In the first part of the paper, from a large set of spectrum data, obtained with the UHF/VHF (INSU/METEO) radars during the FRONTS87 and PYREX experiments, statistical characteristics of the noise power spectral density and behavior of its mean are pointed out. For each range gate and various sets of number of coherent and incoherent integrations, histograms of the noise power spectral density are computed using data from different parts of the campaign. The probability density function, so computed, is asymmetrical for low number of incoherent integrations and tends toward a Gaussian shape as the incoherent integration number increases. This histogram behavior is accounted for by a statistical approach, which assumes that the input noise is a random variable normally distributed and has a white spectrum for UHF as well as for VHF. In the second part of this paper, the results obtained by two different methods are compared and are explained by the shape of the probability density function. In the third part of this paper, it is shown that the mean value of the noise power spectral density is a very sensitive parameter for spectrum quality control. Some examples of its behavior as a function of the range gate, the observing direction, and the total power received are given and interpreted; these examples show how the noise mean power density can reveal malfunctioning of the radar, environmental perturbation, or variation due to the sky noise. ¿ 1997 American Geophysical Union