ALTHOUGH the recent prediction of a near collision between Earth and asteroid XF11 turned out to be inaccurate, hazards from asteroids and other near-Earth objects are out there. After all, just a few years ago, the Shoemaker-Levy comet hurtled onto Jupiter, leaving Earth-sized scars on the planet's face, and a similar event is believed to have caused the extinction of the dinosaurs on Earth. The few nervous moments we Earthlings had over XF11 serve as a reality check on the hazards that await from space.
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).

The intercept geometry, comprising the vehicle positions, target positions, and the changing line of sight between them, is scaled for the indoor table experiment to preserve the intercept geometry of an actual flight. For example, for a successful interception, the line-of-sight rate must approach zero; that is, the vehicle and target must both arrive at the same point at the same time. To preserve that line-of-sight requirement in the test, the test maneuvering distance is scaled down in relation to the target that is projected on the screen. The target location and interception point are predetermined, and these values, used in conjunction with the precise measurements of vehicle position (from the laser measurement system), allow experimenters to determine the ability of the onboard guidance and control software to maneuver the vehicle to the point of interception.

Taking Testing to the Next Steps
The current rectangular, indoor dynamic air-bearing test setup is useful for a variety of experiments. However, the short length of the current glass test surface limits maneuvering distance, thus prohibiting replication of the exact frequency and duration of engine acceleration in actual flight maneuvers. Making the test surface larger and square (10 meters by 10 meters) will enable the performance of a greater range of rendezvous and docking maneuvers, including practicing the circumnavigation of a satellite and determining its spin axis and rotation rate.

To eliminate some of the indoor setup's limitations, an outdoor version of the device is being developed. In this version, the test vehicle "floats" on a smooth rail 100 to 200 meters long and "views" a tilted board on which an incoming target is projected (Figure 3). The rail air-bearing system can move in only one linear direction, but because of its larger scale, it provides an improved replication of flight maneuvers and a more accurate tracking of vehicle position. Both improvements lead to a more precise reconstruction of line-of-sight angles, which is key to correctly predicting the point at which the microsat maneuvers to its target.
The air-bearing team's work on ground testing techniques continues. To date, a 17-meter-long rail has been used to "fly" the newest generation of the microsat vehicle. Longer range outdoor docking experiments that incorporate both an onboard Star Tracker camera, which uses stars to calculate the orientation of the microsats, and a global positioning system receiver are in the planning stages.
--Gloria Wilt

Key Words: AGILE (air-table guided-intercept and line-of-sight experiments), dynamic air-bearing table, dynamic air-bearing rail, ground testing, microsat, microsatellite, spacecraft interceptor, space vehicle.

For further information contact Arno Ledebuhr (925) 423-1184 (

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