ALMA Memo #352
Design and Development of 183 GHz Water Vapour Radiometers

R. Hills, H. Gibson, J. Richer, et al.
March 2001

This memo describes the plans for the development of the prototype 183 GHz radiometers for ALMA. It is planned that such radiometers will be used to correct for the phase errors introduced by water vapour in the atmosphere. One radiometer will be mounted on each of the antennas and will provide real-time measurements of the brightness temperature of the atmosphere at frequencies near the 183 GHz emission line of water. These measurements will be used to estimate the path fluctuations that are caused by the variations in the amount of water vapour along the line of sight from each antenna. Even on a good site corrections for these fluctuations will be needed for much of the time, especially at ALMA higher frequency bands and on long baselines. A substantial amount of work on the design of the radiometers has already been carried out, so this document contains a description of the instruments to be built, at the level of a conceptual design. Two radiometers of somewhat different designs, but with most components in common, will be built and tested. The intention is to develop a cost-effective and reliable final design suitable for production in quantity. It is expected that the prototype radiometers constructed here will subsequently be used for tests in the field, both to sort out compatibility issues with the rest of the ALMA system and to develop the phase-correction techniques further. The project is being carried out as a collaboration between Chalmers and MRAO.

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