![MAKS-2007-Radar](https://upload.wikimedia.org/wikipedia/commons/4/42/MAKS-2007-Radar.jpg)
A pulse-Doppler radar is a
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
system that determines the range to a target using pulse-timing techniques, and uses the
Doppler effect
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
of the returned signal to determine the target object's velocity. It combines the features of pulse radars and
continuous-wave radar
Continuous-wave radar (CW radar) is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. Individual objects can be detected using the Doppler effect, which ...
s, which were formerly separate due to the complexity of the
electronics
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
.
The first operational Pulse Doppler radar was in the
CIM-10 Bomarc
The Boeing CIM-10 BOMARC (Boeing Michigan Aeronautical Research Center) (IM-99 Weapon System prior to September 1962) was a supersonic ramjet powered long-range surface-to-air missile (SAM) used during the Cold War for the air defense of North ...
, an American long range supersonic missile powered by
ramjet
A ramjet, or athodyd (aero thermodynamic duct), is a form of airbreathing jet engine that uses the forward motion of the engine to produce thrust. Since it produces no thrust when stationary (no ram air) ramjet-powered vehicles require an ass ...
engines, and which was armed with a W40 nuclear weapon to destroy entire formations of attacking enemy aircraft. Pulse-Doppler systems were first widely used on
fighter aircraft
Fighter aircraft are fixed-wing military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air superiority of the battlespace. Domination of the airspace above a battlefield ...
starting in the 1960s. Earlier radars had used pulse-timing in order to determine range and the angle of the antenna (or similar means) to determine the bearing. However, this only worked when the radar antenna was not pointed down; in that case the reflection off the ground overwhelmed any returns from other objects. As the ground moves at the same speed but opposite direction of the aircraft, Doppler techniques allow the ground return to be filtered out, revealing aircraft and vehicles. This gives pulse-Doppler radars "
look-down/shoot-down A radar system has look-down/shoot-down capability if it can detect, track and guide a weapon to an air target that (as seen by the radar) is silhouetted against the ground.
Problem and naming
Airborne intercept radar relying exclusively on time ...
" capability. A secondary advantage in military radar is to reduce the transmitted power while achieving acceptable performance for improved safety of stealthy radar.
Pulse-Doppler techniques also find widespread use in
meteorological radars, allowing the radar to determine
wind speed
In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.
Wind speed ...
from the velocity of any precipitation in the air. Pulse-Doppler radar is also the basis of
synthetic aperture radar
Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide fine ...
used in
radar astronomy
Radar astronomy is a technique of observing nearby astronomical objects by reflecting radio waves or microwaves off target objects and analyzing their reflections. Radar astronomy differs from ''radio astronomy'' in that the latter is a passive o ...
,
remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
and mapping. In
air traffic control
Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...
, they are used for discriminating aircraft from clutter. Besides the above conventional surveillance applications, pulse-Doppler radar has been successfully applied in healthcare, such as fall risk assessment and fall detection, for nursing or clinical purposes.
History
The earliest radar systems failed to operate as expected. The reason was traced to Doppler effects that degrade performance of systems not designed to account for moving objects. Fast-moving objects cause a
phase-shift
In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it ...
on the transmit pulse that can produce signal cancellation. Doppler has maximum detrimental effect on
moving target indicator
Moving target indication (MTI) is a mode of operation of a radar to discriminate a target against the clutter. It describes a variety of techniques used for finding moving objects, like an aircraft, and filter out unmoving ones, like hills or trees ...
systems, which must use reverse phase shift for Doppler compensation in the detector.
Doppler weather effects (precipitation) were also found to degrade
conventional radar and moving target indicator radar, which can mask aircraft reflections. This phenomenon was adapted for use with
weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly puls ...
in the 1950s after declassification of some World War II systems.
Pulse-Doppler radar was developed during World War II to overcome limitations by increasing
pulse repetition frequency
The pulse repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit. The term is used within a number of technical disciplines, notably radar.
In radar, a radio signal of a particular carrier frequency is tu ...
. This required the development of the
klystron
A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequenci ...
