It is shown that in many cases the signals that come from sensors installed to control the technical condition of a facility consist of a useful component and additive noise. The noise may be caused by both external factors and the initiation of a defect in the process of facility’s operation. Noise caused by the influence of external factors, as a rule, is not correlated with the useful signal. Noise caused by the initiation of a defect or malfunction is correlated with the useful signal. Therefore, in order to distinguish between the operational and non-operational technical condition of a facility, it is necessary to calculate the correlation coefficient between the noise and the useful signal. However, in practice, it is impossible to isolate the useful component and the noise from the noisy signal. It is shown that the estimates of correlation coefficient and normalized cross-correlation function between the noise and the useful signal serve as indicators of the transition of a technical facility from the operational condition to the non-operational condition. Therefore, determination algorithms of the correlation coefficient and normalized cross-correlation function the between the noise and the useful signal have been developed. It is shown that the estimates of the relay correlation function of the noisy signal are used to calculate these estimates. It is noted that the use of the developed indicators can increase the adequacy and correctness of operation of monitoring systems.