DESIGN CONSIDERATIONS OF MULTIPLE LASER COMMUNICATION LINKS BETWEEN SYNCHRONOUS SATELLITE AND SEVERAL EARTH STATIONS

Abstract : Secure and private megabit-per-second communication links between a satellite in synchronous orbit and several ground stations are desired in certain applications. The highly directional property of laser beams makes laser waves an apt candidate for the carrier of the system. The large potential bandwidth of the laser system can be of value also in providing redundancy for reliability and coding for security. The very directional property of the laser beam, however, presents problems of pointing and acquisition. These problems are particularly severe for the case of simultaneous transmission between the satellite and multiple ground terminals. An additional problem of efficient optical antenna gain arises for this case. Recent advances in high-power, higher-efficiency lasers encourage consideration of a laser transmitter configuration not previously investigated for multiple link communication over synchronous satellite distances. In this configuration, a satellite-borne laser is operated in a relatively high-order mode. Appropriate optics are used to provide multiple links, one lobe of the beam for each link. Also mode switching can provide alternative links. The ground terminals are each equipped with separate lasers. Aside from poor atmospheric seeing, satellite attitude stability appears to be the key performance-limiting element of the system. Sample calculations of requirements and expected performance for the multiple communication link system are presented. (Author)