Plessey AR-3D
The AR-3D was a military air traffic control and early warning radar developed by Plessey and first produced in 1975. It used a pencil beam and simple frequency scanning system known as "squint scan" to produce a low-cost 3D radar system that was also relatively mobile. About 23 were produced in total and found sales around the world into the early 1980s. The frequency scanning system had the disadvantage that a target aircraft would always be "painted" by the same frequency signal, which made the task of jamming the radar simpler. This limited its sales prospects to military users, and shortly after delivering the AR-3D the company began negotiations with the US company ITT-Gillifan to incorporate their multi-frequency scanning with the AR-3D's receivers and display systems to produce the Plessey AR-320. History Squint 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. Through the 1960s, significant effort was put into developing "squintless feeds" that avoided this problem, but the problem also presented an opportunity. Squint is caused by the waveguide system slowing the signal by different amounts depending on the frequency. If the signal exits through a slotted waveguide antenna, this slight shift in timing produces a phase shift of the output. This shift will cause the resulting wavefront to change direction. This is the basis of the phased array radar, which normally accomplishes this using signal delay electronics. As squint causes a similar delay, it can also be used to steer the beam. This leads to the possibility of radar that can scan its beam in a chosen direction, typically vertically, with no moving parts. SQUIRT Another technique that was being widely explored in the late 1950s and early 1960s was the technique of pulse compression. Pulse compression greatly increases the range resolution of a radar, which was formerly defined almost entirely by its pulse width. To get the required resolution in older designs, pulses were very short, typically a few microseconds, which required massive power output in order to produce a detectable reflected signal. Using pulse compression, the pulses could be much longer, at least ten times, meaning the same return signal would be produced with the peak output. Pulse compression works by changing the frequency of the