Table 3-3. Solar Radiation and Illuminance Source Flags  
  
 
  Flag  Definition
A  Post-1976 measured solar radiation data as received from NCDC or 
  other sources
B  Same as "A" except the global horizontal data underwent a 
  calibration correction
C  Pre-1976 measured global horizontal data (direct and diffuse were 
  not measured before 1976), adjusted from solar to local time, usually 
  with a calibration correction
D  Data derived from the other two elements of solar radiation using the 
  relationship, global = diffuse + direct x cosine(zenith)
E  Modeled solar radiation data using inputs of observed sky cover 
  (cloud amount) and aerosol optical depths derived from direct 
  normal data collected at the same location
F  Modeled solar radiation data using interpolated sky cover and 
  aerosol optical depths derived from direct normal data collected at 
  the same location
G  Modeled solar radiation data using observed sky cover and aerosol 
  optical depths estimated from geographical relationships
H  Modeled solar radiation data using interpolated sky cover and 
  estimated aerosol optical depths
I  Modeled illuminance or luminance data derived from measured or 
  modeled solar radiation data
?  Source does not fit any of the above categories. Used for nighttime 
  values and missing data
 
 


http://www.nrel.gov/rredc