TWP/SKYRAD/C2 - Direct Normal reading too low

The NIP (direct_normal shortwave irradiance) measurement was reading too low during this 
time period.  No TWP Ops documentation could be found to determine the cause but data 
review indicate that the NIP appeared to be reading low by about 25W/m2 during this period.  
The NIP data had well defined steps in irradiance when compared to three component checks 
indicating the problem period.

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Direct = (Global - Diffuse)/cos(zenith angle),
where Direct = short_direct_normal
Global = down_short_hemisp
Diffuse = down_short_diffuse_hemisp

See SIRS instrument handbook for details.

• Shortwave direct normal irradiance, pyrheliometer, minima(short_direct_normal_min)
• 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)

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

• (Raw data stream - documentation not supported)