SUPPORT THE WORK

# GetWiki

ARTICLE SUBJECTS
news  →
unix  →
wiki  →
ARTICLE TYPES
feed  →
help  →
wiki  →
ARTICLE ORIGINS
[ temporary import ]
- the content below is remote from Wikipedia
- it has been imported raw for GetWiki
{{Other uses}}{{Use Canadian English|date=August 2016}}{{Use dmy dates|date=August 2016}}{{multiple image| align = right| direction = horizontal| width1 = 170| image1 = Radar antenna.jpgG_A_{r}sigma F^{4}}{{(4pi )}^{2}R_^{2}R_{r}^{2}}where
• Pt = transmitter power
• Gt = gain of the transmitting antenna
• Ar = effective aperture (area) of the receiving antenna; this can also be expressed as {G_rlambda^2}over{4pi}, where

* lambda = transmitted wavelength * Gr = gain of receiving antennaBOOK, Stimson, George, 1998, Introduction to Airborne Radar, SciTech Publishing Inc., 98, 978-1-891121-01-2,
• Ïƒ = radar cross section, or scattering coefficient, of the target
• F = pattern propagation factor
• Rt = distance from the transmitter to the target
• Rr = distance from the target to the receiver.
In the common case where the transmitter and the receiver are at the same location, Rt = Rr and the term RtÂ² RrÂ² can be replaced by R4, where R is the range.This yields:
P_r = {{P_t G_t A_r sigma F^4}over{{(4pi)}^2 R^4}}.
This shows that the received power declines as the fourth power of the range, which means that the received power from distant targets is relatively very small.Additional filtering and pulse integration modifies the radar equation slightly for pulse-Doppler radar performance, which can be used to increase detection range and reduce transmit power.The equation above with F = 1 is a simplification for transmission in a vacuum without interference. The propagation factor accounts for the effects of multipath and shadowing and depends on the details of the environment. In a real-world situation, pathloss effects should also be considered.

### Doppler effect

F_D = 2 times F_T times left (frac {V_R}{C} right).
Passive radar is applicable to electronic countermeasures and radio astronomy as follows:
F_D = F_T times left (frac {V_R}{C} right).
Only the radial component of the velocity is relevant. When the reflector is moving at right angle to the radar beam, it has no relative velocity. Vehicles and weather moving parallel to the radar beam produce the maximum Doppler frequency shift.When the transmit frequency (F_T) is pulsed, using a pulse repeat frequency of F_R, the resulting frequency spectrum will contain harmonic frequencies above and below F_T with a distance of F_R. As a result, the Doppler measurement is only non-ambiguous if the Doppler frequency shift is less than half of F_R, called the Nyquist frequency, since the returned frequency otherwise cannot be distinguished from shifting of a harmonic frequency above or below, thus requiring:
|F_D| < frac {F_R}{2}
Or when substituting with F_D:
|V_R| < frac {F_R times frac {C}{F_T}}{4}
As an example, a Doppler weather radar with a pulse rate of 2 kHz and transmit frequency of 1 GHz can reliably measure weather speed up to at most {{convert|150|m/s|mph|abbr=on}}, thus cannot reliably determine radial velocity of aircraft moving {{convert|1,000|m/s|mph|abbr=on}}.

### Polarization

{{further|Polarization (waves)}}In all electromagnetic radiation, the electric field is perpendicular to the direction of propagation, and the electric field direction is the polarization of the wave. For a transmitted radar signal, the polarization can be controlled to yield different effects. Radars use horizontal, vertical, linear, and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces. Random polarization returns usually indicate a fractal surface, such as rocks or soil, and are used by navigation radars.

### Limiting factors

#### Beam path and range

{{see also|Beam forming|Over-the-horizon radar}}(File:Radar-height.PNG|thumb|upright=1.35|right|Echo heights above ground)The radar beam would follow a linear path in vacuum, but it really follows a somewhat curved path in the atmosphere because of the variation of the refractive index of air, that is called the radar horizon. Even when the beam is emitted parallel to the ground, it will rise above it as the Earth curvature sinks below the horizon. Furthermore, the signal is attenuated by the medium it crosses, and the beam disperses.The maximum range of a conventional radar can be limited by a number of factors:
• Line of sight, which depends on height above ground. This means without a direct line of sight the path of the beam is blocked.
• The maximum non-ambiguous range, which is determined by the pulse repetition frequency. The maximum non-ambiguous range is the distance the pulse could travel and return before the next pulse is emitted.
• Radar sensitivity and power of the return signal as computed in the radar equation. This includes factors such as environmental conditions and the size (or radar cross section) of the target.

