Controllers have restarted on-orbit checkout of the Npoess Preparatory Project (NPP) satellite, which was suspended last year after the Visible Infrared Imaging Radiometer Suite (VIIRS) sensor begin losing sensitivity in four of its channels.
The spacecraft originally was scheduled to become fully operational in December, but its commissioning was put on hold while the VIIRS problem was analyzed.
Engineers found the Raytheon-built instrument’s mirror was contaminated with tungsten oxides, possibly as a result of nonstandard processing when the mirror was coated. The irreversible contamination appears limited to the VIIRS mirror, and managers expect mirror darkening to stop at a level that will permit it to operate within design requirements.
It will take about six weeks to complete commissioning, which resumed on Jan. 18. The Advanced Technology Microwave Sounder, which was the first instrument activated in November, is already generating scientific data for snow and rain studies.
Ultimately VIIRS will collect radiometric imagery of the land, atmosphere and oceans in 22 visible and infrared wavelengths. In addition to the remaining VIIRS channels, NASA says three other instruments remain to be commissioned: the Cross-track Infrared Sounder, the Ozone Mapper Profiler Suite and the Clouds and the Earth’s Radiant Energy System.
Problems with VIIRS contributed to massive projected overruns on the ill-fated Npoess (National Polar-orbiting Operational Environmental Satellite System) civil/military weather satellite program, for which NPP was designed as a pathfinder. Npoess eventually was canceled.