General information
I know nothing about how you might
invest your money in satellite stocks. Please don't ask me.
- Glossary of satellite terms
- If you need to know exactly what Ka-band is, or what FDMA and the rest are, this is a very good, concise, list of terms.
- How satellites work
- A simple introduction to satellites. Part of the Learning from satellites series.
- Introduction to satellites
- A simple introduction to a satellite and its components, suitable for kids.
- Design a satellite
- grossly oversimplified fun for kids and adults.
- Another introduction to satellites
- from William Stallings.
- Basics of spaceflight
- from NASA's Jet Propulsion Lab.
Chapters 3, 4 and 5 will teach you and test you on orbits, while Chapters
10 and 11 cover aspects of communication and power systems relevant to
satellites. There's also the NASA Goddard satellites factsheet and a short history of satellite communications.
For younger viewers, NASA's short course on satellite communication may be of interest. For older readers, there's an orbital mechanics tutorial with problems.
JTrack will
show you the positions of the satellites in the geostationary ring, and the orbits of other satellites. a map of the geostationary ring is provided by the London Satellite Exchange. An overview of geostationary satellites is available.
- Mobile
Satellite Telecommunications Library
- from Brian Mcintosh. A good
introduction to the entire world of satellite communications, although it
has a bias towards
Globalstar. See
also the Satellite 101 page.
- Where it all began
- The very first paper describing the very first constellation, consisting
of three satellites in geostationary orbit. Allegedly the only accurate science-fiction prediction ever.
Authored by the famous
Arthur C. Clarke, before the space race, before Sputnik 1,
and before
Arthur C. Clarke became
a famous author. (There's a mirror of the paper. And now we call it the Clarke orbit, and you can
simulate the original proposal.
David Whalen's short history is also worth reading.
- An introduction to satellite communications
- introduces the basics of geostationary systems. These Questions and answers from Via Satellite may also prove useful.
- Summary
of all proposed commercial satellite systems
- from Analysys
Brief summaries of the main features of many satellite schemes. Here's a
noframes
entry point.
- Satellites operational and in orbit
- A free spreadsheet listing active satellites.
- Satellite failures
- described by the Satellite News Digest.
- Big LEO tables
- This gives a technical overview
of some of the proposed satellite constellations, and attempts
to compare data for vastly different designs, which is a dangerous thing,
unless you know how to interpret the tables and technical information that
is provided.
You can't draw conclusions from this table without knowing a lot about how the
individual systems work, and comparisons between the
networked and non-networked constellations can be particularly misleading,
especially if you forget that
Teledesic
isn't really aimed at
the mobile voice communications market.
Still, the orbital information and diagram give an idea of the scale of
the projects.
There's mounting anecdotal evidence of the effects on health and memory of mobile cellphones. (BBC News, 1 March 1999)
Handheld satellite phones are more powerful, and use higher frequencies, when communicating with satellites - although cost will deter the long periods of
use seen with terrestrial cellphones. Still, I haven't seen any detailed
studies on the effects of health of longterm exposure to satellite phones.
David Jefferies points out that independent scientific evidence
is hard to come by in such a contentious area.
- Material on space law.
- Government wiretapping of satellite phone calls is turning into a
hot potato - although I'm only really aware of US coverage and viewpoints on this problem. (August 1999)
An approach to wiretapping in Iridium is described in this Motorola patent. (A The spy who bugged me, Barry Fox, New Scientist, 11 March 2000.)
- The ITU provides a
set of satellite
features and company position papers (World Telecommunications Policy Forum, October 1996) and an earlier
market
assessment (October 1995).
The ITU is also responsible for running the World Radio Conferences (WRCs) every two years, where frequency and positions
allocations for satellite systems are discussed. You can get an idea of the positions and frequencies allocated to
geostationary and non-geostationary satellites from their tabulated data.
If you're interested in
radio spectrum and the physical layer, the WRC conference papers are essential reading.
The minutes and proposals of past WRCs can be interesting, too.
Here's a report on WRC '97
from the Pacific Telecommunications Review
(December 1997)
Agreeing frequency use with astronomers is also part of the
ITU remit.
ITU recommendations on things such as
charging, billing and accounting principles are grist for the mill.
