By knowing the altitude of release and counting the number of switches from conductor to insulator, the position along the the commutator is known. This in turn is calibrated to give pressure values, from which the altitude may be computed.
This system was not suitable for floating balloons because (1) only 70 to 90 discrete contacts are provided to cover the entire atmospheric pressure range; this means that the best readability obtainable with this system is about ±10 millibars. (2) When the balloon oscillates about a floating level, the frequent changes from one contact to another give ambiguous readings, since the number of discrete resistances used is limited.
For circuit details of this unit, see T.B. Sig. 165 and the Weather Equipment Technician's Manual.
(2) Specially Coded Radiosonde Modulators
To remove the ambiguity of altitudes reported by the system above, extra resistances were introduced into the circuits of those contacts near the floating level; thus, each contact gives a distinctive frequency and each pressure (altitude) can be clearly distinguished.
In this system, there still remains the lack of resolution or sensitivity inherent in the modulator with 70 to 90 contacts.
(3) Olland-Cycle Modulator
To improve the sensitivity of the pressure measurements, an Olland-Cycle (chronometric) pressure modulator was developed. Seen in Figure 30, the modulator contains a standard Signal
[figure]
Figure 30. Olland-Cycle pressure modulator.
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