The AR-320 is a 3D early warning radar developed by the UK's Plessey in partnership with US-based ITT-Gilfillan. The system combined the receiver electronics, computer systems and displays of the earlier Plessey AR-3D with a Gilfillan-developed transmitter and planar array antenna from their S320 series. The main advantage over the AR-3D was the ability to shift frequencies to provide a level of frequency agility and thus improve its resistance to jamming.
The system was designed for sales to the Royal Air Force and a NATO contract for mobile radars on the southern flank of Europe. In 1983, after an extended contest against the Hughes Air Defence Radar, the Royal Air Force ordered six AR-320 systems for their newly forming IUKADGE network under the service name AMES Type 93. The AR-320s were to be held in off-site locations and quickly moved to pre-surveyed areas in case the main radars in the network were attacked. In service, the AR-320 proved to be much less robust than advertised and was ultimately used only in fixed emplacements.
As the radar market grew more demanding and new solid state systems began to replace former vacuum tube systems as high-power microwave sources, the system was extensively redesigned as the AR-325, which, through a series of mergers, became the modern BAe Commander series.
History
Background
During the installation of the first high-power microwave frequency radars using slot antennas, operators of the AMES Type 14 noticed that the apparent angle of the blips on the radar display did not always match the physical angle of the antenna. This was traced to an effect in the waveguides, which had slightly different transmission characteristics based on frequency. When the cavity magnetron warmed and cooled during use, its frequency changed slightly, and a delay was introduced in the waveguide. This caused the signal to no longer exactly match the positioning of the radiators in the antenna, and caused the beam to shift. This effect became known as "squint" and was generally considered annoying, especially on the AMES Type 80 where servicing the magnetron required a lengthy recalibration process to bring the beam back in line with the antenna.
This effect also opened the possibility of steering the beam electronically by deliberately changing the frequency during the period of a long pulse. A magnetron's output is fixed by its physical construction and does not easily allow such shifting. To make practical use of this effect, a much wider bandwidth would be needed, which became possible with the introduction of high-power klystrons and travelling wave tubes in the later 1950s. A number of radar systems began using this technique to provide beam-steering, typically in elevation while the radar spun around its vertical axis to scan in azimuth.
SQUIRT and AR-3D
In 1964, Decca Radar won a contract to study the use of this effect in combination with another emerging technique, pulse compression. In follow-on work they built an experimental system known as SQUIRT which was operational in 1967, the same year Decca's Heavy Radar division was purchased by Plessey. SQUIRT, with modifications made in 1968, demonstrated itself able to scan vertically while providing accurate range measurements, but at the cost of losing accuracy in height finding.
The Ministry of Defence proved uninterested in the result, not only due to the lack of accuracy in altitude but also because a target aircraft would always be "painted" by the same frequency; the vertical angle was a function of frequency, so as long as the aircraft did not change altitude it would always see the same signal. This would make it much easier to jam the radar, making it of limited use in any environment where electronic countermeasures (ECM) was expected.
Plessey felt the concept would still be useful for less demanding roles, notably air traffic control around military bases where the radar would have a secondary early warning role and might not face significant ECM. They developed a commercial model, the Plessey AR-3D, which went on the market in 1975. The system was much less expensive than contemporary systems using phase steering and soon found a number of international sales, with over 20 units eventually being delivered.