Polarization imaging of the moon in support of nighttime remote sensing

Abstract

The Moon is a valuable calibration target for on-orbit satellite instruments because it provides a stable source outside of the atmosphere, but effective lunar calibrations require an accurate characterization of the moonlight in the observation geometry. Similarly, the Moon can be used as the source of illumination in nighttime remote sensing applications, but knowing the moonlight properties improves understanding of the remote sensing data. Radiometric models of the Moon that predict the spectral radiance at a given geometry have been developed from large datasets of lunar radiance measurements. However, for accurate lunar calibrations of polarization-sensitive instruments and for polarimetric nighttime remote sensing, it is critical to account for the polarization state of the moonlight. Moonlight has a polarization state that depends on wavelength and the lunar geometry, but its polarization state has not been characterized as thoroughly as its radiance. This dissertation addressed two knowledge gaps: there were no previously published measurements of disk-averaged moonlight polarization as a function of phase angle for wavelengths longer than 520 nm, and there was no published Moon polarization model that predicted the polarization from the lunar geometry. To address these gaps, a method was developed to record calibrated polarization images of the Moon, and the disk-averaged degree of linear polarization was calculated from the images. The method was initially used to measure the polarization in a broad spectral band (responsive from 400 nm to 1000 nm). Spectral filters were added to the imaging system to measure the moonlight polarization in five spectral bands. The data in 450-nm and 520-nm bands agreed well with previously published measurements, and the data in 650-nm, 800-nm, and 900-nm bands provided the first characterization of polarization as a function of phase angle at these wavelengths. Finally, empirical models were developed using these data and previously published data to predict polarization from the lunar geometry in spectral bands from 336 nm to 900 nm with estimated uncertainties of + or - 0.01 or less. These measurements and the resulting model could enable more accurate lunar calibration for instruments currently on orbit and improve a variety of nighttime remote sensing applications.