Investigation of a nonorthogonal gyroscope model for ICESat-2
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The Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) is a second- generation laser altimeter satellite tasked with accurately measuring the change in ice sheet mass balance . Errors in measurements of ice sheet elevation stem from errors in knowledge of where the spacecraft is pointing, which is a direct result of errors in the knowledge of the spacecraft's attitude. In order to meet the elevation change requirements, very accurate attitude determination is required. One way to improve the accuracy of attitude determination is to improve the error models and the calibration parameters of the sensors on the satellite, particularly the gyroscope. On ICESat-2, the gyroscope is the second-generation space inertial reference unit (SIRU-2). The SIRU-2 consists of four rate integrating hemispherical resonator gyros arranged with their axes forming an octahedral tetrad pyramid shape . This four-axis mounting configuration of the gyroscope is the motivation behind this work. On the original ICESat spacecraft, data from only three axes of the gyroscope was collected and used in the attitude filtering process. Instead of implementing an attitude filter that requires the calibration parameters to be referenced in the spacecraft body frame ,, this work investigates modeling the gyroscope errors in the native gyroscope frame. As a result of this modeling configuration, more data is available; however, more states in the attitude filter need to be estimated. This document develops an Multiplicative Extended Kalman Filter (MEKF) with the gyro parameters in their native frame. After numerical verification of the new model, a sensitivity analysis is performed to determine whether the orthogonal or the sense axis base model is more robust to increased gyroscope noise, increased gyroscope axis bias, scale factor error, and misalignment error, and axis failure. Ultimately, the increased number of states inherent in the sense axis based model introduces an observability issue that does not negatively impact the attitude solution, but also does not allow for current independent prediction of the estimated error parameters. More investigation on ways to separate the estimation of the states is necessary to determine if the added information from this sense axis model can be of use.