MethaneSAT On-Orbit Lunar Calibrations Planning

The MethaneSAT satellite mission aims to detect, quantify, and monitor methane emissions in targets covering over 80% of global oil and gas production. Bridging the gap between existing point-source and global mapping remote sensing satellites, MethaneSAT enables simultaneous emission estimation from discrete sources and from dispersed sources. One of MethaneSAT's calibration activities is a monthly scan of the nearly full moon (a “lunar calibration scan”), which provides a consistent and well-known external light source in the wavelength range that MethaneSAT measures. This paper presents a detailed description of MethaneSAT's lunar calibration activities and example data from an early MethaneSAT lunar calibration scan. MethaneSAT uses a synchronous scanning mode where the spacecraft attitude is fixed with respect to the orbital frame and the instrument array scans perpendicular to the apparent velocity of the moon. MethaneSAT's instruments have a 21.3° field of view while the moon's angular diameter is 0.52°, and so it would require 41 synchronous scans to illuminate the entire field of view. Due to data budget and time limitations MethaneSAT has been performing 5 lunar calibrations scans per lunar cycle when the phase angle is between +5 to +9 degrees. Observations from each lunar calibration scan are to be later compared to the Lunar Irradiance Model of the European Space Agency (LIME)'s disk-integrated lunar irradiance model, which is derived from ground-based observations from the Izana Atmospheric Observatory and Teide Peak. LIME predicts variations in lunar irradiance, accounting for view geometry, phase angle, lunar librations, and the lunar surface albedo distribution of maria and highlands. In August 2024, MethaneSAT successfully scanned the moon, demonstrating the viability of the proposed calibration approach. However, a larger dataset of monthly scans is necessary for trending analysis and comparison against the LIME model. As such, this paper focuses on the planning and methodology for lunar calibration, presenting an example scan as a proof of concept. Further calibration trend analyses will be conducted as more scans are collected in the coming months. Once sufficient data is available, the calibration will be finalized and integrated into satellite retrievals.