Radar Display - A-scope

A-scope

The original radar display, the A-display, shows only the range, not the direction, to targets along a scale. Some people referred to these displays also as R-scope for range scope. An A-scope, a properly configured electronic oscilloscope commonly used with radar in World War II, produces an instantaneous trace of the oscillating voltage signal from the radar receiver on a the screen of a cathode ray tube.

An attached sawtooth voltage generator moves the oscilloscope spot across the screen at a constant fixed speed along the abscissa (horizontal axis). When the antenna sends a radar pulse, the "sweep" starts across the A-display. The sweep reaches the far end of the A-display at the end of the maximum return time of the pulse. The return time of the signal corresponds to the distance to the target divided by the speed of light; therefore, the distance along the abscissa directly indicates the range to any target, usually measured against a scale below the display.

The A-scope displays any detected signal directly on its ordinate (vertical axis), drawing a "blip" (or "pip"), an upward deflection from the abscissa on the cathode ray tube. The magnitude of the blip gives some indication of the number and size of the targets. The A-scope does not indicate the direction to the target.

Aircraft and ships produce coherent echoes on the A-scope, whereas precipitation produces rapidly fluctuating incoherent echoes. Recognition of this phenomenon led to the development of weather radar.

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 abscissa 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, although the naming was not universal.

The display system of A or J scopes was similar to that of an oscilloscope. The cathode ray tube (CRT) was generally an electrostatic type with a circular face. Electrostatic CRT had horizontal (X-axis) and vertical defection (Y-Axis) plates built in. The position of the beam or sweep was based on the applied voltage between the respective horizontal and vertical deflection plates. Size of A-scope displays vary, but 5 to 7 inch diagonal was often used on a radar disply. 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.