Radar System Engineering

SEC. 17.15] A GROUND-TO-GROUND RELAY SYSTEM 727
used to provide the two channels, since, at the time of design, equipment
accommodating subcarriers was not available and weight and power were
not crucial items. The remaining data are combined on these same two
channels as indicated in Fig. 17.20. The mixing and switching of the
signals for the first transmitter is fairly simple. Range markers and the
proper set of angle markers are mixed with each set of video signals, and
the two sets are fed to a video switch, which passes the MTI signals for
the first 50 miles or so and the lower-beam signals thereafter. The
switch is like the circuit of tubes Vl~ and Vlb of Fig. 17”3, but has signals
on both grids.
The synchronizer is similar in function to that of Fig. 17.5 (including
Fig. 17.2), 1 with the addition of a scale-of-three circuit to produce the
beacon switch pulse on every third cycle. The third pulse of the modulator
code is counted down for this purpose and passed to the signal
switching unit. The switching signal to be relayed is delayed 16 ~sec (by
reflection in an 8-~sec delay line) in order that it be clear of the azimuth
pulse at the receiving station.z The three-pulse code, the azimuth pulse,
and the beacon pulse are “mixed” by using them all to trigger a blocking
oscillator.
The video signals to the second transmitter are switched twice.
The upper-beam video and the beacon video are switched cyclically as
described above. The switch is controlled by a flopover circuit (Fig.
13. 16) which remains in the stable state that causes video switch a
to pass the radar signals until a beacon switch-pulse occurs; it then passes
to a second stable state that causes the video switch to pass beacon
signals. At the next modulator pulse the original condition is restored.
A video switch provides for time sharing between the video signals
and the synchronizing pulses in order that the former shall not interfere
with the latter. In its normal position, the flip-flop holds switch b in
the state that passes pulses. Firing of the flip-flop by a pulse delayed by
30 psec from the modulator pulse reverses the video switch allowing echo
signals to pass. The flip-flop returns spontaneously to its original state
shortly before the start of the next radar cycle. Signals from switch b
are combined with range and angle marks in a video mixer, from which
they pass to transmitter No. 2.
At the receiving station, the signals pass through a duplexer to two
receivers. The first delivers the time-shared MTI and lower-beam video
signals, together with markers, directly to the indicators. Signals from
1 The circuit details of the equipment actually tested differ considerably from
those of Figs. 17.2 and 175.
2 In order that pulses passing through the three-pulse coincidence circuit shall not
cause false coincidences, they should follow each other by at least the sum of the code
length and the pulse length—in this case a total of 8 ysec.