Static Gravity Reference Vector Determination for Sensor Calibration
Static gravity reference determination requires site gravimetric surveying and arcsecond stage indexing to isolate true sensor bias from facility tilt errors.
Cardinal Position Tumble
Calibration Sequence
Testing protocol involving the rotation of an inertial sensor through ninety degree increments relative to the local gravity vector determines the scaling and bias parameters. The cardinal position tumble provides a systematic way to expose each axis of an accelerometer to the full force of gravity. By measuring the output at these known orientations, a technician can solve for the unknown calibration constants.
This process assumes that the local gravity value is known and stable.
Rotation Sequence
Procedure begins with the primary axis pointing directly upward and proceeds in a fixed pattern. A cardinal position tumble usually includes six discrete orientations to cover the positive and negative directions of the three orthogonal axes. Each stop allows the sensor to reach a stable state before the data is recorded.
This sequence minimizes the impact of transient noise on the calibration result. Precise mechanical indexing is required to ensure the angles are exactly ninety degrees.
Bias Solution
Comparison between the measured values and the expected gravity vector reveals the offset of the instrument. The cardinal position tumble separates the sensitivity of the sensor from the zero gravity bias. Mathematical models use these data points to calculate a correction factor for every axis.
Accurate bias determination is necessary for the long term stability of navigation systems. These calculated factors are then stored in the non volatile memory of the device.
Fixture Precision
Performance of the test requires a level surface and a vibration free environment. A cardinal position tumble performed on a tilted platform will introduce a projection error into the final coefficients. Specialized granite tables and isolation mounts protect the setup from external disturbances.
The integrity of the calibration depends on the precision of the mechanical fixtures used to hold the sensor. Successful completion of the tumble test confirms the sensor is ready for field deployment.