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For other uses, see Radar (disambiguation).
This long-range radar antenna, known as ALTAIR, is used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on the Kwajalein atoll.[1]

Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for Radio Detection and Ranging. The term has since entered the English language as a standard word, radar, losing the capitalization in the process. Radar was originally called RDF (Radio Direction Finder) in Britain.

A radar system has a transmitter that emits radio waves, that are reflected by the target and detected by a receiver, typically in the same location as the transmitter. Although the radio signal returned is usually very weak, radio signals can easily be amplified. This enables a radar to detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect. Radar is used in many contexts, including meteorological detection of precipitation, measuring ocean surface waves, air traffic control, police detection of speeding traffic, and by the military.

History

Main article: History of radar

Several inventors, scientists, and engineers contributed to the development of radar. The first to use radio waves to detect "the presence of distant metallic objects via radio waves" was Christian Hülsmeyer, who in 1904 demonstrated the feasibility of detecting the presence of a ship in dense fog, but not its distance.[2][3] He received Reichspatent Nr. 165546[4] for his pre-radar device in April 1904, and later patent 169154[5] for a related amendment for ranging. He also received a patent [6] in England for his telemobiloscope on September 22 1904.[2][7]

Nikola Tesla, in August 1917, first established principles regarding frequency and power level for the first primitive radar units.[8] He stated, "[...] by their [standing electromagnetic waves] use we may produce at will, from a sending station, an electrical effect in any particular region of the globe; [with which] we may determine the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the same, or its speed."

Before the Second World War, developments by the Americans (Dr. Robert M. Page tested the first monopulse radar in 1934),[9] the Germans, the French (French Patent n° 788795 in 1934)[10][11] and mainly the British who were the first to fully exploit it as a defence against aircraft attack (British Patent GB593017 by Robert Watson-Watt in 1935)[11][12][13] led to the first real radars. Hungarian Zoltán Bay produced a working model by 1936 at the Tungsram laboratory in the same vein.

In 1934, Émile Girardeau, working with the first French radar systems, stated he was building radar systems "conceived according to the principles stated by Tesla". [1]

The war precipitated research to find better resolution, more portability and more features for the new defence technology. Post-war years have seen the use of radar in fields as diverse as air traffic control, weather monitoring, astrometry and road speed control.

I love radars

Radar signal processing

Distance measurement

Transit time

Pulse radar

One way to measure the distance to an object is to transmit a short pulse of radio signal (electromagnetic radiation), and measure the time it takes for the reflection to return. The distance is one-half the product of round trip time (because the signal has to travel to the target and then back to the receiver) and the speed of the signal. Since radio waves travel at the speed of light (186,000 miles per second or 300,000,000 meters per second), accurate distance measurement requires high-performance electronics.

In most cases, the receiver does not detect the return while the signal is being transmitted. Through the use of a device called a duplexer, the radar switches between transmitting and receiving at a predetermined rate. The minimum range is calculated by measuring the length of the pulse multiplied by the speed of light, divided by two. In order to detect closer targets one must use a shorter pulse length.

A similar effect imposes a maximum range as well. If the return from the target comes in when the next pulse is being sent out, once again the receiver cannot tell the difference. In order to maximize range, one wants to use longer times between pulses, or commonly referred to as a pulse repetition time (PRT).

These two effects tend to be at odds with each other, and it is not easy to combine both good short range and good long range in a single radar. This is because the short pulses needed for a good minimum range broadcast have less total energy, making the returns much smaller and the target harder to detect. This could be offset by using more pulses, but this would shorten the maximum range again. So each radar uses a particular type of signal. Long-range radars tend to use long pulses with long delays between them, and short range radars use smaller pulses with less time between them. This pattern of pulses and pauses is known as the pulse repetition frequency (or PRF), and is one of the main ways to characterize a radar. As electronics have improved many radars now can change their PRF thereby changing their range. The newest radars actually fire 2 pulses during one cell. One for short range (~6 miles) and a separate signal for longer ranges (~60 miles).

The distance resolution and the characteristics of the received signal as compared to noise depends heavily on the shape of the pulse. The pulse is often modulated to achieve better performance thanks to a technique known as pulse compression.

Frequency modulation

Another form of distance measuring radar is based on frequency modulation. Frequency comparison between two signals is considerably more accurate, even with older electronics, than timing the signal. By changing the frequency of the returned signal and comparing that with the original, the difference can be easily measured.

