Atmospheric Fading Probability in Ground-to-Space Feeder Links

Atmospheric turbulence plays an important role in the performance of ground-to-space laser communication feeder link systems. The lead-ahead angle required by the relative satellite velocity results in an uplink data beacon that traverses a different atmospheric path than the downlink beam. This causes anisoplanatic differences between the downlink and uplink wavefront phase when the downlink beam is corrected by an adaptive optics system. This effect is characterized by an isoplanatic patch size [1], g
₀
, which reflects the maximum offset angle for good uplink correction with adaptive optics in the large-aperture (D ≫ r
₀
) limit. The design and analysis of free-space optical communication systems require a fuller development of these concepts; the terrestrial terminals used are far from the large-aperture limit with D/r0 typically on the order of three to five. This results in significantly less phase error, and may use only tip-tilt adaptive optics to achieve adequate correction on the downlink [2]. In addition, the statistical distribution of fading on the uplink is important in the design and evaluation of the overall link budget and performance. Anisoplanatic effects were shown to have small impact on the performance of moderate aperture systems. For all systems it was shown that correction of the uplink beacon using downlink beacon measurements provided net gain and better performance for the uplink, even with point-ahead angles of two to three times the isoplanatic angle.