Collection 5 TERRA MODIS Surface Reflectance Evaluation
2000-2007 time series science analysis
As part of the Collection 5 Terra MODIS science test activities, cutouts of Level 1 and Level 2 surface reflectance
products over active Aeronet sites were collected and used in the evaluation of the test results.
Aerosol Optical Thickness (AOT), aerosol model and column water vapor derived from AERONET measurements were used in 6S
to perform atmospheric correction of Level 1 Top Of Atmosphere reflectance and provide a reference used in assessing
the performance of the MODIS surface reflectance product (MOD09). A surface reflectance value retrieved in MOD09 is considered 'Good'
if it differs from the reference value by an amount less than the published accuracy of MOD09 +/-(0.005+5%) of the reflectance.
For each Aeronet site, a weighted average (by the number of clear observations divided by the cosine of viewing zenith angle - so as
to account for fewer observations offnadir) is computed. The results are displayed on the map below.
The color of the site is function of the percentage of good observations over the site and the size of the disc is function
of the number of clear observations (divided by the cosine of the viewing zenith angle) used in the analysis.
To ensure the use of aeronet measurements representative of the MODIS acquisition conditions,
we filter out sites where the aeronet AOT measurement does not occur within 30 minutes of the
MODIS acquisition or where the aeronet aerosol model inversion is not peroformed within 1 day of the acquisition.
This filter reduces the number of observations used in the comparison.