This technique can be used in continuous wave radar, and is often found in aircraft radar altimeters. In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a sine wave or sawtooth pattern at audio frequencies. The signal is then sent out from one antenna and received on another, typically located on the bottom of the aircraft, and the signal can be continuously compared using a simple beat frequency modulator that produces an audio frequency tone from the returned signal and a portion of the transmitted signal.

Since the signal frequency is changing, by the time the signal returns to the aircraft the broadcast has shifted to some other frequency. The amount of that shift is greater over longer times, so greater frequency differences mean a longer distance, the exact amount being the "ramp speed" selected by the electronics. The amount of shift is therefore directly related to the distance traveled, and can be displayed on an instrument. This signal processing is similar to that used in speed detecting Doppler radar. Example systems using this approach are AZUSA, MISTRAM, and UDOP.

A further advantage is that the radar can operate effectively at relatively low frequencies, comparable to that used by UHF television. This was important in the early development of this type when high frequency signal generation was difficult or expensive.

Speed measurement

Speed is the change in distance to an object with respect to time. Thus the existing system for measuring distance, combined with a memory capacity to see where the target last was, is enough to measure speed. At one time the memory consisted of a user making grease-pencil marks on the radar screen, and then calculating the speed using a slide rule. Modern radar systems perform the equivalent operation faster and more accurately using computers.

However, if the transmitter's output is coherent (phase synchronized), there is another effect that can be used to make almost instant speed measurements (no memory is required), known as the Doppler effect. Most modern radar systems use this principle in the pulse-doppler radar system. Return signals from targets are shifted away from this base frequency via the Doppler effect enabling the calculation of the speed of the object relative to the radar. The Doppler effect is only able to determine the relative speed of the target along the line of sight from the radar to the target. Any component of target velocity perpendicular to the line of sight cannot be determined by using the Doppler effect alone, but it can be determined by tracking the target's azimuth over time. Additional information of the nature of the Doppler returns may be found in the radar signal characteristics article.

It is also possible to make a radar without any pulsing, known as a continuous-wave radar (CW radar), by sending out a very pure signal of a known frequency. CW radar is ideal for determining the radial component of a target's velocity, but it cannot determine the target's range. CW radar is typically used by traffic enforcement to measure vehicle speed quickly and accurately where range is not important.

Reduction of interference effects

Signal processing is employed in radar systems to reduce the interference effects. Signal processing techniques include moving target indication (MTI), pulse doppler, moving target detection (MTD) processors, correlation with secondary surveillance radar (SSR) targets and space-time adaptive processing (STAP). Constant false alarm rate (CFAR) and digital terrain model (DTM) processing are also used in clutter environments.

Plot And Track Extraction

Radar video returns on aircraft can be subjected to a plot extraction process whereby spurious and interfering signals are discarded. A sequence of target returns can be monitored through a device known as a plot extractor. The non relevant real time returns can be removed from the displayed information and a single plot displayed. A sequence of plots can then be monitored and a 'track' formed, thus easing the identification of a genuine aircraft target through unwanted and non relevant radar returns.

Radar engineering

Radar components

A radar has different components:

  • A transmitter that generates the radio signal with an oscillator such as a klystron or a magnetron and controls its duration by a modulator.
  • A waveguide that links the transmitter and the antenna.
  • A duplexer that serves as a switch between the antenna and the transmitter or the receiver for the signal when the antenna is used in both situations.
  • A receiver. Knowing the shape of the desired received signal (a pulse), an optimal receiver can be designed using a matched filter.
  • An electronic section that controls all those devices and the antenna to perform the radar scan ordered by a software.
  • A link to end users.

Antenna design

Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.

Early systems tended to use omni-directional broadcast antennas, with directional receiver antennas which were pointed in various directions. For instance the first system to be deployed, Chain Home, used two straight antennas at right angles for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by rotating the antenna so one display showed a maximum while the other shows a minimum.

One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is a small part of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional.

Parabolic reflector

More modern systems use a steerable parabolic "dish" to create a tight broadcast beam, typically using the same dish as the receiver. Such systems often combine two radar frequencies in the same antenna in order to allow automatic steering, or radar lock.

