On-orbit performance and calibration improvements for the reflective solar bands of Terra and Aqua MODIS

Abstract

Moderate Resolution Imaging Spectroradiometer (MODIS) is the keystone instrument for NASA's EOS Terra and Aqua missions, designed to extend and improve heritage sensor measurements and data records of the land, oceans and atmosphere. The reflective solar bands (RSB) of MODIS covering wavelengths from 0.41 µm to 2.2 µm, are calibrated on-orbit using a solar diffuser (SD), with its on-orbit bi-directional reflectance factor (BRF) changes tracked using a solar diffuser stability monitor (SDSM). MODIS is a scanning radiometer using a two-sided paddlewheel mirror to collect earth view (EV) data over a range of ±55⁰ off instrument nadir. In addition to the solar calibration provided by the SD and SDSM system, lunar observations at nearly constant phase angles are regularly scheduled to monitor the RSB calibration stability. For both Terra and Aqua MODIS, the SD and lunar observations are used together to track the on-orbit changes of RSB response versus scan angle (RVS) as the SD and SV port are viewed at different angles of incidence (AOI) on the scan mirror. The MODIS Level 1B (L1B) Collection 6 (C6) algorithm incorporated several enhancements over its predecessor Collection 5 (C5) algorithm. A notable improvement was the use of the earth-view (EV) response trends from pseudo-invariant desert targets to characterize the on-orbit RVS for select RSB (Terra bands 1-4, 8, 9 and Aqua bands 8, 9) and the time, AOI, and wavelengthdependent uncertainty. The MODIS Characterization Support Team (MCST) has been maintaining and enhancing the C6 algorithm since its first update in November, 2011 for Aqua MODIS, and February, 2012 for Terra MODIS. Several calibration improvements have been incorporated that include extending the EV-based RVS approach to other RSB, additional correction for SD degradation at SWIR wavelengths, and alternative approaches for on-orbit RVS characterization. In addition to the on-orbit performance of the MODIS RSB, this paper also discusses in detail the recent calibration improvements implemented in the MODIS L1B C6.

Analysis of Daytime Cloud Fraction Spatiotemporal Variation over the Arctic from 2000 to 2019 from Multiple Satellite Products

“…The D AM-PM between Cloud-cci Terra and Aqua gradually increased with time, which could be explained by spectral degradation at 0.65 and 0.86 mm, which had a larger impact on Terra MODIS data than on Aqua MODIS data. However, the difference between CERES-SSF Terra and Aqua decreased with time, especially in 2009 and 2017, when Collection 5 was corrected and replaced by Collection 6 (Angal et al 2016;Trepte et al 2019). On the other hand, spectral degradation in the Terra 6.76-and 8.55-mm channels after 2008 might slightly increase CF bias over polar regions.…”

15 Years of Aqua MODIS on-orbit operation, calibration, and performance

“…At the beginning of C6, only a few ocean color bands in Aqua MODIS need to apply earth view data to derive their RVS. Recently, bands 1-4 have also used the EV response trending to improve their RVS characterization and calibration updates [10]. If necessary, same strategy and approach will be extended to other spectral bands.…”

Methods of Earth-view-based calibration of the response versus scan angle of the MODIS reflective solar bands

“…The gains of the MODIS RSB, particularly the short-wavelength visible bands, have experienced significant degradation in the many years since the sensors' launches in 1999 (Terra) and 2002 (Aqua). [1][2][3] To maintain an accurate calibration, the calibration strategies have been updated several times on-orbit. Current methods of calibration of the MODIS RSB combine data from the SD, scheduled lunar observations, and regular observations of pseudo-invariant calibration sites (PICS) on the Earth.…”