Amazingly, you used to have to pay for online copies of this information.
In Swiss Francs, too. The ITU's coming along nicely.
- The European Commission
thought it could make money from licensing frequencies to operators.
Several years after the
FCC and various World Radio Conferences
have done most of the frequency allocation work, too, and so far
only ICO has requested clearance. The legislation wouldn't be ready before
the schemes are, in any case;
the Commission will look rather silly. In
fact, the member
countries already think it's a stupid idea. (nando, March 1996)
The
complete text of
the EC proposal is available as part of the EC legal measures in telecommunications.
It's a surprisingly good read and covers previous
regulation fairly well.
Further clashes between the EU and US over telecom and satellite markets
are likely. Here's an account
of one such clash (nando, April 1996)
The European Union has since given Motorola the go-ahead for Iridium. (news.com, 19 December 1996)
- The
US Department of Transportation expects
up to five LEO systems operational
by 2005. (press release, May 1995)
AST reports on LEO commercial market projections
are worth a look.
(Associate Administrator for Commercial
Space Transportation, now part of the FAA, previously part of the DOT)
Estimates and breakdown of active satellites in orbit (BBC News, December 1999)
- Satellite Communications
Systems and Technology Report, which concluded that the United States
had lost its leading position in satellite communication technology, and
that the market share of the US satellite communications industry was at
risk. (July 1993)
You would never have thought that, given that all the schemes
discussed here originate from US companies, and that
they all seem to be represented in the Washington DC area, by being
members of the
Satellite Industry Association.
- The
Survey of rural information infrastructure technologies includes
Section 4.10.3 summarising the constellations.
This US government survey is no longer maintained.
There has been a some press about TCP/IP, the networking
protocol that the internet and the web rely on, not working properly over
satellite. A lot of media coverage of this resulted from a
Teledesic press
release (Wireless multimedia and internet
via satellite, by Mark Sturza and Farzad Ghazvinian).
That claimed that the TCP/IP reference implementation's 4K buffer
size limits the size of the pipe and the data throughput to only 64kbps.
This was then used to argue that the maximum buffer size of 64K limits
maximum throughput to 1Gbps, so that geostationary
Ka-band competitors to
Teledesic are unsuitable
for high-bandwidth applications, as the increased latency of a GEO connection
decreases the available bandwidth.
(TCP buffers are dimensioned as
bandwidth * delay = buffer size
With a limited
buffer size, a longer end-to-end delay decreases the space available to hold
spare copies of unacknowledged
data for retransmission. This limits the
throughput on a lossless TCP connection.)
However, this completely ignores the work done on larger buffer
sizes for TCP in
RFC 1323,
the "large windows" effort.
(mirror)
This work to expand TCP beyond its original 16-bit buffer space
has been going on for several years, and is already
supported by a number
of versions of unix.
The TCP buffer limit isn't the problem it's made out to be; TCP copes with
GEO delays quite nicely right now, and individual high-bandwidth GEO TCP links
are possible with the right equipment and software - you wouldn't buy the
wrong equipment and expect it to work, would you?
GEO links are suitable for seamless intermediate connections in a TCP
circuit and are already being used for this.
There is nothing stopping you from having many "small"
TCP connections over a broadband link, GEO, fibre or otherwise, and
most broadband internet connections contain a vast number of separate small
pipes.
The real issue with GEO vs LEO is not the alleged inapplicability of TCP, but
the acceptability of the physical delay for
two-way realtime applications where humans are involved, such as telephony or
videoconferencing. And even then, the physical delay of GEO is balanced somewhat
by the increased switching times through a packet-based LEO network. If
you want decent two-way videoconferencing, you'll do the sensible thing
and go to fibre.
Ignore this furore. Anything that talks about TCP/IP but doesn't refer to
Internet Engineering Task Force (IETF) work
or to relevant RFCs should be
questioned. Have the authors done their research?
These papers describe the
situation in greater technical detail, and show that having multiple narrow
TCP pipes across satellite works well. Wide pipes with large buffer sizes can
suffer from the higher bit error rate (BER) of satellites - but that's
something that really needs to be addressed by better symbol coding down at
the satellite's data-link layer for better BER, and not up at
the application layer.
