Radar System Engineering

SEC. 8.9] TRAFFIC CAPACITY 265
different ways in order to give vzmious groups in apparent range on the
indicator. They are instantly readable by rapidly scanning radars, but.
have the disadvantage of being more subject to confusion by overlapping
patterns, when numerous beacons are present at nearly the same bearing,
than are replies that give simpler patterns.
Voice Communication. —Here also, when the repetition rate is high
enough, the relative spacing of pairs of reply pulses or the width of pulses
can be varied at voice frequency to give telephonic communication, usable
when the interrogateor-responsor points steadily at the beacon.
STATISTICAL CONSIDERATIONS
8.9. Tra5c Capacity .-For every beacon there is some limit to the
total amount of energy it can radiate per second, with a corresponding
limitation of the number of its replies per second and of the number of
interrogators that can work with it simultaneously. Beacons have been
made in which the limit to the rate of reply was set by the amount of
interrogation necessary to overheat the beacon until it started a fire, but
this informal system has drawbacks. It is usually desirable to incorporate
an arrangement for limiting the average rate of reply to a safe
value. There are two principal ways of doing this.
In the first method, the sensitivity of the beacon receiver is decreased
as the average number of replies increases. Weaker interrogations are
thus rejected and the replies are limited to stronger interrogatems up to
the maximum number tolerable. For navigational beacons, in general,
this is not a good scheme since it tends to eliminate replies to the more
distant interrogators, which are likely to be the ones most in need of
getting them. For some purposes, where beacons of short range are to
be used in connection with landing systems, this might prove to be a
useful method.
The second method involves reducing the ratio of the number of
replies to the number of interrogations to keep the total number of replies
within the safe limit. The beacon display of all interrogators is somewhat
impaired if there is sufficient overinterrogation to reduce this ratio
markedly, but there is no discrimination against the far-off interrogators.
This result is accomplished by varying the period of insensitivity of the
beacon following the emission of its coded reply. There is always a
finite minimum time between successive beacon replies; the maximum
number of possible replies is thus the reciprocal of this time. Usually
this maximum number is too great for safety unless the “dead” time has
deliberately been made greater than it would naturally be. The simplest
way to protect the beacon against overloading is to arrange that the
receiver will always be dead for the desired time after making a reply and
will then recover sensitivity so quickly that stronger interrogators are