A. Positioning Equipment
(1) SCR-658
The radio direction finding set SCR-658 has been found to be the most useful unit to track a balloon-borne transmitter, within its limited range. If the set is in good condition and the transmitter signal is good, it is possible to receive from a transmitter which 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^{\circ} $ ground reflection of signals makes them nearly meaningless. The azimuth angle and the elevation angle, when above $ 15^{\circ} $ are accurate to about $ 0.5^{\circ} $ . 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 elevation angle is above $ 13^{\circ} $ , 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 unskilled personnel, with limited training. For details of the use of the SCR-658, see War Department publication TMll-1158A.
(2) Theodolite
The meteorological theodolite is useful on daytime flights when skies are clear for ranges up to 100 miles. If radio data are available to give height, the additional information obtained from this instrument--elevation and azimuth angle--will completely fix the balloon's position in three dimensions. When pressure data are known, two theodolites with a base line several miles in length will also uniquely locate the balloon. A third method, 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 altitude 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