A radar display is an electronic device that presents radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and return to the radar system. The receiver converts all received electromagnetic radiation into a continuous electronic analog signal of varying (or oscillating) voltage that can be converted then to a screen display.
Modern systems typically use some sort of raster scan display to produce a map-like image. Early in radar development, however, numerous circumstances made such displays difficult to produce. People developed several different display types.
Oscilloscopes
Oscilloscope attached to two sine-wave voltage sources, producing a circle pattern on the display.
Early radar displays used adapted oscilloscopes with various inputs. An oscilloscope generally receives three channels of varying (or oscillating) voltage as input and displays this information on a cathode ray tube. The oscilloscope amplifies the input voltages and sends them into two deflection magnets and to the electron gun producing a spot on the screen. One magnet displaces the spot horizontally, the other vertically, and the input to the gun increases or decreases the brightness of the spot. A bias voltage source for each of the three channels allows the operator to set a zero point.
In a radar display, the output signal from the radar receiver is fed into one of three input channels in the oscilloscope. Early displays generally sent this information to either X channel or Y channel to displace the spot on the screen to indicate a return. More modern radars typically used a rotating or otherwise moving antenna to cover a greater area of the sky, and in these cases, electronics, slaved to the mechanical motion of the antenna, typically moved the X and Y channels, with the radar signal being fed into the brightness channel.
A-Scope
Chain Home is the canonical A-scope system. This image shows several target "blips" at ranges between 15 and 30 miles from the station. The large blip on the far left is the leftover signal from the radar's own transmitter; targets in this area could not be seen. The signal is inverted to make measurement simpler.
The original radar display, the A-scope or A-display, shows only the range, not the direction, to targets. These are sometimes referred to as R-scopes for range scope. A-scopes were used on the earliest radar systems during World War II, notably the seminal Chain Home (CH) system.
The primary input to the A-scope was the amplified return signal received from the radar, which was sent into the Y-axis of the display. Returns caused the spot to be deflected downward (or upward on some models), drawing vertical lines on the tube. These lines were known as a "blip" (or "pip"). The X-axis input was connected to a sawtooth voltage generator known as a time base generator that swept the spot across the display, timed to match the pulse repetition frequency of the radar. This spread out the blips across the display according to the time they were received. Since the return time of the signal corresponds to twice the distance to the target divided by the speed of light, the distance along the axis directly indicates the range to any target. This was usually measured against a scale above the display.[1]
Chain Home signals were normally received on a pair of antennas arranged at right angles. Using a device known as a radiogoniometer, the operator could determine the bearing of the target, and by combining their range measurement with the bearing, they could determine a target's location in space. The system also had a second set of antennas, displaced vertically along the receiver towers. By selecting a pair of these antennas at different heights and connecting them to the radiogoniometer, they could determine the vertical angle of the target, and thus estimate its altitude. Since the system could measure both range and altitude, it was sometimes known as an HR-scope, from "height-range".
The L-scope was basically two A-scopes placed side by side and rotated vertically. By comparing the signal strength from two antennas, the rough direction of the blip could be determined. In this case there are two blips, a large one roughly centred and a smaller one far to the right.
Early American, Dutch and German radars used the J-scope, which resembled a circular version of the A-scope. These display range as an angle around the display face, as opposed to the linear distance along it. This arrangement allows greater accuracy in reading the range with the same sized display as an A-scope because the trace uses the full circumference rather than just the horizontal distance (so the time base is π times longer. For instance, on a typical .[1] An electro-mechanical version of the J-scope display remained common on consumer boating depth meters until the 1990s.
W. A. S. Butement developed a further adaptation of the J-scope in the "spiral time base", which moved the blip both around the face and outward from the center. This produced a time base that was 7 feet (2.1m) long, allowing very highly accurate measurements of range.
