CHAPTER A2 
RADIOSONDE INSTRUMENTS 
1. Radiosonde Instruments. The radiosonde is a balloon-borne, bat- 
tery-powered instrument usedtogether withthe ground-receiving equip- 
ment to delineate the vertical profile of the atmosphere. Pressure is 
measured by means of a baroswitch whichemploys an expanding aneroid 
pressure cell to move a contact arm across a commutator bar as the 
pressure decreases. Temperature is measured by a thermistor; the 
electrical resistance of the thermistor being a function of temperature. 
Relative humidity is measured by a hygristor; the electrical resistance 
of the hygristor being a function of relative humidity and, to some extent, 
temperature. As the radiosonde ascends, the thermistor and hygristor 
are switched sequentially into the modulator circuit by the baroswitch. 
The frequency of the received signal, therefore, is alternately a function 
of temperature or humidity, and may be any value from 10 to 200 Hz. In 
addition, fixed resistors are connected periodically into the modulator 
circuit by the baroswitch so that the frequency transmitted, when these 
are connected, will show any change that may be occurring in the modu- 
lator characteristic due to variations of circuit parameters. Radiosondes 
Figure A2 - 1. 1680 MHz Radiosonde 
1-1-69 A2-1 FMH#3 
