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The 268-280 GHz radiometer MIRA2 was constructed for ground-based monitoring of vertical profiles of minor stratospheric constituents and has been optimized especially for the observation of the extremely weak chlorine monoxide signature at 278 GHz and its diurnal variation. For calibration an adjustable internal load was developed, which provides any required brightness temperature from cryogenic to ambient temperatures, dependent on mechanical adjustment. In comparison to the well established external beam switching method this new technique results in a higher contrast in particular under critical conditions of the troposphere. For inversion of the measured spectra the modified Constrained Linear Inversion and the Optimal Estimation Method were used alternatively. The extensions of these well established methods include the fit of standing waves within the inversion process itself and the joint retrieval of several spectral lines. The advances in calibration and inversion techniques became obvious during the evaluation ofthe data measured at Kiruna, Sweden, 1996 and 1998, and Ny-Alesund, Svalbard, 1997. Profiles of the trace gases Ozone, N20, HNO3 and ClO could be retrieved. From the data measured in Ny-Alesund a unique time series of ClO- and Ozone has been achieved which shows diurnal and long term variations of ClO and Ozone, respectively.
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