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This paper presents a thermal calibration approach for the accelerometer triad without any temperature-controlled incubator. This approach is implemented by the sequential multiposition observation of the gravity, the raw output, and the temperature values of accelerometers. Based on the fact that the norm of measurement outputs of the accelerometer triad ideally equals to the gravity value in constant thermal conditions, a multiposition least squares estimation procedure is utilized to solve the individual reference-frame thermal parameters, which relaxes the orientation accuracy requirement of the turntable. According to the multiposition observation of a gravity vector in two reference frames, the constrained quaternion optimization is used to solve the relative attitude among different reference frames. Assuming the superposition relation of the gravity vector and the rotation axis direction, the multiposition calibration of the inner triad relationship is implemented for the navigation computation purpose. Comparisons of specific force measurement and navigation position in two experiments illustrate that the thermal drift errors can be greatly reduced in the after-power-on process. It shows that the sensitivity axis of the accelerometer may change as large as five arcseconds in direction.