Researchers here at the Air Force Research Laboratory's (AFRL) Space Vehicles Directorate, and those at the Air Force Office of Scientific Research in Arlington, Va., have rolled out plans for a revolutionary new system of "formation-flying" satellites that can quickly adapt to rapidly changing mission requirements. And they got the idea by watching a flock of birds.

AFRL's intriguing "Technology Satellite of the 21st Century," or the TechSat 21 program, envisions that collaborative clusters of interdependent microsatellites, each weighing about 100 kilograms and flying in close, bird-like formations, may eventually circle the earth, replacing many of today's single, larger satellites.

An artist's illustration of satellites flying in formation. Photo: USAF

"Satellites in a cluster that 'talk' to each other and share data processing, payload, and mission functions now performed by conventional satellites will be smaller, lighter, less expensive to launch, and offer more immediate information-gathering versatility to the warfighter. They will also cost less than current satellites because they can be mass produced and placed in orbit using smaller launch vehicles-perhaps even by a military jet such as an F-15," said Maurice Martin, the Directorate's TechSat 21 program manager.

Most Air Force satellites such as Milstar, Defense Support Program, and Defense Meteorological Satellite Program, are large, heavy, expensive to produce and launch, and generally dedicated to single missions, lacking the technological flexibility to quickly accommodate fast-breaking tactical assignments from theater commanders.

"However, with the advent of ultra-miniature mechanical-electrical devices such as tiny motors, gears, and actuators the size of dust mites, robust microsatellites have entered the realm of possibility, bringing with them greater power, memory, survivability, and computational capacity previously unavailable. 'Microsats' represent truly a giant leap forward in the kind of advanced and affordable satellite technologies the Air Force needs to meet its mission goals in the coming century," said Martin.

"But one of the problems we face in keeping a distributed group of small satellites in relatively close and coordinated proximity to one another is their finite fuel supply. Satellites fire tiny pulse-plasma thrusters to adjust orbital position and attitude; once the fuel is gone, however, the satellite loses maneuverability, and in a cluster, maintaining position is crucial," Martin explained.

Looking to nature for answers, AFRL engineers have studied how flocks of birds cooperate in flight. They learned that when the lead bird tires and slips back in formation, another bird moves in to take its place. Roughly the same cooperating principle applies to the theory of satellite clusters: When the formation's central satellite begins to run low on fuel, another satellite in the same cluster "knows" to take its place, thereby preserving the integrity of the entire group. The "tired" satellite then slips back to a position in the cluster requiring less fuel.

"We also think we have found a solution that addresses the fuel problem," said TechSat 21 technical advisor Dr. Alok Das. "Mathematical theory suggests that a cluster of orbiting satellites will not burn fuel excessively if each satellite in the cluster is carefully placed in a slightly different orbit than its closely hovering neighbors, with small differences in attitude and inclinations. In fact, fuel consumption can be cut by as much as two or three orders of magnitude, which can translate to nearly 1000 times less fuel. Precise orbit placement and this fuel-saving measure are key enablers for the TechSat 21 concept," he said.

Another inherent advantage of TechSat 21 is the notion of operational perpetuity for the warfighter. Current single satellites, once outdated or dead, must be replaced in their entirety

Not TechSat 21.

"With TechSat-21-based architecture, a satellite constellation never dies," explained Das. "Individual microsats in the cluster can be replaced as they wear out, or upgraded as technologies mature through on-orbit servicing (which AFRL is also working on), much like a network of desktop computers linked together -- they can be expanded as needed without throwing out the entire system. TechSat 21 is an exact analogy of that concept."

Autonomous operability is another TechSat 21 mainstay. The satellite cluster itself, working in conjunction with the Global Positioning System, will automatically reshape to optimize its geometry based on the mission assigned the cluster by a ground commander. For example, a "geo-location" mission such as finding a downed pilot would require that one satellite in the cluster move out from the group to a distance of approximately five miles. On the other hand, spotting moving ground targets requires a pattern of closer intervals, maybe several hundred meters, between satellites. Also, additional satellites can be added to an existing cluster to accomplish more diverse missions.

Pinpointing the location of friendly forces, as well as tracking enemy airborne and ground targets, have been recent priorities for Air Force operational units. Consequently, researchers expect that a TechSat 21- based system of distributed satellites, once operational, will focus on one or more of these missions.

By 2003, AFRL and its partners, which include several universities, small businesses, and large aerospace companies, hope to have unraveled the theoretical mathematics of formation flying and launch the first demonstration flight of three or four cooperative microsatellites orbiting in a reconfigurable formation.