Evaluating the Use of TROPICS Pathfinder Observations for Lunar Calibration

Abstract

Traditional two-point in-situ calibration systems for microwave radiometers use large, on-board hot targets [12]. Small satellites such as CubeSats, however, are unable to house these hot targets due to SWaP constraints. Instead, CubeSat radiometers such as the TROPICS Pathfinder use noise diodes as smaller alternative hot calibration targets [18]. Noise diodes can experience calibration drifts that must be characterized and accounted for to maintain the reliability of radiance measurements. Given the stability of lunar radiative transfer models in microwave frequencies, lunar vicarious calibration may be a feasible method to detect calibration drifts. In this thesis, we evaluate the use of TROPICS Pathfinder observations for lunar calibration. We develop a lunar calibration approach that takes TROPICS observations as input, processes TROPICS data for lunar observations, estimates lunar intrusion temperature and scan geometry, and accounts for pointing error. We compare the lunar brightness temperature estimates and measured antenna temperatures to the lunar radiative transfer model developed by Yang and Burgdorf [21]. We test our lunar calibration model on TROPICS Pathfinder lunar observations from November and December 2021. Pathfinder’s antenna temperatures are within 1 K and 2 K of the simulated antenna temperatures for the W/F and G bands, respectively. We find that even though the simulated antenna temperatures generally agree, work remains to improve agreement between measured and modeled lunar brightness temperatures. The antenna temperature differences can fluctuate by ±2K, ±4K, and ±5K for the W, F, and G bands, respectively, so the reliability of this methods needs to be improved further before operational use for calibration. Possible ways to improve lunar calibration results include tuning Yang and Burgdorf’s lunar model, additional pointing error analyses, and lunar calibration model adjustments.