SGP/SIRS/E9 - Downwelling Shortwave values low

Downwelling Shortwave Hemispheric (DSH) irradiance values from the pyranometer instrument 
were frequently below the MIN value of -10 W/m^2 at night.  DSH values also frequently 
disagreed with derived readings (at times well over 50 W/m^2 difference) during the 
daylight hours, especially in the morning. Field techs performed maintenance but could not find 
obvious instrument or connection problems.  

The low DSH values during nighttime were on the threshold of acceptable. Largest offsets 
acceptable with these pyranometers is typically -10 W/m2.  The large derived vs. actual 
DSH differences during the daylight hours are the subject of ongoing ARM investigation. No 
instrument problems could be identified.

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• Down-welling unshaded pyranometer voltage(down_short_hemisp)
• Downwelling shortwave hemispheric irradiance, pyranometer, maxima(down_short_hemisp_max)
• Downwelling Shortwave Hemispheric Irradiance, Ventilated Pyrgeometer, Standard
  Deviation(down_short_hemisp_std)
• Downwelling Shortwave Hemispheric Irradiance, Pyranometer, Minima(down_short_hemisp_min)

• Instantaneous Downwelling Hemispheric Shortwave, Pyranometer Thermopile Voltage(inst_global)

• (Raw data stream - documentation not supported)