Phased array
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''This article is about general theory and electromagnetic phased array.''
''For the ultrasonic and medical imaging application, see [[phased array ultrasonics]].''
[[Image:SSPARS radar, Clear AFB.JPG|thumb|[[PAVE PAWS]] [[Clear AFS]]]]
[[Image:PAVE PAWS Radar Clear AFS Alaska.jpg|thumb|[[PAVE PAWS]] phased array radar in Alaska]]
[[Image:Spliced.fylingdales.jpg|thumb|[[RAF Fylingdales]]]]
[[Image:Cobradane.jpg|thumb|[[Cobra Dane]]]]
[[Image:NuclearWarningSystemMap.png|thumb|[[PAVE PAWS]]]]
[[Image:Mammut Hoarding radar illustration.png|thumb|[[Mammut]] phased array radar [[WW II]]]]
In [[wave theory]], a '''phased array''' is a group of [[antenna (electronics)|antenna]]s in which the relative phases of the respective [[signaling (telecommunication)|signal]]s feeding the antennas are varied in such a way that the effective [[radiation pattern]] of the [[array]] is reinforced in a desired direction and suppressed in undesired directions.<ref>[[Federal Standard 1037C]]. [http://glossary.its.bldrdoc.gov/fs-1037/dir-027/_3979.htm Definition of Phased Array]. Accessed [[April 27]], [[2006]].</ref> This technology was originally developed by future [[Nobel Prize|Nobel]] Laureate [[Luis Alvarez]] during [[World War II]] as a rapidly-steerable radar system for "[[ground-controlled approach]]", a system to aid in the landing of airplanes in England. GEMA in Germany built at the same time the PESA Mammut 1.<ref>http://www.100jahreradar.de/index.html?/gdr_5_deutschefunkmesstechnikim2wk.html Mamut1 first early waring PESA Radar</ref> It was later adapted for [[radio astronomy]], leading to [[Nobel Prize for Physics|Physics Nobel Prizes]] for [[Antony Hewish]] and [[Martin Ryle]] after several large phased arrays were developed at [[University of Cambridge|Cambridge University]]. The design is also used in [[radar]], and is generalized in [[interferometry|interferometric]] radio antennas. Recently, [[DARPA]] researchers announced a 16 element phased array integrated with all necessary circuits to send at 30-50 GHz on a single silicon chip for military purposes.<ref>[http://ucsdnews.ucsd.edu/newsrel/science/10-07PhasedArrayChipDK-L.asp World’s Most Complex Silicon Phased Array Chip Developed at UC San Diego] in UCSD News (reviewed 02. November 2007)</ref>
An '''antenna array''' is a multiple of active antennas coupled to a common source or load to produce a directive radiation pattern. Usually the spatial relationship also contributes to the directivity of the antenna. Use of the term "active antennas" is intended to describe elements whose energy output is modified due to the presence of a source of energy in the element (other than the mere signal energy which passes through the circuit) or an element in which the energy output from a source of energy is controlled by the signal input.
==Usage==
The relative [[amplitude]]s of — and constructive and destructive [[interference]] effects among — the signals radiated by the individual antennas determine the effective [[radiation pattern]] of the array. A phased array may be used to point a fixed radiation pattern, or to [[scan]] rapidly in [[azimuth]] or elevation. Simultaneous electrical scanning in both azimuth and elevation was first demonstrated in a phased array antenna at Hughes Aircraft Company, Culver City, CA, in 1957 (see Joseph Spradley, “A Volumetric Electrically Scanned Two-Dimensional Microwave Antenna Array,” IRE National Convention Record, Part I - Antennas and Propagation; Microwaves, New York: The Institute of Radio Engineers, 1958, 204-212). When phased arrays are used in sonar, it is called [[beamforming]].
The phased array is used for instance in [[optical communication]] as a [[wavelength]]-selective [[demux|splitter]].
For information about active as well as passive phased array radars, see also [[active electronically scanned array]].
===Broadcasting===
In [[broadcast engineering]], phased arrays are required to be used by many [[AM broadcasting|AM broadcast]] [[radio stations]] to enhance [[signal strength]] and therefore coverage in the city of license, while minimizing interference to other areas. Due to the differences between daytime and nighttime [[ionosphere|ionospheric]] [[radio propagation|propagation]] at [[mediumwave]] frequencies, it is common for AM broadcast stations to change between day ([[groundwave]]) and night ([[skywave]]) radiation patterns by switching the [[phase (waves)|phase]] and power levels supplied to the individual antenna elements ([[mast radiator]]s) daily at [[sunrise]] and [[sunset]]. More modest phased array longwire antenna systems may be employed by private radio enthusiasts to receive longwave, mediumwave (AM) and shortwave radio broadcasts from great distances.
