Radar System Engineering

264 RADAR BEACONS [SEC. 88
to be derived by the use of this more complicated system is relative
freedom from random triggering by other radar sets in the same band since
the probability of getting such a group of pulses by accident decreases
rapidly as the complication of the group increases. This matter is discussed
more fully below. All such systems, however, are subject to some
difficulties resulting from the presence of reflected signals that are delayed
because of the longer path traversed.
Two-frequency lntemogation.-Another method related to the use of
groups of pulses is the use of coincident interrogation at two different
frequencies. The two pulses may be actually coincident, or one delayed
with respect to the other by any fixed amount. Both the value of the
second frequency and that of the delay give additional coding possibilityies.
Communication by the Interrogation Link.—So far, methods of interrogation
coding have been discussed which determine whether or not the
beacon will reply when interrogated. More complicated schemes for
coding the interrogating beam enable the transmission of information to
the vehicle carrying the beacon. Such schemes are usually applied only
in cases where the interrogating antenna points continuously at the
beacon. Information in the telegraph code can be conveyed by turning
the interrogator on and off, by varying the width of the pulses, by changing
the spacing betweeh pairs of them, by changing the number of pulses
in groups, and by varying the pulse repetition rate. When the repeti tion
rate is made high enough, the relative spacing of pairs of pulses or
the pulse width can be varied at voice frequencies to give telephonic
communication.
88. Reply Codes. Frequency .—Some coding can be had by use of
several spot frequencies for beacon replies.
Gap Coding.—The beacon transmitter can be turned on and off to
give Morse letters. This system is useful only when the interrogator is
expected to look at the beacon steadily. A variant that has been suggested
for use with scanning radars is a rapid code of this kind which
would break the reply arc on the PPI into dots and dashes. Only simple
codes could be used because of the limitation of beamwidth and the
complete uncertainty as to the part of the coding cycle for which the
broken reply arc would be centered.
Width Coding. —Letters of the Morse code can be transmitted by
changing the width of the pulses in the proper sequence instead of turning
the pulses on and off. Beacons emitting such width-modulated replies
will give a display on scanning radars and have the additional slow gap
code which can be read if the seaming is stopped.
Range Coding. -Codes readily visible on the PPI, as in Figs. 8.2a and
8.2b, are obtained by having the trigger operate a coder that causes the
emission of a number of reply pulses. These may be grouped in time in