Radar System Engineering

238 THE EMPLOYMENT OF RA DAR DATA [SEC, 77
saw service in the European war, but it is based soundly on months of
combat experience with improvised and frequently changed systems and
devices. An actual FDP using one of {he preproduction sets is shown
in position in eastern France in Fig. 7.15. The central antenna is that of
the main radar; the two that flank it are those of British Type 13
heightfinders (Sec. 6.12). The system is largely mounted in and operated
from trucks, and it can all be taken down, loaded on trucks, unloaded
at a new site, and erected and put back on the air in a total time of less
than 24 hr.
7.7. Close Control with SCR-584—Figure 710 shows, attached to
the FDP’s, SCR-584 radar sets. This equipment, designed for accurate
tracking of a single aircraft at a time, in order to permit antiaircraft fire
control, can also be used to provide to a ground controller the information
necessary for highly precise control of the aircraft being tracked. It was
so used by the TAC’S in Europe.
Since the information on target position is normally transmitted to
the antiaircraft computer by means of synchros or accurate potentiometers
(Sec. 13.4), it is necessary to provide a supplementary method of
displaying target position th enable the set to be used for ground control.
This was first done by means of the 180° plotting board shown in use in
Fig. 7.16. .4n arm, pivoted beneath the surface of the board on the side
nearest the controller, swings in azimuth in accordance with the orientation
of the radar antenna. Range information from the set is used to
control the position of a range carriage that runs in and out along the
azimuth arm. The carriage has an optical system which projects a small
spot of light up through the glass surface of the plotting board. Over the
surface of the plotting board is stretched a map of the area surrounding
the radar set, carefully adjusted so that the point on the map occupied by
the radar is directly above the pivot of the moving arm. The scale factor
of the range-carriage mechanism is adjusted to correspond \vith the scale
of the map being used, which may be either 1 to 50,000 or 1 to 100,000.
When the radar is tracking a target, the spot of light from the range carriage
shows up on the map just at the position over which the airplane
is flying at that moment.
The controller can thus issue hk instructions with full and constant
knowledge of the exact position of the aircraft under his control. This is
to be contrasted with the situation that obtains in ground control from a
scanning radar; in that case the controller has only one “look” at his
target per scan—that is, perhaps once in 15 sec. However, he sees all
other targets in the air within range of his radar. The controller in the
SCR-584 sees the position of his aircraft continuously, and in proper
relation to the terrain, but the price paid for this is that he has no knowledge
of the whereabouts of other aircraft.