Radiometric measurements of atmospheric temperature

• Main Author: Taylor, F. W.
• Published: University of Oxford 1970
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.731978

A review of current problems and phenomena requiring further study in Atmospheric Physics is given, with particular reference to the upper atmosphere, defined as the region above 25 km. The conclusion is that an extended program of observations with very wide coverage and high accuracy is required, and that only satellite-borne instruments can do this economically. The particular difficulties associated with making accurate measurements at high levels are discussed, and the possibility of employing a number of conventional spectroscopic techniques is considered and finally rejected. The method of using a cell of carbon dioxide, the pressure of which is periodically modulated, as the spectroscopic element, as suggested by J. T. Houghton and C. D. Rodgers in 1965, is evaluated theoretically and shown to be capable of a temperature discrimination of 1°K with adequate height resolution at altitudes up to 70 km. The rest of the thesis is concerned mainly with the design of suitable instrumentation. Three different prototypes using different pressure-modulation techniques and different optical systems were tried until a sufficiently sensitive and reliable design was achieved, suitable for a satellite instrument. This consists of an optical system using a germanium window and one germanium field lens together with a conical light-pipe and a wide band interference filter to isolate the band; an experimental pyroelectric detector developed by Mullard Ltd. is used. Pressure modulation is achieved by the use of a light alloy piston mounted on beryllium-copper spider springs and magnetically driven by cross-connected Helmholtz coils. Special electronics are used to maintain a constant amplitude of the piston. Modulating the pressure of the absorbing gas also modifies its absorption coefficient, chopping the centres of spectral lines in the selected band, and generating an a.c. signal at the detector. The intensity of this signal is related to the temperature of the emitting gas. The distribution of the emitting gas along the line of sight can be computed by the use of spectral models and data on the band, but it was also measured directly by an experiment based on the Curtis-Godson approximation and using a simulated atmosphere in a large tank. The final design was incorporated into a flight prototype designed to be carried to 40 km. by a large balloon. The balloon radiometer had an in-flight calibration system together with voltage, current and temperature 'housekeeping' measurements all multiplexed for use with single channel eight-bit digital telemetry. The final two chapters deal with the flights of the balloon instruments, and the reduction of the data and its comparison with simultaneous measurements made by conventional radio-sonde. The results confirm the predictions of the theory, and it is concluded that a powerful new technique is available for the study of the Upper Stratosphere and Mesosphere.