Comparison of Rayleigh and sodium laser guide stars for improved ground-to-GEO communication

Atmospheric turbulence limits satellite-to-ground free space optical communication. Adaptive optics at the ground station can correct for turbulence, but anisoplanatism limits the correction along the uplink, point-ahead angle, to the satellite. A laser guide star (LGS) can be used to measure the turbulence along the uplink path. This paper investigates pre-compensation of the uplink communication to a geo-stationary satellite using the two types of LGS used in astronomy (sodium and Rayleigh). The evaluation includes an error budget and a model analysis under different realistic atmospheric turbulence conditions. LGS based pre-compensation provides higher coupled flux and reduced fades, by ∼ 4 dB, versus downlink pre-compensation. While a high power (80 W) sodium LGS can result in higher mean coupled flux to the satellite, a lower power (15 W) Rayleigh LGS performs as well in terms of the probability of uplink transmission (i.e. reduced fades). We conclude that the uplink performance improvement, versus likely cost, with Rayleigh LGS is significant and warrants further exploration.