An airborne millimetre-wave radiometer at 183 GHz: receiver development and stratospheric water vapour measurements

The subject of this Ph.D. thesis is the redesign of an airborne water-vapour radiometer AMSOS (Airborne Millimetreand Submillimetre Observing System) and measurements conducted with the new receiver. The thesis consists of three parts and four publications. The publications are placed within the thesis after appropriate chapters. The first part is an introduction which addresses the scientific motivation of this thesis: why is water vapour in the stratosphere important and why are we studying it at all? An answer to this is made by emphasising the three main roles of stratospheric water vapour: its role in the ozone chemistry connected with ozone depletion, its greenhouse effect contributing to the global warming of the atmosphere and its long lifetime which makes H2O a good tracer. Another question arises logically: how can we measure water vapour in the stratosphere? This is also answered in the first part, where we discuss the physics behind absorption and emission in the millimetre-wave part of the spectrum, radiative transfer and line broadening and how they can be used for retrieval of vertical H2O-profiles. The principles of operation and calibration of a microwave radiometer are also presented here. The second part of the thesis deals with experimental aspects and describes design of the AMSOS radiometer and especially of the radiometer’s quasioptics. After a short introduction to Gaussian optics, a new layout of the radiometer frontend is presented. All components are described according to their role in the quasioptics: first, a beam-shaping section which consists of a feed horn, a parabolic and an elliptic mirror; then a new quasioptical λ/4isolator for reduction of internal reflections (which cause baseline ripple in observed spectra); and finally a Martin-Puplett interferometer for sideband filtering. The theoretical calculations and methods used in the design of every component are presented together with numerical simulations. However, special attention was paid to measurements of the quasioptical behaviour of every component and their artifacts. Measurements of the amplitude, phase and polarisation were done using the AB Millimetre vector network analyser, mainly at the radiometer’s operational frequency of 183 GHz. Several components were also tested at other frequencies. The beam-shaping section generated a regular output beam with a half power beam width of around 1◦, which was one of our main goals. The new λ/4-isolator performed well and ac-