Gravity-Mapping Satellite's Communications Glitch to Be Fixed Soon

PARIS ? Ground controllers are gradually raising the orbit of Europe's GOCE gravity-measuring satellite in preparation for a series of procedures they hope will permit the satellite to recover its data-transmission function, which failed in July.

The procedures, designed by a team of engineers from the European Space Agency (ESA) and GOCE's industrial contractors, had not been approved for use as of Aug. 24. But Mark R. Drinkwater, head of ESA's Earth observation mission science division, said some variant of the proposed data-restoration plan would be approved, and that by mid-September, GOCE controllers should have an idea of whether the plan is working.

GOCE, or the Gravity and steady-state Ocean Circulation Explorer, was launched in March 2009 on a two-year mission. It has produced its first global maps of the minor differences in gravitational attraction of different parts of the Earth. ESA officials say GOCE is the first satellite to produce such maps.

The satellite has already performed two-thirds of its mission assignments. Its managers nonetheless had hoped to continue the mission beyond 2011 given the fact that the relatively low solar activity in the past year has permitted GOCE to fly at an exceptionally low orbit and use relatively little fuel to eliminate the atmospheric drag that inhibits flight at GOCE's 256-kilometer operating altitude.

"We have four of a planned six mappings completed, and we have fuel in the bank," Drinkwater said in an Aug. 24 interview. "There is no reason we couldn't operate until the end of 2012, if our member states grant us approval."

GOCE's main computer failed in February in what mission managers describe as a rare and still-unexplained failure of a computer chip that has flown many times on satellite missions.

With the main computer out of service, the July failure prevented GOCE from sending science data altogether. A board of inquiry has been established to examine the July incident, but Drinkwater said it is already clear that it bears no relation to the February computer chip failure.

"The present anomaly manifests itself in a different manner from the previous problem, as the processor itself remains healthy," ESA said in a statement on the GOCE telemetry issue.

It remains unclear exactly what component suffered the glitch that stopped the flow of science data. The 18-nation ESA, in July 22 and Aug. 3 statements about GOCE's status, said the problem appears to reside either in the backup computer's processor board or in the satellite's telemetry module. Several attempts to reboot the backup computer have not overcome the data-transmission problem.

The proposed work-around includes returning GOCE's nominal computer to partial service, and combining that capacity with similar capacity available on the backup computer despite the July glitch.

Drinkwater said GOCE continues to respond to commands and shows no sign of having lost any of its data-generating ability.

Ground teams have sent software commands to GOCE that have forced the satellite to deliver more telemetry than it normally does, enabling controllers to verify the status of all the satellite's systems in preparation for the recovery operation.

As part of that preparation, GOCE's orbit is being raised by about 9 kilometers, to 265 kilometers, in anticipation of the fact that during the recovery, it may lose the use of its xenon-ion electric propulsion system, which permits GOCE to compensate for atmospheric drag as it flies. Also helping the satellite fly is its peculiar aerodynamic design, resembling an arrowhead.

The temporary loss of the propulsion system would cause GOCE to lose altitude almost immediately.

Many of its components have never flown before, including its gradiometer payload, composed of three pairs of accelerometers that will produce a global map of gravity's differing pull in different regions of Earth, which were built by France's Onera aerospace research institute, and its xenon-ion thrusters, built by Qinetiq of Britain. But it is not these systems that have caused GOCE's problems so far.

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