Radar System Engineering

268 RADAR BEACONS [SEC. 8.10
of sensitivity totriggering byasecond pulse of amount 2T. If Npulses
arrive per second on the average, the average sensitive time per second
will be 2TN. This is also the fraction of the time that the beacon is sensitive,
or the probability that a random pulse will find it sensitive and
trigger it. Since N random pulses arrive per second the number of
triggering per second will be N times this probability, or 2TN2. Thus,
the ratio of the number of beacon replies to the number of random pulses
per second is 2TN. If, for example, T = 5 psec and N = 500 pps, the
product 2TN has a value of ~~, the number of random triggerings per
second being reduced from 500 to 2.5. If a three-pulse code is used, a
similar argument applies for the third pulse and an additional factor of
2TN will come in to give 4T2N3 as the number of random replies per
second.
The argument also applies to two-frequency interrogation. IIere
the number of random interrogations will be 2TNIN2 if NI and Nz are
the respective random rates at the two frequencies, and the beacon is
made so that the two pulses have to arrive in a certain order for triggering
to occur. If the order is immaterial, the rate of random interrogation
will be twice as great, namely, ATNIN2.
In making a comparison between the two systems, it is necessary to
keep in mind that one must use estimates of the numbers of random
pulses that will be present after the system has been introduced. Changing
radar sets to give double pulses increases the number at the radar
frequency; adding coincident interrogating pulses at a second frequency
increases the number at that frequency.
8.10. Unsynchronized Replies.-JVhen a beacon is being interrogated
by several radars at the same time it replies to all, and the receiver of each
interrogator-responsor will respond to all of these signals. Replies to the
interrogation by a particular radar are synchronized and give a stationary
pattern on its indicator as do its radar echoes; the replies to interrogations
by the other radar sets appear to be unsynchronized and thus show at
random on the screen and are similar to random noise from other causes.
Figure 8.12 shows several beacon signals appearing on the PPI of a 3-cm
radar; one of them is sufficiently overinterrogated to show unsynchronized
replies. This effect is displeasing aesthetically, but experience has shown
that a great amount of overinterrogation is necessary to interfere seriously
with recognition of the beacon. The problem is more troublesome when
numerous adj scent ground or ship interrogators are interrogating numerous
airborne beacons. Unsynchronized replies coming from the multiple
interrogation of a near-by beacon may interfere with detection of the
signals from one farther away at the same azimuth. It is difficult to
make a definite statement about the extent of the interference because it
depends so much on the characteristics of the radar set. The use of