A. Positioning Equipment
(l) SCR-658
The radio direction tinding set SCR-658 has been
found to be the most useful unit to track a balloon-
borne transmitter, within its .limited range. I£
the set is in good condition and the transmitter
signal is good, it is possible to receive from a
transmitter whiQh is 150 miles away at an altitude
of 50,000 feet. At this distance, the elevation
angle is usually not high enough to be reliable,
since below angles of 13~ ground reflection of sig-
nals makes them nearly meaningless. The azimuth
angle and the elevation angle, when above 15~ are
accurate to about 0.5°. It is thus necessary to
use two such sets on about a 100-mile base line
to give a position fix. If the elevation of the
balloon is determined independently, and the eleva-
tion angle is above 13°, it is possible to locate
the balloon-borne transmitter with one SCR-658.
The installation and maintenance of SCR-658 requires
the services of a specially trained man, while the
operation procedure may be made by relatively un-
skilled personnel, with lim:i ted training. For
details of the use of the SCR-658, see War Department
publication TMll-1158!.
( 2) Theodolite
The meteorological theod9lite is useful on daytime
flights when skies are clear for ranges up to 100
miles. If radio datalre available to give height,
the additional information obtained from this in-
&trument--elevation and azimuth angle-JWill completely
fix the balloon's position in three dimensions.
When pressure data are known, two theodolites
with a base line several mil-es in length will also
uniquely locate the balloon. A third metho~. less
accurate but still useful, is the method of stadia
measurements. By carefully measuring, prior to
release, the distance between two distinctive portions
of the train and then noting the angular distance
subtended during flight by these instruments, the alti-
tude and hence all coordinates of the balloon may be
determined.
Regular and frequent checks must be made of the scale
adjustments of the instruments and of the base plate
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