Types of scan

  1. Primary Scan: A scanning technique where the main antenna aerial is moved to produce a scanning beam, examples include circular scan, sector scan etc
  2. Secondary Scan: A scanning technique where the antenna feed is moved to produce a scanning beam, example include conical scan, unidirectional sector scan, lobe switching etc.
  3. Palmer Scan: A scanning technique that produces a scanning beam by moving the main antenna and its feed. A Palmer Scan is a combination of a Primary Scan and a Secondary Scan.
Phased array: Not all radar antennas must rotate to scan the sky.

Slotted waveguide

Main article: Slotted waveguide

Applied similarly to the parabolic reflector the slotted waveguide is moved mechanically to scan and is particularly suitable for non-tracking surface scan systems, where the vertical pattern may remain constant. Owing to lower cost and less wind exposure, shipboard, airport surface, and harbour surveillance radars now use this in preference to the parabolic antenna.

Phased array

Main article: Phased array

Another method of steering is used in a phased array radar. This uses an array of similar aerials suitably spaced, the phase of the signal to each individual aerial being controlled so that the signal is reinforced in the desired direction and cancels in other directions. If the individual aerials are in one plane and the signal is fed to each aerial in phase with all others then the signal will reinforce in a direction perpendicular to that plane. By altering the relative phase of the signal fed to each aerial the direction of the beam can be moved because the direction of constructive interference will move. Because phased array radars require no physical movement the beam can scan at thousands of degrees per second, fast enough to irradiate and track many individual targets, and still run a wide-ranging search periodically. By simply turning some of the antennas on or off, the beam can be spread for searching, narrowed for tracking, or even split into two or more virtual radars. However, the beam cannot be effectively steered at small angles to the plane of the array, so for full coverage multiple arrays are required, typically disposed on the faces of a triangular pyramid (see picture).

Phased array radars have been in use since the earliest years of radar use in World War II, but limitations of the electronics led to fairly poor accuracy. Phased array radars were originally used for missile defense. They are the heart of the ship-borne Aegis combat system, and the Patriot Missile System, and are increasingly used in other areas because the lack of moving parts makes them more reliable, and sometimes permits a much larger effective antenna, useful in fighter aircraft applications that offer only confined space for mechanical scanning.

As the price of electronics has fallen, phased array radars have become more and more common. Almost all modern military radar systems are based on phased arrays, where the small additional cost is far offset by the improved reliability of a system with no moving parts. Traditional moving-antenna designs are still widely used in roles where cost is a significant factor such as air traffic surveillance, weather radars and similar systems.

Phased array radars are also valued for use in aircraft, since they can track multiple targets. The first aircraft to use a phased array radar is the B-1B Lancer. The first aircraft fighter to use phased array radar was the Mikoyan MiG-31. The MiG-31M's SBI-16 Zaslon phased array radar is considered to be the world's most powerful fighter radar [2]. Phased-array interferometry or, aperture synthesis techniques, using an array of separate dishes that are phased into a single effective aperture, are not typically used for radar applications, although they are widely used in radio astronomy. Because of the Thinned array curse, such arrays of multiple apertures, when used in transmitters, result in narrow beams at the expense of reducing the total power transmitted to the target. In principle, such techniques used could increase the spatial resolution, but the lower power means that this is generally not effective. Aperture synthesis by post-processing of motion data from a single moving source, on the other hand, is widely used in space and airborne radar systems (see Synthetic aperture radar).

Frequency bands

The traditional band names originated as code-names during World War II and are still in military and aviation use throughout the world in the 21st century. They have been adopted in the United States by the IEEE, and internationally by the ITU. Most countries have additional regulations to control which parts of each band are available for civilian or military use.

Other users of the radio spectrum, such as the broadcasting and electronic countermeasures (ECM) industries, have replaced the traditional military designations with their own systems.

