High latitude optical satellite communications – cloud coverage in Norway

Summary Wireless satellite communications has traditionally utilized modulated radio waves to transfer information to, from and between satellites. Optical communications utilize similar principles with electromagnetic waves (photons) at significantly higher frequencies (THz). The increased frequency implies that hydrometeors in form of clouds and fog severely attenuate optical signals. In this study we investigate the amount of cloud coverage within the Norwegian High North to be able to estimate the optical communications availability of a ground station communicating with satellites. We have utilized twelve months of cloud coverage images obtained from weather satellites in 2014, and processed these to obtain the percentage of images having cloud coverage less (or exceeding) a given threshold. The results are in forms of numerical maps for parts of Norway north of 620N including Svalbard and surrounding sea area. Manual cloud observation data from the same time period has been used to validate the results. In addition we have utilized numerical weather models from the Norwegian Meteorological Institute (AROME MEPS og Arctic) covering the complete Norwegian area for the year 2018. Significant differences were observed, especially in cases with low cloud coverage. However, it should be noted that the image grid (1.6/2.5 km grid, depending on dataset) is not directly comparable with the manual observations of cloud coverage of the (hemispherical) sky. The satellite and model datasets also cover different years. The numerical cloud data was utilized to estimate to what degree diversity with two ground stations improves the probability of having clear sky. In general, less cloud cover is observed over land, and islands such as Svalbard and Greenland have significantly lower cloud coverage compared to areas in the open sea. The satellite images from 2014 indicate a maximum of about 59% with cloud-free conditions. At the potential Norwegian ground locations selected for study, we observe less than about 33% cloud-free conditions. Values given by the AROME prediction models for 2018 are significantly lower than those from satellite images, especially at the island locations. The highest percentage of cloud free conditions is found in the southern Norway with values around 25%. Two-station diversity has the potential of improving the availability, and a combination of two sites increases the probability of either one (or both) having cloud-free images to between 39 and 54%, depending on the dataset. This is significantly less than an objective of having optical access to satellites during all passes for low earth orbiting satellites or continuous communications with geostationary satellites. A combination of more than two diversity stations, and preferably also utilizing radio frequency communications, is expected to improve the access to satellites in the High North. Cooperation with optical Earth stations in other countries may be required to ensure successful downloading of observation data in each pass.