Radar System Engineering

730 RADAR RELAY [SEC. 17.15
the second receiver contain the coded modulator pulse, the azimuth pulse,
and the beacon switch pulse. They pass through a switch which excludes
all signals while video is being transmitted but opens shortly before the
arrival of a trigger pulse. This switching, the decoding of the modulator
pulse, and the derivation of the scanner information from the azimuth
pulse, are done by methods similar to those of Figs. 17.2 and 17.5. The
upper-beam video signals and the beacon video signals are separated by a
video switch which consists essentially of a pair of out-of-phase switches
similar to that made up by tubes V1~ and Vlb in Fig. 17”3. The video
switch is controlled by a flopover which is triggered to the beacon position
by the beacon switch pulse and to the radar position by the next modulator
pulse. The switch pulse is singled out by coincidence with a pulse
derived locally (N) at the proper time by delaying the decoded modulator
pulse.
The time occupied by the pulse code results in the triggering of the
indicators 8 ~sec too late. This produces a slight distortion in the displays,
but there is no error in range measurements since accurate range
markers are transmitted with the video signals. The display distortion
is unimportant because short-range displays are not used. Similarly, the
fact that the indicators must be blanked out for the first 30 ~sec is of no
importance since targets at such close range are practically never of
interest.
Several r-f equipments, including the three described in earlier
sections, were tried experimentally in this application. All operated
with reasonable satisfaction, maximum range being limited in every case
only by the line of sight. On the whole, the microwave equipment is
considerably superior to the others because of its compactness, the small
power involved, the narrowness of the beam, and the greater freedom
from interference. However, the fact that the use of frequency modulation
did not permit the pulses to be transmitted at higher level than the
video signals was a definite handicap for reasons described below.
The equipment as a whole operated about as anticipated. Comparative
PPI photographs taken simultaneously at the two stations are shown
in Fig. 17.21. The only difficulties of consequence involved occasional
loss of synchronization, usually because of pulse interference picked up
on the radio link or on the radar set. The direct results of spurious
triggers on either the angle data or the sweep triggering were not appreciable,
but loss of the trigger occasionally upset the sequencing with
unfortunate results. Once the proper chain is broken, it can be spuriously
started by interference or by video signals and remain in error for several
cycles. Both the indicator sweeps and the azimuth data are then in
error, sometimes by as much as 5° or 10°. The resulting angular error
persists until it is manually removed. When amplitude selection of the