Radar frequency bands
Band Name Frequency Range Wavelength Range Notes
HF 3–30 MHz 10–100 m coastal radar systems, over-the-horizon radar (OTH) radars; 'high frequency'
P < 300 MHz 1 m+ 'P' for 'previous', applied retrospectively to early radar systems
VHF 50–330 MHz 0.9–6 m very long range, ground penetrating; 'very high frequency'
UHF 300–1000 MHz 0.3–1 m very long range (e.g. ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency'
L 1–2 GHz 15–30 cm long range air traffic control and surveillance; 'L' for 'long'
S 2–4 GHz 7.5–15 cm terminal air traffic control, long-range weather, marine radar; 'S' for 'short'
C 4–8 GHz 3.75–7.5 cm Satellite transponders; a compromise (hence 'C') between X and S bands; weather
X 8–12 GHz 2.5–3.75 cm missile guidance, marine radar, weather, medium-resolution mapping and ground surveillance; in the USA the narrow range 10.525 GHz ±25 MHz is used for airport radar. Named X band because the frequency was a secret during WW2.
Ku 12–18 GHz 1.67–2.5 cm high-resolution mapping, satellite altimetry; frequency just under K band (hence 'u')
K 18–27 GHz 1.11–1.67 cm from German kurz, meaning 'short'; limited use due to absorption by water vapour, so Ku and Ka were used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band radar guns operate at 24.150 ± 0.100 GHz.
Ka 27–40 GHz 0.75–1.11 cm mapping, short range, airport surveillance; frequency just above K band (hence 'a') Photo radar, used to trigger cameras which take pictures of license plates of cars running red lights, operates at 34.300 ± 0.100 GHz.
mm 40–300 GHz 7.5 mm – 1 mm millimetre band, subdivided as below. The letter designators appear to be random, and the frequency ranges dependent on waveguide size. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.
Q 40–60 GHz 7.5 mm – 5 mm Used for Military communication.
V 50–75 GHz 6.0–4 mm Very strongly absorbed by the atmosphere.
E 60–90 GHz 6.0–3.33 mm
W 75–110 GHz 2.7 – 4.0 mm used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.

Radar modulators

Modulators, also called pulse forming networks or line (PFNs) act to provide the short pulses of power to the magnetron. This technology is known as Pulsed power. In this way, the transmitted pulse of RF radiation is kept to a defined, and usually, very short duration. Modulators consist of a high voltage pulse generator formed from a HV supply, and a high voltage switch such as a thyratron.

A klystron tube may also be used as a modulator because it is an amplifier, so it can be modulated by its low power input signal.

Radar coolant

Coolanol and PAO (poly-alpha olefin) are the two main coolants used to cool airborne radar equipment today.[citation needed]

The U.S. Navy has instituted a program named Pollution Prevention (P2) to reduce or eliminate the volume and toxicity of waste, air emissions, and effluent discharges. Because of this Coolanol is used less often today.

PAO is a synthetic lubricant composition is a blend of a polyol ester admixed with effective amounts of an antioxidant, yellow metal pacifier and rust inhibitors. The polyol ester blend includes a major proportion of poly(neopentyl polyol) ester blend formed by reacting poly(pentaerythritol) partial esters with at least one C7 to C12 carboxylic acid mixed with an ester formed by reacting a polyol having at least two hydroxyl groups and at least one C8-C10 carboxylic acid. Preferably, the acids are linear and avoid those which can cause odours during use. Effective additives include secondary arylamine antioxidants, triazole derivative yellow metal pacifier and an amino acid derivative and substituted primary and secondary amine and/or diamine rust inhibitor.

A synthetic coolant/lubricant composition, comprising an ester mixture of 50 to 80 weight percent of poly(neopentyl polyol) ester formed by reacting a poly(neopentyl polyol) partial ester and at least one linear monocarboxylic acid having from 6 to 12 carbon atoms, and 20 to 50 weight percent of a polyol ester formed by reacting a polyol having 5 to 8 carbon atoms and at least two hydroxyl groups with at least one linear monocarboxylic acid having from 7 to 12 carbon atoms, the weight percents based on the total weight of the composition.

Radar functions and roles

Surface search radar display commonly found on ships

Detection and search radars

Threat radars

Missile guidance systems

Battlefield and reconnaissance radar

Military map marking symbol Radar as of NATO standard APP-6a

Air Traffic Control and navigation

Air traffic control radar at London Heathrow Airport

Space and range instrumentation radar systems

  • Space (SP) Tracking Systems
  • Range Instrumentation (RI) Systems
  • Video Relay/Downlink Systems
  • Space-Based Radar

Weather-sensing Radar systems

  • Storm front reflectivities on a Weather radar screen (NOAA)
    Storm front reflectivities on a Weather radar screen (NOAA)
  • Wind profiling radar
    Wind profiling radar

Radars for biological research

Through The Wall Radar Systems

Radar systems which operate using Ultra Wideband technology can sense a human behind walls. This is possible since the reflective characteristics of humans are generally greater than those of the typical materials used in construction. However, since humans reflect far less radar energy than metal does, these systems require sophisticated technology to isolate human targets and moreover to process any sort of detailed image.

