BOSTON — Officials overseeing development of a key but problematic sensor for the next generation of U.S. government polar-orbiting weather satellites believe they are close to resolving a new series of issues that cropped up during recent hardware testing.

But these officials cautioned that despite progress on the Visible Infrared Imager Radiometer Suite (VIIRS) for the civil-military National Polar-orbiting Operational Environmental Satellite System (NPOESS), it is too early to declare victory.

The VIIRS instrument, being developed by Raytheon Space and Airborne Systems of El Segundo, Calif., is intended to provide imagery, monitor sea-surface temperature and color, and make other measurements . Technical difficulties in its development have been cited as a major factor in the cost growth that prompted U.S. government officials to restructure the NPOESS program this year.

The NPOESS satellites, to be owned and operated jointly by the U.S. Air Force and U.S. National Oceanic and Atmospheric Administration, are slated to start launching around 2013. Northrop Grumman Space Technology of Redondo Beach, Calif., is the prime contractor on the program, which was restructured after its projected cost soared from $6.8 billion to more than $11 billion.

The new NPOESS program plan was unveiled this spring. The VIIRS development effort encountered additional snags shortly thereafter during thermal vacuum testing of an engineering model.

Air Force Col. Dan Stockton, NPOESS system program director, said he would not describe any of the issues as minor, but added that finding problems at the hardware testing stage of any program is to be expected.

The problems encountered during the VIIRS testing included a slight warping in the sensor’s mirrors caused by a reaction between its nickel and aluminum components , Stockton said. This distorted the data collected by the sensor, he said. Program officials are confident that this problem can be taken care of by adjusting the mirror design to account for the slight bending, or by finding a way to isolate the aluminum in the mirror, according to another NPOESS program official.

The testing also revealed some “cross-talk” on the VIIRS sensor, Stockton said. The other program official, who did not want to be identified, described the problem as interference between different light-gathering channels on the sensor and said the issue will likely be resolved in the next several weeks.

Brian Arnold, a retired Air Force three-star general who oversaw military space procurement as commander of the service’s Space and Missile Systems Center in Los Angeles , said experience has taught him not to declare a troubled sensor or satellite out of the woods until it has launched.

But Arnold, now vice president for strategic programs at Raytheon Space and Airborne Systems, said the latest VIIRS problems are not a major cause for concern. Problems uncovered during assembly, integration and testing of an engineering model are much easier to deal with than those discovered when a flight unit is nearing completion , Arnold said.

None of the recent issues with VIIRS are expected to delay the planned September 2009 launch of the NPOESS Preparatory Project, an experimental spacecraft that will host several key NPOESS instruments, Stockton said .

Arnold said Raytheon plans to deliver the VIIRS engineering prototype Dec. 9 to Ball Aerospace and Technologies Corp. of Boulder, Colo., which is building the platform for the NPOESS Preparatory Project . Ball will integrate all of the prototype sensors for the mission on the actual spacecraft platform for additional testing prior to delivery of the flight unit, he said.

The first VIIRS flight unit is expected to be completed by July 2008 , Stockton said. Raytheon already has begun construction of the hardware, and things have gone smoothly thus far, Arnold said.