Background
What is a satellite constellation?
Anyone can put a satellite into orbit, and many people have done so.
We
have.
But your lone satellite is only over part of the world at any time. It can't cover the entire world at once. If you need global or near-global coverage, you need to launch
a number of satellites so that at least one satellite can be seen from every
point on the earth at once. Or nearly every point, anyway.
And, once you've figured out the geometry and the orbits of your
satellites so that their coverage areas (or footprints) overlay the
world, you have a satellite constellation, with coordinated coverage and control.
Why would I need a satellite constellation?
Your own satellite constellation used to seem to be the essential
must-have accessory if you were planning on being a major player in global
telecommunications in the 21st century.
A number of constellation-based schemes were racing to try and be profitable in two areas:
- Voice communications
- Iridium appears to
have squandered an eighteen-month
lead, having begun, ended and restarted service with a large number of launched satellites. Behind it are Globalstar,
which has begun service,
ICO, and a host of other 'big LEO' schemes that were planned. These focused on voice as
the market to go for, but can give fax, paging, and modem-level
data speeds too. Unfortunately, telephony is a legacy application, and GSM has most
of the planet tied up; finding a market proved to be a problem. ICO seemed to be reinventing itself as a data service before turning into a tax write-off.
- Satellite radio
-
Yes, radio. Sirius and XM Radio are competing to give North
American drivers something to listen to.
An area that is currently on hold and increasingly unlikely to be built is:
- Broadband networking
- This is a much newer area than voice; the established players included
Teledesic and Skybridge,
Spaceway and
a large number of geostationary Ka-band constellations. Many of these never got beyond
the paper planning stage.
These satellite-based schemes received competition from some
very different schemes:
worldwide undersea fibre networks and stratospheric
balloons.
Constellations are already being used for:
- Messaging
- Orbcomm
has already launched much of its constellation, and other schemes are planned. These
'little LEO' schemes don't get the same level of publicity as the 'big
LEO' schemes, as tracking your company's trucks around their routes or other
mundane day-to-day applications don't
warrant attention in the same way that the thought of being able to phone
your significant other while skiing down a mountain does.
- Geodesy and navigation
- By now, everyone has heard of
GPS,
although it's not the be-all and end-all of navigation; how far do you
trust the US military? Cue Galileo.
- Remote sensing
- Taking pictures and scientific measurements of the Earth is useful, and with a constellation of satellites with similar capabilities you can get readings of specific areas of the Earth more quickly.
Why are you interested in constellations?
It all started when I did
my masters
thesis at TELECOM
Paris in Toulouse. I spent five months doing nothing but studying the
topology of satellite network constellations, and the interest
stayed. Since then I've generated
other
publications on satellite constellations, and even spent five years writing
a PhD thesis on internetworking with satellite constellations.
Satellite network constellations?
It's a lousy term, but it was the best that I could come up with
to describe satellite constellations where the satellites could talk to
each other directly via inter-satellite links (ISLs), without the signal
hopping to and from an earth station.
Of the voice-oriented constellations, only
Iridium falls
into this 'networked' category. Of the broadband data constellations, only
Teledesic and the
Motorola proposals it subsumed really belong here,
although a number of the GEO Ka-band constellations could be considered as
simple self-contained ring networks.
As a result of this, my masters thesis talked about
Iridium and
Teledesic rather a lot,
as at the time these were the only proposed commercial constellations with ISLs and
therefore really interesting. By the time my PhD thesis was done,
a number of other proposals had appeared, and optical ISLs were all the rage
(and commercial expectations had crashed, making it very unlikely that
such systems would ever get built).
Why do ISLs make constellations more interesting?
Well, in having the satellites talking to each other you create what
is effectively a homogeneous very-very-very wide-area computer
network. The widest network ever - at least until the Interplanetary Internet gets underway. And networking is my thing. Well,
our thing.
Lloyd Wood
(L.Wood@society.surrey.ac.uk)
this page last updated 27 November 2005
