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From Global Positioning Systems, Inertial Navigation, and Integration

9.4.4 Gimbal Attitude Implementations

The primary function of gimbals is to isolate the ISA from vehicle rotations, but they are also used for other INS functions.

9.4.4.1 Accelerometer Recalibration Navigation accuracy is very sensitive to accelerometer biases, which can shift as a result of thermal transients in turnon/turnoff cycles, and can also drift randomly over time. Fortunately, the gimbals can be used to calibrate accelerometer biases in a stationary 1-g environment. In fact, both bias and scale factor can be determined by using the gimbals to point the accelerometer input axis straight up and straight down, and recording the respective accelerometer outputs aup and adown. Then the bias abias = (aup + adown) /2 and scale factor s = (aup - adown) /2glocal, where glocal is the local gravitational acceleration.

9.4.4.2 Gyrocompass Alignment This is the process of determining the orientation of the ISA with respect to locally level coordinates (e.g., NED or ENU). Gyrocompassing allows the ISA to be oriented with its sensor axes aligned parallel to the north, east, and vertical directions. It is accomplished using three servo loops. The two "leveling"


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