For TCP, implementing Selective Acknowledgements
(RFC2018) and
fast recovery (RFC2581) also improves performance in the face of errors, even before link-local
optimisations such as split TCP connections (from e.g. Mentat) are considered.
Take a look at work undertaken by the TCP over satellite working group: RFC2488 and RFC2760.
The world's first laser data intersatellite link is established. SILEX is a
LEO-to-GEO laser communications project being worked on by
Matra Marconi Space for
esa. It's mentioned in this
SPIE report
on free-space lasers.
Even though there are very few existing operational intersatellite links,
and most of those are Iridium crosslinks, radio and even phased-array intersatellite links
are passe. Everyone's looking at lasers for higher bandwidth;
M-Star was the first to propose
to use them in a commercial constellation.
An esa bulletin is available discussing this.
NASA's GOLD (Ground/Orbiter Lasercomm Demonstration) experiments have tested
laser communications from ground to
satellite.
For information on academic research in this area, papers on free-space laser
communication from SPIE - the international society for optical engineering are worth looking at.
With all the satellites and debris flying around in orbit, there is a small but
increasing risk of orbital collisions. There's also the doomsday
scenario of these collisions creating debris that flies off and collides with
other satellites, which breaks into debris that... this chain reaction is known as the cascade
effect, and it raises the possibility of denying access to space. This
sort of consideration has affected the satellite insurance sector.
The impact (pun intended) of the Leonid meteor shower that the Earth passes through every November is of concern;
several years have been very heavy.
In the meantime, we can worry about an increasing number of
close encounters, and
new hazards for a new age.
Proposed technical solution: Tethers Unlimited suggest a deorbiting
method based on a tethered weight; here's a description of the
proposal (BBC News, 15 January 1999) and a description from New Scientist.
Interesting idea, but it really needs a watchdog that is constantly reset to
cope with satellite failure; I can't see this being commercially viable unless
deorbiting is legislated for. There might be an analogy with the introduction
of airbags in cars here.
- Speak Finnish?
- Here's an article comparing satellite
mobile phone systems.
- Speak Russian?
- If you can read Cyrillic, here's some material on satellites and constellations.
- Speak Polish?
- Here's some more material on satellites and constellations.
- Speak French?
- Here's another article.
Information
on Africa-focused satellite telecoms.
- Speak Dutch?
- Here's a student project with amusing conclusions.
Locally
- ATM over satellite
-
The ATM over satellite page is maintained by my former colleague Tolga.
If you believe
that a protocol suite developed for a tree of permanent, high-bandwidth, fixed, error-free
fibre links is really useful in lower-bandwidth satellite constellations
with rapidly changing geometries in bursty-error space I wish you lots of
luck. ATM has to be hacked
heavily, to the point where it will
hardly be recognisable as ATM, in order to truly support satellite links.
As if ATM wasn't complex enough already. A number of constellation proposals
are planning on combining two or more ATM cells in a frame, with CRC and error
coding. Ugh.
I don't believe in ATM, and this Netheads vs Bellheads
article by Steve G. Steinberg will give you some of the background that
underlies such heresy.
- Small satellites
-
Information on many micro- and mini-satellites from Alex da-Silva Curiel, over in
our Surrey Space Centre. Here we build, launch and operate
our own small
satellites. Very little to do with constellations (apart from our
proposed LEqO and Tsinghua and
now E-Sat), but SSC do know a lot
about small satellites. Which is being leveraged in constellations.
- Centre for
Communication Systems Research
-
Here we study satellite-related and mobile communication technologies of all kinds.
In the United Kingdom
Outside the UK commercial sector, the British National Space
Centre is as good as it gets. Alas,
very little funding; anything happening commercially is at UK Space. The IEE Satellite Systems Professional Group can have interesting material, but you'll need to create an account to view it.
You're better off learning about the companies in the UK space industry instead. Satellite Links and related sites are useful.
Internationally
I recommend looking at esa (particularly
esa telecommunication and satellite applications)
or NASA for space-related information.
Lloyd Wood
(L.Wood@society.surrey.ac.uk)
this page last updated 19 December 2004