, the
traveling wave tube
A traveling-wave tube (TWT, pronounced "twit") or traveling-wave tube amplifier (TWTA, pronounced "tweeta") is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range. The TWT belongs t ...
, and solid state devices. Early pulse-dopplers were incompatible with other high power microwave amplification devices that are not
coherent
Coherence, coherency, or coherent may refer to the following:
Physics
* Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference
* Coherence (units of measurement), a deri ...
, but more sophisticated techniques were developed that record the phase of each transmitted pulse for comparison to returned echoes.
Early examples of military systems includes the
AN/SPG-51
The AN/SPG-51 is an American tracking / illumination fire-control radar for RIM-24 Tartar and RIM-66 Standard missiles. It is used for target tracking and Surface-to-air missile guidance on s, s, and s.
The Italian Navy used it aboard their ''Au ...
B developed during the 1950s specifically for the purpose of operating in hurricane conditions with no performance degradation.
The
Hughes AN/ASG-18 Fire Control System
The Hughes AN/ASG-18 Fire Control System was a prototype airborne fire control radar system for the planned North American XF-108 Rapier interceptor aircraft, and the Lockheed YF-12 for the United States Air Force. It was the US's first Pulse-Dop ...
was a prototype airborne radar/combination system for the planned
North American XF-108 Rapier
The North American XF-108 Rapier was a proposed long-range, high-speed interceptor aircraft designed by North American Aviation intended to defend the United States from supersonic Soviet strategic bombers. The aircraft would have cruised at ...
interceptor aircraft for the United States Air Force, and later for the
Lockheed YF-12
The Lockheed YF-12 was an American Mach 3+ capable, high-altitude interceptor prototype, developed and manufactured by American aerospace company Lockheed Corporation.
It was developed during the late 1950s and early 1960s as a potenti ...
. The US's first pulse-Doppler radar, the system had
look-down/shoot-down A radar system has look-down/shoot-down capability if it can detect, track and guide a weapon to an air target that (as seen by the radar) is silhouetted against the ground.
Problem and naming
Airborne intercept radar relying exclusively on time ...
capability and could track one target at a time.
Weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
,
chaff
Chaff (; ) is the dry, scaly protective casing of the seeds of cereal grains or similar fine, dry, scaly plant material (such as scaly parts of flowers or finely chopped straw). Chaff is indigestible by humans, but livestock can eat it. In agri ...
,
terrain
Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin word ...
,
flying techniques, and
stealth are common tactics used to hide aircraft from radar. Pulse-Doppler radar eliminates these weaknesses.
It became possible to use pulse-Doppler radar on aircraft after digital computers were incorporated in the design. Pulse-Doppler provided
look-down/shoot-down A radar system has look-down/shoot-down capability if it can detect, track and guide a weapon to an air target that (as seen by the radar) is silhouetted against the ground.
Problem and naming
Airborne intercept radar relying exclusively on time ...
capability to support air-to-air missile systems in most modern military aircraft by the mid 1970s.
Principle
Range measurement
![Radarops](https://upload.wikimedia.org/wikipedia/commons/0/07/Radarops.gif)
Pulse-Doppler systems measure the range to objects by measuring the elapsed time between sending a pulse of radio energy and receiving a reflection of the object. Radio waves travel at the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
, so the distance to the object is the elapsed time multiplied by the speed of light, divided by two - there and back.
Velocity measurement
![Doppler effect diagrammatic](https://upload.wikimedia.org/wikipedia/commons/f/f7/Doppler_effect_diagrammatic.png)
Pulse-Doppler radar is based on the
Doppler effect
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
, where movement in range produces frequency shift on the signal reflected from the target.
:
Radial velocity
The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...
is essential for pulse-Doppler radar operation. As the reflector moves between each transmit pulse, the returned signal has a
phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
difference, or ''phase shift'', from pulse to pulse. This causes the reflector to produce Doppler modulation on the reflected signal.
Pulse-Doppler radars exploit this phenomenon to improve performance.
The amplitude of the successively returning pulse from the same scanned volume is
:
where
:
is the distance radar to target,
:
is the radar wavelength,
:
is the time between two pulses.
