U) ABSTRACT (U) With the increased interest in multiple node space networks such as Brilliant Pebbles, Brilliant Eyes and Iridium', satellite-to-satellite communication connectivity via crosslinks over short ranges is an important consideration. Simultaneous link connectivity requires either several terminals per satellite or time division multiplexing approaches. This paper introduces a concept for integrating several small lightweight crosslink communication terminals to provide the desired simultaneous communication capability .(U) A suite of three laser communication crosslink terminals which have impact on satellite size, weight, and power is discussed. The total three terminal suite weight and power are given as 28 pounds and 58 watts maximum, respectively. The design provides one megabit per second point to point connectivity over 9,000 kilometers range with a bit error rate, and includes subsystem modal control.(U) The communication channel, link acquisition and cooperative tracking of the optical energy are discussed. The major components are identified and the technology risk associated with each is assessed. Risk is shown to be minimal. (U) INTRODUCTION(U) Multiple node distributed satellite networks are being considered for a number of applications in both the military and commercial arenas. With the large number of low earth orbit (LEO) satellites required for these systems, the costs associated with launch (related directly to weight) are extremely important.Light weight, low cost, and low power satellites are required to achieve the low life cycle costs demanded in fielding these large space based networks.(U) To achieve the global coverage desired by these systems, multiple orbital planes of multiple satellites per plane are envisioned2. One concept is illustrated for three planes in Figure 1. Full duplex communication connectivity between satellites is established fore and aft to form a ring network in-plane.(U) Cross-planeconnectivity is establishedvia selected nodes to UNCLASSIFIEC Figure 1: (U) Distributed Network Connectivity tions, a minimum of three terminals (fore, aft and across) are required per satellite.(U) Optical communication crosslink terminals offer many advantages in the above scenarios. Because of the high frequencies associated with optical links, narrow beam divergences, small field of views, and small aperture sizes can be used. These advantages work directly to reduce spacecraft design complexity. Narrow viewfields and beam divergences allow use of a single frequency source for both transmit and receive functions. This terminal simplicity is carried to the constellation level where only a single frequency is required. Complex frequency management is not required to mitigate self interference. High order network management functions are not necessary to allocate frequency or time slots to the crosslink function as would be necessary with conventional RF approaches.(U) Because of the greater than three orders of magnitude difference in frequency between optical crosslinks and...
Trade swdies and close examination of direct detection laser crosslinks for a number of applications have shown that the development risk is comparable to or lower than that of RF systems. Since laser crosslinks offer significant advantages ovez RF crosslinks for the same applications, the specification of laser crosslinks is a good choice. The reason that the risk is low is examined in this paper which details the technologies and components which comprise current, second generation design laser crosslinks. The specifics which are examined include diode power summing, diode life, data rates of a Gbps and beyond, detector sensitivity/radiation effects, telescopes, gimbals and optics, alignment stability, acquisition, pointing and tracking, electronics and the spsee qualification of assemblies and systems. Differences between first and current (second generation) system designs and technologies are examined. An assessment of the maturity of the current direct detection technology is made and conclusions formulated. INTRODUCFIONThe crosslink designer is faced with the task of evaluating the pros and cons of a particular crosslink approach and justifying his decision in selecting between system approaches. Trade studies set the issues in perspective and provide insight into the advantages and disadvantages of each approach. High on the priority list is the risk of pursuing a particular approach whether it be RF or lasercom technology. As the designer evaluates laser communications technology he must include an assessment of key technologies. The purpose of this paper is to realistically evaluate the maturity of technology on the way to ascertaining risk. The maturity will be assessed by an examination of the key technologies and assemblies which comprise current generation direct detection lasercom crosslink systems. TECHNOLOGY DISCUSSIONSince the first generation system was designed some ten years ago a great deal of component and technology development has taken place and continues today. A second generation system is currently being developed which uses these new technology developments. Technology risk has been mitigated by the build and test of key components/ assemblies. Environmental tests have established the stability and survivability of key elements.What follows is an examination of these developments and an assessment of their maturity. First and second generation system differencesThe first generation laser crosslink, designed in the early eighties, is currently in production. McDonnell Douglas Aerospace (MDA) is under contract to produce eight (8) systems. This crosslink is designated the Laser Crosslink Subsystem (LCS) and is used on a geosynchronous satellite. It is expected that the first unit will be launched in the near future. See Reference 1.The second generation laser crosslink, currently under development, is designated the Crosslink Set (CLS). It is called a set because the satellite crosslink suite consists of three optical units with a common electronics package.The first generation LCS is ...
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