Radar System Engineering

SEC.8.9]
TRAFFIC
CAPACITY
267
which transforms
to give
n=
W’7–
W+l.
w,
(9)
Figure 8.11 gives a plot of Eq. (9) for two values of ~. For existing radars
and beacons r is ordinarily of the order of 0.1.
It is clear from the foregoing that there is no simple way to estimate
the traffic capacity of a beacon system since it depends on the lowest value
of W tolerable for satisfactory operation, and that in turn depends to a
considerable extent on the indicators of the interrogator-responsor. For
certain actual microwave radars and beacons, something like five radars
giving steady interrogations are allowable without serious impairment
of the presentation. Where necessary, this number can be increased by
designing the beacon to have a shorter dead time.
So far in this discussion, the results of scanning by the interrogating
radars have been ignored. If a
radar scans through 360° and interrogates
the beacon through only 6°~
it will load the beacon only + as
much as if it pointed at it steadily.
The traffic capacity of the beacons
in such a system is increased by a
factor of about 60. Actually, it is
necessary to take account of the
statistical fluctuations of the interrogation.
Therefore, the number
of interrogating radars should be
n
FIG.S.11.—The fraction of interrogating
pulses, W, getting replies as a function of
the number of the interrogating radar
sets, n.
substantially less than would be allowable if they remained uniformly
spaced in their intervals of interrogation. Interrogating planes near the
beacon tend to interrogate through much greater sectors of their scan and
thus load the beacon disproportionately. For this reason, control of the
width of the reply arcs by decreasing the interrogating power would be
desirable.
In any case, if large traffic is to be expected, it is desirable to use some
sort of interrogation coding to ensure that beacons will be triggered only
by those radar sets that want to look at them, and not by ones uninterested
in getting replies from the beacon.
The effectiveness of multiple-pulse interrogation codes in cutting
down unwanted beacon replies can be seen as follows. Let us assume
that N random pulses per second are received by a beacon, and that the
beacon is triggered if any pulse is followed by a second one that arrives
later by an interval of time between D – T and D + T. D is the delay,
and ~ T is the tolerance. Each pulse thus produces a following interval