#### Interference

Radar systems must overcome unwanted signals in order to focus on the targets of interest. These unwanted signals may originate from internal and external sources, both passive and active. The ability of the radar system to overcome these unwanted signals defines its signal-to-noise ratio (SNR). SNR is defined as the ratio of the signal power to the noise power within the desired signal; it compares the level of a desired target signal to the level of background noise (atmospheric noise and noise generated within the receiver). The higher a system's SNR the better it is at discriminating actual targets from noise signals.

#### Clutter

* Moving target indication, which integrates successive pulses and * Doppler processing, which uses filters to separate clutter from desirable signals.

### Reduction of interference effects

Signal processing is employed in radar systems to reduce the radar interference effects. Signal processing techniques include moving target indication, Pulse-Doppler signal processing, moving target detection processors, correlation with secondary surveillance radar targets, space-time adaptive processing, and track-before-detect. Constant false alarm rate and digital terrain model processing are also used in clutter environments.

### Plot and track extraction

A Track algorithm is a radar performance enhancement strategy. Tracking algorithms provide the ability to predict future position of multiple moving objects based on the history of the individual positions being reported by sensor systems.Historical information is accumulated and used to predict future position for use with air traffic control, threat estimation, combat system doctrine, gun aiming, and missile guidance. Position data is accumulated by radar sensors over the span of a few minutes.There are four common track algorithms.WEB,weblink Fundamentals of Radar Tracking, Applied Technology Institute, yes,weblink" title="web.archive.org/web/20110824221707weblink">weblink 24 August 2011,
Radar video returns from 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. In some radar systems, or alternatively in the command and control system to which the radar is connected, a radar tracker is used to associate the sequence of plots belonging to individual targets and estimate the targets' headings and speeds.

## Engineering

• 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.
• A display processor to produce signals for human readable output devices.
• An electronic section that controls all those devices and the antenna to perform the radar scan ordered by software.
• A link to end user devices and displays.

### Antenna design

(File:Electronics Technician - Volume 7 - Figure 2-48.jpg|thumb|AS-3263/SPS-49(V) antenna. (US Navy))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 omnidirectional 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 showed 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.

#### Types of scan

• Primary Scan: A scanning technique where the main antenna aerial is moved to produce a scanning beam, examples include circular scan, sector scan, etc.
• Secondary Scan: A scanning technique where the antenna feed is moved to produce a scanning beam, examples include conical scan, unidirectional sector scan, lobe switching, etc.
• 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.
• Conical scanning: The radar beam is rotated in a small circle around the "boresight" axis, which is pointed at the target.

#### Slotted waveguide

(File:Radar antennas on USS Theodore Roosevelt SPS-64.jpg|right|thumb|Slotted waveguide antenna)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 its lower cost and less wind exposure, shipboard, airport surface, and harbour surveillance radars now use this approach in preference to a parabolic antenna.

### 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. They have been adopted in the United States by the Institute of Electrical and Electronics Engineers and internationally by the International Telecommunication Union. 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 industries, have replaced the traditional military designations with their own systems.{| class="wikitable"|+ Radar frequency bands style="background:#ccc;"!Band name!!Frequency range!!Wavelength range!!Notes

## Principles

### Illumination

Radar relies on its own transmissions rather than light from the Sun or the Moon, or from electromagnetic waves emitted by the objects themselves, such as infrared wavelengths (heat). This process of directing artificial radio waves towards objects is called illumination, although radio waves are invisible to the human eye or optical cameras.

