 IPP's with a fixed 50 Range Gates to produce 128 [[Doppler effect|Doppler]] filters using a FFT. The variable number of Range Gates from its five PRF's were all less than 50. Within [[radar]] technology PRF is important since it determines the maximum target range (''R''<sub>max</sub>) and maximum Doppler velocity (''V''<sub>max</sub>) that can be accurately determined by the radar.<ref>{{Cite web|url=http://www.radartutorial.eu/18.explanations/ex36.en.html|title=Pulse Repetition Frequency|publisher=Radartutorial}}</ref> Conversely, a high PRR/PRF can enhance target discrimination of nearer objects, such as a periscope or fast moving missile. This leads to use of low PRRs for search radar, and very high PRFs for fire control radars. Many dual-purpose and navigation radars—especially naval designs with variable PRRs—allow a skilled operator to adjust PRR to enhance and clarify the 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).
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| ==Definition==
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| Pulse repetition frequency (PRF) is the number of times a pulsed activity occurs every second.
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| This is similar to [[cycle per second]] used to describe other types of waveforms.
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| PRF has a pulse period, <math> \Tau </math>, which is the time between pulses.
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| :<math> \Tau = \frac{1}{\text{PRF}}</math>
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| PRF is usually associated with pulse spacing, which is the distance that the pulse travels before the next pulse occurs.
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| :<math> \text{Pulse Spacing} = \frac{\text{Propagation Speed}}{\text{PRF}}</math>
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| ==Physics==
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| PRF is crucial to perform measurements for certain physics phenomenon.
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| For example, a [[tachometer]] may use a [[strobe light]] with an adjustable PRF to measure rotational velocity. The PRF for the strobe light is adjusted upward from a low value until the rotating object appears to stand still. The PRF of the tachometer would then match the speed of the rotating object.
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| Other types of measurements involve distance using the delay time for reflected echo pulses from light, microwaves, and sound transmissions.
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| ==Measurement==
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| PRF is crucial for systems and devices that measure distance.
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| * [[Radar]]
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| * [[Laser range finder]]
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| * [[Sonar]]
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| Different PRF allow systems to perform very different functions.
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| ===Radar===
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| PRF is required for [[radar]] operation. This is the rate at which transmitter pulses are sent into air or space.
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| ====Range ambiguity====
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| A radar system determines range through the time delay between pulse transmission and reception by the relation:
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| :<math>\text{Range} = \frac{c\tau}{2}</math>
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| For accurate range determination a pulse must be transmitted and reflected before the next pulse is transmitted. This gives rise to the maximum unambiguous range limit:
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| :<math>\text{Max Range} = \frac{c\tau_\text{PRT}}{2} = \frac{c}{2\,\text{PRF}} \qquad \begin{cases} \tau_\text{PRT} = \frac{1}{\text{PRF}} \end{cases}</math>
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| The maximum range also defines a range ambiguity for all detected targets. Because of the periodic nature of pulsed radar systems, it is impossible for some 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. | |
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| The [[Range ambiguity resolution|range ambiguity resolution process]] is used to identify true range when PRF is above this limit.
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| ====Low PRF====
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| Systems using PRF below 3 kHz are considered low PRF because direct range can be measured to a distance of at least 50 km. Radar systems using low PRF typically produce unambiguous range.
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| Unambiguous Doppler processing becomes an increasing challenge due to coherency limitations as PRF falls below 3 kHz.
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| For example, an [[L-Band]] radar with 500 Hz pulse rate produces [[frequency ambiguity resolution|ambiguous velocity]] above 75 m/s (170 mile/hour), while detecting true range up to 300 km. This combination is appropriate for civilian aircraft radar and [[weather radar]].
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| :<math>\text{300 km range} = \frac{C}{2 \times 500}</math>
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| :<math>\text{75 m/s velocity} = \frac{500 \times C}{2 \times 10^9}</math> | |
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| Low PRF radar have reduced sensitivity in the presence of low-velocity clutter that interfere with aircraft detection near terrain. [[Moving target indicator]] is generally required for acceptable performance near terrain, but this introduces [[radar scalloping]] issues that complicate the receiver. Low PRF radar intended for aircraft and spacecraft detection are heavily degraded by weather phenomenon, which cannot be compensated using moving target indicator.
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| ====Medium PRF====
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| Range and velocity can both be identified using medium PRF, but neither one can be identified directly. Medium PRF is from 3 kHz to 30 kHz, which corresponds with radar range from 5 km to 50 km. This is the ambiguous range, which is much smaller than the maximum range. [[Range ambiguity resolution]] is used to determine true range in medium PRF radar.
