Analyses of a 4.5 year SNO (Simultaneous Nadir Overpass) time series between AVHRR on NOAA-16 and -17 suggest
that the AVHRR observations based on operational vicarious calibration have become very consistent since mid 2004.
This study also suggests that the SNO method has reached a high level of relative accuracy (~1.5%, 1 sigma) for both the
0.63 and 0.84 μm bands, which outperforms many other vicarious methods for satellite radiometer calibration.
Meanwhile, for AVHRR and MODIS, a 3.5 year SNO time series suggests that the SNO method has achieved a 0.9%
relative accuracy (1 sigma) for the 0.63 μm band, while the relative accuracy for the 0.84 um band is on the order of +/-
5% and significantly affected by the spectral response differences between AVHRR and MODIS.
Although the AVHRR observations from NOAA-16 and -17 agree well, they significantly disagree with MODIS
observations according to the SNO time series. A 9% difference was found for the 0.63 μm band (estimated uncertainty
of 0.9%, 1 sigma), and the difference is even larger if the spectral response differences are taken into account. Similar
bias for the 0.84 μm band is also found with a larger uncertainty due to major differences in the spectral response
functions between MODIS and AVHRR. It is expected that further studies with Hyperion observations at the SNOs
would help us estimate the biases and uncertainty due to spectral differences between AVHRR and MODIS.
It is expected that in the near future, the calibration of the AVHRR type of instruments can be made consistent through
rigorous cross-calibration using the SNO method. These efforts will contribute to the generation of fundamental climate
data records (FCDRs) from the nearly 30 years of AVHRR data for a variety of geophysical products including aerosol,
vegetation, and surface albedo, in support of global climate change detection studies.