User:Marshallsumter/Radiation astronomy1/Motion

Sensors
Def. "a device or organ that detects certain external stimuli and responds in a distinctive manner" is called a sensor.

Many sensors generate outputs that reflect the rate of change in attitude. These require a known initial attitude, or external information to use them to determine attitude. Many of this class of sensor have some noise, leading to inaccuracies if not corrected by absolute attitude sensors.

Motion sensors
Gyroscopes are devices that sense rotation in three-dimensional space without reliance on the observation of external objects. Classically, a gyroscope consists of a spinning mass, but there are also "Laser Gyros" utilizing coherent light reflected around a closed path. Another type of "gyro" is a hemispherical resonator gyro where a crystal cup shaped like a wine glass can be driven into oscillation just as a wine glass "sings" as a finger is rubbed around its rim. The orientation of the oscillation is fixed in inertial space, so measuring the orientation of the oscillation relative to the spacecraft can be used to sense the motion of the spacecraft with respect to inertial space.

Motion Reference Units are single- or multi-axis motion sensors. They utilize Micro-Electro-Mechanical-Structure (MEMS) sensor technology. These sensors are revolutionizing inertial sensor technology by bringing together micro-electronics with micro-machining technology, to make complete systems-on-a-chip with high accuracy.

Horizon sensors
A horizon sensor is an optical instrument that detects light from the 'limb' of the Earth's atmosphere, i.e., at the horizon. Thermal Infrared sensing is often used, which senses the comparative warmth of the atmosphere, compared to the much colder cosmic background. This sensor provides orientation with respect to the earth about two orthogonal axes. It tends to be less precise than sensors based on stellar observation. Sometimes referred to as an Earth Sensor.

Orbital gyrocompasses
Similar to the way that a terrestrial gyrocompass uses a pendulum to sense local gravity and force its gyro into alignment with earth's spin vector, and therefore point north, an orbital gyrocompass uses a horizon sensor to sense the direction to earth's center, and a gyro to sense rotation about an axis normal to the orbit plane. Thus, the horizon sensor provides pitch and roll measurements, and the gyro provides yaw. See Tait-Bryan angles.

Inertial navigation systems
An inertial navigation system (INS) is a navigation device that uses a computer, motion sensors (accelerometers) and rotation sensors (gyroscopes) to continuously calculate by dead reckoning the position, the orientation, and the velocity (direction and speed of movement) of a moving object without the need for external references. Often the inertial sensors are supplemented by a barometric altimeter and sometimes by magnetic sensors (magnetometers) and/or speed measuring devices. INSs are used on mobile robots and on vehicles such as ships, aircraft, submarines, guided missiles, and spacecraft.