Deep Space Optical Communication Via Relay Satellite

Abstract
A study of the possible utilization of optical communications for a deep space link via an Earth-orbiting relay satellite is presented. The optical link is used primarily for high rate data transmission from a deep space vehicle to the relay, while RF links are envisioned for the relay to Earth link. This type of hybrid system combines the advantages of optical frequencies for the free space channel to the relay with the advantages of RF links for atmospheric transmission. Auxiliary optical beacon and low rate RF links may be included for aiding pointing, tracking and timing operations.* A preliminary link analysis is presented for initial sizing of optical components and power levels, in terms of achievable data rates at various distances. The potential advantage of an optical link over present RF deep space link capabilities is shown. The problems of pointing with narrow optical beams are discussed, and the performance degradation due to pointing errors is evaluated. Since the modulation formats of the optical link to the relay and the RF link from the relay may be significantly different, an interface problem may arise in interconnecting the two links at the relay. This requires reformatting the detected optical signals to conform to the retransmitted RF link. The interface problem is discussed and alternative implementations are examined.