So
:
This allows the radar to separate the reflections from multiple objects located in the same volume of space by separating the objects using a
spread spectrum
In telecommunication and radio communication, spread-spectrum techniques are methods by which a signal (e.g., an electrical, electromagnetic, or acoustic signal) generated with a particular bandwidth is deliberately spread in the frequency dom ...
to segregate different signals:
:
where
is the phase shift induced by range motion.
Benefits
Rejection speed is selectable on pulse-Doppler aircraft-detection systems so nothing below that speed will be detected. A one degree antenna beam illuminates millions of square feet of terrain at range, and this produces thousands of detections at or below the horizon if Doppler is not used.
Pulse-Doppler radar uses the following signal processing criteria to exclude unwanted signals from slow-moving objects. This is also known as clutter rejection. Rejection velocity is usually set just above the prevailing wind speed (10 to 100 mile/hour or 15 to 150 km/hour). The velocity threshold is much lower for
weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly puls ...
.
:
In airborne pulse-Doppler radar, the velocity threshold is offset by the speed of the aircraft relative to the ground.
:
where
is the angle offset between the antenna position and the aircraft flight trajectory.
Surface reflections appear in almost all radar. Ground clutter generally appears in a circular region within a radius of about near ground-based radar. This distance extends much further in airborne and space radar. Clutter results from radio energy being reflected from the earth surface, buildings, and vegetation. Clutter includes weather in radar intended to detect and report aircraft and spacecraft.
Clutter creates a vulnerability region in
pulse-amplitude time-domain radar. Non-Doppler radar systems cannot be pointed directly at the ground due to excessive false alarms, which overwhelm computers and operators. Sensitivity must be reduced near clutter to avoid overload. This vulnerability begins in the low-elevation region several beam widths above the horizon, and extends downward. This also exists throughout the volume of moving air associated with weather phenomenon.
Pulse-Doppler radar corrects this as follows.
* Allows the radar antenna to be pointed directly at the ground without overwhelming the computer and without reducing sensitivity.
* Fills in the vulnerability region associated with
pulse-amplitude time-domain radar for small object detection near terrain and weather.
* Increases detection range by 300% or more in comparison to
moving target indication
Moving target indication (MTI) is a mode of operation of a radar to discriminate a target against the clutter. It describes a variety of techniques used for finding moving objects, like an aircraft, and filter out unmoving ones, like hills or trees ...
(MTI) by improving sub-clutter visibility.
Clutter rejection capability of about 60 dB is needed for
look-down/shoot-down A radar system has look-down/shoot-down capability if it can detect, track and guide a weapon to an air target that (as seen by the radar) is silhouetted against the ground.
Problem and naming
Airborne intercept radar relying exclusively on time ...
capability, and pulse-Doppler is the only strategy that can satisfy this requirement. This eliminates vulnerabilities associated with the low-elevation and below-horizon environment.
Pulse compression Pulse compression is a signal processing technique commonly used by radar, sonar and echography to increase the range resolution as well as the signal to noise ratio. This is achieved by modulating the transmitted pulse and then correlating th ...
, and
moving target indicator
Moving target indication (MTI) is a mode of operation of a radar to discriminate a target against the clutter. It describes a variety of techniques used for finding moving objects, like an aircraft, and filter out unmoving ones, like hills or trees ...
(MTI) provide up to 25 dB sub-clutter visibility. MTI antenna beam is aimed above horizon to avoid excessive false alarm rate, which renders systems vulnerable. Aircraft and some missiles exploit this weakness using a technique called flying below the radar to avoid detection (
Nap-of-the-earth
Nap-of-the-earth (NOE) is a type of very low-altitude flight course used by military aircraft to avoid enemy detection and attack in a high-threat environment. Other, mostly older terms include "ground-hugging", "terrain masking", "flying under t ...
). This flying technique is ineffective against pulse-Doppler radar.
Pulse-Doppler provides an advantage when attempting to detect missiles and
low observability aircraft flying near terrain, sea surface, and weather.
