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

Tue May 25 13:16:00 PDT 2010

At 02:03 PM 5/25/2010 -0400, kg4zlb at gmail.com wrote:
>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.

Sometimes I wonder, as AO-40's AOP precesses over time if it won't become a 
hazard to other satellites some day.

KB7ADL