May 25, 2010

Satellites Prepare For Zombie-Sat Interference

Two spacecraft are about to begin an unusual orbit above the Pacific Ocean in order to try and evade the interference from a third, failed satellite.

Intelsat lost control of its Galaxy-15 platform in April.  It will not take commands from the ground and is in a drift towards a neighboring satellite.

The nearest satellite, AMC-11, will now be eased out of its path.

SES World Skies, the satellite's operator, said that customers should be unaffected.

The intricacy of the maneuvers of the satellites is thought to be unprecedented in the commercial telecommunications sector.

"We have to do slight maneuvers with our spacecraft pretty regularly, but a maneuver of this nature and complexity - no, I'm not aware of anyone having done this before," Alan Young, chief technology officer with SES World Skies, told BBC News.

"We need to make sure Americans can continue to watch their television. They love their TV and it's important," he added.

Galaxy-15 launched in 2005 to re-distribute TV services to cable companies throughout North and Central America.  The satellite also sent navigation data to airplanes to improve the accuracy of their GPS receivers.

The satellite experienced a major fault on April 5 and its services were switched to a back-up spacecraft.  Engineers have not yet determined the cause, but damage from a solar storm is one possibility being investigated.

Even though Intelsat cannot communicate with Galaxy-15, its electronic payload remains fully functional, and it is capable of re-transmitting on full power any signal it receives.

This means that if Galaxy-15 were to get too close to another spacecraft it could then start retransmitting their signals and seriously interfere with their services.

This "dead-but-alive" condition earned the Intelsat platform the unfortunate nickname of "zombie-sat" in some quarters.

The satellite was positioned at 133 degrees West, and about 22,000 miles above the Pacific.  It now drifts east into the slot occupied by AMC-11, a satellite operated by competitor SES World Skies.

SES plans to initiate a drift in AMC-11 on Tuesday to match that of Galaxy-15 in order to ensure that its TV customers experience no loss or degradation of service.  It has also already commanded a second satellite, SES-1, to come in behind the damaged platform.

Services will then be switched between the satellites until Galaxy-15 passes through the orbital slot and AMC-11 can return to its normal position and duties.

"We've moved customers on AMC-11 on to a very large uplink antenna," Young told BBC.

"This means we can very finely discriminate between the two spacecraft so that we can direct all of the energy into AMC-11 and as little energy as possible into Galaxy-15. If you don't put anything into Galaxy-15, you won't get anything out."

Intelsat told BBC that its engineers would continue to try and regain control of Galaxy-15, while it cooperates fully with other operators to minimize disruption.

Collisions with other satellites are highly unlikely.  Galaxy-15 should be separated from nearby spacecraft by many miles as it passes through other orbital slots.

Ultimately, it is assumed that the Intelsat platform will lose the ability to point its solar panels at the Sun and experience total power failure.  This could take months.

It will eventually make its way to the gravity libration points where orbital debris has a tendency to aggregate.


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