The Foster scanner, or Variable Path scanner, is a type of radar system that produces a narrow beam that rapidly scans an area in front of it. Foster scanners were widely used in post-World War II radar systems used for artillery and mortar spotting. Modern radars in this role normally use electronic scanning in place of a Foster scanner for this purpose.
The Foster scanner consists of two parts; a box-shaped antenna and the "scanner" itself. These are normally placed in front of a shaped reflector.
The antenna consists of a thin rectangular section of a parabolic antenna, as if one cut the sides away from a conventional parabolic antenna, leaving only the thin strip where the feed horn is mounted. Rectangular plates are placed on either side of the "cut", extending forward in the direction of broadcast. The resulting system has the basic size and shape of a large pizza box with the rear side rounded. In the UK, these are known as "cheese" antennas as they resemble a section cut from a wheel of cheese. The feed to the antenna is via a rectangular slot in a waveguide running the length of the box in front of the parabolic section.
The scanner itself consists of a section of rectangular waveguide formed into a hollow cone, like an ice cream cone. A second metal cone is placed within the first, leaving a gap between the two that forms a section of waveguide. Small metal "teeth" at the point where the input waveguide meets the cone causes the radar signal to be reflected 90 degrees into the slot between the two cones. Used alone, this system causes the radar signal to travel a different total path length at the narrow end of the cone as opposed to the wide end. This causes the wavefront of the signal to be rotated with respect to the antenna.
The inner cone is not solid, but has a slot cut through the center to form another section of waveguide. Teeth on the outside of this cone pick up the signal from the slot between the cones, reflects it through its center slot, and then a second set of teeth sends it back into the slot between the two cones. The inner cone is allowed to rotate within the outer. As the inner cone rotates, the total signal length changes, more at the wide end than the narrow. When sent to the antenna, this causes the wavefront to scan across the area between the two horizontal plates, "snapping" to the initial position when the inner cone rotates back to the original point.
The scanner allows scans at the rotational speed of the inner cone, which is easily built in a fashion allowing very rapid scanning. Typical angles are +/- 20 degrees.
The Foster scanner was used in a number of post-World War II counterbattery radar systems, the first examples reaching service in the late 1950s and early 1960s. In these systems the scanner was mounted horizontally, in order to produce a signal that scanned back and forth horizontally in front of the radar set. Typical horizontal scanning ranges were between 40 and 50 degrees, with a 1 degree beamwidth.
In typical systems, the scanner would be mounted so that it could be quickly pointed at two different vertical angles. This could be accomplished by moving the main antenna reflector behind the scanner, or through a variety of electronic means. The system would rapidly switch the scanner between these two angles, while the scanner itself continued to deflect the signal left and right. The location of the shell horizontally was revealed by the timing of the returned signal compared to the rotation of the scanner at that instant. Using traditional pulse timing methods the range of the shell could be determined at the same instant. Simple trigonometry using the range and the known fixed vertical angle of the beam revealed the shell's vertical location. The time it took to cross one beam and then the other revealed the shell's speed.
Early systems included the US AN/MPQ-4, which used lobe switching to switch between two angles about 35 mils apart (about 2 degrees). Reflected signals were displayed on two B-scopes. Measurements from the B-scopes were copied into a ballistic calculator, which produced the X and Y location of the launcher. The UK Green Archer radar used a manual switch to flip between the vertical angles. The radar was normally left in the "lower" angle setting, while the operator waited for returns. When one was seen the switch was moved to the upper position, and the second return plotted. Ballistic calculation was performed by plotting the two points on an X-Y chart paper, and then moving mechanical cursors over the points. The rest of the calculation was carried out electronically.
The Foster scanner was originally designed by John Stuart Foster while working at the MIT Radiation Laboratory as part of a technical exchange from the Canadian National Research Council. It was quickly adapted for artillery spotting use, particularly for mortars. In this role the scanner would be used to make two or more rapid observations of the shell in flight, and then plotted to calculate the initial position.
Radar is a detection system that uses radio waves to determine the distance (ranging), angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it has high directivity. It functions similarly to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, meaning that they can produce the narrowest beamwidths, of any antenna type. In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave (SHF) frequencies, at which the wavelengths are small enough that conveniently-sized reflectors can be used.
A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. They are used as feed antennas for larger antenna structures such as parabolic antennas, as standard calibration antennas to measure the gain of other antennas, and as directive antennas for such devices as radar guns, automatic door openers, and microwave radiometers. Their advantages are moderate directivity, low standing wave ratio (SWR), broad bandwidth, and simple construction and adjustment.
A counter-battery radar is a radar system that detects artillery projectiles fired by one or more guns, howitzers, mortars or rocket launchers and, from their trajectories, locates the position on the ground of the weapon that fired it. Such radars are a subclass of the wider class of target acquisition radars.
