Radar System Engineering

684 RADAR RELAY [SEC. 17.3
staggering of two trains, or the lapse of time between a “basic”
pulse which is one of a train and a second pulse occurring a controlled
time later in the same cycle.
Omitting for the moment all questions of amplitude versus frequency
modulation, and all problems of external interference, the choice of a
data-transmission system involves three intricately related questions:
(1) how to avoid interference with the video and trigger signals and
mutual interference among the various data signals; (2) whether to use
c-w or pulse methods; (3) which geometrical quantities among those
descriptive of the scanner motion can best be chosen for transmission.
Since the video signals contain frequencies from nearly zero up to a
few megacycles per second it is not feasible to separate scanner data
signals and radar echo signals on ‘a basis of their frequency components.
This leaves the two alternatives of time-sharing within the radar pulse
cycle, or the use of one or more subcarriers.
In the timi-sharing method the scanner data are sent during the
“rest” interval at the end of the radar cycle when the indicators are idle
and the video signals are not useful. The interval can be occupied by as
many signals as are necessary to carry the information. Pulse-timing
techniques are usually used with time sharing since they are somewhat
simpler and probably more accurate than those involving interrupted
continuous waves. It is usually possible to allow some of the pulses to
take part in the transmission of more than one piece of data. Often the
modulator pulse itself is used as part of this timing system.
When the subcarrier method is adopted, it is customary to use continuous
waves rather than pulses, partly for reasons of simplicity and
partly because of smaller bandwidth requirements. When c-w signals are
used, the various components of the scanner data can be distinguished
from one another by sending each on a separate subcarrier, or by using a
different audio frequency for each signal and transmitting them together
on a single subcarrier.
The geometrical quantities transmitted can be chosen in various ways,
depending on the application. The most important are:
1. Changes in the orientation of the scanner can be transmitted by
means of a wave train in which one cycle represents an advance of
the scanner through a given small increment of angle. If the
scanner velocity is constant or changes only slowly, the wave train
can be a sinusoid, or convertible into a sinusoid; hence a synchronous
motor may be used in the scan converter. Some method of
adjusting the initial phase of the converter relative to the scanner
nnd of recognizing alignment must be provided. This general