Quantifying Gyro Noise Coefficients from Allan Variance Data
Quantify gyro noise coefficients by fitting specific logarithmic slope asymptotes to overlapped Allan deviation curves gathered in thermally stabilized static rigs.
Earth Rate Coupling
Rotational Sensitivity
Gyroscopic sensor performance depends on the ability to isolate inertial sensing from the angular velocity of the planet. Earth rate coupling represents the parasitic input of the planetary rotation vector into a non-axis-aligned sensor frame. This leakage occurs whenever the sensitive axis of a device does not maintain perfect orthogonality with the rotation axis of the globe.
System designers minimize this effect through precise mechanical leveling and software-based bias compensation during static operation.
Coordinate Alignment
Hardware manufacturers define the inertial reference frame relative to the geographic north and the local gravity vector. Earth rate coupling arises if the sensor package tilt introduces a component of the fifteen degree per hour planetary rotation into the sensitive axis. High-end tactical grade units employ secondary leveling sensors to detect these installation errors.
Error budgets include this contribution to ensure the long term stability of navigation solutions.
Mathematical Correction
Calibration algorithms calculate the projection of the planetary rotation vector onto the sensor sensitive axes. Mathematical models rotate the sensed inertial input by the latitude-dependent magnitude of the earth rate to remove the unwanted signal. Failure to account for this projection produces a cumulative drift in orientation outputs over time.
Precision relies on the accurate entry of geographical coordinates into the navigation processor.
Operational Variance
Environmental conditions change the physical relationship between the sensor and the planetary frame. Thermal expansion of mounting brackets shifts the alignment of the sensitive axis relative to the horizon. Constant monitoring of the residual bias allows the internal compensation loop to adjust for these mechanical distortions.
Modern inertial systems maintain angular accuracy by rejecting the signal component that matches the frequency and magnitude of the diurnal rotation.