SGP/SIRS/E21 - NIP data intermittently missing or incorrect

The short_direct_normal data was showing negative values (<-50 W/m^2)(during first time 
range) and missing value (-9999)(during second time range) each day a little after sunrise. 
Primarily between the hours of 11:00 - 14:00 GMT. Site Ops repaired a broken wire in the 
NIP cable and data appeared OK after the repair.

NIP data are not recoverable. Users can calculate short_direct normal component using:
short_direct_normal = (down_short_hemisp - down_short_diffuse_hemisp)/cos(zenith angle)

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

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