Results of a Study for Temporal Structures of Cloud Radiation in MWIR and LWIR Ranges

The results for experimental studies of the temporal structures of cloud atmosphere radiation in the mid-wave infrared (MWIR) range, with a wavelength of (3–5) μm, and the long-wave infrared (LWIR) range, with a wavelength of (8–13) μm, are presented. The initial data used were discrete signals (DS), values for fluctuations of atmosphere radiation brightness energy at the output of a two-channel radiometer operating in the MWIR and LWIR ranges, with 60 s duration, with a sampling frequency of 8 kHz for each channel. Accordingly, each measurement provided the acquisition for DS arrays from 480 thousand counts at a fixed azimuthal direction in directions different in elevation angle in the range of (10–26) ° (in every 2 °). In the studying process the indicated DS arrays, autocorrelation functions and time intervals (correlation radii) were estimated, during which the autocorrelation coefficients R(t) for each channel decreased from 1 to 0.5, as well as the power spectral density (PSD) for fluctuations of atmosphere radiation brightness energy, the correlogram method having been used. It has been established that time intervals (correlation radii) decrease with increasing observation angle due to a decrease in the effect of clouds shielding each other and the appearance of a greater number of gaps between them, as well as the fact that autocorrelation intervals in the MWIR range are larger than in the LWIR range. Time intervals (correlation radii) has been established to decrease with increasing observation angle due to a decrease in the effect of clouds shielding each other and the appearance of a greater number of gaps between them, as well as the fact that autocorrelation intervals in the MWIR range are larger than in the LWIR one. This indicates that the frequency of fluctuations in the energy brightness of atmosphere radiation in the MWIR range is less than in the LWIR one. The Analysis for the spectral diagrams of the specified PSD showed that the main fluctuations energy for the energy of brightness (radiance) is concentrated in the low-frequency region, which lies significantly below the frame-rate deflection of thermal imaging equipment. Consequently, it is possible to select the optimal frame rate from the video stream for subsequent processing by a passive optical-electronic system for detecting an unmanned aerial vehicle against an atmospheric background at frequencies significantly lower (up to 10 Hz) than the frame frequency (50 Hz) of the video stream.