Abstract
In recent years, the study of microwave radiation from the Moon’s surface has been of interest to the astronomy and remote sensing communities. Due to the stable geophysical properties of the Moon’s surface, microwave lunar radiation is highly predictable and can be accurately modeled, given sufficient observations from reliable instruments. Specifically, for microwave remote sensing study, if International System of Unit (SI) traceable observations of the Moon are available, the Moon can thus be used as an SI traceable calibration reference for microwave instruments to evaluate their calibration accuracies and assess their long-term calibration stabilities. Major challenges of using the Moon as a radiometric source standard for microwave sensors include the uncertainties in antenna pattern measurements, the reliability of measurements of brightness temperature (Tb) in the microwave spectrum of the lunar surface, and knowledge of the lunar phase lag because of penetration depths at different detection frequencies. Most microwave-sounding instruments can collect lunar radiation data from space-view observations during so-called lunar intrusion events that usually occur several days each month. Addressed in this work based on Moon observations from the Advanced Technology Microwave Sounder and the Advanced Microwave Sounding Unit/Microwave Humidity Sounder are two major issues in lunar calibration: the lunar surface microwave Tb spectrum and phase lag. The scientific objective of this study is to present our most recent progress on the study of lunar microwave radiation based on satellite observations. Reported here are the lunar microwave Tb spectrum and phase lag from 23 to 183 GHz based on observations of microwave-sounding instruments onboard different satellite platforms. For current Moon microwave radiation research, this study can help toward better understanding lunar microwave radiation features over a wide spectrum range, laying a solid foundation for future lunar microwave calibration efforts.
Highlights
Lunar radiation is highly stable in the microwave spectrum because of the stable geophysical properties of the Moon’s surface
Addressed in this work based on Moon observations from the Advanced Technology Microwave Sounder and the Advanced Microwave Sounding Unit/Microwave Humidity Sounder are two major issues in lunar calibration: the lunar surface microwave Tb spectrum and phase lag
The scientific objective of this study is to present our most recent progress on the study of lunar microwave radiation based on satellite observations
Summary
Lunar radiation is highly stable in the microwave spectrum because of the stable geophysical properties of the Moon’s surface. Based on lunar surface geophysical parameters derived from satellite observations, including the bulk density, thermal and dielectric profiles, and heat conduction, microwave Tbs at millimeter and submillimeter wavelengths were simulated using lunar regolith temperature profiles derived from DLRE lunar surface temperatures by solving the heating conduction equation using the finite-difference time-domain method [18,19]. Based on well-calibrated lunar antenna temperature datasets, a physical model can be developed to derive the disk-integrated lunar surface microwave Tb spectrum with frequencies from 23 to 183 GHz. Table 1 lists the microwave Tb retrieval results at the full-moon phase for each ATMS frequency. The retrieved Moon-disk-averaged Tb spectrum shows a strong frequency-dependent feature, i.e., increasing from 270 K at 23 GHz to 300 K at 183 GHz at the full-moon phase
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