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satellite constellations Lloyd's satellite constellations
Introduction | Background | Overview | General | References | Media | Comments?

General information

I know nothing about how you might invest your money in satellite stocks. Please don't ask me.

An introduction to satellites

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.

Comparison of technical details

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.

Sundry stuff

Health

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.

Political stuff

TCP/IP over satellite

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.

Laser intersatellite links

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.

Orbital debris

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.

Industry magazines

Non-English material

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.

Other satellite-related material

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@surrey.ac.uk)
this page last updated 19 December 2004