Radar System Engineering

SEC. 87] INTERR~A TION CODES 263
with time. The intersections of the horizontal lines with the sawtooth
curves give the times at which replies are received by a radar tuned to the
frequency corresponding to the horizontal line. In case a all interrogators
at different frequency get regularly spaced pulses, in case b they come
in pairs, the grouping depending on the frequency. This use of a single
tube for receiver and transmitter is convenient especially because it tends
to mitigate the effects of antenna mismatch often encountered in installations
in the field. The shift of frequency for both transmitter and
receiver is automatically almost exactly the same. Separate receivers
and transmitters whose tuning controls have been ganged can be used
if desired but this method requires more care in design and installation.
CODING
The simplest type of beacon replies ~vith a single pulse to every pulse
of sufficient strength received within a certain band of frequency. For
purposes of reducing interrogation by confining it to only those interr~
gators that are intentionally seeking beacon replies, coding of the
interrogation may be used. Likewise, the replies may be made more
complicated in a variety of ways for the purpose either of identifying
the beacon or of using it as a part of an auxiliary communication system.
8.7. Interrogation Codes. Frequency. -If the beacon is one that
replies to all radars of a given type, the frequency for interrogation is not
highly characteristic. If it is one to be used in a system with specially
designed interrogator-responsors, certain discrete interrogation frequencies
can be used as part of a code characterizing the particular
beacon.
Pulse Length.—In order to avoid excess interrogation of ground
beacons by airborne radars not interested in beacon replies, some beacons
have been designed so that they are triggered only by pulses longer than
those used for ordinary radar search. Careful design of the beacon
receiver is required to prevent the stretching of strong, short pulses to
give the same effects as longer ones of medium strength, but this problem
has been solved satisfactorily. The beacon reply is obviously delayed
since it cannot occur until after the lapse of a time longer than the duration
of the search pulse. This delay must be allowed for in estimating
the range to the beacon.
Multiple Pulses.—The beacon can be made to include a decoder so
that a trigger is developed only upon reception of a pair of pulses having
the proper separation, or it can be made so that it will reply only upon
reception of a sbill more complicated group of properly spaced pulses.
A considerable time delay may be involved, but in many cases this can
be obviated by having the sweep of the interrogating radar started by the
last of the group of interrogating pulses. One of the prinrip:d benefits