by Bill Ritchie: writchie@bigfoot.com

Webmasters Note: I didn't think I would actually be writing an article for this Edition of the Journal, but I recently attended a lecture on the IRIDIUM communications system. Whilst there I decided that it would make an interesting article so I took copious notes. I am now in the process of finishing off the article itself, but in the meantime I wanted to get this copy of the Journal posted, so I have decided to go ahead and do that - then put up the article in a few days time. I suppose this is another demonstration of the flexibility of using the INTERNET. Had this been a paper Journal I would have missed the deadline sometime ago!

The article on IRIDIUM will, therefore, follow. . . . . . .and here it is!! . Bill 29/1/99

A man telephoned a friend at two o' clock in the morning. "I hope I haven't disturbed you," he said cheerily. "Oh no" the friend replied, "that's quite all right, I had to get up to answer the phone anyway." - Carl Brandt

IT'S GOOD TO TALK

It has long been the dream of communicators to be able to use a phone from literally anywhere in the world. The rapid growth of mobile telephones began to address that, but of course the day to day mobile needs to be within range of a network, and that's not always possible in third world countries or remote areas. The introduction of the Iridium system has, however, changed all that. It is now literally possible to make a phone call from virtually anywhere in the world - barring bunkers and deep jungles!

Iridium is a telephone system, which uses Low Earth Orbit (LEO) satellites to provide a telecommunications system spanning the globe. First conceived in the mid 1980's the name is derived from the Iridium element which has 77 electrons circling its nucleus, and during the initial planning phase it was anticipated that 77 satellites would be required to provide global coverage. Subsequent modelling revealed that only 66 would be required, but the element 66 is called Dysprosium - which is rather difficult to get the tongue around - so the Iridium handle stayed.

The Iridium system uses a total of 66 LEO satellites in six orbital planes - so that's eleven in each orbital plane. On top of that each orbital plane has a spare satellite in it - to cater for redundancy. The satellites orbit at a height of 485 miles (780 Kilometres) and complete a single orbit in just over 1 Hour 40 Minutes.

WHY LEO - WHY NOT GEO?

Now you may be thinking - why don't we just use satellites, which are higher, and remain in a stationary position relative to the Earth - Geosynchronous Earth Orbit (GEO)? Well there are a number of reasons:

• GEO satellites orbit at a height of 22,000 miles - 36,000 Km above the Earth - over the equator. At that distance signals to and from the satellite suffer tremendous losses due to what's called - Free Space Loss. This is overcome with the use of very high gain antennas - like a TV satellite dish - or by using large amounts of power. Obviously it's not practical to carry such devices around with a mobile phone, and power output is also limited.

• Because they orbit over the equator, GEO satellites cannot provide cover over the more northern and southern regions - because their footprint does not extend that far due to the curvature of the earth itself. Indeed the further away from the equator, the weaker the signal within the footprint, so even higher gain antennas are required.

With a much lower orbital height, LEO satellites are able to pick up weaker signals, and, at the same time, concentrate their spot beams more effectively. The rub is of course, that because they are covering the ground so fast, you need more of them, and a number of orbital paths. Not only that, but the satellites themselves need to be intelligent, and have the ability to route calls between themselves - they are, therefore, mini telephone exchanges in space. Another advantage of LEO communications is that users do not experience the delays inherent in GEO systems. We suffer delays in GEO systems due to the sheer distances involved. Even if I were on the equator directly underneath a GEO satellite it would be 22 thousand miles away. If the person I was talking to, through the satellite, was in the next building across the street, that's 44 thousand miles our conversation has to travel. As light travels at 186 thousand miles per second, that's just under a quarter of a second, and that becomes noticeable. If I'm not on the equator and directly underneath the satellite then the distance becomes even greater depending on my look angle in relation to the Bird (satellite). It is possible that total distance can get to sixty-odd thousand miles which is about one third of a second - even more noticeable.

With a LEO satellite it's never going to be much more than a thousand or so miles away. Even with relaying through other satellites a conversation to the other side of the world is only going to be about 13-14 thousand miles or so - a significant difference to that of a GEO system. A single Iridium satellite puts out 48 spot beams, which overlap, and a single spot beam covers an area of approximately 40 Square Km.

FLEXABILITY

Iridium is a flexible system, as the mobile phone is intelligent enough to detect land based networks and use them first in preference to locking onto the closest orbiting satellite. The overall system is also intelligent enough to direct a call through the most economical route - so there are a number of options

• Iridium phone to Iridium phone:

Firstly the system will check to see if the other subscriber is accessible over terrestrial means. If so then the call will be set up over terrestrial networks and the Public Switched Telephone Network (PSTN) - and the calling phone will also use the terrestrial system if it is available.

