Pulse repetition frequency
Encyclopedia
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 (Rmax) and maximum Doppler
velocity (Vmax) 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 a 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).
This is similar to cycle per second
used to describe other types of waveforms.
PRF has a pulse period,
, which is the length of time between pulses.
PRF is usually associated with pulse spacing, which is the distance that the pulse travels before the next pulse occurs.
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.
Other types of measurements involve distance using the delay time for reflected echo pulses from light, microwaves, and sound transmissions.
Different PRF allow systems to perform very different functions.
operation. This is the rate at which transmitter pulses are sent into air or space.
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:
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.
The range ambiguity resolution process
is used to identify true range when PRF is above this limit.
Unambiguous Doppler processing becomes an increasing challenge due to coherency limitations as PRF falls below 3 kHz.
For example, an L-Band radar with 500 Hz pulse rate produces 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
.
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.
is used to determine true range in medium PRF radar.
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.
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.
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).
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 will admit the signal (1,500/0.033).
Medium PRF has unique radar scalloping
issues that require redundant detection schemes.
, but range resolution becomes problematic.
High PRF is limited to systems that require close-in performance, like proximity fuses and law enforcement radar
.
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.
It becomes increasingly difficult to take multiple samples between transmit pulses at these pulse frequencies, so range measurements are limited to short distances.
Signals propagate at the speed of sound
, 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 ultrasound images
of the human body should be less than about 2 kHz (1,497/0.5).
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.
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
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
Electronic warfare
Electronic warfare refers to any action involving the use of the electromagnetic spectrum or directed energy to control the spectrum, attack an enemy, or impede enemy assaults via the spectrum. The purpose of electronic warfare is to deny the opponent the advantage of, and ensure friendly...
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
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
and some sciences), and the latter (PRR) more commonly used in military-aerospace terminology
Terminology
Terminology is the study of terms and their use. Terms are words and compound words that in specific contexts are given specific meanings, meanings that may deviate from the meaning the same words have in other contexts and in everyday language. The discipline Terminology studies among other...
(especially United States armed forces terminologies) and equipment specifications such as training and technical manuals for radar and sonar systems.
The reciprocal
Multiplicative inverse
In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by 1/x or x−1, is a number which when multiplied by x yields the multiplicative identity, 1. The multiplicative inverse of a fraction a/b is b/a. For the multiplicative inverse of a real number, divide 1 by the...
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
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
technology PRF is important since it determines the maximum target range (Rmax) and maximum Doppler
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
velocity (Vmax) 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 a 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).
Definition
Pulse repetition frequency (PRF) is the number of times a pulsed activity occurs every second.This is similar to cycle per second
Cycle per second
The cycle per second was a once-common unit of frequency.With the organization of the International System of Units in 1960, the cycle per second was officially replaced by the hertz, or reciprocal second—i.e. the cycle in 'cycle per second' was dropped...
used to describe other types of waveforms.
PRF has a pulse period,
, which is the length of time between pulses.PRF is usually associated with pulse spacing, which is the distance that the pulse travels before the next pulse occurs.
Physics
PRF is crucial to perform measurements for certain physics phenomenon.For example, a tachometer
Tachometer
A tachometer is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute on a calibrated analogue dial, but digital displays are increasingly common...
may use a strobe light
Strobe light
A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope...
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.
Other types of measurements involve distance using the delay time for reflected echo pulses from light, microwaves, and sound transmissions.
Measurement
PRF is crucial for systems and devices that measure distance.- RadarRadarRadar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
- Laser range finder
- SonarSonarSonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...
Different PRF allow systems to perform very different functions.
Radar
PRF is required for radarRadar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
operation. This is the rate at which transmitter pulses are sent into air or space.
Range ambiguity
A radar system determines range through the time delay between pulse transmission and reception by the relation: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:
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.
The range ambiguity resolution process
Range ambiguity resolution
Range ambiguity resolution is a technique used with medium Pulse repetition frequency radar to obtain range information for distances that exceed the distance between transmit pulses.This signal processing technique is required with pulse-Doppler radar....
is used to identify true range when PRF is above this limit.
Low PRF
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.Unambiguous Doppler processing becomes an increasing challenge due to coherency limitations as PRF falls below 3 kHz.
For example, an L-Band radar with 500 Hz pulse rate produces ambiguous velocity
Frequency ambiguity resolution
Frequency ambiguity resolution is used to find true target velocity for medium pulse repetition frequency radar systems.This is used with pulse-Doppler radar.-Definition:...
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
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
.
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
Radar scalloping
Scalloping is a radar phenomenon that reduces sensitivity for certain distance and velocity combinations.The name is derived from the appearance of areas that are scooped out of graphs that indicate radar sensitivity....
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.
Medium PRF
Range and velocity can both be identified using medium PRF, but neither one can be identified directly. Medium PRF is from 3kHz to 30kHz, 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 resolutionRange ambiguity resolution
Range ambiguity resolution is a technique used with medium Pulse repetition frequency radar to obtain range information for distances that exceed the distance between transmit pulses.This signal processing technique is required with pulse-Doppler radar....
is used to determine true range in medium PRF radar.
Medium PRF is used with Pulse-Doppler radar
Pulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...
, which is required for look-down/shoot-down
Look-down/shoot-down
Look-down/shoot-down is a capability a radar system is said to possess if it is able to detect, track and put a weapon onto an air target moving below the horizon as seen by the radar...
capability in military systems. Doppler radar return is generally not ambiguous until velocity exceeds the speed of sound.
A technique called ambiguity resolution
Ambiguity resolution
Ambiguity resolution is used to find the value of a measurement that requires modulo sampling.This is required for pulse-Doppler radar signal processing.-Measurements:...
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.
For example, an L band
L band
L band refers to four different bands of the electromagnetic spectrum: 40 to 60 GHz , 1 to 2 GHz , 1565 nm to 1625 nm , and around 3.5 micrometres .-NATO 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).
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 will admit the signal (1,500/0.033).
Medium PRF has unique radar scalloping
Radar scalloping
Scalloping is a radar phenomenon that reduces sensitivity for certain distance and velocity combinations.The name is derived from the appearance of areas that are scooped out of graphs that indicate radar sensitivity....
issues that require redundant detection schemes.
High PRF
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 bandL band
L band refers to four different bands of the electromagnetic spectrum: 40 to 60 GHz , 1 to 2 GHz , 1565 nm to 1625 nm , and around 3.5 micrometres .-NATO L band:...
, but range resolution becomes problematic.
High PRF is limited to systems that require close-in performance, like proximity fuses and law enforcement radar
Radar gun
A radar speed gun is a small doppler radar unit used to measure the speed of moving objects, including vehicles, pitched baseballs, runners and other moving objects. Radar speed guns may be hand-held, vehicle-mounted or static...
.
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.
It becomes increasingly difficult to take multiple samples between transmit pulses at these pulse frequencies, so range measurements are limited to short distances.
Sonar
Sonar systems operate much like radar, except that the medium is liquid or air.Signals propagate at the speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
, 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 ultrasound images
Medical ultrasonography
Diagnostic sonography is an ultrasound-based diagnostic imaging technique used for visualizing subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions...
of the human body should be less than about 2 kHz (1,497/0.5).
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.