Overcoming atmospheric scintillation, wavefront distortion, and cloud extinction during low-elevation quantum key distribution passes from LEO satellites to ground optical stations.
Atmospheric turbulence distorts the optical phase front within the lower 10 km of the atmosphere, degrading Strehl ratios to less than 0.05. Ground stations employ Shack-Hartmann wavefront sensors and 128-actuator deformable mirrors operating at 2 kHz to restore wavefront coherence, boosting fiber coupling efficiency by 18 dB.
Daytime QKD operations suffer from intense solar Rayleigh scattering. By inserting ultra-narrowband Faraday anomalous dispersion optical filters (FADOF) with sub-picometer passbands and sub-nanosecond gating single-photon detectors, daylight background noise is suppressed below dark count levels.