Radar System Engineering

486 1’IIE RECh’I J’I.YG SI”.57’E.JI—I.VI) IC.4 TORS [SEC. 13.4
beyond the point ~vhere the resolving po\ver of the eye ceases to be the
limiting factor (5- to 7-in. magnetic tubes, 3- to 5-in. electrostatic tubes).
On the other hand, if geometrical measurements or estimates are to be
made, or if plotting is to be done, it is desirable to have as much dispersion
as possible; for such use the larger sizes are definitely preferable.
This requirement must be balanced against the available space, particularly
in airborne equipment.
For many purposes even the largest available tubes (lZ-in. diameter)
are still too small, and it is desirable to provide an enlarged presentation.
Since at most scanning rates phosphorescent screens do not provide
enough light for projection, it is necessary to devise special methods. Of
those used to date, one involves the projection of a photograph of the
PPI or other scope, which is developed and projected in a few seconds by
special techniques; the other involves opaque projection from the skiatron
screen (see Sec. 7.3).
COORDINATION WITH THE SCANNER
Under various circumstances several different sorts of electrical
information must be delivered from the scanner to the indicator. (Some
of these have already been illustrated in Sec. 12.1. )
1. Signals capable of controlling an electromechanical repeater. The
repeater can bc used to position a cathode-ray-tube coil or to
provide a dummy scanner shaft to which the final data transmitters
are attached.
2. Slowly varying voltage proportional to the scan angle, to be used
in the cartesian display of angle as in type B and type C displays.
The voltages can be obtained directly from a potcntiomctcr of
proper characteristics or from the envelope of a carrier which has
been properly modulated. In many cases where the angle displayed
is small, sin 0 can be substituted for 0.
3. Signals produced by passing a range sweep voltage (or current)
through a resolver to produce an electronic 1’1’1 or an RH 1.
4. Slowly varying voltages proportional respecti\rellf to the sine and
cosine of the scanner angle, which are used to control sa\vtooth
generators in such a way that they produce s\veep components
equivalent to (3). This is spoken of as “ pre-time-base” resolution.
The voltages themselves can be transmitted by a potentiometer,
or a modulated carrier can be used as in (2). The
approximations sin d = 6 and cos 6 = 1 am often {Ised.
13.4. Angle-data Transmitters.—Thc devices that provide the
scanner data are known as “ (angle) data transmitters.” ‘~hey in-