It is important to test the attitude control systems on satellites before they are launched in space. Traditionally this has been done by dropping the satellite, and firing the thrusters before the satellite makes a soft landing in a net. This method only allows a few seconds of testing and does not lend itself to the measurement of pointing accuracy. A better method is to mount the satellite on a spherical air bearing. This provides a frictionless pivot with three degrees of freedom. Servo loop stability, response time, and pointing accuracy can be determined. In order for this concept to work, the composite center of gravity of the rotating system must be coincident with the center of rotation of the air bearing. There are three basic techniques for accomplishing this: (1) counterbalancing the satellite, (2) using a shallow spherical bearing with a center of rotation considerably above the mounting surface, and (3) splitting the satellite in half and placing each half on the side of the sphere which is supported on a film of air. This paper describes a number of spherical air bearing “space simulators” in detail, and outlines the fundamental concepts of this type of testing.
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