Ground Alignment Error Bounds for Inertial Navigation Initialization
Ground alignment error bounds depend on accelerometer turn-on bias for leveling and East gyro bias stability divided by cosine latitude for heading accuracy.
Specific Force
Acceleration Sum
Sum of all non-gravitational accelerations acting on a unit mass as measured by an accelerometer. This specific force represents the actual contact forces, such as thrust or drag, that cause a vehicle to move relative to a freefalling frame. The data is the fundamental input for the velocity update equations in an inertial navigation computer.
Measurement Principle
Accelerometers respond only to the forces exerted by the internal structure on the proof mass. While the gravity of the earth acts on every atom of the sensor, it is the specific force that creates the measurable displacement within the transducer. In a state of freefall, the measured specific force is zero despite the presence of a strong gravitational field.
Navigational Integration
Velocity is calculated by integrating the measured signal over time after accounting for the local gravity vector. The specific force data must be transformed from the body frame of the vehicle into a stable geographic frame before this integration can occur. Errors in this transformation or in the initial measurement lead to a quadratic growth of position error in the navigation solution.
Precise time-stamping of these measurements ensures that the integration remains synchronized with the rotational data from the gyroscopes.
Frame Reference
Distinction between coordinate systems is essential for the correct interpretation of the sensor output.