Calibration improvements for MODIS and VIIRS SWIR spectral bands

Abstract

Both MODIS and VIIRS use a solar diffuser (SD) to calibrate their reflective solar bands (RSB), covering wavelengths from 0.41 to 2.3 µm. On-orbit changes of the SD bi-directional reflectance factor (BRF) are tracked by an on-board solar diffuser stability monitor (SDSM). The current SDSM design only covers the spectral range from 0.41 to 0.93 µm. In general, the SD degradation is strongly wavelength-dependent with larger degradation occurring at shorter wavelengths, and the degradation in the SWIR region is expected to be extremely small. As each mission continues, however, the impact due to SD degradation at SWIR needs to be carefully examined and the correction if necessary should be applied in order to maintain the calibration quality. For Terra MODIS, alternative approaches have been developed and used to estimate the SD degradation for its band 5 at 1.24 µm and a time-dependent correction has already been applied to the current level 1B (L1B) collection 6 (C6). In this paper, we present different methodologies that can be used to examine the SD degradation and their applications for both Terra and Aqua MODIS and S-NPP VIIRS SWIR calibration. These methodologies include but not limited to the use of lunar observations, Pseudo Invariant Calibration Sites (PICS), and deep convective clouds (DCC). A brief description of relative approaches and their use is also provided in this paper.

VIIRS Reflective Solar Bands Calibration Progress and Its Impact on Ocean Color Products

“…The SD degradation for the shortwave infrared (SWIR) bands, or wavelengths longer than 935 nm, is beyond the spectral coverage of the SDSM, as seen in Table 1, and typically is assumed to be null for wavelengths beyond this range. The SD may have non-negligible SD degradation at the wavelength of the shortest SWIR band, M8 (1238 nm), and the degradation is estimated to be around 0.5% [8,34,35]. The VIIRS SD on-orbit degradation, h(λ), is tracked by the on-board SDSM calibration, as mentioned previously [6][7][8].…”

Suomi NPP VIIRS Reflective Solar Bands Operational Calibration Reprocessing

“…In this way, the H factors are directly used to scale the calibration coefficients for the visible and near-infrared bands (M1-M7 and I1-I2). Degradation of the solar diffuser reflectance is considered negligible for the short-wave infrared bands (I3 and M8-M11) [12].…”