The goal of the project MicroRAT (Microwave Radiometry of the Atmosphere) is to deploy passive microwave radiometry to determine the distribution of trace gases in earth's atmosphere. At the moment the work is focused on instrumental developments for soundings of atmospheric water vapor, liquid water and opacity.

Due to its strong influence on the propagation of electromagnetic waves water vapor is the principal atmospheric constituent that determines atmospheric opacity. Water vapor is distributed in earth's atmosphere highly inhomogeneous, both in space and time. In order to get reliable data about the quality of radioastronomical observations that can be expected at a certain site, opacity has to be measured or modeled from measured parameters like the total column amount of water vapor.

Furthermore water vapor is the key gas that dominates weather and climate. It plays an important role in the dynamics, the radiative balance and the chemistry of the atmosphere ("ozone hole", "global change"). Measurements gathered by microwave radiometry can contribute substantially to the field of aeronomy.

The rotational line of water vapor with the lowest frequency is centered at 22.2351 GHz. This line is well suited for ground-based observations as earth's atmosphere shows relatively high transmission at this frequency and the line is not saturated. MicroRAT intends to observe this line with different instruments:

. The Itapetinga Radio Observatory at Atibaia is equipped with a low noise receiver at 22.2 GHz that is capable of performing high spectral resolution measurements of the water vapor line. Up to now it never was used for measurements of water vapor in earth's atmosphere.

. MOPS (Microwave Opacity Sounder), a dual channel radiometer (22.2 GHz / 31.6 GHz) that is currently assembled and will be applied for site testing at the observatory El Leoncito (Argentina).

. A high-performance radiometer at 22.2 GHz for the retrieval of stratospheric and mesospheric altitude profiles of water vapor will be developed in future.