Tracking Terra MODIS on-orbit polarization sensitivity using pseudo-invariant desert sites

The Moderate-Resolution Imaging Spectroradiometer (MODIS) is currently flying on NASA's Earth Observing System (EOS) Terra and Aqua satellites, launched in 1999 and 2002, respectively. MODIS reflective solar bands (RSB) in the visible wavelength range are known to be sensitive to polarized light based on prelaunch polarization sensitivity tests. The polarization impact is dependent on scan angle and mirror side. After about five years of on-orbit operation, it is found that a few shortest-wavelength bands of Terra MODIS show increased polarization sensitivity. In this study, we examine the impact of polarization on measured top-of-atmosphere (TOA) reflectances over pseudo-invariant desert sites. The standard polarization correction equation is used in combination with simulated at-sensor radiances by the Second Simulation of a Satellite Signal in the Solar Spectrum (6SV), Vector Radiative Transfer Code. Key Mueller matrix elements describing the polarization and gain correction of these bands are derived over the mission lifetime. Results indicate that the polarization sensitivity increases with scan mirror’s angle of incidence (AOI) and relatively large impact is observed from mirror side 2. At the end of 2009, it reaches a peak at approximately 30% at 0.41 μm and stabilizes since then.