SGP/SIRS/E21 - Tracker failure

First apparent on 03/26, the short_direct_normal values were incorrect after approximately 
1700GMT each day. The tracker was found to have excessive clock drift and was reset by 
site ops on 05/04/2010 and returned to service at 1935GMT. The down_short_diffuse_hemisp 
values appear unaffected by the tracker misalignment.

The short_direct_normal data values are suspect after approximately 1800GMT each day from 
03/26 to 04/26 and after 1700GMT from 04/27 to the end of the period.  The 
short_direct_normal data are not recoverable but can be calculated indirectly using the following 
relationship for each minute of data:
down_short_hemsip = short_direct_normal * cos(zenith angle) + down_short_diffuse

• 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)