On [[VHF]], phased arrays are used extensively for [[FM broadcasting]]. These greatly increase the [[antenna gain]], magnifying the emitted RF energy toward the [[horizon]], which in turn greatly increases a station's [[broadcast range]]. In these situations, the distance to each element from the transmitter is identical, or is one (or other [[integer]]) wavelength apart. Phasing the array such that the lower elements are slightly delayed (by making the distance to them longer) causes a downward [[beam tilt]], which is very useful if the antenna is quite high on a [[radio tower]].
Other phasing adjustments can increase the downward radiation in the [[far field]] without tiliting the main [[lobe]], creating [[null fill]] to compensate for extremely high [[mountain]]top locations, or decrease it in the [[near field]], to prevent excessive exposure to those workers or even nearby homeowners on the ground. The latter effect is also achieved by half-wave spacing – inserting additional elements halfway between existing elements with full-wave spacing. This phasing achieves roughly the same horizontal gain as the full-wave spacing; that is, a five-element full-wave-spaced array equals a nine- or ten-element half-wave-spaced array.
===Naval usage===
[[Image:USSMasonDDG-87.jpg|thumb|upright|Port and starboard octagonal panels are the phased array radar, AN/SPY-1D, on the [[USS Mason (DDG-87)|''USS Mason'' (DDG-87)]].]]
Phased array radar systems are also used by [[warships]] of several navies including the Chinese, Japanese, Norwegian, Spanish, Korean and United States' navies in the [[Aegis combat system]]. Phased array radars allow a warship to use one [[radar]] system for surface detection and tracking (finding ships), air detection and tracking (finding aircraft and missiles) and missile uplink capabilities. Prior to using these systems, each [[surface-to-air missile]] in flight required a dedicated [[fire-control radar]], which meant that ships could only engage a small number of simultaneous targets. Phased array systems can be used to control missiles during the mid-course phase of the missile's flight. During the terminal portion of the flight, [[continuous-wave]] fire control directors provide the final guidance to the target. Because the radar beam is electronically steered, phased array systems can direct radar beams fast enough to maintain a [[fire-control system|fire control quality]] track on many targets simultaneously while also controlling several in-flight missiles. The [[AN/SPY-1]] phased array radar, part of the [[Aegis combat system]] deployed on modern U.S. [[cruisers]] and [[destroyers]], "is able to perform search, track and missile guidance functions simultaneously with a capability of over 100 targets."<ref>{{cite web | last = | first = | authorlink = http://www.janes.com/company/about/| coauthors = | title = AEGIS Weapon System MK-7| work = | publisher = [[Jane's Information Group]] | date = 2001-04-25| url = http://www.janes.com/defence/naval_forces/news/misc/aegis010425.shtml| format = | doi = | accessdate = 2006-08-10 }}.</ref> Likewise, the [[Thales|Thales Herakles]] phased array multi-function radar onboard the [[Formidable class frigate]]s of the [[Republic of Singapore Navy]] has a track capacity of 200 targets and is able to achieve automatic target detection, confirmation and track initiation in a single scan, while simultaneously providing mid-course guidance updates to the [[MBDA Aster]] missiles launched from the ship.<ref>{{cite journal |last=Scott |first=Richard |year=2006 |month=April |title=Singapore Moves to Realise Its Formidable Ambitions |journal=Jane's Navy International |volume=111 |issue=4 |pages=42–49}}</ref>
The [[German Navy]] and the [[Dutch Navy]] have developed the [[Active Phased Array Radar]] System (APAR).
[[Image:APAR.jpg|thumb|[[Active Phased Array Radar]] mounted on top of [[Sachsen class frigate]] F220 ''Hamburg's'' superstructure of the [[German Navy]].]]
:''See also: [[Active Electronically Scanned Array]], [[Aegis combat system]] and [[AN/SPY-1]]''
===Space probe communication===
The [[MESSENGER]] spacecraft is a mission to the planet [[Mercury (planet)|Mercury]] (arrival [[18 March]] [[2011]]). This spacecraft is the first deep-space mission to use a phased-array antenna for [[telecommunication|communication]]s. The radiating elements are [[linear polarization|linearly-polarized]], slotted [[waveguide]]s. The antenna, which uses the [[X band]], uses 26 radiative elements but can gracefully downgrade.<ref>[http://ieeexplore.ieee.org/iel5/7416/20159/00931694.pdf Phased-Array Antenna for the MESSENGER Deep Space Mission]</ref>
===Weather research usage===
[[Image:Par installation.jpg|thumb|left|AN/SPY-1A radar installation at [[NSSL]], Norman, OK.]]
