Atmospheric phenomena fundamentally influence the laser beam properties along the atmospheric propagation path. Atmospheric effects reduce the beam intensity, diminish the positioning accuracy of the laser beam, and impair the quality of transferred information. The evaluation of the long-distance laser performance includes the knowledge of the corresponding optical turbulence and atmospheric transmissivity. Weather measuring techniques in accordance with the demands of the propagation experiment and a continuously recorded optical turbulence are intended to permit the locally derived data to be evaluated in comparison to national and international standards.
The parameter of the refractive structure function Cn2 is derived by a commercial scintillometer (1 mW @ 670 nm) designed for measuring distances between 50 m and 250 m. The optical turbulence is recorded for a standard distance of 126 m and a standard height of 1m above ground corresponding to the standard height of the propagated laser beam.
The visual range due to precipitation, fog, and aerosols is measured at the same height. The influence of precipitation is monitored by a laser distrometer (0.5 mW @ 785 nm). Fog and aerosol effects are considered by the provoked extinction of a super-broadband white light diode. Both systems indicate a visual range defined as the distance, where the radiance of the measuring beam is reduced to 2% of the initial value.
Atmospheric properties like pressure, temperatures, relative humidity, wind, and global radiation are measured conventionally. The measuring positions are located at different heights and sections along the propagation path. This arrangement provides additional information on the homogeneity of the atmospheric phenomena on the optical test range. The temperature of the asphalt surface is also measured at several positions along the propagation path. The knowledge of local air and ground temperatures allows estimations of the convective processes.
Atmospheric data are continuously measured and recorded standardized. The weather measurements accompanying the propagation experiments are performed simultaneous to the propagation tests and close by to the propagation path in the intention to describe the interaction between atmosphere and laser radiation most exactly and reliably.