WASHINGTON — The sensor blamed for holding up development of the U.S. civil-military National Polar-orbiting Operational Environmental Satellite System (NPOESS) has entered what is expected to be its final three months of environmental testing.

NPOESS prime contractor Northrop Grumman Aerospace Systems of Redondo Beach, Calif., announced May 19 that the Visible Infrared Imager Radiometer Suite (VIIRS) has entered thermal vacuum testing at the facilities of its builder, Raytheon Space and Airborne Systems of El Segundo, Calif. For nearly 100 days the instrument will be subjected to the extreme hot and cold cycles it is expected to encounter in space.

Raytheon is due to deliver the VIIRS instrument, dubbed flight unit 1, this fall for integration with the NPOESS Preparatory Project satellite, an NPOESS precursor mission NASA expects to launch in January 2011.

Budget documents NASA released in May blamed a seven-month slip in the precursor satellite’s launch schedule on VIIRS development woes. The same sensor has also taken much of the blame for the cost overruns and delays to the much larger NPOESS program.

Raytheon spokesman John Barksdale said the sensor entered thermal vacuum testing the first week of May and has been performing as expected. “Everything is going great,” Barksdale said May 29. “We are very happy with it right now.”

VIIRS is designed to collect measurements of the atmosphere, clouds, land and sea in nearly two dozen spectral bands. The flight unit now undergoing testing, however, will lack some of the ocean-color monitoring capabilities originally intended for the sensor.

Barksdale said the VIIRS sensor in development for the first NPOESS satellite has undergone design modifications that will allow it to meet the full set of ocean-color requirements. That flight unit is on track for delivery in 2011, he said. The first NPOESS satellite is slated to launch in May 2014.

Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...