Scientists and engineers at Lawrence Livermore have been engineering small, agile satellites that can help deal with potential space calamities. Called microsatellites (microsats, for short), they are an outgrowth of research performed for the Laboratory's Clementine satellite program, which mapped the moon and then discovered the first evidence that water may exist there. The microsatellites are envisioned as operating autonomously in orbit to serve a variety of future space-exploration needs in addition to probing near-Earth asteroids. Microsatellites would be able to strike or probe the potentially hazardous objects that threaten Earth. In addition, they might be handy rescue vehicles used to inspect disabled satellites and relay observations about them to ground stations; they might also dock with and repair satellites. Microsatellites could also be part of a control system that protects and defends U.S. assets in space.The capability for such uses will come through integrating a complex array of advanced technologies in the microsatellite vehicle. Sensors, guidance and navigation controls, avionics, and power and propulsion systems--all must perform precisely and in concert so the vehicles can find, track, lock onto, and rendezvous with their targets, even though those targets are also on the move. The rigorous ground testing of microsatellites' integrated technologies is essential; these tests produce data needed for effective flight testing.The best ground-testing environment is one that mimics, as much as possible, the free-floating environment of a space flight. Finding a way to emulate such an environment was one of the important tasks facing microsatellite developers.
Inspired by a Game
Traditionally, space vehicles have been ground tested on a stationary hemispherical air bearing, a device that floats a test vehicle with high-pressure air. The air bearing provides the vehicle with three angular degrees of freedom. The stationary air bearing is useful for testing the stability of a space vehicle in orbit. But because microsats will be performing precision maneuvers in space that involve translation--that is, parallel, sideways motions--its testing must also account for linear dynamics.
Clementine II program leader Arno Ledebuhr, engineering group leader Larry Ng, and mechanical engineers Jeff Robinson and Bill Taylor came up with the idea for a dynamic air-bearing device that provides five degrees of freedom (three rotational, or angular, and two translational, or linear, motions). Their inspiration came from the game of air hockey, which uses air pushed out of a table to float hockey pucks. In the dynamic air bearing, this configuration is inverted--the air is pushed out of the pucks. Three such air pucks are used to support a traditional air bearing on a fixture that also includes an air supply--from high-pressure nitrogen tanks (Figure 1). As the air pucks release the high-pressure air, the whole device is lifted off the surface on which it has been sitting. Because the three air pucks, equally distributed on a 19-centimeter-radius circle, can support a total weight of more than 150 kilograms, it capably floats itself (5 kilograms) and a microsat test vehicle (25 kilograms). It thus allows the test vehicle to move linearly as if in a near-zero-gravity space environment.
Scaling Down Space Maneuvers
The Livermore team is using the dynamic air-bearing device in a series of experiments called AGILE, for air-table guided-intercept and line-of-sight experiments. These experiments will evaluate a vehicle's ability to "divert," that is, maneuver in space while keeping track of a moving target (such as an incoming asteroid) and then close in to intercept it. The objective of these experiments is to quantify the distances by which the microsats miss intercepting the target, thus allowing microsat developers to identify hardware and software deficiencies.
For a vehicle to accomplish an interception, its sensors and measurement, navigation, and control systems must work together to continually calculate vehicle speed and position in relation to the target. They must calculate the point at which the target can be intercepted and get the vehicle to that point at precisely the same time as the target. Because both the vehicle and target are moving, the line of sight to the target continually changes, and therefore, vehicle acceleration and position must be constantly adjusted. Further complicating these calculations are the many other factors that can affect maneuvering precision, such as changing vehicle mass due to fuel expenditure, vehicle acceleration capability, and minor misalignment of hardware components.
The interception experiments use a test vehicle that can move with five degrees of freedom. The vehicle sits on the dynamic air bearing, which in turn is borne on two large, smooth glass plates resting side by side on a table. The glass plates form a rectangular test table approximately 1.5 by 7.2 meters. A laser projects a target onto a wall-mounted target board parallel to the long side of the glass test table. A precision measurement system, consisting of a laser and a camera, accurately measures and records the test vehicle's position (Figure 2).
