[amsat-bb] 'Zombie-sat' and the clever orbital dance

Tue May 25 11:03:08 PDT 2010

The "dead-but-alive" telecommunications satellite, Galaxy-15 
<http://www.orbital.com/SatellitesSpace/Communications/PanAmSat/>, has 
begun to enter the space of neighbouring craft, and their operators are 
planning evasive action.

"Zombie-sat" has captured the imagination of the internet space forums 
these past few weeks. It's probably the nickname that's done it.

When we sit on the sofa skipping across the smorgasbord of channels with 
our remote-controls, we don't usually give much thought to the "bent 
pipes" that sit 36,000km above our heads, delivering the televisual feast.

Intelsat's Galaxy-15 
<http://www.intelsat.com/resources/galaxy-15/operational-status.asp> 
satellite was put in geostationary orbit five years ago to re-distribute 
TV services to cable companies across North America, and also to send 
navigation data to aeroplanes to improve the accuracy of their GPS 
receivers.

But the "bird" experienced a major hiccup at the beginning of April.

It's not known precisely what happened. One possibility is that it was 
damaged by high-speed particles billowing off the Sun in a solar storm - 
an ever-present danger for orbiting electronics.

The satellite is still operational: it's still "on", but Intelsat cannot 
control it. Any signal it receives, it re-transmits at high power. It's 
a very unusual situation.

What doesn't help is the fact that Galaxy-15, which is supposed to sit 
at 133 degrees West (over the eastern Pacific), is drifting slowing 
eastwards by about 0.05 degrees a day. This will take it into the path 
of other satellites, and first to have an issue is AMC-11 
<http://www.ses-worldskies.com/worldskies/satellites/01_amc-fleet/amc-11/index.php>, 
another TV services spacecraft operated by SES World Skies 
<http://www.ses-worldskies.com/worldskies/index.php>.

If SES were to do nothing, Zombie-sat would soon start picking up and 
retransmitting signals sent to AMC 11. To users on the ground who depend 
on AMC 11 for their daily dose of MTV, this could lead to a horrendous 
mash-up.

It would be like trying to listen to two people who are shouting the 
same conversation at you.

So, SES World Skies will today begin a delicate orbital dance, in which 
they will allow AMC 11 to drift in tandem with Galaxy-15 while at the 
same time sneaking up another satellite behind the pair.

The plan is for the SES controllers to then leapfrog many of the 
services on AMC-11 across to this other satellite, known as SES-1, 
thereby minimising the disruption to customers.

The manoeuvres are unprecedented, says Alan Young, the chief technology 
officer with SES World Skies.

    "The closest AMC-11 and Galaxy-15 will come is measured in
    kilometres, and in space terms that's quite close. But the risk here
    is not one of collision; we're not at all concerned about that. The
    problem is that they're so close when viewed from Earth that it's
    not easy to distinguish between the two satellites and seeing as
    they both operate in the same frequency band, there will be
    interference if we're not careful.

    "We've gone to a number of measures, including moving customers on
    AMC-11 on to a very large uplink antenna. 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."

AMC-11 will eventually be moved back to its orbital slot to resume 
normal operations once the zombie has passed through, which should be 7 
June.

All satellite operators and comms companies will have to work out what 
Galaxy-15 means to them. Here at the BBC, we've had to consider how some 
of our international services like the BBC World News channel 
<http://www.bbcworldnews.com> might be affected.

This channel is fed through Intelsat's Galaxy-13 platform. The most 
recent calculations suggest everything should be fine.

Anyone sitting on their sofa in North America should be oblivious to the 
space waltz that is about to take place.

There are some wider issues, however. For satellite manufacturers, there 
will be keen interest in understanding exactly what happened to Galaxy-15.

Satellites have redundant, or back-up systems 
<http://news.bbc.co.uk/1/hi/sci/tech/8458203.stm>; and when they have 
major upsets, there are usually modes that will completely re-boot the 
spacecraft automatically after a period of time.

Galaxy-15 was made by Orbital Sciences <http://www.orbital.com/>, but 
Patrick Wood, the chief technical officer for EADS Astrium satellites 
<http://www.astrium.eads.net/>, told me the entire industry had an 
interest in finding out what went wrong:

    "Part of our design review process is to check through the
    architecture to ensure there isn't a single point that, were it to
    fail, we'd lose complete control of the spacecraft. Clearly
    Galaxy-15 has had a major event and most organisations will want to
    understand what happened. From an industrial point of view, the
    surprising thing is that Galaxy-15 is locked on full power. This
    tends to suggest the control/tele-command side of the spacecraft has
    failed and left the spacecraft in whatever mode it was in when it
    was last commanded. It's a very unusual case."

And, of course, the whole episode raises once again the issue of orbital 
space debris <http://news.bbc.co.uk/1/hi/sci/tech/7916582.stm>. Galaxy 
15 will likely end its days in one of the two great "garbage patches" in 
the sky.

These libration points 
<http://en.wikipedia.org/wiki/Geostationary_orbit#Earth_orbital_libration_points>, 
as they are known, are located at roughly 105 degrees West and 75 
degrees East. They are gravitational "sweet-spots" where drifting 
objects will naturally coalesce.

The two libration points now contain more than 150 defunct satellites 
[395Kb PDF] 
<http://www.secureworldfoundation.org/siteadmin/images/files/file_460.pdf>.

Satellite operators are urged to put their geostationary spacecraft in a 
"graveyard orbit" once their missions are complete. This usually means 
pushing the platforms even higher into the sky.

But of the 21 spacecraft which reached end of life in 2009, only 11 were 
disposed of in accordance with the Inter-Agency Space Debris 
Coordination Committee's (IADC) re-orbiting guidelines [99 Kb PDF] 
<http://www.iadc-online.org/Documents/Docu/IADC_Mitigation_Guidelines_Rev1_Sep07.pdf>. 

We may all love our satellite TV, but we're starting to build a problem 
for ourselves.

Watch this space.