The [[National Severe Storms Laboratory]] has been using a SPY-1A phased array antenna, provided by the US Navy, for weather research at its [[Norman, Oklahoma]] facility since [[April 23]], [[2003]]. It is hoped that research will lead to a better understanding of thunderstorms and tornadoes, eventually leading to increased warning times and enhanced prediction of tornadoes. Project participants include the National Severe Storms Laboratory and National Weather Service Radar Operations Center, [[Lockheed Martin]], [[United States Navy]], [[University of Oklahoma]] School of Meteorology and School of Electrical and Computer Engineering, Oklahoma State Regents for Higher Education, the [[Federal Aviation Administration]], and [http://www.bcisse.com/ Basic Commerce and Industries]. The project includes [[research and development]], future [[technology transfer]] and potential deployment of the system throughout the United States. It is expected to take 10 to 15 years to complete and initial construction was approximately $25 million.<ref>[[National Oceanic and Atmospheric Administration]]. [http://www.norman.noaa.gov/publicaffairs/backgrounders/backgrounder_par.html PAR Backgrounder]. Accessed [[April 6]], [[2006]].</ref>
===Optics===
Within the visible or infrared spectrum of electromagnetic waves it is also possible to construct [[phased array optics|optical phased arrays]]. They are used in wavelength multiplexers and filters for telecommunication purposes,<ref>P. D. Trinh, S. Yegnanarayanan, F. Coppinger and B. Jalali [http://www.ee.ucla.edu/~oecs/comp_pub/intr_opt/Optics23.pdf Silicon-on-Insulator (SOI) Phased-Array Wavelength Multi/Demultiplexer with Extremely Low-Polarization Sensitivity] IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 7, JULY 1997</ref> laser beam steering, and holography.
===Radio-Frequency Identification===
Recently phased array antennas have been included in [[RFID]] systems in order to significantly boost the reading capability of passive UHF tags passing from 30 feet to 600 feet.<ref>[http://www.rfidradio.com/?p=25 Mojix Redefines the Range for Passive RFID], RFID Radio</ref>
==Mathematical perspective and formulae==
A phased array is an example of ''N''-slit [[diffraction]]. It may also be viewed as the coherent addition of ''N'' [[line source]]s. Since each individual antenna acts as a slit, emitting radio waves, their diffraction pattern can be calculated by adding the phase shift φ to the fringing term.
We will begin from the ''N''-slit diffraction pattern derived on the [[diffraction]] page.
:<math>
\psi ={{\psi }_0}\left[\frac{\sin \left(\frac{{\pi a}}{\lambda }\sin\theta \right)}{\frac{{\pi a}}{\lambda }\sin\theta}\right]\left[\frac{\sin
\left(\frac{N}{2}{kd}\sin\theta\right)}{\sin \left(\frac{{kd}}{2}\sin\theta \right)}\right]
</math>
Now, adding a φ term to the <math>\begin{matrix}kd\sin\theta\,\end{matrix}</math> fringe effect in the second term yields:
:<math>\psi ={{\psi }_0}\left[\frac{\sin \left(\frac{{\pi a}}{\lambda }\sin \theta\right)}{\frac{{\pi a}}{\lambda }\sin\theta}\right]\left[\frac{\sin
\left(\frac{N}{2}\big(\frac{{2\pi d}}{\lambda }\sin\theta + \phi \big)\right)}{\sin \left(\frac{{\pi d}}{\lambda }\sin\theta +\phi \right)}\right]
</math>
Taking the square of the wave function gives us the intensity of the wave.
:<math>I = I_0{{\left[\frac{\sin \left(\frac{\pi a}{\lambda }\sin\theta\right)}{\frac{{\pi a}}{\lambda } \sin [\theta
]}\right]}^2}{{\left[\frac{\sin \left(\frac{N}{2}(\frac{2\pi d}{\lambda} \sin\theta+\phi )\right)}{\sin \left(\frac{{\pi d}}{\lambda
} \sin\theta+\phi \right)}\right]}^2}
</math>
:<math>
I =I_0{{\left[\frac{\sin \left(\frac{{\pi a}}{\lambda } \sin\theta\right)}{\frac{{\pi a}}{\lambda }
\sin\theta}\right]}^2}{{\left[\frac{\sin \left(\frac{\pi }{\lambda } N d \sin\theta+\frac{N}{2} \phi \right)}{\sin
\left(\frac{{\pi d}}{\lambda } \sin\theta+\phi \right)}\right]}^2}
</math>
Now space the emitters a distance <math> d=\begin{matrix}\frac{\lambda}{4}\end{matrix}</math> apart. This distance is chosen for simplicity of calculation but can be adjusted as any scalar fraction of the wavelength.
