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During the remainder of the 1980's and early 1990's, many technology improvements will be made to achieve high data rate satellite-to-satellite communications. Programs such as the Strategic Defense Initiative Organization's SDS (Strategic Defense System) have projected architecture requirements for data transfer rates ranging from hundreds of kilobits per second to hundreds of megabits per second on satellite crosslinks. The advanced technology required to realize these rates is being investigated even now, but a general solution for the communications problem, defined in part by unique operating environ-ments ranging from benign to highly perturbed, is not possible. This paper focuses on one aspect of the overall satellite communications problem. It begins by briefly comparing radio frequency (RF) with direct detection laser communications (lasercom) for application to high data rate SDS low and medium earth orbit crosslinks. This comparison is based on near term technology suitable for early deployment in the SDS. The RF system is a millimeter-wave system operating in the 60 GHz region of the electromagnetic spectrum. The Lasercom system is considered to operate in the near infrared, from 0.8 to 1.1 micrometers. The trades consider operation in benign as well as the very stressing environments postulated for the Strategic Defense System which includes low and high altitude nuclear bursts as well as the threat of spaced based jamming sources and directed energy weapons. Results of the trades are presented to show the advantages and disadvantages of each type of system. The advantages and disadvantages of the RF and Lasercom systems are of sufficient magnitude to consider combining the best features of each to create a truly new communication system with capabilities far exceeding those of either system. For example, maximum survivabilty results from carrying both systems (RF & Lasercom) on the satellite. However, this approach is costly in terms of weight, power, and real estate. A better approach would be to carry the capabilities of both sys-tems. Such an approach is proposed in this paper. The combination of systems, termed the Dual Mode Communications System (DMCS), is the principal thrust of the remainder of the paper.
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