Audible Doppler and target size support passive vehicle type classification when
identification friend or foe
Identification, friend or foe (IFF) is an identification system designed for command and control. It uses a transponder that listens for an ''interrogation'' signal and then sends a ''response'' that identifies the broadcaster. IFF systems usual ...
is not available from a
transponder signal. Medium
pulse repetition frequency
The pulse repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit. The term is used within a number of technical disciplines, notably radar.
In radar, a radio signal of a particular carrier frequency is tu ...
(PRF) reflected microwave signals fall between 1,500 and 15,000 cycle per second, which is audible. This means a
helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...
sounds like a helicopter, a
jet sounds like a jet, and
propeller aircraft
A powered aircraft is an aircraft that uses onboard propulsion with mechanical power generated by an aircraft engine of some kind.
Aircraft propulsion nearly always uses either a type of propeller, or a form of jet propulsion. Other potential ...
sound like propellers.
Aircraft with no moving parts produce a tone. The actual size of the target can be calculated using the audible signal.
Detriments
Ambiguity processing is required when target range is above the red line in the graphic, which increases scan time.
Scan time is a critical factor for some systems because vehicles moving at or above the speed of sound can travel one mile (1.6 km) every few seconds, like the
Exocet
The Exocet () is a French-built anti-ship missile whose various versions can be launched from surface vessels, submarines, helicopters and fixed-wing aircraft.
Etymology
The missile's name was given by M. Guillot, then the technical director ...
,
Harpoon
A harpoon is a long spear-like instrument and tool used in fishing, whaling, seal hunting, sealing, and other marine hunting to catch and injure large fish or marine mammals such as seals and whales. It accomplishes this task by impaling the t ...
,
Kitchen
A kitchen is a room or part of a room used for cooking and food preparation in a dwelling or in a commercial establishment. A modern middle-class residential kitchen is typically equipped with a stove, a sink with hot and cold running water, a ...
, and
Air-to-air missile
The newest and the oldest member of Rafael's Python family of AAM for comparisons, Python-5 (displayed lower-front) and Shafrir-1 (upper-back)
An air-to-air missile (AAM) is a missile fired from an aircraft for the purpose of destroying a ...
. The maximum time to scan the entire volume of the sky must be on the order of a dozen seconds or less for systems operating in that environment.
Pulse-Doppler radar by itself can be too slow to cover the entire volume of space above the horizon unless fan beam is used. This approach is used with the AN/SPS 49(V)5 Very Long Range Air Surveillance Radar, which sacrifices elevation measurement to gain speed.
Pulse-Doppler antenna motion must be slow enough so that all the return signals from at least 3 different PRFs can be processed out to the maximum anticipated detection range. This is known as
dwell time. Antenna motion for pulse-Doppler must be as slow as radar using
MTI.
Search radar that include pulse-Doppler are usually dual mode because best overall performance is achieved when pulse-Doppler is used for areas with high false alarm rates (horizon or below and weather), while conventional radar will scan faster in free-space where false alarm rate is low (above horizon with clear skies).
The antenna type is an important consideration for multi-mode radar because undesirable phase shift introduced by the radar antenna can degrade
performance measurement
Performance measurement is the process of collecting, analyzing and/or reporting information regarding the performance of an individual, group, organization, system or component.
Definitions of performance measurement tend to be predicated upon an ...
s for sub-clutter visibility.
Signal processing
The signal processing enhancement of pulse-Doppler allows small high-speed objects to be detected in close proximity to large slow moving reflectors. To achieve this, the transmitter must be coherent and should produce low
phase noise
In signal processing, phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity (jitter). Generally speaking, radio-frequency engineers ...
during the detection interval, and the receiver must have large instantaneous
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...
.
*
Pulse-Doppler signal processing detailed explanation
Pulse-Doppler signal processing also includes ambiguity resolution to identify true range and velocity.
*
Ambiguity resolution detailed explanation
The received signals from multiple PRF are compared to determine true range using the range ambiguity resolution process.
*
Range ambiguity resolution detailed explanation
The received signals are also compared using the frequency ambiguity resolution process.
