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SEC.17]
GENERAL COMPARISON OF BASIC TECHNIQUES
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the direction of the destination, but this requires accurate knowledge of
the drift.
Homing may beaccomplished inthe general case, cutting across the
two coordinate families, by steering so that the two instrument readings,
which indicate the present position of the craft, approach their destina-
tion values at rates approximately proportional to the amounts by which
they respectively differ from those values. (For example, if the present
Gee coordinates of an aircraft are 6.4 and 38.5 and the destination is at
16.6, 35.1, the craft should be steered so that the first coordinate increases
about three times as fast as the second diminishes. ) In a system like
Gee, where both the destination readings can be set into the indicator
beforehand, an almost direct homing track maybe held by eye estimation
of the motion of the pips on the screen.
Strict proportionality of change
of coordinates could be maintained by the use of rather simple auxiliary
equipment, which would direct the craft along a definite track to the
destination. Such equipment has not been developed as yet, but sug-
gestions regarding its form and use in the Loran system are given in
Chap. 4. The technique might be adapted to any system in which
position is referred to a coordinate grid based upon instrument readings.
1.7. General Comparison of Basic Techniques.—Some comparison
of the basic techniques can be made without entering into details of
instrumentation or operation.
1. The direction-finding and hyperbolic systems, which do not use the
echo or bounce-back principle of radar, have a great advantage in
range or coverage, both because they are not subject to the two-
fold dispersal of a signal traveling over a return path and because
they can operate with long waves, for which attenuation is much
less than it is for short waves and for which range maybe extended
by ionospheric reflection. Radar systems must use ultrahigh or
microwave frequencies in order to get definition in the optical
sense.
2. Systems that measure distance by timing’ of signals give greater
line-of-position accuracy than direct angle-measuring systems.
With radar systems, accuracy of the circular lines of position is
independent of distance or direction. With angle systems, accu-
racy is inversely proportional to distance. With hyperbolic sys-
tems (and the Sonne direction-finding beacon) accuracy depends
on both distance and direction.
At long distances the hyperbolic
systems effectively determine angle by distance-measuring tech-
nique, with a considerable advantage in precision.
Both angle
and hyperbolic systems are affected at long ranges by errors due
to variations in the ionosphere, but these affect the direction of