key sensor aboard the National Oceanic and Atmospheric Administration’s (NOAA) N Prime weather satellite suffered no internal damage in a factory accident last April, NOAA and NASA officials have concluded after months of computer modeling and functional tests. The launch date for N Prime remains February 2009, government

officials said.

N Prime is the next spacecraft in the U.S. series of polar-orbiting weather satellites. The accident “could have had a severe impact on the schedule. Fortunately, none of that came true,” said Gary Davis, director of the Office of Systems Development within NOAA’s satellite division in Silver Spring, Md. “We would have held the launch until we repaired the instrument.

Lockheed Martin engineers were shifting the position of N Prime at the company’s factory in Sunnyvale, Calif., April 14, when the craft’s search-and-rescue antenna broke loose from a nylon tether and banged into the Advanced Microwave Sounding Unit (AMSU) A1 instrument.

AMSU A1 is tuned to measure air temperatures, especially inside thunderclouds and hurricanes. A companion instrument on N

Prime, AMSU A2, is designed to measure

atmospheric moisture. NASA purchased the instruments as a set for $18 million.

The impact

cracked a thermal reflector on the

housing of AMSU A1 but left no visible damage to the instrument, government and contractor officials said at the time.

NASA, which is managing development of the satellite for NOAA, needed to rule out the possibility of internal damage that could have required unbolting the instrument for testing and repairs. That process might have delayed the launch and threatened the quality of weather forecasts, NOAA scientists said.

NOAA added N Prime to its satellite plan to reduce the chances of a gap in forecasting data between the final NOAA polar orbiter, NOAA N launched in 2005, and the new satellites of the National Polar-

orbiting Operational Environmental Satellite System or NPOESS, which are scheduled to begin launching in 2013.

“If NOAA N failed before [N Prime] went up, we would have run the risk of a break in continuity,” Davis said.

After the accident, Lockheed Martin engineers calculated the severity of the shock that the AMSU instrument received in the accident. They gave that model to engineers at Northrop Grumman in Azusa, Calif., who built the AMSU A1 instrument

under a contract with NASA.

Northrop Grumman engineers then modeled the possible effect on AMSU’s electronics, an N Prime official explained.

Though the final modeling results are not complete, the Lockheed Martin shock numbers showed that “the instrument didn’t see any vibrations beyond the levels it was qualified at,” Davis said.

Davis said engineers also conducted extensive functional tests on the spacecraft, including the AMSU A1 instrument. “Going through a very long process, we exonerated the instrument, and it’s now been declared flyable,” he said. Engineers are preparing N Prime for thermal vacuum testing at Sunnyvale, he said.

N Prime will continue the AMSU measurements made by NOAA’s three existing, operational polar orbiters. Microwave energy emitted by Earth passes through clouds, which means the AMSU sensors can make measurements inside towering thunderstorms and hurricanes. Infrared sensors can only measure the temperatures at the tops of clouds, and NOAA’s geosynchronous weather satellites are too far from Earth to accurately sense the soft microwave signals, NOAA scientists said.

The AMSU instruments have had a big impact in hurricane forecasting, said Ralph Ferraro, an AMSU expert at the College Park, Md.,

office of NOAA’s Center for Satellite Applications and Research in Camp Springs, M


Some hurricanes gain strength surprising quickly, and one of the first signs of that rapid intensification is when warm air pushes higher into the atmosphere at the storm’s core, Ferraro said.

“AMSU has been instrumental in identifying these rapidly intensifying hurricanes,” he said.

After the accident, NASA and NOAA officials ordered a safety stand down on N Prime but lifted the order when an initial review found no evidence of negligence or foul play. N Prime had been

damaged severely in a 2003 factory accident when it fell onto a concrete floor.

A final report about the most recent accident, written by

a NASA-led Mishap Investigation Team, is due to be released Sept. 21, Davis said.

NASA managers have asked Lockheed Martin and Northrop Grumman to provide estimates of the cost of the extra modeling and functional tests. Davis said the government would be required to pay those costs because of the risk-sharing nature of N Prime’s cost-plus contract.

A $34-million addition to the contract announced Aug. 3 was unrelated to the April accident. It stemmed from the NPOESS delay announced two years ago, Davis said. With NOAA N working well in orbit, NOAA decided to keep N Prime on the ground a little longer to extend its operations closer to the first NPOESS spacecraft. The extra money will cover five months of storage at Sunnyvale and associated personnel costs, Davis said.