Radar display

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Modern radar 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 ultimately developed several different display types.

The radar system transmits a single pulse of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and return to the radar system. The radar receiver converts all received electromagnetic radiation into a continuous electronic analog signal of varying (or oscillating) voltage.

Oscilloscopes[edit]

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 input "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 the input 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 radar receiver provides one of three input channels to 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.

A-Scope[edit]

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 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.

The original radar display, the A-scope or A-display, shows only the range, not the direction, to targets. Some people referred to these displays also as R-scope 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 upward (or in CH, downward), 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 that swept the spot across the display, timed to match the pulse repetition frequency of the radar. This spread out the vertical traces 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 below the display.

Early American and German radars used the J-scope, which resembled a version of the A-scope. These circular J-scopes display range as an angle around the display face. 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 line is π times longer). An electro-mechanical version of the J-scope display remained common on consumer boating depth meters until recently.

Some early radars, notably versions of the Chain Home system, used the HR-scope, a modified A-scope, which displayed the return from two antennas on the same display with the antennas displaced vertically. By comparing the strength of the two "blips", the elevation could be estimated with some degree of accuracy. The name refers to "height-range".

A similarly modified version of the A-scope display was commonly used for ground-search radars, notably in ASV radars - (Air-Surface Vessel). In this case, two receiver antennas were used in front of a common reflector, pointed slightly to the left and right of the aircraft centerline. Reception from both, using lobe switching, was sent to the left and right sides of a vertically oriented A-scope, and range could be measured as before. However, displacement of the target to the sides of the aircraft would result in the return being stronger on one side than the other, causing the "blip" on that side to be larger. This allowed the radar operator to easily indicate what direction to turn to intercept the target. These types of displays were sometimes referred to as ASV-scopes or L-scopes, although the naming was not universal.

Size of A-scope displays vary, but 5 to 7 inch diagonal was often used on a radar display. The 7JPx series of CRT's (7JP1, 7JP4 and 7JP7) was originally designed as an A-scope display CRT. Since the job of the A scope was to track an object, the phosphor used in the CRT has to have a long persistence (the trace remained on the display longer) from sweep to sweep. The P7 phosphor has the longest persistence so the 7JP7 was used.

B-Scope[edit]

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). 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 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. 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 was rotated to point up, and then 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. 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. 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[edit]

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", the moving spot being the target blip. Range is typically displayed separately in these cases, often as a number at the side of the display.

Almost identical to the C-scope is the G-scope, which overlays a graphical representation of the range to the target. This is typically represented by a horizontal line that "grows" out from the target indicator "blip" to form a wing-like diagram. The wings grew in length at shorter distances to indicate the target was closer. 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.

Plan Position Indicator[edit]

This images 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. 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, physically spinning the deflection magnets, but the electronics needed to do this in a "solid-state" fashion were not particularly complex, and were in use in the early 1940s.

Beta Scan Scope[edit]

A Beta Scan display. The long lines running diagonally across the screen represents the middle of the glideslope, the vertical bars are distance markers, and the dashed lines (just visible) represent the "maximum safe distance" an aircraft can stray from the glideslope. The distance between these markers and the centerline decreases closer to the touchdown point.

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.

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