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The signature simulations were carried out for evening time situation when the sun is below the horizon.
The mobile camouflage system significantly reduced the infrared signature by shielding the absolute hottest structure's radiation, and directing the exhaust and hot air flows from visible structures, especially from the front and side directions. The changes in signature of the MBT were successfully simulate with TAIThermIR and the models of the MBT and camouflage that had been developed. The tool does not allow the calculation of the exhaust gas radiance contribution to the signature.
The transmission measurements were made along slightly inclined path of 1830 m during the period from mid- November 2018 to mid-January 2019. The transmission values were collected together with weather data including measurement of ozone content in the atmosphere.
A simple model for ultraviolet transmission that takes into account both visibility and measured ozone content has been developed. This model shows a difference to the model found in the Modtran program, often used to predict atmospheric transmissions in several wavelength bands including ultraviolet wavelengths. Overall, the new model shows a weaker connection between ultraviolet extinction, ozone content and visibility compared to Modtran.
Data from the countrywide ozone measurements as well as data collected in connection with transmission measurement showed variations in the atmospheric ground-level ozone content that are not easily explained from weather parameters alone. Reports, see for instance [2], state that it is volatile hydrocarbons and other molecules, both natural and anthropogenic, along with solar radiation that influence the amount of ozone. The variation of both volatile hydrocarbons and solar radiation together with a variation in wind direction and speed have an impact of varying transmission.
In practice all measured ozone values during conditions with low relative humidity were relatively high. There was also a high correlation between high ozone values and high wind velocities. The relation between ultraviolet extinction deduced from measurements and the corresponding values for the visual wavelength range and corresponding values calculated using Modtran show different relationships. The transmission measurements in the ultraviolet wavelengths range showed that the amount of aerosols has no major significance for the ultraviolet extinction at visual ranges above 15 km.
Due to the short period of data collection, mostly fall-like weather conditions, the conclusions that can be drawn from the measurements are limited. Furthermore, the ground-level horizontal measurements make it very hard to draw conclusions about the ultraviolet transmission at higher air layers.
The optical turbulence is generated by fluctuations (variations) in refractive index of the atmosphere. These fluctuations are caused in turn by changes in atmospheric temperature and humidity. The structure function of refractive index, Cn2, is the single most important parameter in the description of turbulence effects on the propagation of electromagnetic radiation. In the boundary layer, the lowest part of the atmosphere where the ground directly influence the atmosphere, is the variation of Cn2 in Sweden between about 10-17 and 10-12 m-2/3, see Bergström et al. [5]. Along a horizontal path is the Cn 2 often assumed to be constant. The variation of the Cn2 along a slant path is described by the Tatarski model as function of height to the power of -4/3 or -2/3, depending on day or night conditions.
The hazard of laser damage of eye is calculated for a long slant path downward. The probability of exceeding the maximum permissible exposure (MPE) level is given as a function of distance in comparison with nominal ocular hazard distance (NOHD) for adopted levels of turbulence. Furthermore, calculations are carried out for a laser pointer or a designator laser from a high altitude and long distance down to a ground target. The used example shows that there is an 10% risk of exceeding the MPE at a distance 2 km beyond the NOHD, in this example 48 km, due to turbulence level of 5·10-15 m-2/3 at ground height. The turbulence influence on a laser beam along horizontal path on NOHD have been shown before by Zilberman et al. [4].
The measurement was concentrated on low clouds, mostly of the cumulus type. We found that these clouds between 0-2 km often showed a layered structure and that they often indicated a limited optical density probably allowing for observation through the cloud. This information is hard to achieve from a passive EO sensor only. This was supported both from the simulation of the lidar response from thin clouds and from inverting the measured lidar waveform.
The comparison between the camera image intensities and the integrated range corrected lidar signals showed both negative and positive correlations. The highest positive correlation was obtained from comparing the lidar signal with the cloud temperature as derived from the FLIR camera. However, there were many cases when one or two of the camera intensities correlated negatively with the lidar signal. We could for example observe that under certain conditions the cloud which was dark in the SWIR appeared as white in the visible camera and vice versa. Example of lidar and image data will be presented and analyzed.
Lidar measurement as support to the ocular hazard distance calculation using atmospheric attenuation
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