See also

Template:Nautical portal

Notes

  1. ^ Ronald Reagan Test Site on the Kwajalein atoll
  2. ^ a b Christian Hülsmeyer by Radar World
  3. ^ (German) Christian Hülsmeyer Biografie
  4. ^ DE 165546  Verfahren, um metallische Gegenstände mittels elektrischer Wellen einem Beobachter zu melden.
  5. ^ DE 169154  Verfahren zur Bestimmung der Entfernung von metallischen Gegenständen (Schiffen o. dgl.), deren Gegenwart durch das Verfahren nach Patent 16556 festgestellt wird.
  6. ^ GB 13170  Telemobiloscope
  7. ^ (German) 100. Jahre Radar Improve­ment in Hertzian-wave Projecting and Receiving Apparatus for Locating the Position of Distant Metal Objects in 100 years of radar a German publication
  8. ^ The Electrical Experimenter, 1917
  9. ^ Goebel, Greg (2007-01-01). "The Wizard War: WW2 & The Origins Of Radar, Chapter 1: The British Invention of Radar". Retrieved 2007-03-24.
  10. ^ FR 788795  Nouveau système de repérage d'obstacles et ses applications
  11. ^ a b Template:Fr icon Copy of Patents for the invention of radar on www.radar-france.fr
  12. ^ British man first to patent radar official site of the Patent Office
  13. ^ GB 593017  Improvements in or relating to wireless systems

References

Further reading

  • Buderi, Robert, The invention that changed the world: the story of radar from war to peace, Simon & Schuster, 1996. ISBN 0-349-11068-9 ISBN 0-316-90715-4
  • Hall, P.S., T.K. Garland-Collins, R.S. Picton and R.G. Lee, Radar, Brassey's (UK) Ltd., 1991, Land Warfare Series: Vol 9, ISBN 0-08-037711-4.
  • Kaiser, Gerald, Chapter 10 in "A Friendly Guide to Wavelets", Birkhauser, Boston, 1994.
  • Jones, R.V., Most Secret War, ISBN 1-85326-699-X. R.V. Jones' account of his part in British Scientific Intelligence between 1939 and 1945, working to anticipate the German's radar, radio navigation and V1/V2 developments.
  • Le Chevalier, François, Principles of Radar and Sonar Signal Processing, Artech House, Boston, London, 2002. ISBN 1-58053-338-8.
  • Skolnik, Merrill I., Introduction to Radar Systems, McGraw-Hill (1st ed., 1962; 2nd ed., 1980; 3rd ed., 2001), ISBN 0-07-066572-9. The de-facto radar introduction bible.
  • Skolnik, Merrill I., Radar Handbook. ISBN 0-07-057913-X widely used in the US since the 1970s. New 3rd Edition, February 2008, ISBN 0-07-148547-3; 978-0-07-148547-0
  • Stimson, George W., Introduction to Airborne Radar, SciTech Publishing (2nd edition, 1998), ISBN 1-891121-01-4. Written for the non-specialist. The first half of the book on radar fundamentals is also applicable to ground- and sea-based radar.
  • Bragg, Michael., RDF1 The Location of Aircraft by Radio Methods 1935–1945, Hawkhead Publishing, Paisley 1988 ISBN 0-9531544-0-8 The history of ground radar in the UK during World War II
  • Latham, Colin & Stobbs, Anne., Radar A Wartime Miracle, Sutton Publishing Ltd, Stroud 1996 ISBN 0-7509-1643-5 A history of radar in the UK during World War II told by the men and women who worked on it.
  • Pritchard, David., The Radar War Germany's Pioneering Achievement 1904–1945 Patrick Stephens Ltd, Wellingborough 1989., ISBN 1-85260-246-5
  • Zimmerman, David., Britain's Shield Radar and the Defeat of the Luftwaffe, Sutton Publishing Ltd, Stroud, 2001., ISBN 0-7509-1799-7
  • Brown, Louis., A Radar History of World War II, Institute of Physics Publishing, Bristol, 1999., ISBN 0-7503-0659-9
  • Bowen, E.G., Radar Days, Institute of Physics Publishing, Bristol, 1987., ISBN 0-7503-0586-X
  • Howse, Derek, Radar At Sea The Royal Navy in World War 2, Naval Institute Press, Annapolis, Maryland, USA, 1993, ISBN 1-55750-704-X
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