Radar System Engineering

SEC. 7.4] THE RELAY OF RADAR DISPLAYS 225
or of any deliberate or accidental deformation or distortion. The
accuracy is determined entirely by the precision of the scanning PPI and
by the geometrical relationship between the PPI and the map. Slight
changes in parallax between the center and the edge of the tube can be
compensated by using a suitably nonlinear range sweep.
If the radar site is moving, a compensating motion must be applied
to the map in order to maintain the proper relationship on the final
displays.
This method can be applied to any type of index or marker, and has
obvious uses as a substitute for elaborate computing circuits under some
circumstances.
7.4. The Relay of Radar Displays.—It is common for the best location
of the radar station and the optimum location of a control center to be
different. The radar site is chosen from the standpoint of good coverage,
freedom fl om permanent echoes, and the like; the criteria entering into
the choice of site for a control center are usually entirely different. Also,
a control center should receive supplementary information from other
radar installations located elsewhere, even though a single radar equipment
may provide the primary data for control of operations. This will
enable the coverage of the primary radar to be supplemented by information
from neighboring sectors, and will provide coverage of possible
“blind spots” of the primary radar.
The telling of plots by telephone land line was the technique first used
(Sec. 7.5) for the transmission of radar data from one point to another.
Substantial errors and delays are inherent in this procedure. Far more
important, when the information to be transmitted has been @hered
by a modern long-range, high-definition radar, is the low traffic-handling
capacity of the system of telling and plotting. Literally hundreds of
targets may show at a given time on the indicator of such a radar as the
first one described in Chap. 15; an attempt to convey with adequate
accuracy and speed the information provided at a rate of four sweeps per
minute is hopeless under such conditions.
Considerations such as these led to the development of means for
reproducing radar displays at a distant point by transmitting the radar
video signals and appropriate synchronizing information by more or less
conventional radio practice. The technical problems of “radar- relay, ”
as this is called, have been worked out, and systems for the purpose are
discussed in detail in Chap. 17.
Radar relay is mentioned here to emphasize its usefulness as one
element in the creation of an organization for the use of radar data. An
example of an operational system in which this technique is important
is given in Sec. 7“8.