Lunar phase angle impact on Aqua and Terra MODIS PV LWIR crosstalk coefficients calculation

The MODIS instruments onboard the Terra and Aqua satellites have provided continuous global observations for science research and applications for more than 24 and 22 years, respectively. The sensors’ calibration accuracy and product quality have been maintained. Electronic crosstalk in the photovoltaic (PV) longwave infrared (LWIR) bands is a known issue. The crosstalk coefficients are derived using scheduled monthly lunar observations. Crosstalk corrections for the Terra MODIS PV LWIR bands have been applied in both Collection 6.1 (C6.1) and the upcoming Collection 7 (C7) Level 1B products. The correction for the Aqua PV LWIR bands crosstalk has also been applied to C7 for the entire mission and C6.1 after the March 2022 safe mode. The lunar phase of the scheduled lunar events is around -55 for Aqua and 55 for Terra over the mission until the Aqua orbit started drifting in December 2021 and the Terra Constellation Exit Maneuvers (CEM) were performed in October 2022. To meet the constraint of the 0-20° spacecraft roll maneuvers, the phase angle range for the scheduled moon events is lower after the de-orbiting and CEM. Since unscheduled lunar observations cover a larger range of phase angles, the coefficients derived from those lunar observations can be used to assess the phase angle impact on the crosstalk coefficients calculation. A comparison between the crosstalk coefficients from scheduled and unscheduled lunar observations is also performed. The dependency on the phase angle is small except for the cases when Terra band 30 is the sending band. This analysis is performed over a wide range of phase angles covered by the unscheduled lunar observations. The assessment is useful for future PV LWIR bands crosstalk corrections and to maintain the L1B product quality for both Terra and Aqua MODIS.