RSS Overhaul and Redeployment at SGP in August 2009

Scott Stierle NOAA/ESRL/Global Monitoring Division/G-RAD
Peter Kiedron National Oceanic and Atmospheric Administration

Category: Instruments

An overhaul of the rotating shadowband spectroradiometer (RSS) was approved after it was discovered that its responsivity had periodic time and wavelength dependencies. After finding the problem, we increased the frequency of radiometric calibrations to bi-weekly calibrations to reduce the measured irradiance errors due to responsivity changes. We speculated that the nature of the responsivity behavior was either from etaloning caused by the spacing between two filters used for responsivity compression or from etaloning caused by the variable thickness in the coating that was cumulating on the chilled CCD due to outgassing from the casting that served as the main optical chamber. During the overhaul we resolved that the outgassing was responsible for the instability. We considered several approaches to fixing the problem within the limits of the RSS’s design. Eventually, we decided to build a vacuum chamber around the CCD that could be inserted into the optical chamber. We were able to achieve a vacuum in the chamber of less than 2 x 10-7 Torr. This also improved the thermoelectric cooling, because heat dissipation by convection was reduced. The RSS was redeployed in August 2009 and has operated continuously since. Prior to the redeployment, the RSS was carefully re-characterized at the NOAA Central Ultraviolet Calibration Facility, which can execute both UV and visible calibrations and characterizations. Currently, the RSS signal and processed data are closely monitored. New QC plots have been generated to facilitate our diagnostic abilities. They are available on ARM’s RSS website. While it is too early to evaluate the degree of success of the overhaul, all external parameters are within the specifications. We expect to apply Langley-generated calibrations to the new RSS data within the next few months.

This poster will be displayed at ASR Science Team Meeting.