Following a few recent conversations, I’ve been reading about satellites. Of course, this all relates back to Internet access – we were discussing satellites that provide communications with South Pole, and now I have plenty Internet access to read up on satellites with.
My understanding at this point is that there are essentially four independent satellite systems that are relevant to communication with Pole:
- GOES-3 – Originally a weather satellite, but after a decade of service it was repurposed as a communications satellite… in 1989.
- Iridium – Private network of satellites, used to provide satellite phone service anywhere in the world.
- TDRSS – NASA system for communicating with American stuff in space.
- SKYNET-4C – British military communications satellite, launched in 1990.
GOES is what most folks on station use to communicate with the outside world via Internet and VoIP (telephone) access, and SKYNET is being looked into, to replace GOES when it eventually dies. Unfortunately, these satellites aren’t available to Polies all the time. We talk to the satellites using high frequency radio, which means you need a clear path between the antenna at either end, and the satellite.
GOES-3 and SKYNET-4C are in geostationary orbits, with enough inclination that they are sometimes visible from the South Pole. I made up this helpful diagram, which illustrates the situation. The figure-eights are the paths followed by the two satellites over time, from a viewpoint kept above the same point on Earth. It’s slightly confusing in that the satellite paths are shown over time (a full day), but the stars and sun are just at one time (in December, so there’s sunlight shining on Antarctica). Imagine that the satellites each follow their figure-eight, while the eights and Earth spin through space together.
An additional problem with GOES is that it’s batteries died long ago, so it only works when the sun is shining on it. As we just learned, GOES stays more-or-less above the equator, at the same longitude (roughly 104deg W), which means it gets eclipsed by Earth every day (and the moon on occasion) and loses power.
Fortunately, most folks don’t need to deal with the mathy details – there’s a GOES access schedule. Just don’t forget to download it when you have Internet access!
TDRSS is a network of geosynchronous NASA satellites, three of which are occasionally visible from Pole. The NSF has an agreement with NASA to use a certain amount of bandwidth on TDRSS, and that that bandwidth is used mainly for uploading science data from projects at South Pole, like IceCube! My impression is that I won’t have much to do with TDRSS, except making sure the IceCube data to be uploaded gets put in the queue.
Iridium is used for important communications when the other satellites aren’t visible – mainly voice calls, but some limited data transfer (a few kbps) can happen too. It’s quite a different system from the geosynchronous satellites above – in those, data is essentially bounced off a satellite by two ground stations, which leads to the line-of-sight problem.
The 66 Iridium satellites are in polar orbits, much lower (therefore faster) than the geostationary satellites like GOES or SKYNET. Iridium transfers data between satellites to get around the curvature of Earth – more complicated, but gives much better coverage, especially at the poles. They might be of more interest in several months, since they provide for regularly scheduled flares.