Heterodyne coherent tracking technology for inter-satellite laser link during direct sunlight

Abstract There is a possibility that the angle of the laser link between two geosynchronous orbit satellites and the incident direction of sunlight will be small during the periods of the vernal equinox and autumnal equinox. At these times, the sunlight is incident to the receiving detector along the receiving channel of the laser terminal on the satellite, which easily causes the signal laser to be submerged in the background light. This study analyzed the influence of direct sunlight on a laser terminal, and a tracking scheme for heterodyne coherent detection using two four-quadrant detectors was designed. The explanations of the analysis and experimental results show that this scheme has a sufficient signal-to-noise ratio for tracking during direct sunlight. Experimental verification was carried out, with a solar simulator used to simulate the solar illumination. The laser terminal could track and communicate stably under direct irradiation by the sun, with a tracking accuracy that was better than 1 μrad (1 σ), and a bit error rate that was better than 10-7 at 1 Gbps. These research results could be applied to different link scenarios such as an inclined geosynchronous orbit and low earth orbit constellation.