Radar display

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.

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.

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.

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.

Early American, Dutch and German radars used the J-scope, which resembled a circular version of the A-scope.

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.

[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.

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 "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.

[1] This is typically represented by a horizontal line that "grows" out from the target indicator blip to form a wing-like shape.

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.

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.

Radar cathode ray tubes such as the 7JP4 used for PPI displays had a circular screen and scanned the beam from the center outwards.

In the vertical, the two diagonal lines show the desired glideslope (upper) and minimum altitude approach (lower).

An airport surveillance radar display
Oscilloscope attached to two sine-wave voltage sources, producing a circle pattern on the display.
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.
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.
C-scope display. The target is above and to the right of the radar, but the range is not displayed.
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.
A Beta Scan display.