How do you test the functionality of a wholly new type of satellite docking system in the weightlessness of space, without shooting round after round of prototype into orbit? If you're DARPA, you float the satellites—air hockey-style—on top of a 37.5-ton slab of granite.
Conventional satellites to this point have generally employed specially-designed, one-off docking systems—and that's only if the satellite is designed to be serviced or maintained in the future. It's not like you can just shoot any service satellite into orbit and expect it to successfully rendezvous and dock with another satellite already in the sky. But that's exactly what DARPA is trying to build. The US agency has funded the Front-end Robotics Enabling Near-term Demonstration (FREND), a project aiming to create a fully autonomous docking capability for satellites that weren't built to be serviced.
FREND will also allow a satellite to reposition its own orbital trajectory. With this ability, fleets of satellites could be moved, say to improve television coverage in times of national crisis. Individual satellites could last longer saving by using this add-on system (rather than their own fuel supplies) to move into safe orbits. This could also destroy dead satellites altogether by plunging them into the atmosphere.
But before the FREND system can be tested in space, it needs to prove its mettle here on Earth. To do that, researchers at the U.S. Naval Research Laboratory Spacecraft Engineering Department, recently built a Gravity Offset Table, the only one of its kind. The table itself is a 20-foot by 15-foot by 1.5-foot single granite slab pulled from Raymond Granite Quarry in Clovis, Ca and honed to a tolerance of +/- 0.0018 inches flat across the entirety of its surface. Prototypes are suspended above the slab using powerful jets known as air bearings. The ultra-smooth, ultra-flat face of the granite accurately reproduces the inertial forces and orbital dynamics found in space, allowing the prototypes a full six degrees of freedom when moving about.
"We accomplish this by floating models of spacecraft and other resident space objects on air bearings—similar to the dynamics of an upside-down air hockey table," Dr. Gregory P. Scott, space robotics scientist, said in a press statement. "Based on the inertia of the 'floating' system, a realistic spacecraft response can be measured when testing thrusters, attitude control algorithms, and responses to contact with other objects."
The Gravity Offset Table testbed is also equipped with a precision optical measurement system to track satellites' positions and orientations, which allows researchers to optimize the approach and docking procedure in the lab. [NRL 1, 2 - AZRobotics - Top Image: isaravut / Shutterstock - Side Image: U.S. Naval Research Laboratory]