Abstract
A technique for monitoring and evaluating the performance of on-orbit calibration for satellite ocean color sensors has been developed. The method is based on the sensor on-orbit vicarious calibration approach using in situ ocean optics measurements and radiative transfer simulations to predict (calculate) sensor-measured top-of-atmosphere spectral radiances. Using this monitoring method with in situ normalized water-leaving radiance nLw(λ) data from the Marine Optical Buoy (MOBY) in waters off Hawaii, we show that the root-cause for an abnormal inter-annual difference of chlorophyll-a data over global oligotrophic waters between 2012 and 2013 from the Visible Infrared Imaging Radiometer Suite (VIIRS) is primarily due to the VIIRS on-orbit calibration performance. In particular, VIIRS-produced Sensor Data Records (SDR) (or Level-1B data) are biased low by ~1% at the wavelength of 551 nm in 2013 compared with those in 2012. The VIIRS calibration uncertainty led to biased low chlorophyll-a data in 2013 by ~30-40% over global oligotrophic waters. The methodology developed in this study can be implemented for the routine monitoring of on-orbit satellite sensor performance (such as VIIRS). Particularly, long-term Chl-a data over open oceans can also be used as an additional source to evaluate ocean color satellite sensor performance. We show that accurate long-term and consistent MOBY in situ measurements can be used not only for the required system vicarious calibration for satellite ocean color data processing, but also can be used to characterize and monitor both the short-term and long-term sensor on-orbit performances.
Highlights
Satellite ocean color sensors, such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) [1], the Moderate Resolution Imaging Spectroradiometer (MODIS) [2], the MediumResolution Imaging Spectrometer (MERIS) [3], and the Visible Infrared Imaging Radiometer Suite (VIIRS) [4], measure the top-of-atmosphere (TOA) reflectance ρt(λ) (or radiance Lt(λ)) at multiple wavelengths from the visible to the near-infrared (NIR) and shortwave infrared (SWIR)
We show that Marine Optical Buoy (MOBY) measurements can be used to vicariously calibrate the satellite ocean color sensors [43,44,45,46,47], such as SeaWiFS, MODIS, MERIS, and VIIRS, in order to derive accurate satellite ocean color products, and can be used to characterize and monitor the miniscule changes of satellite sensors in order to produce long-term high quality satellite ocean color products
VIIRS-measured global Level-3 ocean color product images and some validation results can be found at the website
Summary
Satellite ocean color sensors, such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) [1], the Moderate Resolution Imaging Spectroradiometer (MODIS) [2], the MediumResolution Imaging Spectrometer (MERIS) [3], and the Visible Infrared Imaging Radiometer Suite (VIIRS) [4], measure the top-of-atmosphere (TOA) reflectance ρt(λ) (or radiance Lt(λ)) at multiple wavelengths from the visible to the near-infrared (NIR) and shortwave infrared (SWIR). VIIRS ocean color EDR products were significantly improved with better sensor on-orbit calibration and became consistent with those from MODIS-Aqua in 2012 This demonstrates that it is critical to accurately characterize the sensor instrument performance and carry out on-orbit radiometric calibration correctly in order to achieve high data quality in the SDR and EDR. We show that MOBY measurements can be used to vicariously calibrate the satellite ocean color sensors [43,44,45,46,47], such as SeaWiFS, MODIS, MERIS, and VIIRS, in order to derive accurate satellite ocean color products, and can be used to characterize and monitor the miniscule changes (in relative short-term) of satellite sensors in order to produce long-term high quality satellite ocean color products
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