mass-production methods and quality control associated with standard dry batteries gave greater assurance of satisfactory performance.
Subsequent measurements made of the temperature inside the transmitter battery pack showed that the temperature can be maintained above -10°C if the transmitter and batteries are housed in a box insulated with one- to two-inch walls of Styrofoam. This insulation is effective even through long nighttime periods when no solar heating is added.
One type of battery tested in flight was a light-weight wet cell (Burgess Type AM) of the "dunk" type, (magnesium + silver chloride). These cells were vacuum-packed to provide indefinite shelf-life. Activated by immersion in water just before release, they were expected to produce a constant voltage over a period of 6 hours to overcome cold temperature effects. Those units used proved to be rather unsatisfactory and subject to erratic behavior. Furthermore the cost of the cells was very great compared with ordinary cells.
E. Radio Direction-Finding
For ground stations, when the balloon-borne transmitter is a T-69, the SCR-658 RDF set has been used. With such a set the radio signal can be picked up at distances up to 150 miles and good azimuth bearing may be obtained (accurate to less than one degree). Although the elevation angle may be obtained with equal accuracy when free from distortion, angles of less than 13 degrees are usually affected by ground reflection to such an extent as to render them valueless.
To extend the range over which such sets were effective, two or more usually were used, positioned along the expected track of the balloon at intervals of about 100 miles. With two sets giving crossed azimuth "fixes" the position may be determined. If the elevation angle is above 13 degrees, it is possible to fix the balloon with one SCR-658 (assuming the pressure altitude is known).
For details of the maintenance and use of the SCR-658, see War Department publication T.M. 11-1158A.
When aircraft are used to follow and position the balloon, the use of a radio-compass is found to be feasible, using the AM-1 transmitter at a frequency that is within the limits of the compass receiver. By homing on the signal and flying along the indicated bearing until the compass needle reverses, the balloon's position may be found from initial distances of up to 500 miles. No appreciable cone of silence has been observed in recent flights which used a transmitter operating at 1746 kc.
Radio compass equipment, AN/ARN-7, is described in U. S. A. A. F. publication T. O. 68-10.
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