*
Frequency ambiguity resolution detailed explanation
Range resolution
The range resolution is the minimal range separation between two objects traveling at the same speed before the radar can detect two discrete reflections:
:
In addition to this sampling limit, the duration of the transmitted pulse could mean that returns from two targets will be received simultaneously from different parts of the pulse.
Velocity resolution
The velocity resolution is the minimal radial velocity difference between two objects traveling at the same range before the radar can detect two discrete reflections:
:
Special consideration
Pulse-Doppler radar has special requirements that must be satisfied to achieve acceptable performance.
Pulse repetition frequency
Pulse-Doppler typically uses
medium pulse repetition frequency
The pulse repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit. The term is used within a number of technical disciplines, notably radar.
In radar, a radio signal of a particular carrier frequency is tu ...
(PRF) from about 3 kHz to 30 kHz. The range between transmit pulses is 5 km to 50 km.
Range and velocity cannot be measured directly using medium PRF, and ambiguity resolution is required to identify true range and speed. Doppler signals are generally above 1 kHz, which is audible, so audio signals from medium-PRF systems can be used for passive target classification.
Angular measurement
Radar systems require angular measurement. Transponders are not normally associated with pulse-Doppler radar, so sidelobe suppression is required for practical operation.
Tracking radar systems use angle error to improve accuracy by producing measurements perpendicular to the radar antenna beam. Angular measurements are averaged over a span of time and combined with radial movement to develop information suitable to predict target position for a short time into the future.
The two angle error techniques used with tracking radar are monopulse and conical scan.
*
Monopulse radar
Monopulse radar is a radar system that uses additional encoding of the radio signal to provide accurate directional information. The name refers to its ability to extract range and direction from a single signal pulse.
Monopulse radar avoids prob ...
*
Conical scanning
Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target. Conical scanning is similar in concept to the earlier lobe switching concept used ...
Coherency
Pulse-Doppler radar requires a
coherent oscillator with very little noise.
Phase noise
In signal processing, phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity (jitter). Generally speaking, radio-frequency engineers ...
reduces sub-clutter visibility performance by producing apparent motion on stationary objects.
Cavity magnetron
The cavity magnetron is a high-power vacuum tube used in early radar systems and currently in microwave ovens and linear particle accelerators. It generates microwaves using the interaction of a stream of electrons with a magnetic field while ...
and
crossed-field amplifier
A crossed-field amplifier (CFA) is a specialized vacuum tube, first introduced in the mid-1950s and frequently used as a microwave amplifier in very-high-power transmitters.
Raytheon engineer William C. Brown's work to adapt magnetron principles ...
are not appropriate because noise introduced by these devices interfere with detection performance. The only amplification devices suitable for pulse-Doppler are
klystron
A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequenci ...
,
traveling wave tube
A traveling-wave tube (TWT, pronounced "twit") or traveling-wave tube amplifier (TWTA, pronounced "tweeta") is a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range. The TWT belongs t ...
, and solid state devices.
Scalloping
Pulse-Doppler signal processing introduces a phenomenon called scalloping. The name is associated with a series of holes that are scooped-out of the detection performance.
Scalloping for pulse-Doppler radar involves blind velocities created by the clutter rejection filter. Every volume of space must be scanned using 3 or more different PRF. A two PRF detection scheme will have detection gaps with a pattern of discrete ranges, each of which has a blind velocity.
Windowing
Ringing artifacts pose a problem with search, detection, and ambiguity resolution in pulse-Doppler radar.
Ringing is reduced in two ways.
First, the
shape of the transmit pulse is adjusted to smooth the leading edge and trailing edge so that RF power is increased and decreased without an abrupt change. This creates a transmit pulse with smooth ends instead of a square wave, which reduces ringing phenomenon that is otherwise associated with target reflection.
Second, the shape of the receive pulse is adjusted using a
window function
In signal processing and statistics, a window function (also known as an apodization function or tapering function) is a mathematical function that is zero-valued outside of some chosen interval, normally symmetric around the middle of the inte ...
that minimizes ringing that occurs any time pulses are applied to a filter. In a digital system, this adjusts the phase and/or amplitude of each sample before it is applied to the
fast Fourier transform
A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). Fourier analysis converts a signal from its original domain (often time or space) to a representation in th ...
