Abstract
The Visible Infrared Imaging Radiometer Suite (VIIRS) is being used to continue the record of Earth Science observations and data products produced routinely from National Aeronautics and Space Administration (NASA) Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. However, the absolute calibration of VIIRS's reflected solar bands is thought to be biased, leading to offsets in derived data products such as aerosol optical depth (AOD) as compared to when similar algorithms are applied to different sensors. This study presents a cross-calibration of these VIIRS bands against MODIS Aqua over dark water scenes, finding corrections to the NASA VIIRS Level 1 (version 2) reflectances between approximately +1 % and -7 % (dependent on band) are needed to bring the two into alignment (after accounting for expected differences resulting from different band spectral response functions), and indications of relative trending of up to ^0.35 % per year in some bands. The derived calibration gain corrections are also applied to the VIIRS reflectance and then used in an AOD retrieval, and are shown to decrease the bias and total error in AOD across the midvisible spectral region compared to the standard VIIRS NASA reflectance calibration. The resulting AOD bias characteristics are similar to those of NASA MODIS AOD data products, which is encouraging in terms of multisensor data continuity.
Highlights
Launched in late 2011, the Suomi National Polar-Orbiting Partnership (S-NPP) satellite is a precursor to the Joint Polar Satellite System (JPSS), which represents the generation of the USA’s operational Earth observation satellites
One of the instruments aboard S-NPP is the Visible Infrared Imaging Radiometer Suite (VIIRS; Cao et al, 2013, 2014), designed to continue the types of observations made by the Defence Meteorological Satellite Program (DMSP) Advanced Very High Resolution Radiometers (AVHRR, data record 1978 onwards) and Earth Observing System (EOS) sensors such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS, 1997– 2010) and Terra/Aqua Moderate Resolution Imaging Spectroradiometers (MODIS, 2000 onwards)
All corrections presented represent scaling factors which should be applied for National Aeronautics and Space Administration (NASA) VIIRS Level 1 (L1) data to make it radiometrically consistent with NASA MODIS Collection 6 L1 data and account for the fact that the sensors have different spectral response functions. The steps of this exercise are listed here and outlined in detail in the following subsections: 1. selection of appropriate MODIS/VIIRS pixels to consider; 2. correction for the effects of absorption by trace gases in the atmosphere; 3. forward radiative transfer modelling to predict the top of atmosphere (TOA) reflectance which should be observed by VIIRS, given that observed by MODIS; 4. aggregation of results to a monthly timescale and derivation of cross-calibration scaling coefficients
Summary
Launched in late 2011, the Suomi National Polar-Orbiting Partnership (S-NPP) satellite is a precursor to the Joint Polar Satellite System (JPSS), which represents the generation of the USA’s operational Earth observation satellites. Even if MODIS Aqua’s calibration is not perfect, tying VIIRS to MODIS Aqua does mean that any calibration-related biases in derived data sets should look similar in both sensors, increasing the level of data product consistency between the two These studies have typically only considered a subset of VIIRS RSBs, excluding some which are required for the AOD retrieval algorithms which have been applied to EOS-era measurements. They used NOAA L1 data from the first few years of the VIIRS mission; these results may not necessarily be transferable to the NASA calibration, or to more recent years of observations, since the underlying L1 source data are not the same (and, as mentioned, various updates to the NOAA IDPS products have been made in forward-processing).
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