Since its launch from Vandenberg Air Force Base in California on Oct. 28, 2011, the NPP spacecraft and its five instruments have been undergoing initial checkout before starting regular science observations. Activation and checkout of the full suite of instruments was delayed for several weeks after an anomaly was discovered in the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument.
Following an extensive investigation that successfully identified the cause of the VIIRS instrument anomaly, the checkout and activation process for the full set of NPP instruments was resumed on Jan. 18. The remaining commissioning activities for NPP are expected to take approximately six weeks.
All NPP instruments were originally scheduled to be operating and collecting data by mid-December. The Advanced Technology Microwave Sounder (ATMS) instrument was activated first and acquired its first measurements on Nov. 8. ATMS is working well and providing high-spatial-resolution microwave data to take measurements of temperature and humidity in both clear and cloudy conditions. Scientists are already using the ATMS measurements in research studies of rain and snow.
VIIRS acquired its first measurements on Nov. 21. VIIRS is designed to collect radiometric imagery of the Earth's land, atmosphere and oceans using 22 visible and infrared wavelengths. During checkout, engineers detected a larger than expected decrease in sensor sensitivity in four of VIIRS's near-infrared and visible channels.
As a precaution, activation and checkout of the remaining VIIRS channels and the Cross-track Infrared Sounder (CrIS), the Ozone Mapper Profiler Suite (OMPS) and the Clouds and the Earth's Radiant Energy System (CERES) instruments was delayed until the VIIRS anomaly was better understood.
Investigation of the VIIRS anomaly identified tungsten oxides, an anomalous material on the surface of the mirror. Tungsten oxides could cause the surface of the mirror to darken. The investigation discovered a non-standard process that occurred during the mirror coating as a potential source of tungsten oxide contamination on the VIIRS mirrors.
This evidence suggests that the cause of the contamination is limited to the VIIRS instrument, and is not a concern for other NPP instruments. While likely irreversible, the darkening of the VIIRS mirror caused by the contamination is expected to reach a plateau and remain at that level for the life of the mission. Although testing on this issue is continuing, NPP mission managers expect this plateau to still provide sufficient margins to allow VIIRS to meet its design requirements.
Information from this investigation will be applied to the VIIRS instrument currently in development for the National Oceanic and Atmospheric Administration's Joint Polar Satellite System.
After commissioning and a calibration/validation period, NPP data will be available from NOAA's Comprehensive Large Array-data Stewardship System.