A slot antenna consists of a metal surface, usually a flat plate, with one or more holes or slots cut out. When the plate is driven as an antenna by an applied radio frequency current, the slot radiates electromagnetic waves in a way similar to a dipole antenna. The shape and size of the slot, as well as the driving frequency, determine the radiation pattern. Slot antennas are usually used at UHF and microwave frequencies at which wavelengths are small enough that the plate and slot are conveniently small. At these frequencies, the radio waves are often conducted by a waveguide, and the antenna consists of slots in the waveguide; this is called a slotted waveguide antenna. Multiple slots act as a directive array antenna and can emit a narrow fan-shaped beam of microwaves. They are used in standard laboratory microwave sources used for research, UHF television transmitting antennas, antennas on missiles and aircraft, sector antennas for cellular base stations, and particularly marine radar antennas. A slot antenna's main advantages are its size, design simplicity, and convenient adaptation to mass production using either waveguide or PC board technology.
Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target. Conical scanning is similar in concept to the earlier lobe switching concept used on some of the earliest radars, and many examples of lobe switching sets were modified in the field to conical scanning during World War II, notably the German Würzburg radar. Antenna guidance can be made entirely automatic, as in the American SCR-584. Potential failure modes and susceptibility to deception jamming led to the replacement of conical scan systems with monopulse radar sets. They are still used by the Deep Space Network for maintaining communications links to space probes. The spin-stabilized Pioneer 10 and Pioneer 11 probes used onboard conical scanning maneuvers to track Earth in its orbit.
Monopulse radar is a radar system that uses additional encoding of the radio signal to provide accurate directional information. The name refers to its ability to extract range and direction from a single signal pulse.
A radar display is an electronic device to present radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets and return to the radar system. The receiver converts all received electromagnetic radiation into a continuous electronic analog signal of varying voltage that can be converted then to a screen display.
Radar engineering details are technical details pertaining to the components of a radar and their ability to detect the return energy from moving scatterers — determining an object's position or obstruction in the environment. This includes field of view in terms of solid angle and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. Radar sensors are classified by application, architecture, radar mode, platform, and propagation window.
Green Archer, also called Radar, Field Artillery, No 8 was a widely used British mortar locating radar operating in the X band using a Foster scanner. Developed by EMI after an experimental model by the Royal Radar Establishment, it was in British service from 1962 until 1975 with the Royal Artillery. A self-propelled version was designated FV436 or Radar, FA, No 8 Mk 2. It was replaced by Cymbeline starting in 1975.
A Primary radar is a conventional radar sensor that illuminates a large portion of space with an electromagnetic wave and receives back the reflected waves from targets within that space. The term thus refers to a radar system used to detect and localize potentially non-cooperative targets. It is specific to the field of air traffic control where it is opposed to the secondary radar which receives additional information from the target's transponder.
The AN/APQ-7, or Eagle, was a radar bombsight system developed by the US Army Air Force. Early studies started in late 1941 under the direction of Luis Alvarez at the MIT Radiation Laboratory, but full-scale development did not begin until April 1943. By this time US-built, higher frequency systems promising better performance over the existing British H2S radar were entering production. Eagle's even higher resolution was considered important to Air Force planners who preferred precision bombing but were failing to deliver it, and high hopes were put on the system's abilities to directly attack small targets like docks and bridges.
The Radar Set AN/MPQ-4 was a US Army counter-battery radar primarily used to find the location of enemy mortars and larger artillery in a secondary role. Built by General Electric, it first entered service in 1958, replacing the earlier and much simpler AN/MPQ-10. The MPQ-4 could determine the location of an enemy mortar in as little as 20 seconds by observing a single round, whereas the MPQ-10 required several rounds to be launched and could take 4 to 5 minutes to take a "fix". The MPQ-4 remained one of the primary US counter-battery systems through the late 1970s until it was replaced by passive electronically scanned array radars like the AN/TPQ-36.
The Type 277 was a surface search and secondary aircraft early warning radar used by the Royal Navy and allies during World War II and the post-war era. It was a major update of the earlier Type 271 radar, offering much more power, better signal processing, new displays, and new antennas with greatly improved performance and much simpler mounting requirements. It allowed a radar with performance formerly found only on cruisers and battleships to be fitted even to the smallest corvettes. It began to replace the 271 in 1943 and was widespread by the end of the year.
The AMES Type 82, also widely known by its rainbow codename Orange Yeoman, was an S-band 3D radar built by Marconi and used by the Royal Air Force (RAF), initially for tactical control and later for air traffic control (ATC).
An organ-pipe scanner is a system used in some radar systems to provide scanning in azimuth or elevation without moving the antenna. It consists of a series of waveguides and feed horns arranged in front of a shaped reflector, each one positioned to reflect the beam in a different direction. The wave guides meet at a central point where a small rotating waveguide feeds the microwave signal into each of the horns in turn as it rotates past them.
In radio systems, many different antenna types are used with specialized properties for particular applications. Antennas can be classified in various ways. The list below groups together antennas under common operating principles, following the way antennas are classified in many engineering textbooks.
Martello is a family of phased array radar systems developed by Marconi Electronic Systems in the 1970s and introduced operationally in the early 1980s. They provided long-range early warning capabilities but also had the accuracy needed for interception plotting and "putting on" of other weapons systems like surface-to-air missiles. The name comes from the Martello towers that provided defence in earlier years.
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 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.