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Europe's GOCE satellite set to receive software patches

Posted: August 29, 2010

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Engineers expect to know in September whether a series of software patches will restore functionality in two faulty computers on GOCE, a European gravity-mapping satellite that stopped transmitting science data in July.

Artist's concept of the GOCE satellite. Credit: ESA
Two apparently unrelated glitches in February and July crippled the arrow-shaped satellite's pair of redundant flight computers, leaving GOCE unable to return data on Earth's gravitational field for more than a month.

European Space Agency officials hope to link working parts of the two computers through software patches, paving the way for GOCE to resume its $444 million mission this fall.

"When we start to initiate the proposed patches for linking the two computers together, we'll be in a more robust situation," said Mark Drinkwater, manager of ESA's Earth observation mission science division. "We hope to have some news about whether we're able to successfully uplink those patches in September."

GOCE's A-side computer malfunctioned in February, and engineers have traced that problem to an unexplained chip failure. Officials transitioned the satellite to a backup computer, but it suffered a mishap in July between a processor and the module that prepares telemetry to be sent back to Earth.

The July glitch left GOCE's precious gravity measurements stranded on-board the satellite, but controllers have access to housekeeping engineering data and full control of the spacecraft, according to Drinkwater.

The software updates, which are now being finalized, will combine chains of both flight computers to restore the system to full operations.

In the meantime, engineers at the European Space Operations Center have commanded GOCE's ion engine to boost the spacecraft's orbit by about 10 kilometers. Officials say the maneuver is a hedge against a further computer failure that could affect the satellite's navigation and piloting functions.

Drinkwater said GOCE's orbit is being raised to 265 kilometers, or nearly 165 miles.

"That gives us some safe operating margin while we're doing the patching," Drinkwater said Friday. "If we're not operating the ion system, we decay very slowly. The 10 kilometers gives us about a month of safe margin to give us a window in time in which we can try to successfully try to resume nominal telemetry."

GOCE's ion thruster maintains the satellite's unusually low orbit, which is susceptible to subtle changes in Earth's atmosphere caused by variations in solar activity. Without the ion system, officials say GOCE could drop from orbit and burn up in the atmosphere within weeks.

GOCE's first gravity map was unveiled in June. Credit: ESA
But an extended period of quiet solar activity has lessened the atmospheric drag on the spacecraft, allowing engineers to save enough xenon propellant to operate GOCE through the end of 2012, well beyond the anticipated end of its mission next April.

"Having more fuel on-board means we're not in a hurry right now to try to institute a quick fix because we've got the fuel to troubleshoot the problem and figure out how best to implement the solution," Drinkwater said.

GOCE launched in March 2009, and Drinkwater said the rest of the spacecraft is in good health.

The mission requires the low altitude to make sensitive observations of Earth's gravity field. GOCE is designed to detect tiny fluctuations in the planet's gravitational pull, collecting data for scientists to create global gravity maps.

Designed to produce gravity measurements 100 times better than its predecessors, GOCE has already gathered about two-thirds of the planned nominal mission data. GOCE has completed four of six planned mapping phases, according to Drinkwater.

"We have a significant degree of data in the bank from the nominal mission," Drinkwater said. "We're making huge scientific progress with the data we've already got."

Scientists released the first global gravity model from GOCE in June. Officials say more mapping campaigns will sharpen the resolution of the results.

GOCE, which stands for the Gravity field and steady-state Ocean Circulation Explorer, is creating an exceptionally accurate map of the planet's geoid, a global model illustrating subtle variations in the gravity field if oceans were motionless.

Scientists use the geoid's reference surface to weigh against measurements of ocean activity. The comparisons allow scientists to more accurately study ocean circulation and sea level changes.