To improve the accuracy of angle measurements, the concept of lobe switching became common in early radars. In this system, two antennas are used, pointed slightly left and right, or above and below, the boresight of the system. The received signal would differ in strength depending on which of the two antennas was more closely pointed at the target, and be equal when the antenna was properly aligned. To display this, both antennas were connected to a mechanical switch that rapidly switched between the two, producing two blips in the display. In order to differentiate them, one of the two receivers had a delay so it would appear slightly to the right of the other. The operator would then swing the antenna back and forth until both blips were the same height. This was sometimes known as a K-scope.[2]
A slightly modified version of the K-scope was commonly used for air-to-air (AI) and air-to-surface-vessel (ASV) radars. In these systems, the K-scope was turned 90 degrees so longer distances were further up the scope instead of further to the right. The output of one of the two antennas was sent through an inverter instead of a delay. The result was that the two blips were displaced on either side of the vertical baseline, both at the same indicated range. This allowed the operator to instantly see which direction to turn; if the blip on the right was shorter, they needed to turn to the right. These types of displays were sometimes referred to as ASV-scopes or L-scopes, although the naming was not universal.[1]
Size of A-scope displays vary, but 5 to 7 inch diagonal was often used on a radar display. The 7JPx series of CRTs (7JP1, 7JP4 and 7JP7) was originally designed as an A-scope display CRT.
B-Scope
E-scope on the left and B-scope on the right. The E-scope shows two blips at slightly different altitudes, the top one being slightly closer as well. The B-scope shows three blips, the closest being head on, a second just to its right and slightly longer range, and a third near the right edge of the scanning pattern.
A B-scope or b-scan provides a 2-D "top down" representation of space, with the vertical axis typically representing range and the horizontal axis azimuth (angle).[1] The B-scope's display represented a horizontal "slice" of the airspace on both sides of the aircraft out to the tracking angles of the radar. B-scope displays were common in airborne radars in the 1950s and 60s, which were mechanically scanned from side to side, and sometimes up and down as well.
The spot was swept up the Y-axis in a fashion similar to the A-scope's X-axis, with distances "up" the display indicating greater range. This signal was mixed with a varying voltage being generated by a mechanical device that depended on the current horizontal angle of the antenna. The result was essentially an A-scope whose range line axis rotated back and forth about a zero point at the bottom of the display. The radio signal was sent into the intensity channel, producing a bright spot on the display indicating returns.
An E-scope is essentially a B-scope displaying range vs. elevation, rather than range vs. azimuth.[1] They are identical in operation to the B-scope, the name simply indicating "elevation". E-scopes are typically used with height finding radars, which are similar to airborne radars but turned to scan vertically instead of horizontally, they are also sometimes referred to as "nodding radars" due to their antenna's motion. The display tube was generally rotated 90 degrees to put the elevation axis vertical in order to provide a more obvious correlation between the display and the "real world". These displays are also referred to as a Range-Height Indicator, or RHI, but were also commonly referred to (confusingly) as a B-scope as well.
The H-scope is another modification of the B-scope concept, but displays elevation as well as azimuth and range. The elevation information is displayed by drawing a second "blip" offset from the target indicator by a short distance, the slope of the line between the two blips indicates the elevation relative to the radar.[1] For instance, if the blip were displaced directly to the right this would indicate that the target is at the same elevation as the radar. The offset is created by dividing the radio signal into two, then slightly delaying one of the signals so it appears offset on the display. The angle was adjusted by delaying the time of the signal via a delay, the length of the delay being controlled by a voltage varying with the vertical position of the antenna. This sort of elevation display could be added to almost any of the other displays, and was often referred to as a "double dot" display.
C-Scope
C-scope display. The target is above and to the right of the radar, but the range is not displayed.
A C-scope displays a "bullseye" view of azimuth vs. elevation. The "blip" was displayed indicating the direction of the target off the centreline axis of the radar, or more commonly, the aircraft or gun it was attached to. They were also known as "moving spot indicators" or "flying spot indicators" in the UK, the moving spot being the target blip. Range is typically displayed separately in these cases, often using a second display as an L-scope.[1]
Almost identical to the C-scope is the G-scope, which overlays a graphical representation of the range to the target.[1] This is typically represented by a horizontal line that "grows" out from the target indicator blip to form a wing-like shape. The wings grew in length at shorter distances to indicate the target was closer, as does the aircraft's wings when seen visually. A "shoot now" range indicator is often supplied as well, typically consisting of two short vertical lines centered on either side of the middle of the display. To make an interception, the pilot guides his aircraft until the blip is centered, then approaches until the "wings" fill the area between the range markers. This display recreated a system commonly used on gunsights, where the pilot would dial in a target's wingspan and then fire when the wings filled the area inside a circle in their sight. This system allowed the pilot to estimate the range to the target. In this case, however, the range is being measured directly by the radar, and the display was mimicking the optical system to retain commonality between the two systems.
