Microsatellites -- weighing less than 100 kg -- have traditionally had limited capability (power, memory, computing capability, etc.) and have played less of a role in Air Force missions. However, new technologies for lightweight and low-cost satellites and Micro-Electro-Mechanical Systems (MEMS) are enabling highly capable microsatellites.

One new way to perform missions from space is the concept of clusters of microsatellites that operate cooperatively to perform the function of a larger, single satellite. Each smaller satellite communicates with the others and shares the processing, communications, and payload or mission functions. Thus, the cluster of satellites forms a "virtual satellite," an idea characterized by the Air Force Scientific Advisory Board Space Technology Panel as the technology that will lead to new exploitation of capabilities in space.

This concept promises many benefits, including greater utility and flexibility by permitting the cluster to reconfigure and optimize its geometry for a given mission, enhanced survivability, and increased reliability. It is expected that clusters will reduce life cycle cost by using mass-produced satellites and minimizing the launch cost by optimizing the launch vehicle's cargo capacity.

The cluster concept also eases performance upgrades by allowing upgraded satellites to join a cluster, increasing the overall performance of the virtual satellite rather than replacing a single, large satellite or the entire cluster.

The Air Force Research Laboratory is exploring this new paradigm for performing space missions in a coordinated effort dubbed TechSat 21 (Technology Satellite of the 21st Century).

Under this effort, a variety of application missions is being considered including surveillance, passive radiometry, terrain mapping, navigation, and communications.

The space- based radar mission for Ground Moving Target Indication was chosen as a stressing case and is the focus of the initial investigation. An innovative concept has been devised that can potentially satisfy most or all of the desired mission requirements while minimizing weight, power, and cost.

The key to this concept is a cluster of microsatellites orbiting in close formation, each with a receiver to detect coherently not only the return from its own transmitter, but the bistatic responses received from the orthogonal transmit signals of the other satellites as well.

This allows for a wealth of independently sampled angle-of-arrival data to be collected as the constellation forms a large but sparse coherent array. By virtue of its sparseness, the independent apertures look through different parts of the ionosphere, thus temporal and spatial variations on the scale of their separation could adversely affect their operation.

Furthermore, the concept is not viable unless each micro-satellite has extremely low weight and cost, while being very capable. For this concept to work and be cost effective, it is imperative that fundamental questions be answered:

• How can the information sensed by a number of smaller radar apertures be combined to increase the performance of the whole? And the related question, what are optimal geometric formations of the smaller apertures?
• What orbits or orbital control techniques are possible to maintain the desired configuration?
• What is the spatial and temporal structure of the ionosphere on the scale of the spatial separation of the satellites and the duration of observation, respectively? What are the effects of these variations on the operation of a sparse aperture radar?
• How can small, low-cost satellites be devised which have significant capability? What role does MEMS play in enabling these low-cost satellites? What are the key impediments and challenges to using MEMS for microsatellites?

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