Artist's concept of a child's spinning top.

Gyroscopes sense the slightest change in rate about an axis of rotation. They are "inertial reference" devices that keep the spacecraft stable. Inertial reference is a state of matter that has no external forces acting on it. That is the baseline, or point of comparison that will be used to maintain the stability of the spacecraft. Reference gyros measure the forces acting on them. In deep space, the force will be proportional to the ship's velocity (speed). In a spacecraft, gyroscopes tell the onboard computer when the craft has changed its attitude (direction and pointing). The computer then sends the information to the spacecraft's stabilization device, which can make corrections.

In the past all gyroscopes used spinning masses. The simple gyroscope we are all most familiar with the child's top. In recent years other types of gyros have been used in spacecraft: laser ring gyros, hemispherical resonator gyros, and microelectromechanical system (MEMS) gyros——the type Space Technology 6 (ST6) will use. MEMS are a combination of tiny electrical and mechanical devices etched in silicon much like computer chips.

Three Axis MEMs

Compass uses a MEMS 3-axis assembly that incoporates "tuning fork" gyro sensors and mixed signal application-specific integrated circuits (ASIC). These gyro electronics are designed to operate with approximately 12 off-chip components at a power draw of 75mWatts.

The Compass MEMS assembly includes data acquisition electronics that will provide angular rate, temperature, and health and status data to the processor. These components are small in volume, mass, and have high resistance to radiation and vibration.

Together, the Compass gyroscopes and star camera keep the spacecraft stable and oriented in the right direction in space.