### Reflection

The power Pr returning to the receiving antenna is given by the equation:
P_{r}=frac{P_
VHF>|Very long range, ground penetrating; 'very high frequency'
|'P' for 'previous', applied retrospectively to early radar systems; essentially HF + VHF
UHF>Ballistic Missile Early Warning System>ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency'
L band>L1â€“2 Gigahertz>centimetre>cmLong range air traffic control and surveillance; 'L' for 'long'
S band>S2â€“4 GHz7.5â€“15 cmModerate range surveillance, Terminal air traffic control, long-range weather, marine radar; 'S' for 'short'
C band (IEEE)>C4â€“8 GHz3.75â€“7.5 cmSatellite transponders; a compromise (hence 'C') between X and S bands; weather; long range tracking
X band>X8â€“12 GHz2.5â€“3.75 cmMissile guidance, marine radar, weather, medium-resolution mapping and ground surveillance; in the United States the narrow range 10.525 GHz Â±25 MHz is used for airport radar; short range tracking. Named X band because the frequency was a secret during WW2.
Ku band>|High-resolution, also used for satellite transponders, frequency under K band (hence 'u')
K band (IEEE)>K18â€“24 GHz1.11â€“1.67 cmFrom German language kurz, meaning 'short'; limited use due to absorption by water vapor>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 Â± 0.100 GHz.
Ka band>Ka24â€“40 GHz0.75â€“1.11 cmMapping, 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 Â± 0.100 GHz.
millimetre>mm Millimetre band, subdivided as below. The frequency ranges depend 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.
V band>V40â€“75 GHz4.0â€“7.5 mm Very strongly absorbed by atmospheric oxygen, which resonates at 60 GHz.
W band>W75â€“110 GHz2.7â€“4.0 mmUsed as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.

Modulators act to provide the waveform of the RF-pulse. There are two different radar modulator designs:
• High voltage switch for non-coherent keyed power-oscillatorsWEB,weblink Radar Modulator, radartutorial.eu, These modulators consist of a high voltage pulse generator formed from a high voltage supply, a pulse forming network, and a high voltage switch such as a thyratron. They generate short pulses of power to feed, e.g., the magnetron, a special type of vacuum tube that converts DC (usually pulsed) into microwaves. 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.
• Hybrid mixers,WEB,weblink Fully Coherent Radar, radartutorial.eu, fed by a waveform generator and an exciter for a complex but coherent waveform. This waveform can be generated by low power/low-voltage input signals. In this case the radar transmitter must be a power-amplifier, e.g., a klystron or a solid state transmitter. In this way, the transmitted pulse is intrapulse-modulated and the radar receiver must use pulse compression techniques.

Coherent microwave amplifiers operating above 1,000 watts microwave output, like travelling wave tubes and klystrons, require liquid coolant. The electron beam must contain 5 to 10 times more power than the microwave output, which can produce enough heat to generate plasma. This plasma flows from the collector toward the cathode. The same magnetic focusing that guides the electron beam forces the plasma into the path of the electron beam but flowing in the opposite direction. This introduces FM modulation which degrades Doppler performance. To prevent this, liquid coolant with minimum pressure and flow rate is required, and deionized water is normally used in most high power surface radar systems that utilize Doppler processing.WEB,weblink J.L. de Segovia, Physics of Outgassing, Instituto de FÃ­sica Aplicada, CETEF â€œL. Torres Quevedoâ€, CSIC, Madrid, Spain, 2012-08-12, Coolanol (silicate ester) was used in several military radars in the 1970s. However, it is hygroscopic, leading to hydrolysis and formation of highly flammable alcohol. The loss of a U.S. Navy aircraft in 1978 was attributed to a silicate ester fire.WEB,weblink Stropki, Michael A., 1992, Polyalphaolefins: A New Improved Cost Effective Aircraft Radar Coolant, Aeronautical Research Laboratory, Defense Science and Technology Organisation, Department of Defense, Melbourne, Australia, 2010-03-18, Coolanol is also expensive and toxic. The U.S. Navy has instituted a program named Pollution Prevention (P2) to eliminate or reduce the volume and toxicity of waste, air emissions, and effluent discharges. Because of this, Coolanol is used less often today.

