MODIS data is inter-compared with data collected from the AERONET worldwide network of ground stations (See http://aeronet.gsfc.nasa.gov). The MODIS and AERONET data are temporally and spatially matched using the following criteria:
Temporal: All AERONET observations made within +/- 30 minutes of a MODIS overpass at an AERONET site.
Spatial: All MODIS data retrieved within a 25 Km radius of an AERONET location for the 10 Km product and within 7.5 Km for the 3 Km product.
A minimum of 20% of possible MODIS pixels and two AERONET measurements meeting the above criteria are required to include the point in the validation data base.
For more information on this method see “A spatio-temporal approach for global validation and analysis of MODIS aerosol products” by Ichoku et. al.(2002) and “Multi-sensor Aerosol Products Sampling System (MAPSS)” by Petrenko et al. (2012).
For a more detailed answer see the Validation section of our website.