Radar System Engineering

682 RADAR RELAY [SEC. 172
transmitter. At the receiving station, the receiver amplifies and demodulates
the incoming signals and delivers the results to an “analyzer.” The
latter performs the necessary sorting into video signals, trigger pulse, and
scanner data. The video and trigger are delivered immediately to the
indicator system. The scanner data must usually be modified in form
before being passed on either to the indicators for direct use in electrical
display synthesis, or to the “scan converter.” The scan converter uses
these data to construct a duplicate of the scanner motion that can be
used to drive a position-data transmitter associated with the indicators.
Since the requirements of the actual transmission and reception are
very similar to those of television, slightly modified television transmitters
and receivers can be used. Together with the antennas, etc., they will
be referred to as the “radio-frequency” (r-f) equipment. The remaining
components will be called the “terminal” equipment.
Except, perhaps, when microwave frequencies are used in the radio
link, the ultimate limit of sensitivity is usually set by the degree of outside
interference rather than by the inherent signal-to-noise ratio of the
receiver. Many factors must be considered in trying to minimize the
effects of this interference.
1. The strength of the desired signals at the receiver input terminals
should be made as high as feasible compared with that of the
interference. This is chiefly a matter of making proper choice of
frequency, transmitter power, and antenna characteristics.
2. The data signals should, as far as possible, be made unlike the
expected interference in signal characteristics, and every advantage
should be taken of these differences in the receiving equipment.
3. In certain cases a favorable signal-to-interference ratio can be
enhanced by techniques such as the use of wide deviation ratios
with frequency modulation.
METHODS OF SCANNER-DATA TRANSMISSION
In even the simplest situation, the relay link must transmit the radar
video signals, the trigger pulse, signals descriptive of the scanning, and
sometimes range and angle markers. 1 In more complex cases some or all
of these items may be duplicated, and additional data such as beacon or
Identification of Friend or Foe, IFF, signals maybe involved. One of the
major problems of radar relay is to find economical methods of carrying
all this information at one time. In the next few sections it will be
assumed that a single transmitter is to relay one set of data of each
1Range markers need be transmitted only if some error is unavoidably present in
the timing of the modulator trigger pulse. Angle markers, on the other hand, furnish
a convenient check on the accuracy with which the scanner motion is followed.