Preparing Envisat for Mission Extension a Major Undertaking

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PARIS — Moving the large Envisat environmental satellite into a lower orbit in October to minimize fuel use and extend the satellite’s already longer-than-planned service life by three years will be equivalent to starting a new mission in terms of instrument calibration and ground communications, European Space Agency (ESA) officials said.

In addition to retuning the 8,000-kilogram Envisat’s 10 observing instruments to adjust to the new orbit, ESA ground controllers will need to adapt their ground communications network to function with the new orbital parameters to ensure that Envisat’s signals do not interfere with signals from the ERS-2 satellite.

Envisat, launched in 2002, is perhaps the largest civil Earth observation satellite ever built and, at 2.3 billion euros ($2.9 billion), one of the most expensive.

The satellite appears to have delivered on many of its promises, creating a global community of users in Europe and elsewhere that use the satellite’s sensors to generate a huge variety of data on environmental phenomena.

But Envisat was launched with a hydrazine fuel tank that is not much larger than that used for its much smaller predecessor, ERS-2, which was launched in 1995. Envisat was designed to operate for five years, and ERS-2 for up to five years.

Both are still operational. ERS-2 will be retired in 2011, while Envisat will continue its mission, from a lower orbit, through 2013 and perhaps a bit longer.

In a series of presentations June 30 and July 1 at the Living Planet symposium in Bergen, Norway, ESA officials described how they will use Envisat’s fuel just about to the last drop. If all goes as planned, they said, they will add three years to Envisat’s life, with almost all its instruments unaffected by the orbit change.

When it was launched, Envisat had 314 kilograms of fuel. Initial orbit-raising and regular maneuvers to correct the satellite’s position — plus seven collision-avoidance maneuvers to steer clear of orbital debris — have left the satellite with an estimated 81 kilograms of fuel today.

“This is what we think we have on board,” said Frank-Juergen Diekmann, Envisat flight manager at ESA’s European Space Operations Centre in Darmstadt, Germany. “In fact, we don’t know. There is no direct means to measure the fuel left.”

By studying the experience of the French space agency, CNES, with the Spot series of Earth observation satellites over the last 30 years, ESA has used telemetry and a method that involves counting the pulses of Envisat’s thruster engines to calculate the remaining fuel.

Diekmann said these methods are by their nature conservative, and viewed as accurate to within 10 percent. ESA needs to err on the side of caution, especially since the frequency of collision-avoidance maneuvers is increasing at Envisat’s nearly 800-kilometer orbit, and Envisat is likely to need to perform such maneuvers between now and its retirement, Diekmann said.

Starting Oct. 22, Envisat will shut down most of its operations and begin a 10-day series of maneuvers to lower its orbit from 799.8 kilometers to 782.4 kilometers. Six separate repositioning moves are planned, and these are estimated to consume slightly more than 33 kilograms of fuel.

That will leave the satellite with between 37 and 44 kilograms of fuel for its final three years in orbit. To save fuel, Envisat will no longer correct its orbit for the gradual change in inclination relative to the equator.

The option of de-orbiting the satellite — meaning lowering the orbit enough so that the satellite burns in the Earth’s atmosphere within 25 years, as international debris-mitigation guidelines urge — was never realistic for Envisat because of the satellite’s size relative to its fuel reserves.

Pierre Vogel, a principal engineer at ESA’s European Space Research and Technology Centre in Noordwijk, Netherlands, said lowering Envisat’s orbit consumes 2 kilograms of fuel for every kilometer.

“Envisat was designed before these guidelines were proposed, and given the size of the fuel tank, de-orbiting it would have meant stopping the mission just a few weeks after it started,” said Henri Laur, a director in ESA’s European Space Research Institute (ESRIN) Earth observation center in Frascati, Italy.

Envisat’s entire ground segment will need to be reconfigured to adapt to the new, lower orbit. “We don’t want to scare people, but it’s a major modification and to a certain extent it’s a new mission” that starts Nov. 2 and resumes completely in December, said Sergio Vazzana of ESRIN. “Most of the components were designed more than 10 years ago with only one orbit.”

Of particular concern is how to send commands to Envisat, and receive telemetry data from the satellite, as it passes over its designated ground station in Kiruna, Sweden.

Because of its lower orbit, Envisat will be circling the Earth at a slightly higher speed than ERS-2. For some Kiriuna passes, the two satellites will be sending data at the same time and in the same frequency. Avoiding that with new antennas will be a challenge, as will be coordinating use of the two Kiruna antennas, which in addition to ERS-2 and Envisat are being used for ESA’s Goce and Cryosat Earth observation spacecraft.

“We will need to invent some [interference] mitigation measures to prevent miscommanding of the satellites,” Diekmann said. In addition, a new antenna will be needed for Kiruna unless a decision is made to stop ERS-2 downlinks occasionally. “We hope to make the best of it in the coming months.”