:<math>I =I_0{{\left[\frac{\sin \left(\frac{\pi }{\lambda } a \theta \right)}{\frac{\pi }{\lambda } a
\theta }\right]}^2}{{\left[\frac{\sin \left(\frac{\pi }{4} N \sin\theta+\frac{N}{2} \phi \right)}{\sin \left(\frac{\pi }{4}
\sin\theta+ \phi \right)}\right]}^2}</math>
Sin achieves its maximum at <math>\begin{matrix}\frac{\pi}{2}\end{matrix}</math> so we set the numerator of the second term = 1.
:<math>
\frac{\pi }{4} N \sin\theta+\frac{N}{2} \phi = \frac{\pi }{2}
</math>
:<math>
\sin\theta=\Big(\frac{\pi }{2} - \frac{N}{2} \phi \Big)\frac{4}{N \pi }
</math>
:<math>
\sin\theta=\frac{2}{N}-\frac{2\phi }{\pi }
</math>
Thus as ''N'' gets large, the term will be dominated by the <math>\begin{matrix}\frac{2\phi}{\pi}\end{matrix}</math> term. As sin can oscillate between −1 and 1, we can see that setting <math>\phi=-\begin{matrix}\frac{\pi}{2}\end{matrix}</math> will send the maximum energy on an angle given by
:<math>\theta = \sin^{-1}(1) = \begin{matrix}\frac{\pi}{2}\end{matrix} = 90^{\circ}</math>
Additionally, we can see that if we wish to adjust the angle at which the maximum energy is emitted, we need only to adjust the phase shift φ between successive antennas. Indeed the phase shift corresponds to the negative angle of maximum signal.
A similar calculation will show that the denominator is minimized by the same factor.
== Different types of phased arrays ==
There are two main different types of phased arrays, also called beamformers. There are [[time domain]] beamformers and [[frequency domain]] beamformers.
A time domain beamformer works, as the name says, by doing time-based operations. The basic operation is called "delay and sum". It delays the incoming signal from each array element by a certain amount of time, and then adds them together. Sometimes a multiplication with a window across the array is done to increase the mainlobe/sidelobe ratio, and to insert zeroes in the characteristic.
There are two different types of frequency domain beamformers. The first type separates the different frequency components that are present in the received signal into different frequency bins (using either an [[FFT]] or a [[filterbank]]). When different delay and sum beamformers are applied to each frequency bin, it is possible to point the main lobe to different directions for different frequencies. This can be an advantage for communication links.
The other type of frequency domain beamformers makes use of so called Spatial Frequency. This means that an FFT is taken across the different array elements, not in time. The output of the N point FFT are N channels, which are evenly divided in space. This approach makes a simple implementation of several beamformers at the same time possible, but this approach is not flexible, because the different directions are fixed.
== See also ==
* [[Active Phased Array Radar]]
* [[Aperture synthesis]]
* [[Beamforming]]
* [[Synthetic aperture radar]]
* [[Synthetic aperture sonar]]
* [[Inverse synthetic aperture radar]] (ISAR)
* [[Side-scan sonar]]
* [[Smart antenna]]
* [[Active Electronically Scanned Array]]
* [[MIL-STD-188]]
* [[Phased array optics]]
* [[Phased array ultrasonics]]
* [[Radar MASINT]]
* [[Thinned array curse]]
* [[Tikhomirov Scientific Research Institute of Instrument Design|NIIP]] and [[Phazotron|NIIR]], [[Russia]]n developers of phased arrays
==References==
{{Refbegin}}
{{Refend}}
{{Reflist}}
==External links==
*[http://www.nssl.noaa.gov/par/ Radar Research and Development - Phased Array Radar] - [[National Severe Storms Laboratory]] experimental phased array radar, using a former US Navy SPY-1A antenna.
*[http://www.harpoonhq.com/waypoint/articles/Article_044.pdf Shipboard Phased Array Radars]
*[http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870018450_1987018450.pdf NASA Report: MMICs For Multiple Scanning Beam Antennas for Space Applications]
*[http://americas.kyocera.com/kai/semiparts/products/trmods.cfm?page=28 Kyocera America, Inc. TR Modules for Phased Array Radars]
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