. The
Dolph-Chebyshev window is the most effective because it produces a flat processing floor with no ringing that would otherwise cause false alarms.
Antenna
Pulse-Doppler radar is generally limited to mechanically aimed antennas and active phase array.
Mechanical RF components, such as wave-guide, can produce Doppler modulation due to phase shift induced by vibration. This introduces a requirement to perform full spectrum operational tests using shake tables that can produce high power mechanical vibration across all anticipated audio frequencies.
Doppler is incompatible with most electronically steered phase-array antenna. This is because the phase-shifter elements in the antenna are non-reciprocal and the phase shift must be adjusted before and after each transmit pulse. Spurious phase shift is produced by the sudden impulse of the phase shift, and settling during the receive period between transmit pulses places Doppler modulation onto stationary clutter. That receive modulation corrupts the
measure of performance
Performance measurement is the process of collecting, analyzing and/or reporting information regarding the performance of an individual, group, organization, system or component.
Definitions of performance measurement tend to be predicated upon a ...
for sub-clutter visibility. Phase shifter settling time on the order of 50ns is required. Start of receiver sampling needs to be postponed at least 1 phase-shifter settling time-constant (or more) for each 20 dB of sub-clutter visibility.
Most antenna phase shifters operating at PRF above 1 kHz introduce spurious phase shift unless special provisions are made, such as reducing phase shifter settling time to a few dozen nanoseconds.
The following gives the maximum permissible settling time for antenna
phase shift module
A phase shift module is a microwave network module which provides a controllable phase shift of the RF signal. Phase shifters are used in phased arrays.
Classification
Active versus passive
Active phase shifters provide gain, while passive ...
s.
:
where
:''T'' = phase shifter settling time,
:''SCV'' = sub-clutter visibility in
dB,
:''S'' = number of range samples between each transmit pulse,
:''PRF'' = maximal design pulse repetition frequency.
The antenna type and scan performance is a practical consideration for multi-mode radar systems.
Diffraction
Choppy surfaces, like waves and trees, form a diffraction grating suitable for bending microwave signals. Pulse-Doppler can be so sensitive that
diffraction
Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
from mountains, buildings or wave tops can be used to detect fast moving objects otherwise blocked by solid obstruction along the line of sight. This is a very lossy phenomenon that only becomes possible when radar has significant excess sub-clutter visibility.
Refraction and ducting use transmit frequency at
L-band
The L band is the Institute of Electrical and Electronics Engineers (IEEE) designation for the range of frequencies in the radio spectrum from 1 to 2 gigahertz (GHz). This is at the top end of the ultra high frequency (UHF) band, at the lower en ...
or lower to extend the horizon, which is very different from diffraction.
Refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
for
over-the-horizon radar
Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is ...
uses variable density in the air column above the surface of the earth to bend RF signals. An inversion layer can produce a transien
troposphere ductthat traps RF signals in a thin layer of air like a wave-guide.
Subclutter visibility
Subclutter visibility involves the maximum ratio of clutter power to target power, which is proportional to dynamic range. This determines performance in heavy weather and near the earth surface.
:
Subclutter visibility is the ratio of the smallest signal that can be detected in the presence of a larger signal.
:
A small fast-moving target reflection can be detected in the presence of larger slow-moving clutter reflections when the following is true:
:
Performance
The pulse-Doppler radar equation can be used to understand trade-offs between different design constraints, like power consumption, detection range, and microwave safety hazards. This is a very simple form of modeling that allows performance to be evaluated in a sterile environment.
The theoretical range performance is as follows.
:
where
:''R'' = distance to the target,
:''P''
t = transmitter power,
:''G''
t =
gain
Gain or GAIN may refer to:
Science and technology
* Gain (electronics), an electronics and signal processing term
* Antenna gain
* Gain (laser), the amplification involved in laser emission
* Gain (projection screens)
* Information gain in de ...
of the transmitting antenna,
:''A''
r = effective aperture (area) of the receiving antenna,
:''σ'' =
radar cross section
Radar cross-section (RCS), also called radar signature, is a measure of how detectable an object is by radar. A larger RCS indicates that an object is more easily detected.