This image shows a modern PPI display in use, with the islands and ground surrounding the ship in green. The current direction of the radar can be seen as the dotted line pointing northwest.
The PPI display provides a 2-D "all round" display of the airspace around a radar site. The distance out from the center of the display indicates range, and the angle around the display is the azimuth to the target. The current position of the radar antenna is typically indicated by a line extending from the center to the outside of the display, which rotates along with the antenna in realtime.[1] It is essentially a B-scope extended to 360 degrees. The PPI display is typically what people think of as a radar display in general, and was widely used in air traffic control until the introduction of raster displays in the 1990s.
PPI displays are actually quite similar to A-scopes in operation, and appeared fairly quickly after the introduction of radar. As with most 2D radar displays, the output of the radio receiver was attached to the intensity channel to produce a bright dot indicating returns. In the A-scope a sawtooth voltage generator attached to the X-axis moves the spot across the screen, whereas in the PPI the output of two such generators is used to rotate the line around the screen. Some early systems were mechanical, using a rotating deflection coil around the neck of the display tube, but the electronics needed to do this using a pair of stationary deflection coils were not particularly complex, and were in use in the early 1940s.
Radar cathode ray tubes such as the 7JP4 used for PPI displays had a circular screen and scanned the beam from the center outwards. The deflection yoke rotated, causing the beam to rotate in a circular fashion.[3] The screen often had two colors, often a bright short persistence color that only appeared as the beam scanned the display and a long persistence phosphor afterglow. When the beam strikes the phosphor, the phosphor brightly illuminates, and when the beam leaves, the dimmer long persistence afterglow would remain lit where the beam struck the phosphor, alongside the radar targets that were "written" by the beam, until the beam re-struck the phosphor.[4][5]
Beta Scan Scope
A Beta Scan display.
The specialist Beta Scan Scope was used for precision approach radar systems. It displays two lines on the same display, the upper one (typically) displaying the vertical approach (the glideslope), and the lower one the horizontal approach. A marker indicates the desired touchdown point on the runway, and often the lines are angled towards the middle of the screen to indicate this location. A single aircraft's "blip" is also displayed, superimposed over both lines, the signals being generated from separate antennas. Deviation from the centerline of the approach can be seen and easily relayed to the pilot.
In the image, the upper portion of the display shows the vertical situation, and the lower portion the horizontal. In the vertical, the two diagonal lines show the desired glideslope (upper) and minimum altitude approach (lower). The aircraft began its approach below the glideslope and captured it just before landing. The proper landing point is shown by the horizontal line at the left end. The lower display shows the aircraft starting to the left of the approach line and then being guided toward it.
Terrain-following radar (TFR) is a military aerospace technology that allows a very-low-flying aircraft to automatically maintain a relatively constant altitude above ground level and therefore make detection by enemy radar more difficult. It is sometimes referred to as ground hugging or terrain hugging flight. The term nap-of-the-earth flight may also apply but is more commonly used in relation to low-flying military helicopters, which typically do not use terrain-following radar.
The SCR-268 was the United States Army's first radar system. Introduced in 1940, it was developed to provide accurate aiming information for antiaircraft artillery and was also used for gun laying systems and directing searchlights against aircraft. The radar was widely utilized by both Army and Marine Corps air defense and early warning units during World War II. By the end of World War II the system was already considered out of date, having been replaced by the much smaller and more accurate SCR-584 microwave-based system.
