Radar System Engineering

726 RADAR RELAY [SEC. 17.15
RADAR RELAY SYSTEMS
Two complete radar relay systems will now be described; their general
features are sufficiently applicable to future requirements to make this
worth while, even though better systems could now be designed. The
requirements for the two systems are quite different. The first involves
the transmission between fixed ground stations of three sets of radar echo
signals and one set of beacon signals, all resulting from antennas mounted
on a single continuously rotating scanner. Although the number of sets of
video signals is large, the use of directional antennas eases the r-f problem,
and the continuous scan renders scanner synchronization relatively
simple.
In the second example, one set of radar signals and one set of beacon
signals are relayed to ground from a long-range airborne radar set
arranged to permit sector scanning. In contrast to the former case, the
“picture” data are relatively simple, but the requirement of large
angular coverage forces the use of low-gain antennas and puts a severe
requirement on the r-f system. Sector scan and the turning of the
aircraft greatly complicate the scanner-synchronization problem.
17.15. A Ground-to-ground Relay System.—The radar set originating
the data in this example is a ground-based microwave set (see Chap.
15) in which the scanner, rotating at either 2 or 4 rpm, carries two
radar antennas and one beacon antenna. One of the radar antennas provides
long-range low-angle coverage; the other provides coverage at high
angles. Both regular video signals and MTI (Chap. 16) video signals are
derived from the upper-beam signals; this beam is chosen for MTI because
it is the one predominantly used at the shorter ranges where the clutter is
worse.
Thus four sets of video signals must be transmitted: lower-beam
radar echoes, upper-beam radar echoes, upper-beam MTI video signals,
and beacon responses. Time sharing is used to put two. sets of video
signals on each of two carriers. One channel is shared between the MTI
video signals and the lower-beam video signals, MTI video being transmitted
for a time interval corresponding to the first 30 to 50 miles of
range from the radar, and the lower-beam video for the remainder of the
radar cycle. A second channel is shared between the upper-beam radar
echoes and the beacon signals; since the data from these two are simultaneous,
switching must be done on a whole-cycle, rather than on a fractional-cycle,
basis. Two pulse cycles are allotted to radar, then one to
beacon, and so on, the unequal division being used because signal sensitivity
is more critical in radar than in beacon operation. The resultant
loss in sensitivity is 0.5 db for radar signals and 1 db for beacon returns.
Two transmitters, feeding a common antenna through a duplexer, are