An object reflects a limited amount of radar energy back to the source. ...
, or scattering coefficient, of the target,
:''F'' =
antenna pattern propagation factor,
:''D'' = Doppler filter size (transmit pulses in each
Fast Fourier transform
A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). Fourier analysis converts a signal from its original domain (often time or space) to a representation in th ...
),
:''K''
b =
Boltzmann's constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...
,
:''T'' = absolute temperature,
:''B'' =
receiver bandwidth (band-pass filter),
:''N'' =
noise figure Noise figure (NF) and noise factor (''F'') are figures of merit that indicate degradation of the signal-to-noise ratio (SNR) that is caused by components in a signal chain. These figures of merit are used to evaluate the performance of an amplifier ...
.
This equation is derived by combining the
radar equation
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, ...
with the
noise equation and accounting for in-band noise distribution across multiple detection filters. The value ''D'' is added to the standard radar range equation to account for both
pulse-Doppler signal processing
Pulse-Doppler signal processing is a radar and CEUS performance enhancement strategy that allows small high-speed objects to be detected in close proximity to large slow moving objects. Detection improvements on the order of 1,000,000:1 are comm ...
and
transmitter FM noise reduction.
Detection range is increased proportional to the fourth root of the number of filters for a given power consumption. Alternatively, power consumption is reduced by the number of filers for a given detection range.
Pulse-Doppler signal processing
Pulse-Doppler signal processing is a radar and CEUS performance enhancement strategy that allows small high-speed objects to be detected in close proximity to large slow moving objects. Detection improvements on the order of 1,000,000:1 are comm ...
integrates all of the energy from all of the individual reflected pulses that enter the filter. This means a
pulse-Doppler signal processing
Pulse-Doppler signal processing is a radar and CEUS performance enhancement strategy that allows small high-speed objects to be detected in close proximity to large slow moving objects. Detection improvements on the order of 1,000,000:1 are comm ...
system with 1024 elements provides 30.103 dB of improvement due to the type of signal processing that must be used with pulse-Doppler radar. The energy of all of the individual pulses from the object are added together by the filtering process.
Signal processing for a 1024-point filter improves performance by 30.103 dB, assuming compatible transmitter and antenna. This corresponds to 562% increase in maximal distance.
These improvements are the reason pulse-Doppler is essential for military and astronomy.
Aircraft tracking uses
Pulse-Doppler radar for aircraft detection has two modes.
* Scan
* Track
Scan mode involves frequency filtering, amplitude thresholding, and ambiguity resolution. Once a reflection has been
detected and
resolved, the pulse-Doppler radar automatically transitions to tracking mode for the volume of space surrounding the track.
Track mode works like a
phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
, where Doppler velocity is compared with the range movement on successive scans. Lock indicates the difference between the two measurements is below a threshold, which can only occur with an object that satisfies
Newtonian mechanics
Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows:
# A body remains at rest, or in motion ...
. Other types of electronic signals cannot produce a lock. Lock exists in no other type of radar.
The lock criterion needs to be satisfied during normal operation.
*
Lock eliminates the need for human intervention with the exception of helicopters and
electronic jamming.
Weather phenomenon obey
adiabatic process
In thermodynamics, an adiabatic process (Greek: ''adiábatos'', "impassable") is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment. Unlike an isothermal process, ...
associated with
air mass
In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to la ...
and not
Newtonian mechanics
Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows:
# A body remains at rest, or in motion ...
, so the lock criterion is not normally used for weather radar.
Pulse-Doppler signal processing
Pulse-Doppler signal processing is a radar and CEUS performance enhancement strategy that allows small high-speed objects to be detected in close proximity to large slow moving objects. Detection improvements on the order of 1,000,000:1 are comm ...
selectively excludes low-velocity reflections so that no detections occurs below a threshold velocity. This eliminates terrain, weather, biologicals, and
mechanical jamming with the exception of decoy aircraft.
