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Software Fix Puts Acrimsat Back On Track

By Paul Hoversten Washington Bureau Chief

15 June 2000 WASHINGTON -- It took more than four frustrating months of adjustment by ground controllers, but a sun-watching satellite that was off kilter since launch last year is working at last.

The $8 million Active Cavity Radiometer Irradiance Monitor satellite known as Acrimsat is to be turned over to NASA next week from builder Orbital Sciences Corp. of Dulles, Virginia, to begin five years of operations.

"We're all ready to go," said Roger Helizon, a project scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "We've had a month of trouble-free data coming in from it since early April."

Solar Probe The European Space Agency recently approved an extended mission for Ulysses, a scientific spacecraft focused on the sun. Want to read more?

Helizon and other Acrimsat managers were in Washington this week to provide a progress report to NASA Headquarters in advance of next week's hand-over of spacecraft operations from Orbital.

Acrimsat will study the amount of sunlight falling on Earth's atmosphere, oceans and land to help scientists improve predictions of long-term climate change. A brightening of the sun, for example, could add to the so-called "greenhouse effect" on Earth in which carbon dioxide and other gases are trapped in the atmosphere, warming the planet.

Wrong angle

The 253-pound (115-kilogram) spacecraft was launched on December 20 aboard an Orbital Taurus rocket from Vandenberg Air Force Base, California.

But because of an error in its software commands, Acrimsat wound up pointing 16 degrees away from the sun in a higher-than-expected orbit of 443 miles (714 kilometers) above Earth.

The misalignment meant the spacecraft was unable to look directly at the sun's center to record the amount of radiation streaming toward Earth. To accurately measure sunlight, Acrimsat needed to be pointed to within a quarter of a degree of dead center.

It took about four and a half months to fix Acrimsat. Ground controllers from Orbital using NASA data first had to slow the spacecraft's spin in order to transmit a series of software "patches" and then gradually speed it up to see how well the commands worked.

"We're all ready to go." --Roger Helizon, project scientist, JPL

By April 5, Acrimsat's pointing was corrected to within a 10th of a degree of the target, allowing it to perform useful science. That rate of accuracy was better even than what the mission required.

"When you look back, I never believed the satellite was ever in danger," Helizon said. "I always believed it would get pointed right."

Still there are useful lessons to be learned, said JPL program manager Ron Zenone.

Pressures to meet the launch schedule had interfered with the amount of time managers needed to fully test Acrimsat before launch, he said. Acrimsat was flying as a secondary payload on the Taurus, which also carried a Korean remote-sensing satellite.

"They did the best job they could," Zenone said. "I dont think anything was shortchanged. But we need to put more emphasis on software testing."

Because the Acrimsat contract was awarded on a fixed-cost basis -- at $27 million, including launch -- Orbital is not liable for any penalty. In fact, the company stands to win an additional $100,000 performance fee, depending on the health of the spacecraft a year after its launch.

http://www.space.com/missionlaunches/missions/acrimsat_000614.html

-- (Dee360Degree@aol.com), June 16, 2000