Radar System Engineering

732 RADAR RELAY [SEC. 17.10
pulses is possible (with the amplitude-modulated equipment ), these
effects are greatly reduced. Amplitude selection also provides suppression
of weak interference picked up on the link itself. With this protection
interruptions are extremely brief and cause little difficulty.
17.16. Relay System for Airborne Radar.-In the system just
described, the scanner synchronization was rendered fairly simple by the
continuous scanning, and the problem of obtaining adequate signal
strength was simplified by the use of dkectional antennas. The principal
complications were those involved in the simultaneous transmission of
several sets of video data. The present section will describe briefly
the arrangements used to solve a far more difficult problem, in which the
data originate from a l~ng-range airborne set equipped for sector scanning.
The scanner synchronization, diffi~ult in any case, is rendered far more
so by the fact that the omnidirectional antennas required give so little
gain that the interference problem is severe. Every possible device must
be used to provide a maximum of power from the transmitter, to reject
interference in the receiver, and to protect the synchronization pulses by
coding, by switching, and so on.
The video data involved are simple, consisting merely of radar signals
and of signals from a separate beacon receiver. In order that the two
sets of video signals may be accommodated, cyclical time sharing is used
during the intervals of beacon use, the modulator trigger serving as the
signal that radar is being transmitted on a given cycle.
The design was built around the sine-cosine synchronization method
of Sec. 17”9 and the 300-M c/sec amplitude-modulated r-f equipment of
Sec. 1712. Much experimentation was done, however, with the phaseshifted
pulse method of synchronization (Sec. 17.6), and with the
100-Mc/sec frequent y-modulated r-f equipment of Sec. 1713. The
former combination is outlined in Fig. 1722, in which some parts peculiar
to this system and not heretofore described are shown.
It is necessary to provide the azimuth data in terms of compass
directions rather than aircraft heading. To accomplish this, an a-f wave
is passed through a two-phase synchro on the scanner and a two-phase
differential synchro controlled by a compass so that the two resulting
signals have amplitudes proportional to sin 0 and cos @ respectively,
where 6 is the scanner orientation with respect to true north (Sec. 13.4).
Each of these signals is passed through a phase-sensitive rectifier keyed
by the audio oscillation in order to develop the slowly varying voltages
necessary to control the sine and cosine delay circuits (Sec. 17.8).
The remainder of the synchronizer is like that shown in Fig. 17.14
except for the provision for radar-beacon switching on alternate cycles.
During periods of beacon use, the relay of Fig. 17.14 is to the right,
diverting the modulator pulse from the coder to the scale-of-two multi-