Radar System Engineering

SEC. 13.11] ANGLE INDICES 517
the second synchro to introduce the dilTerential setting. The two nulls
will result in two signal markers 180° apart. If this is undesirable one
of them can be removed by a method involving the use of a multiphase
rotor in the second synchro. A second modulation envelope, 90° out of
phase with the first, is obtained and used to blank one of the markers
(see Fig. 4.5, Vol. 22).
The production of a movable index by the contactor or photocell
method requires that the differential setting be made mechanically.
This method is seldom used unless the indicator contains a mechanical
repetition of the scanner motion.
Movable electronic indices appear on the indicator only when the
cathode-ray beam is on a particular part of the tube face. To set an
index on an echo, it is essential to be able to observe the results of moving
the index. If the scan is rapid, the o~erator has freauent chances to do
so, but if it is slow he cannot correct
an error for several seconds.
This is not too serious in the case
of the movable range marker,
because it appears at all azimuths
and can be’ 6steered” in the proper
direction as it approaches the persistent
echo remaining from the
previous scan.
An angle index, on the other
hand, if formed by the methods of
@w
),1
1
Data
transmitter
* Operator’s control
Display
circuits
FIG. 13.35.—Substitution method for movable
angle index.
the preceding paragraph will “flash up” only as the scanner passes by.
The operator has no chance to make a correction before the next scan, and
even then the correction must be an educated guess. It is very difficult
to make accurate settings within the space of a few scans, especially if
there is relative motion between radar set and target. 1
It is, therefore, essential on slow scans to use some other method. For
all displays except rotating-coil PPI’s, the so-called “substitution”
method illustrated in Fig. 13.35 can be used. From time to time control
of the display is switched from the regular angle data transmitter to one
that can be set by the operator. During these intervals the direction
taken by the range sweep is determined by the setting of the “ substitution”
transmitter and is thus under the control of the operator. The
switching may be made to occur automatically on a certain fraction of the
range sweep cycles equally distributed throughout the scan (which
requires rapid electronic switching), or it may occur continuous y over an
I The problem is greatly simplified if aided tracking is used—that is, if come am%of
computer moves the index in accordance with the relative motion, using knowledge
acquired by making the best settings possible in the past.