A review of this system by members of the University staff has been published in "Transactions of the American Geophysical Union(2)". Earlier work on this development has been reported by members of this Research Division in"Technical Report 93.01(3) and in the "Journal of the American Meteorological Society"(4).
A manual for those interested in making use of balloon systems of this type has also been published as "Section II, Operations" of our Technical Report 93.02"(1). This report consists of a discussion of instrumentation for balloon systems, techniques for launching and trading, and telemetering from balloons as developed and tested at New York University.
C. Telemetering From Balloon Systems
The second requirement of this project was the investigation, development, and testing of balloon borne telemetering systems. The development was completed in June of 1948 and a final report(7) of work accomplished and recommendations made to the sponsor at that time.
Two types of transmitter units were suggested as a means of accomplishing the telemetering of data from a balloon to ground station receivers. A high frequency system, making use of line-of-sight transmission allows for accurate positioning of the balloon system from two ground stations. The line-of-sight characteristic, however, limits the range of this type transmitter, and ranges in excess of 250 miles are not to be expected with a balloon system floating at 40,000 ft.
Three line of sight transmitters were designed for use in balloon work. The first, the FM-1, was designed to operate at 72 mc, using a conventional reactance tube modulator. Several stages were included to deliver 1 watt output at the design frequency. The unit was quite complicated and the required input power large due to the requirement for several stages to transmit at the high frequency. Fig. 4 is a schematic of the FM-1 transmitter.
In order to overcome this limitation of FM sets, a two tube transmitter was developed (Fig. 5). Variation in vacuum tube resistance is used to modulate the oscillator plate voltage of a self-excited oscillator in accordance with the audio signal. This provides the frequency modulation desired. In order to maintain a stable center frequency and render the oscillator insensitive to changes in supply voltage, a neon tube voltage regulator was included.