Radar System Engineering

218 THE EMPLOYMENT OF RA DAR DATA [SEC.73
index will continue to stay on the target echo, provided that the correct
value of wind has been set into the device by means of the two “wind”
knobs shown. Even after the index and the target echo have moved off
the face of the radar indicator, the dials will still show the target position
relative to the aircraft.
The determination of wind requires that two fix operations like that
just described be made on the same echo (which need not correspond to a
target whose identity is known). The major correction which will have
to be made at the second fix is that arising from the effect of wind. The
wind and fix knobs of the GPI are arranged so that they can be gripped
together and turned together by the same amount. A proper choice of
scale factors will enable the operation of taking the second fix to correct
the wind velocity at the same time. If the wind error is to be taken out
exactly by this process of double-gripping the wind and fix knobs, account
must be taken of the fact that the proper ratio between the scale factors
of these two knobs is a function of the time elapsed since the first fix was
made. The GPI is arranged with a ratio of scale factors which changes
with time in the proper way, so that double-gripping will remove wind
errors entirely at the second fix, providing this second fix is made at any
time up to six minutes after the
4/+
Nonreflecting surface
I
10 to 20 % reflection
Lamp
Observer
+
~-L
Edge4ighted
s
scale
FIW 7.4.—Method of superimposinganedgelighted
scale on a CRT pattern.
first.
7.3. Aids to Plotting and Control.—The
problem of controlling
aircraft on the basis of radar information
involves careful plotting of
the signals seen on an indicator.
The controller desires to know the
position of a given signal with
respect to a map or grid—which
must therefore be somehow superposed
on the display—and hc
wishes to record the position of a
target on successive sweeps, to determine the direction and approximate
speed of its motion.
Optical Superposition. —Optical devices of the same general character
as the chart projector mentioned in the last section are helpful for these
purposes. Two schemes involving the use of a partially reflecting mirror
to place a virtual image of a screen in optical superposition with the display
are illustrated in Figs. 7.4 and 75.
The device of Fig. 7.4 is identical in principle \vith the chart projector
shown as Fig. 7.1. The virtual image formed in this case, however, is
that of an edge-lighted screen \vhich can be engraved \vitha scale or a set
of indices. Multiple sets of indices with different scale factors ran be