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 raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems. The word raster comes from the Latin word rastrum, which is derived from radere ; see also rastrum, an instrument for drawing musical staff lines. The pattern left by the lines of a rake, when drawn straight, resembles the parallel lines of a raster: this line-by-line scanning is what creates a raster. It is a systematic process of covering the area progressively, one line at a time. Although often a great deal faster, it is similar in the most general sense to how one's gaze travels when one reads lines of text.
In radar systems, the blip-to-scan ratio, or blip/scan, is the ratio of the number of times a target appears on a radar display to the number of times it theoretically could be displayed. Alternately it can be defined as the ratio of the number of scans in which an accurate return is received to the total number of scans.
Track-while-scan (TWS) is a mode of radar operation in which the radar allocates part of its power to tracking a target or targets while part of its power is allocated to scanning. It is similar to but functions differently in comparison to its counterparts range-while-search (RWS), long range search (LRS), air combat mode (ACM), velocity search with ranging (VSR) and combined radar mode (CRM). In track-while-scan mode the radar has the ability to acquire and lock/track multiple targets while simultaneously providing a view of the surrounding airspace, which in turn aids the pilot and or operator in maintaining better situational awareness.
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.
A time base generator is a special type of function generator, an electronic circuit that generates a varying voltage to produce a particular waveform. Time base generators produce very high frequency sawtooth waves specifically designed to deflect the beam of a cathode ray tube (CRT) smoothly across the face of the tube and then return it to its starting position.
Radar, Gun Laying, Mark I, or GL Mk. I for short, was an early radar system developed by the British Army to provide range information to associated anti-aircraft artillery. There were two upgrades to the same basic system, GL/EF and GL Mk. II, both of which added the ability to accurately determine bearing and elevation.
A primary radar or primary surveillance radar (PSR) 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 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.
Searchlight Control, SLC for short but nicknamed "Elsie", was a British Army VHF-band radar system that provided aiming guidance to an attached searchlight. By combining a searchlight with a radar, the radar did not have to be particularly accurate, it only had to be good enough to get the searchlight beam on the target. Once the target was lit, normal optical instruments could be used to guide the associated anti-aircraft artillery. This allowed the radar to be much smaller, simpler and less expensive than a system with enough accuracy to directly aim the guns, like the large and complex GL Mk. II radar. In 1943 the system was officially designated Radar, AA, No. 2, although this name is rarely used.
The AMES Type 82, also widely known by its rainbow codename Orange Yeoman, was an S-band 3D radar built by the Marconi Company and used by the Royal Air Force (RAF), initially for tactical control and later for air traffic control (ATC).
The AMES Type 7, also known as the Final GCI, was a ground-based radar system introduced during World War II by the Royal Air Force (RAF). The Type 7 was the first truly modern radar used by the Allies, providing a 360 degree view of the airspace around the station out to a distance of about 90 miles (140 km). It allowed fighter interceptions to be plotted directly from the radar display, a concept known as ground controlled intercept, or GCI.
The HF200 is a height finder radar designed and first built by Decca Radar in 1957, and continuing sales into the 1970s after the division was purchased by Plessey in 1965. It was one of the company's successful heavy radar projects, winning the contract for many of the ROTOR stations in the UK and additional sales around the world with a total production run of about 40 examples. These served into the 1980s, and in one case, 1993, before 3D radars removed the need for separate height-finders.
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.
Red Steer, also known as ARI 5919 and ARI 5952 depending on the version, was a tail warning radar used on the British V bomber force. Built by EKCO, it was developed from the experimental AI.20 radar for the English Electric Lightning. The Lightning required its radar to be remotely installed in the nose of the aircraft, and this made the set equally suitable for remote mounting in the tail of the bombers.
References
1 2 3 4 5 6 7 8 9 "Glossary of Terms". Radar - Operational Characteristics of Radar Classified by Tactical Application. pp.109–114. Retrieved April 1, 2016.
Raju, G. S. N. (2008). Radar engineering and fundamentals of navigational aids. New Delhi: I. K. International Publishing House Pvt Ltd. pp.54, 237, 241, 252–259. ISBN978-81-906942-1-6.
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