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).[1] An electro-mechanical version of the J-scope display remained common on consumer boating depth meters until the 1990s.
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 and ground-search radars, notably in AI radars and ASV radars - (Air-Surface Vessel). 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.
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.
Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination.
The low-UHF band Würzburg radar was the primary ground-based tracking radar for the Wehrmacht's Luftwaffe and Kriegsmarine during World War II. Initial development took place before the war and the apparatus entered service in 1940. Eventually, over 4,000 Würzburgs of various models were produced. It took its name from the city of Würzburg.
The microwave landing system (MLS) is an all-weather, precision radio guidance system intended to be installed at large airports to assist aircraft in landing, including 'blind landings'. MLS enables an approaching aircraft to determine when it is aligned with the destination runway and on the correct glidepath for a safe landing. MLS was intended to replace or supplement the instrument landing systems (ILS). MLS has a number of operational advantages over ILS, including a wider selection of channels to avoid interference with nearby installations, excellent performance in all weather, a small "footprint" at the airports, and wide vertical and horizontal "capture" angles that allowed approaches from wider areas around the airport.
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.
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 height finder is a ground-based aircraft altitude measuring device. Early height finders were optical range finder devices combined with simple mechanical computers, while later systems migrated to radar devices. The unique vertical oscillating motion of height finder radars led to them also being known as nodding radar. Devices combining both optics and radar were deployed by the U.S. Military.
The AN/FPS-16 is a highly accurate ground-based monopulse single object tracking radar (SOTR), used extensively by the NASA crewed space program, the U.S. Air Force and the U.S. Army. The accuracy of Radar Set AN/FPS-16 is such that the position data obtained from point-source targets has azimuth and elevation angular errors of less than 0.1 milliradian and range errors of less than 5 yards (5 m) with a signal-to-noise ratio of 20 decibels or greater.
An oscilloscope is a type of electronic test instrument that graphically displays varying voltages of one or more signals as a function of time. The main purpose is capture information on electrical signals for debugging, analysis, or characterization. The displayed waveform can then be analyzed for properties such as amplitude, frequency, rise time, time interval, distortion, and others. Originally, calculation of these values required manually measuring the waveform against the scales built into the screen of the instrument. Modern digital instruments may calculate and display these properties directly.
This is a subdivision of the Oscilloscope article, discussing the various types and models of oscilloscopes in greater detail.
The history of the oscilloscope was fundamental to science because an oscilloscope is a device for viewing waveform oscillations, as of electrical voltage or current, in order to measure frequency and other wave characteristics. This was important in developing electromagnetic theory. The first recordings of waveforms were with a galvanometer coupled to a mechanical drawing system dating from the second decade of the 19th century. The modern day digital oscilloscope is a consequence of multiple generations of development of the oscillograph, cathode-ray tubes, analog oscilloscopes, and digital electronics.
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 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.
Klein Heidelberg (KH) was a passive radar system deployed by the Germans during World War II. It used the signals broadcast by the British Chain Home system as its transmitter, and a series of six stations along the western coast of continental Europe as passive receivers. In modern terminology, the system was a bistatic radar. Because the system sent no signals of its own, the allies were unaware of its presence, and did not learn of the system until well after the D-Day invasion. The system is referred to as Klein Heidelberg Parasit in some references.
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.
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 Marconi and used by the Royal Air Force (RAF), initially for tactical control and later for air traffic control (ATC).
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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