Calibration of the in-orbit center-of-mass of TaiJi-1

TL;DR

This paper introduces a novel estimation method combining Extended Kalman Filter and Rauch-Tung-Striebel Smoother to accurately determine the satellite's center-of-mass offset, improving measurement precision for gravitational wave detection.

Contribution

The paper presents a new combined estimation approach for in-orbit center-of-mass calibration, enhancing accuracy and reliability over previous methods.

Findings

Estimated COM offsets: dx ≈ -0.19 mm, dy ≈ 0.64 mm, dz ≈ -0.82 mm.

Significant noise reduction in GRS after calibration.

Method provides consistent results with crosscheck algorithms.

Abstract

Taiji program is a space mission aiming to detect gravitational waves in the low frequency band. Taiji-1 is the first technology demonstration satellite of the Taiji Program in Space, with the gravitational reference sensor (GRS) serving as one of its key scientific payloads. For accurate accelerometer measurements, the test-mass center of the GRS must be positioned precisely at the center of gravity of the satellite to avoid measurement disturbances caused by angular acceleration and gradient. Due to installation and measurement errors, fuel consumption during in-flight phase, and other factors, the offset between the test-mass center and the center-of-mass (COM) of the satellite can be significant, degrading the measurement accuracy of the accelerometer. Therefore, the offset needs to be estimated and controlled within the required range by the center-of-mass adjustment mechanism during the satellite's lifetime. In this paper, we present a novel method, the Extended Kalman Filter combined with Rauch-Tung-Striebel Smoother, to estimate the offset, while utilizing the chi-square test to eliminate outliers. Additionally, the nonlinear Least Squares estimation algorithm is employed as a crosscheck to estimate the offset of COM. The two methods are shown to give consistent results, with the offset estimated to be $dx \approx $$-$$0.19$ mm, $dy \approx 0.64$ mm, and $dz \approx $$-$$0.82$ mm. The results indicate a significant improvement on the noise level of GRS after the COM calibration, which will be of great help for the future Taiji program.