A phased array is a type of electromagnetic wave detection system usually associated with radar that is based on the transmission of airborne radio waves. It can also be built on the concept of sonar for underwater scanning of objects with sound waves, and is being researched as of 2011 using optical wave fronts as well. The concept is based upon earlier versions of radio antenna and follows the same fundamental principle where the reflection of radio waves off of objects is used to determine their location and direction of movement. The primary difference between a phased array radar versus a standard radar dish is that a phased system does not have to be physically moved or rotated to scan an object traveling across the sky.
Radar signals diminish in effectiveness outside of a limited angle of projection, so early dish antenna were placed along a line to extend their overall view of the sky. One of the earliest forms of this developed during the Cold War and preceded phased array technology itself, known as the US Distant Early Warning (DEW) Line of radar installations in the Arctic and Canada. When phased array technology was being perfected in 1958, Russia developed one of the first versions of working phased systems in the early 1960s, codenamed by the North Atlantic Treaty Organization (NATO) as the installations of Dog House, Cat House, and Hen House. The equipment consisted of radar installations that could effectively scan at least one-third of the Russian frontier where it bordered Europe for incoming missile attacks, along with automated nuclear missile interceptor systems to destroy any possible targets.
The most advanced phased array radar system as of 2006 is the Sea-Based X-Band Radar (SBX) developed by the US military to track ballistic missiles and other fast-moving objects in flight through the atmosphere or space surrounding the Earth. The SBX contains 45,000 radiating elements that are individual antenna that each transmit a radio signal. Precise timing of each antenna signal and how it overlaps with its nearest neighbors allows the SBX to create a wave front that can actively scan objects moving across its Field of View (FOV). This encompasses a cone of space spanning 120°, so the SBX system incorporates four radar units to cover an entire hemisphere of the globe simultaneously.
Phased array technology for radar systems is very complex and requires computer controls that are fast and reliable. The SBX system has to change the direction of the overall radar beam once every 0.000020th of a second, or once every 20 microseconds to be effective. This makes advanced phased array systems very expensive as compared to traditionally linked radars, with the SBX system costing nearly $900,000,000 US Dollars (USD) to complete.
More modest types of phased array technology include phased array ultrasound used in medical imaging and to scan the interior of metal structures for defects. Sound waves are overlapped to enhance the overall signal and change its direction of scan to look for interior features. The phased array transducer used in such equipment has from 16 to 256 individually transmitting sound wave probes that are activated in groups of 4 to 32 to enhance the quality of the image.
Phased Array Optics (PAO), while only theoretical as of 2011, is being researched for the ability that it would have to produce three-dimensional holographic landscapes that would be indistinguishable to the naked eye from that of the real world. The technology would have to be able to manipulate light waves for constructive and destructive interference, as is done with radio waves, at a level that is smaller than the natural wavelength of the light itself. The systems that would be necessary to do this would include advanced computers for rapid processing of the signals and a spatial light modulator (SLM) to control when and how each wavelength of light was manipulated. Projections are that, by the middle of the 21st century, such PAO systems will be possible.