A microwave radiometer has been developed for long‐term measurements of strato‐mesospheric ozone profiles at midlatitudes. The instrument located at the Bordeaux Observatory, France (45°N) is operated in the framework of the French atmospheric station, of the European Alpine station belonging to the Network for the Detection of Stratospheric Change. The instrument detects an ozone spectral line at 110.836 GHz, enabling the retrieval of ozone profiles from ∼25 to ∼75 km. The technical parameters are described as the observing technique, calibration procedures, and data acquisition. Two retrieval methods are used: the Backus‐Gilbert method and the optimal estimation method. Strong emphasis is placed on the error analysis including a detailed characterization of errors. To improve the retrieval, we discuss which errors have to be taken into account in the retrieval process. Three cases have been selected for which retrieval errors due to time stochastic and time systematic errors are calculated. First, only the thermal noise is introduced in the retrieval code; second, all errors are introduced; and finally, a selection among errors is searched in order to provide a good compromise between the retrieval error and the vertical resolution. Diurnal variations of mesospheric O3 are compared with results from a photochemical model. The agreement is within 5–10%. During the February‐March 1996 period the temporal evolution of ozone at an altitude of 30 km is in good agreement with both satellite and three‐dimensional model data. There is a good correlation with the potential vorticity field calculated at the 850 K isentropic temperature. A crossing of the polar vortex over the site is associated with an O3 decrease observed in the three data sets.