In free-space optical (FSO) communications, conditions may be met when laser links suffer from solar background radiation (SBR).
There are four types of such conditions
Direct sunlight hitting a photodetector
Reflected sunlight (glints)
Sunlight scattered by hydrometeors
Sunlight scattered by surrounding objects (walls, etc.)
Direct sunlight may cause total break of communications (link outage), and thus affect the link availability. However, experiments prove that the sunlight does not cause irreversible degradation of semiconductor photodetectors used in FSO systems.
Estimations are made of the link outage periods duration for various types of SBR conditions, also other effects caused by SBR have been considered. Recommendations are presented for the link directivity optimization to avoid (or to minimize the probability of) communication interrupts caused by SBR. A nomographic chart has been developed to forecast periods of time when direct or scattered solar radiation may cause link outage. With this chart, a user in any point of the globe, knowing the link orientation (azimuth and elevation angles), can see when and for how long (if at all) may the link operation be affected by unfavorable SBR conditions, also in many cases it is possible to recommend insignificant modifications in the link orientation causing material improvement in FSO system performance.