The target Doppler signal from the detection is converted from
frequency domain
In physics, electronics, control systems engineering, and statistics, the frequency domain refers to the analysis of mathematical functions or signals with respect to frequency, rather than time. Put simply, a time-domain graph shows how a signa ...
back into
time domain
Time domain refers to the analysis of mathematical functions, physical signals or time series of economic or environmental data, with respect to time. In the time domain, the signal or function's value is known for all real numbers, for the cas ...
sound for the operator in track mode on some radar systems. The operator uses this sound for passive target classification, such as recognizing helicopters and electronic jamming.
Helicopters
Special consideration is required for aircraft with large moving parts because pulse-Doppler radar operates like a
phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
. Blade tips moving near the speed of sound produce the only signal that can be detected when a
helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...
is moving slow near terrain and weather.
Helicopters appears like a rapidly pulsing noise emitter except in a clear environment free from clutter. An audible signal is produced for passive identification of the type of airborne object. Microwave Doppler frequency shift produced by reflector motion falls into the audible sound range for human beings (), which is used for target classification in addition to the kinds of conventional
radar display
A radar display is an electronic device to present radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and retur ...
used for that purpose, like A-scope, B-scope, C-scope, and RHI indicator. The human ear may be able to tell the difference better than electronic equipment.
A special mode is required because the Doppler velocity feedback information must be unlinked from radial movement so that the system can transition from scan to track with no lock.
Similar techniques are required to develop track information for jamming signals and interference that cannot satisfy the lock criterion.
Multi-mode
Pulse-Doppler radar must be multi-mode to handle aircraft turning and crossing trajectory.
Once in track mode, pulse-Doppler radar must include a way to modify Doppler filtering for the volume of space surrounding a track when radial velocity falls below the minimum detection velocity. Doppler filter adjustment must be linked with a
radar track function to automatically adjust Doppler rejection speed within the volume of space surrounding the track.
Tracking will cease without this feature because the target signal will otherwise be rejected by the Doppler filter when radial velocity approaches zero because there is no change in frequency.
Multi-mode operation may also include continuous wave illumination for
semi-active radar homing
Semi-active radar homing (SARH) is a common type of missile guidance system, perhaps the most common type for longer-range Air-to-air missile, air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is ...
.
See also
*
Radar signal characteristics
A radar system uses a radio-frequency electromagnetic signal reflected from a target to determine information about that target. In any radar system, the signal transmitted and received will exhibit many of the characteristics described below.
I ...
(fundamentals of the radar signal)
*
Doppler radar
A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the f ...
(non-pulsed; used for navigation systems)
*
Weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly puls ...
(pulsed with Doppler processing)
*
Continuous-wave radar
Continuous-wave radar (CW radar) is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. Individual objects can be detected using the Doppler effect, which ...
(non-pulsed, pure Doppler processing)
*
Fm-cw radar
Continuous-wave radar (CW radar) is a type of radar system where a known stable frequency continuous wave radio energy is transmitted and then received from any reflecting objects. Individual objects can be detected using the Doppler effect, which ...
(non-pulsed, swept frequency, range and Doppler processing)
*
Aliasing
In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampled. It also often refers to the distortion or artifact that results when a ...
- the reason for ambiguous velocity estimates
*
Doppler sonography
Doppler ultrasonography is medical ultrasonography that employs the Doppler effect to perform imaging of the movement of tissues and body fluids (usually blood), and their relative velocity to the probe. By calculating the frequency shift of ...
- velocity measurements in medical ultrasound. Based on the same principle
External links
Doppler radarpresentation, which highlights the advantages of using the
autocorrelation technique
Pulse-Doppler radarhandouts from ''Introduction to Principles and Applications of Radar'' course at University of Iowa
* Modern Radar Systems by Hamish Meikle ()
* Advanced Radar Techniques and Systems edited by Gaspare Galati ()
References
Bibliography
*
{{DEFAULTSORT:Pulse-Doppler Radar
Radar
Radar signal processing
Measuring instruments
Microwave technology
Navigational equipment
Air traffic control
Science and technology during World War II
Targeting (warfare)