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| Medium PRF is used with [[Pulse-Doppler radar]], which is required for [[look-down/shoot-down]] capability in military systems. Doppler radar return is generally not ambiguous until velocity exceeds the speed of sound.
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| A technique called [[ambiguity resolution]] is required to identify true range and speed. Doppler signals fall between 1.5 kHz, and 15 kHz, which is audible, so audio signals from medium-PRF radar systems can be used for passive target classification.
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| For example, an [[L band]] radar system using a PRF of 10 kHz with a duty cycle of 3.3% can identify true range to a distance of 450 km (30 * C / 10,000 km/s). This is the '''instrumented range'''. Unambiguous velocity is 1,500 m/s (3,300 mile/hour).
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| :<math>\text{450 km} = \frac{C}{0.033 \times 2 \times 10,000}</math>
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| :<math>\text{1,500 m/s} = \frac{10,000 \times C}{2 \times 10^9}</math>
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| The unambiguous velocity of an [[L-Band]] radar using a PRF of 10 kHz would be 1,500 m/s (3,300 mile/hour) (10,000 x C / (2 x 10^9)). True velocity can be found for objects moving under 45,000 m/s if the band pass filter admits the signal (1,500/0.033).
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| Medium PRF has unique [[radar scalloping]] issues that require redundant detection schemes.
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| ====High PRF====
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| Systems using PRF above 30 kHz function better known as interrupted continuous-wave (ICW) radar because direct velocity can be measured up to 4.5 km/s at [[L band]], but range resolution becomes problematic.
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| High PRF is limited to systems that require close-in performance, like proximity fuses and [[Radar gun|law enforcement radar]].
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| For example, if 30 samples are taken during the quiescent phase between transmit pulses using a 30 kHz PRF, then true range can be determined to a maximum of 150 km using 1 microsecond samples (30 x C / 30,000 km/s). Reflectors beyond this range might be detectable, but the true range cannot be identified.
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| :<math>\text{150 km} = \frac{30 \times C}{2 \times 30,000}</math>
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| :<math>\text{4,500 m/s} = \frac{30,000 \times C}{2 \times 10^9}</math>
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| It becomes increasingly difficult to take multiple samples between transmit pulses at these pulse frequencies, so range measurements are limited to short distances.<ref name=cwi>{{cite web|url=http://www.crcnetbase.com/doi/pdf/10.1201/9781420006711.ch14|title=Continuous Wave Radar|accessdate=January 29, 2011}}</ref>
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| ===Sonar===
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| Sonar systems operate much like radar, except that the medium is liquid or air, and the frequency of the signal is either audio or ultra-sonic. Like radar, lower frequencies propagate relatively higher energies longer distances with less resolving ability. Higher frequencies, which damp out faster, provide increased resolution of nearby objects.
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| Signals propagate at the [[speed of sound]] in the medium (almost always water), and maximum PRF depends upon the size of the object being examined. For example, the speed of sound in water is 1,497 m/s, and the human body is about 0.5 m thick, so the PRF for [[Medical ultrasonography|ultrasound images]] of the human body should be less than about 2 kHz (1,497/0.5).
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| As another example, ocean depth is approximately 2 km, so sound takes over a second to return from the sea floor. Sonar is a very slow technology with very low PRF for this reason.
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| ===Laser===
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| {{main|LIDAR}}
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| Light waves can be used as radar frequencies, in which case the system is known as LIDAR, an acronym meaning '''Li'''ght '''D'''etection '''A'''nd '''R'''anging.
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| Laser range or other light signal frequency range finders operate just like radar at much higher frequencies. Non-laser light detection is utilized extensively in automated machine control systems (e.g. electric eyes controlling a garage door, conveyor sorting gates, etc.), and those that use pulse rate detection and ranging are at heart, the same type of system as a radar—without the bells and whistles of the human interface.
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| Unlike lower radio signal frequencies, light does not bend around the curve of the earth or reflect off the ionosphere like C-band search radar signals, and so [[LIDAR]] is useful only in line of sight applications like higher frequency radar systems.
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| ==See also==
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| * [[Radar]]
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| * [[Pulse-Doppler radar]]
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| * [[Weather radar]]
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| ==References==
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| {{Reflist}}
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| {{DEFAULTSORT:Pulse Repetition Frequency}}
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| [[Category:Radar theory]]
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| [[Category:Units of frequency]]
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