Inertial space is a reference system for measuring acceleration or change in motion. It is self-contained and not subject to external forces. An Inertial Navigation System (INS) uses motion sensors and a computer to determine speed and position. The accuracy of an INS can be improved with more sophisticated devices such as the fiber optic gyroscope. A gyro compass is used on ships to indicate the geographic North Pole.
Inertial space is a reference system against which acceleration or change in motion is measured. Within an inertial frame of reference, objects experience constant relative motion and appear to be at rest in reference to each other; this defines the inertia of space and serves as a background against which the change in motion of an object is measured. The results of measurements made in one inertial frame can be converted into another by a simple mathematical calculation.
A property of an inertial frame is that the behavior of its objects is not subject to forces external to that frame of reference. In Newtonian physics, the fixed stars were considered an inertial frame of reference; it is now known that stars are not fixed but have their own relative motions in galaxies, as do galaxies in larger group structures. Using stars as if their relative motion defined an inertial space introduces few errors.
A spinning gyroscope without rotational acceleration will maintain its orientation in inertial space; if it rotates at a constant speed, it will continue to point in the same direction relative to the fixed stars. You can measure motion changes related to the orientation of the gyroscope and use the data to calculate speed and position. This is the basis for an Inertial Navigation System (INS), which determines a vehicle’s speed and position solely from reference to a position in inertial space.
An INS typically consists of motion sensors, such as gyroscopes and accelerometers, and a computer. The system is provided with the initial speed and position, and then calculates the future position and speed in real time from the sensor data. The linear and angular acceleration changes are measured with reference to the alignment of the gyroscope to the inertial space. Regardless of its initial conditions, an INS is completely self-contained and is not subject to jamming or other interference.
Accumulated error from measurement and calculation tends to make an INS less accurate over an extended period of time. This deficiency has been somewhat compensated for by more sophisticated devices such as the fiber optic gyroscope, which is based on the Sagnac effect. In this type of device, counter-rotating lasers produce an interference pattern from which changes in angular velocity with respect to a position in inertial space can be calculated.
On ships, a gyro compass is used to indicate the geographic North Pole. The device uses the properties of a gyroscope to maintain a fixed orientation with respect to inertial space and a pendulum to align it with the Earth’s axis of rotation. As long as the gyroscope rotor is parallel to the earth’s axis, there is no torque, or angular resistance, from the earth’s rotation. Misalignment is self-correcting by forces due to planetary rotation.
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