SGP/SWS/C1 - Spectrometer heater controller failed

A heater controller failed on the Shortwave Spectrometer instrument (SWS). The heater 
maintained the Silicon diode array spectrometer temperature at 27 +/-0.2 Celsius. Operatation 
of the instrument is not dependent on the proper operation of the heater, therefore the 
SWS was run during the 10 day period in which the problem was diagnosed and repaired.  
However, up to a 4 percent error in radiance values can be expected in the wavelength range 
from 900 nm to 981 nm. 

The SWS data spans a wavelength range of 350 nm to 2150 nm with a typical error in 
radiance value of 2 percent.  From our analysis, the data with errors outside of the typical, 
only spans a small portion of the total wavelength range.  An instrument calibration with 
the heater malfunctioning was performed on September 26, 2007, and a response function was 
generated by the SWS mentor. If necessary, data users can contact the mentor to obtain 
the response function which will improve but not eliminate completely the errors in 
absolute spectral radiance values measured.

• Calibrated spectrum(zen_spec_calib)

SGP/SWS/C1 - Instrument offline for annual calibration

SWS instrument was taken offline for an annual calibration.  

Comparision of the December 19, 2007 annual calibration with the previous annual 
calibration done on November 8, 2006 shows very little change in the instrument sensitivity over 
2007.  From our analysis, there is effectively no change in the silicon spectrometer 
sensitivity from 400 to 981 nanometers.  The InGaAs spectrometer shows a slight decrease in 
sensitivity of 1 to 2% over the wavelength range of 982 to 2150 nanometers, which is within 
our expected error range.  Characterization of the change in sensitivity from 350 to 400 
nanometers over the year is a bit more difficult to determine and maybe greater than our 
expected error of 2%.  The science team at the University of Colorado is currently 
working on this problem.

• time
• base_time
• shutter_closed
• time_offset
• Wavelength of spectrum data(wavelength)
• lon
• alt
• Calibrated spectrum(zen_spec_calib)
• lat

• SI_spect_temp
• pc104_neg12v
• internal_temp
• ps2_12v
• pc104_pos12v
• alt
• lon
• time_offset
• pc104_5v
• base_time
• time
• ps2_5v
• lat
• InGaAs_spect_temp

SGP/SWS/C1 - Contamination by scattered light on optics.

It has been observed that the zenith radiance measurement is susceptible to contamination 
from light scattered off of the internal optics. This problem is most serious when 
measuring clear sky radiance in the presence of direct sun.  Under this worst case condition, 
it affects short wavelengths (less than 500 nm) at a few percent level, moderate 
wavelengths (500-800nm) between a few percent and 10-20%, and longer wavelengths up to 80% or more.

Chui and Marshak have demonstrated some success at identifying isolated affected data 
using the TSI to find cases of scattered clouds.  In general though, one must treat the 
zenith radiance data with considerable caution whenever direct solar radiance is likely to be 
present.  It is anticipated that the SWS zenith radiance for overcast skies is not 
significantly affected.

The instrument has been renovated by adding a sun shade to prevent direct sunlight from 
striking the optics.  An additional set of baffles further restricts the field of view, 
further reducing the potential for even off-axis bright clouds to affect the zenith 
measurement.  Data taken after June 2009 is considered to be free of contamination.

• Calibrated spectrum(zen_spec_calib)