Constellations, clusters, and communication technology: Expanding small satellite access to space

The trend toward small-sized spacecraft continues in government applications and is even increasing in commercial space endeavors that are funded by venture capital. Small spacecraft, including nanosatellites, microsatellites, and small satellites (smallsats), are an attractive alternative to traditional, larger spacecraft due to reduced development costs, decreased launch costs, and increased launch opportunities. A significant disadvantage, however, of a small spacecraft is its reduced or limited capabilities. The physical size of the small spacecraft reduces the size of the payload and/or the number of payloads that it can host, its propulsion capabilities, and its power. Small spacecraft are most commonly used in low Earth orbit, limiting the number of observation opportunities for a particular area of the Earth or space and the number of ground station downlink opportunities for stored data. These constraints affect the complexity and types of applications that small spacecraft can serve. Using multiple spacecraft that work together can overcome many of these limitations and expand the utility of small spacecraft. Two concepts for cooperative groups of spacecraft are constellations and clusters. A key technical challenge for small spacecraft, constellations, and clusters is communication of data. Communication challenges exist for accommodating varying numbers of users, serving high user densities in a given geographical area, and providing a consistent quality of service for different types of applications (e.g., Internet access, voice communication, machine-to-machine). This paper explores concepts for emerging communication technologies for space-ground and inter-satellite communication, while citing examples of existing and new constellation and cluster concepts. Several aspects of the communication systems are examined in terms of frequency bands, data rates, multiple access methods, and accommodation on spacecraft.