## Regulations

Definitions

Application

Hardware

Similar detection and ranging methods

{{Reflist}}

## Bibliography

### General

• BOOK, Reg Batt, The radar army: winning the war of the airwaves, 1991, 978-0-7090-4508-3,
• BOOK, E.G. Bowen, Radar Days, 1998-01-01, Taylor & Francis, 978-0-7503-0586-0,
• BOOK, Michael Bragg, RDF1: The Location of Aircraft by Radio Methods 1935â€“1945, 2002-05-01, Twayne Publishers, 978-0-9531544-0-1,
• BOOK, Louis Brown, A radar history of World War II: technical and military imperatives, 1999, Taylor & Francis, 978-0-7503-0659-1,
• BOOK, Robert Buderi, The invention that changed the world: how a small group of radar pioneers won the Second World War and launched a technological revolution, 1996, 978-0-684-81021-8,
• Burch, David F., Radar For Mariners, McGraw Hill, 2005, {{ISBN|978-0-07-139867-1}}.
• BOOK, Ian Goult, Secret Location: A witness to the Birth of Radar and its Postwar Influence, 2011, History Press, 978-0-7524-5776-5,
• BOOK, Peter S. Hall, Radar, March 1991, Potomac Books Inc, 978-0-08-037711-7,
• BOOK, Derek Howse, Naval Radar Trust, Radar at sea: the royal Navy in World War 2, February 1993, Naval Institute Press, 978-1-55750-704-4,
• BOOK, R.V. Jones, Most Secret War, August 1998, Wordsworth Editions Ltd, 978-1-85326-699-7,
• Kaiser, Gerald, Chapter 10 in "A Friendly Guide to Wavelets", Birkhauser, Boston, 1994.
• Kouemou, Guy (Ed.): Radar Technology. InTech, 2010, {{ISBN|978-953-307-029-2}}, (Radar Technology - Free Open Access Book | InTechOpen).
• BOOK, Colin Latham, Anne Stobbs, Radar: A Wartime Miracle, January 1997, Sutton Pub Ltd, 978-0-7509-1643-1,
• BOOK, FranÃ§ois Le Chevalier, Principles of radar and sonar signal processing, 2002, Artech House Publishers, 978-1-58053-338-6,
• BOOK, David Pritchard, The radar war: Germany's pioneering achievement 1904-45, August 1989, Harpercollins, 978-1-85260-246-8,
• BOOK, Merrill Ivan Skolnik, Introduction to radar systems, 1980-12-01, 978-0-07-066572-9,
• BOOK, Merrill Ivan Skolnik, Radar handbook, 1990, McGraw-Hill Professional, 978-0-07-057913-2,
• BOOK, George W. Stimson, Introduction to airborne radar, 1998, SciTech Publishing, 978-1-891121-01-2,
• Younghusband, Eileen., Not an Ordinary Life. How Changing Times Brought Historical Events into my Life, Cardiff Centre for Lifelong Learning, Cardiff, 2009., {{ISBN|978-0-9561156-9-0}} (Pages 36â€“67 contain the experiences of a WAAF radar plotter in WWII.)
• Younghusband, Eileen. One Woman's War. Cardiff. Candy Jar Books. 2011. {{ISBN|978-0-9566826-2-8}}
• BOOK, David Zimmerman, Britain's shield: radar and the defeat of the Luftwaffe, February 2001, Sutton Pub Ltd, 978-0-7509-1799-5,

• Skolnik, M.I. Radar Handbook. McGraw-Hill, 1970.
• Hao He, Jian Li, and Petre Stoica. Waveform design for active sensing systems: a computational approach. Cambridge University Press, 2012.
• Solomon W. Golomb, and Guang Gong. Signal design for good correlation: for wireless communication, cryptography, and radar. Cambridge University Press, 2005.
• M. Soltanalian. Signal Design for Active Sensing and Communications. Uppsala Dissertations from the Faculty of Science and Technology (printed by Elanders Sverige AB), 2014.
• Fulvio Gini, Antonio De Maio, and Lee Patton, eds. Waveform design and diversity for advanced radar systems. Institution of engineering and technology, 2012.
• E. Fishler, A. Haimovich, R. Blum, D. Chizhik, L. Cimini, R. Valenzuela, "MIMO radar: an idea whose time has come," IEEE Radar Conference, 2004.
• Mark R. Bell, "Information theory and radar waveform design." IEEE Transactions on Information Theory, 39.5 (1993): 1578â€“1597.
• Robert Calderbank, S. Howard, and Bill Moran. "Waveform diversity in radar signal processing." IEEE Signal Processing Magazine, 26.1 (2009): 32â€“41.

{{Wiktionary}}{{Commons}}
{{Prone to spam|date=November 2014}}{{Z148}}{{Authority control}}

- content above as imported from Wikipedia
- "radar" does not exist on GetWiki (yet)
- time: 10:29pm EDT - Sun, Apr 21 2019
[ this remote article is provided by Wikipedia ]
LATEST EDITS [ see all ]
GETWIKI 09 MAY 2016
GETWIKI 18 OCT 2015
M.R.M. Parrott
Biographies
GETWIKI 20 AUG 2014
GETWIKI 19 AUG 2014
GETWIKI 18 AUG 2014
Wikinfo
Culture
CONNECT