ARCHITECTURAL DESIGN OF A GROUND-BASED DEEP-SPACE OPTICAL RECEPTION ANTENNA

An architectural design of a ground-based antenna (telescope) for receiving optical communications from deep space is presented. A channel capacity of 100 kbits/s from Saturn or 5 Mbits/s from Mars requires a 30-cm-diameter transmitter and a 10-m-diameter reception antenna. The f/0.5 primary mirror will be hexagonally segmented and will have a surface roughness tolerance of 2-mu-m rms to effect a substantial savings relative to the cost of an astronomical optical imaging telescope of the same diameter. The antenna will receive communications even when the deep-space laser source appears to be located within a small angle of the sun (small solar elongation). Instead of a long, unwiedly, conventional sunshade, a sunshade consisting of hexagonal tubes will be mounted in precise alignment with the primary mirror segmentation. The ends of the tubes will be trimmed so that both the sunshade and the antenna will fit within a more-than-hemispherical dome whose diameter clears a sphere only six-fifths the diameter of the primary reflector. This sunshade permits reception when solar elongations are as small as 12-degrees. Additional vanes may be inserted in the hexagonal tubes to permit reception at 6-degrees or 3-degrees. The frequency-doubled output of the Nd:YAG source laser will be tuned dynamically to lie within a Fraunhofer line (a spectral interval of reduced solar emission) to carry the signal with reduced interference from sunlight. The source laser and the Fraunhofer filter (a narrowband predetection optical filter) will be tuned to match the Doppler shifts of the source and background. Typical Doppler shifts are less than 0.05 nm or 53 GHz. A typical Saturn-to-Earth data link can reduce its source power requirement from 8.8 W to 2 W of laser output by employing a Fraunhofer filter instead of a conventional multilayer dielectric filter.