SGP/SIRS/E16 - Direct normal instrument failure

The short_direct_normal/NIP data spiked a few times and then completely failed resulting 
in missing values (-9999) for this time period.  

The short_direct_normal data during this time period is not recoverable. However, it can 
be reconstructed using the following shortwave instrument relationship for each minute of 
data:

short_direct_normal = (down_short_hemisp - down_short_diffuse)/cos(zenith angle).

• Instantaneous direct normal shortwave irradiance, pyrheliometer thermopile
  voltage(inst_direct_normal)

• Shortwave direct normal irradiance, pyrheliometer, minima(short_direct_normal_min)
• Shortwave direct normal irradiance, pyrheliometer(short_direct_normal)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• Shortwave Direct Normal Irradiance, Pyrgeometer, Standard Deviation(short_direct_normal_std)