Millimetre wave remote sensing of the atmosphere

Abstract

Recent advances in millimetre wave technology has opened up a new region of the spectrum to remote sensing from artificial satellites. The main part of this work involves a millimetre wave proving experiment for a satellite borne millimetre wave active sounder to measure surface pressure over the oceans. The Microwave Pressure Sounder is a6 channel, low power radar operating in the spectral range from 24 to 75 GHz. The strength of the return echoes from the sea surface determines the amount of oxygen in the path which can be directly related to the surface pressure to an accuracy of 1 mb, when corrected for sea surface reflectivity and atmospheric temperature and water content by this multichannel instrument. Measurements of atmospheric attenuation along a horizontal path were related to atmospheric pressure changes by a millimetre wave instrument built at Heriot-Watt University. The transmissometer measured the differential absorption between two frequencies (54 and 58 GHz) over a 650 metre path. The deduced atmospheric pressure was found to compare with the barometric pressure with a standard deviation of two millibars for the best data set. These results demonstrate that atmospheric attenuation can be measured with sufficient precision for a satellite borne instrument to determine the surface atmospheric pressure over the oceans to an accuracy of approximately one millibar. This accuracy would lead to significant improvements in the modelling of the atmosphere and weather forecasting. Various other techniques to remotely sense surface atmospheric pressure are reviewed. Recently, increased awareness of the sensitivity of the environment and evidence of the effects of man-made pollutants has given rise to an increased awareness in the health of the Earth and led to several instruments being developed to monitor our planet. One of these instruments, the Microwave Limb Sounder to be flown on the Upper Atmosphere Research Satellite (launch October 1991) is described. This instrument uses millimetre wave radiometers at 63 GHz, 183 GHz and 205 GHz to measure the amount of chlorine oxide, ozone and water vapour in the upper atmosphere. These gases are important in understanding the photochemistry of the mesosphere. Global distributions of the gases will be produced and changes in concentration will be monitored during the three year mission.