Using the monthly data sets during 1951-2008 from standard upper-air stations, the spatial and seasonal distributions of upper turbulence over China are studied, and the atmospheric turbulence intensity at the pressure levels of 200hPa, 100hPa and 50hPa are given in this paper, providing scientific reference for relevant experiments. The intensity of atmospheric turbulence is closely related to height; at 200hPa pressure level, a downward trend of turbulence intensity from north to south is shown over China; at 100hPa, the turbulence is decreasing from south to north, weak turbulence occurs in the area north to 40°N; at 50hPa in near space, the turbulence in the west is slightly weaker than that in middle and east of the country. Influenced by seasonal variations of the mean circulations at each altitude, strong turbulence always occurs in winter while weak one in summer; but at 100hPa, the distribution of turbulence is evenly all over the country in summer, while weak turbulence occurs above the Tibetan Plateau and north of 40°N in winter; at 50hPa, the turbulence is weak in autumn, and a bit strong in summer. On the whole, the results could reflect the characteristic distributions of atmospheric optical turbulence in most general circumstances, and the most important value is to give the relative regional distributions of turbulence, to evaluate regional optical conditions on a macro scale.
The earth atmosphere is an unavoidable part of the satellite-ground laser communication link, which has a serious impact on the laser signal transmission. Poor weather conditions will cause the equipment at ground sites to be disabled, heavy cloud cover will interrupt the communication line, as well as the attenuation caused by atmospheric absorption and scattering will introduce an average reduction in signal power. Moreover, the irradiance scintillations introduced by atmospheric turbulence will cause the jitter of the received power and the deterioration of the bit error rate of laser communication. The speckle effect of laser caused by phase distortion will cause the decrease of tracking accuracy and the fiber coupling efficiency of PAT (pointing, capturing, tracking) unit. The sky background brightness, direct sunlight, etc., can cause the overall performance of the acquisition, tracking, and communication system to decline. The atmospheric polarization disturbance will decrease the mixing efficiency of coherent laser communication system, and the deflection effect of atmosphere will affect the alignment accuracy of PAT unit, and so on. All of these can seriously affect the transmission and communication of laser signals, and become the technical bottleneck restricting the development and application of high-speed satellite-ground laser communication. In this study, based on several typical optical stations in different geographical areas including the Ali site in Tibet, the Delingha site in Qinghai and so on, the research on the availability of atmospheric channel for laser communication is carried out. The characteristics of atmospheric channel of ground stations are analyzed, including the cloud cover, atmospheric transmittance, irradiance, atmospheric turbulence and so on, and the availabilities of single station and multi-station are discussed, aims to establish an evaluation system for the study of atmospheric channel availability of ground sites, as well as to put forward the availability criteria for atmospheric channel, and the results will provide scientific basic data for the performance of the ground optical receiving network for laser communication.
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