The “Chirp” Digital Radiosonde

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Abstract
This paper reports on a digital measurement (“chirp”) system which has application for a wide range of meteorological and earth satellite measurements. The system employs a simple concept in which a voltage pulse, proportional to a sensor voltage, is used to generate a burst of pulses from a voltage controlled oscillator (VCO). A count of the high frequency oscillations which make up the “chirp” provides the digital measurement. The system is adapted to multiple sensor use with a multiplexer. The system has the advantage in that one has the option of selecting an ac amplifier for low level signals in conjunction with a variety of multiplexers and VCO for the desired measurement. One particular combination of multiplexer and VCO was used to demonstrate its use as a digital radiosonde. A flight test of the digital radiosonde was obtained. Results clearly show fine structural detail in the temperature profile without any need for subjective interpretation by the operator. Numerous isothermal and inversion layers less than 100 m in thickness were observed. The digital radiosonde used conventional (U.S. Weather Bureau) temperature and humidity sensors. Temperature resolution is about 0.1C and relative humidity is about 0.1 per cent. The system resolution is 0.1 per cent.

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