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
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
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. 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".
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 . 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 (2.1 m) long, allowing very highly accurate measurements of range. This was used with coast artillery units, allowing them to lay their guns on even small boats entirely through the radar. 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. 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. 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
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