In-Orbit Temperature Calibration of Long-Term Korea Pathfinder Lunar Orbiter Magnetometer Data

The Korea Pathfinder Lunar Orbiter (KPLO) magnetometer (KMAG) is a triaxial racetrack fluxgate magnetometer with a 1.2-meter-long boom for the KPLO mission. Previous studies on KMAG data processing and calibration have demonstrated that the quality of processed data is sufficient for general scientific research. However, additional calibration is necessary to remove residual noise fields that make it difficult to distinguish small-amplitude signals. As some magnetic interferences induced by the space environment are not fully covered by ground-based calibration, which is conducted before launch, in-orbit calibration performed during the mission phase is necessary. Although we applied several in-orbit calibration procedures, residual interference noise fields that follow a similar periodic trend of sensor temperature prominently remain during orbital phases with large temperature variations. In this study, we conducted in-orbit temperature calibration based on the fluxgate sensor characteristics, in which the measured field has an offset related to temperature variations. The variations in the observed magnetic field and sensor temperature of the KMAG measurements followed similar trends not only in the relatively short time interval, but also in the long term more than two years. We applied three approaches based on this similarity and validated the calibration performance of the best one. The temperature coefficient from the ground test was 0.2 nT/°C, while a larger value of up to 1.2 nT/°C was obtained in this study. Despite this difference, residual noise significantly reduced by applying the newly derived in-orbit temperature coefficient to the actual data. This demonstrates that it can be applied to the in-orbit calibration of fluxgate magnetometers.