If both subscribers do not have access over a terrestrial system, then the call will be routed through the satellite system direct. In this case communications would be established directly between Iridium phones without touching any terrestrial system whatsoever.

• Iridium phone to standard number - the call can be set up a number of ways:
  
  Firstly the phone will look to see if a terrestrial system is available. If it is it will direct the call through that.
  
  If no terrestrial system is available, the call will be directed through the satellite system and brought down to earth at the Ground-Station closest to the called number. From there it is routed through the terrestrial PSTN. A number of Ground-Stations have been set up around the world, and interestingly the main one for Europe is in Milan - Italy.

Currently the Iridium hand-held phone itself is quite large - something akin to the old bulky mobiles, but there are plans to use new technology to make them smaller. I doubt, however, that they will ever reach the minuscule sizes we are now encountering with standard mobiles.

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PRODUCING THE SATELLITES AND GETTING THEM THERE

The production of the satellites themselves interested me. They cost approximately $7 million to make, and launch - and about twenty percent of that total cost is made up of insurance! Satellites it seems are now being built on production lines, and a single satellite takes about 4 to 5 days to complete. Doubtless that will get quicker and indeed cheaper!

Three types of launch systems have been used to date:

• The Boeing (Delta II) - capable of sending up five satellites per launch.
  
  
• The Khrunichev (Proton) - Capable of sending up seven satellites per launch.
  
  
• The China Great Wall (Long March 2C) - Capable of sending up two satellites per launch.

The average satellite has a life expectancy of 4 to 5 years, after which its fuel load is not sufficient to boost its orbit. Once the orbit decays it is slowed causing it to re-enter the earth's atmosphere and vaporise.

TECHNICAL DETAIL

I have included some technical information below for the techie types:

• Frequency bands used are as follows:
  
  Telephone and messaging service links: 1616-1626.5 MHz (L-Band).
  
  Satellite Cross-Links: 23.18-23.38 GHz (K_A Band).
  
  Ground Segment Links:
  
  Down-links: 19.4-19.6 GHz (K_A Band).
  
  Up-links: 29.1-29.3 GHz (K_A Band).

• Signalling:
  
  Gateway to International Switching Centre: Pulse Code Modulation (PCM) Transmission.
  
  Iridium Telephone: Frequency Division Multiplexing/Time Division Multiplexing (FDMA/TDMA) Quadarture Phase Shift Keying (QPSK).

• Transmission Rates: 2.4 Kilobits Per Second (KBps).

• Typical Link Margin - 16 Decibels.

WHO WILL USE IRIDIUM?

A typical Iridium phone call costs about $4.00 per minute - not bad when compared to Inmersat Phones which have to be lugged around in a briefcase type device, aligned with a GEO satellite, and cost about $9.00 per minute. Most businessmen, however, tend to operate in areas which are by now saturated with terrestrial mobile telephone networks, so they are not going to be too interested in Iridium. There are, however, still lots of people on this planet who need communications wherever they are:

• Shipping and rescue services.
  
  
• Aircraft - which can use Iridium providing they don't exceed about Mach 2 at which point the orbiting satellites, can get confused!
  
  
• Companies looking for minerals or oil in remote areas.
  
  
• Expeditions.
  
  
• The Military - a large potential user.
  
  
• Businessmen who do have to regularly operate in third world countries which have a poor communications infrastructure.

IS THIS OUR INTERNET DREAM?

So, are we going to be surfing the Internet on Iridium? Well if you look at the transmission rate therein lies your answer - NO. Iridium was designed as a voice system; the future of Data in orbit lies with a planned system called Teledesic, which is due in service in about 2003. Teledesic will have 288 satellites - twelve orbital planes with 24 satellites in each plane. This system is essentially going to provide the Internet in the sky because it will offer blistering data asymmetric speeds:

• 64 Mbps on a down-link.
  
  
• Up to 2 Mbps on the up-link.
  
  
• Broadband terminals with 64 Mbps on two way links.

If you want to get your head round those speeds - well they are about 2,000 times faster than your average 28.8 Kbps modem!

OH - and there are no points for guessing who is one of the major investors in Teledesic - yes no other than our friend Bill Gates. I'm not sure what the overall cost of Teledesic is running at, but I am informed that Iridium cost $9 Billion. As Teledesic is over 4 times the size we are probably looking at $40-50 Billion. Uncle Bill will probably draw that out of his coffee fund!

Well I hope that's given you some idea of just what's going on above your heads! If you lie on you back looking at a clear night sky, you shouldn't be able to miss seeing several satellites zapping by!

For more information, the following websites are worth a visit:

www.iridium.com

www.teledesic.com

Bye for now. Bill

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