The display

The object of the display unit, which is sometimes called the indicator, is to present to the observer a clear and accurate picture of the area around the ship, showing all objects in their correct po­sitions in range and azimuth. Assuming that the receiver has ampli­fied the echo pulse without destroying its essential shape, the main task of the display unit is to permit the time interval between trans­mitted pulse and returning echo to be measured in some simple and accurate way. The time taken by the wave travelling to and return­ing from a target 50 yards away is about 0.3 microseconds. To meas­ure this is clearly beyond the capacity of mechanical devices and some kind of "electronic stop watch" is needed.

The stop watch of the radar display is the cathode-ray tube (CRT) and its associated circuits. Essentially the CRT consists of a spe­cially coated screen and an electron gun. The gun fires a very narrow stream of electrons of the screen, which fluoresces, or glows brightly, at the point of impact when the stream is strong enough. The electron stream is the "hand" of the stopwatch and the small bright area is called the spot. The electron stream and therefore the spot can be deflected across the screen at very high, predetermined and constant speeds when suitable electric or magnetic forces are applied to it, while the brightness of the spot can be varied by altering the intensity of the stream.

The form of display used in marine radar is known as the plan position indicator, or PPI. In this, the spot when at rest is in the centre of the circular screen and is deflected radially, its direction of movement being made always to coincide with the direction of the rotating scanner. The deflection commences at the moment a pulse is transmitted and shows as a radial line, known as the trace or sweep. (Fig. 24)

The speed of radial deflection is arranged to suit the maximum range of the display at the time. If this range should be 10 miles, the spot is required to reach the edge of the screen at the moment an echo would return from such a target. As the time interval would be that for a wave to travel 20 miles, the spot must move from centre to edge in this time (124 microseconds). As it moves at a constant speed, its distance from the centre at any instant represents the range ot a target whose echo arrives at that instant. The echo pulse is used

(5)Later trace showing echo paint ana fauung afterglow due to rotation

Note. Read captions in the order given

Deflector colts

Electron gun

\Echo paint

Centre spot

}(6)Recelpt — _tr0, (of echo causes lnten-{\) \slucatLon of elect- *^ir^ ~=%ib )ron stream.

Fluorescent screen PP.1

{2)Radial bending of electron stream by magnetic field of deflector coils in syn­chronism with each trans­mitted pulse

(4)Future positions^

of trace conform- с ₀ a

~* log with scanner rotation. There may be 2000 traces per revolution

(3) Trace made by spot as electron stream is deflec­ted radially (afterglow omitted). Spot Is invisible during return to centre

Fig. 24

to brighten the spot momentarily and thus it leaves a paint or a small area of greater brightness in the correct position on the screen, from which the bearing and range of the target can be read directly. The material used for coating the screen is such that the bright paint takes an appreciable time to fade. This quality is known as afterglow or persistence.

As the echoes from all targets will be shown in this manner, if they are within the maximum range chosen and they return echo pulses of sufficient strength, the observer will see a plan view of the objects in the area around his ship.