The main problem with the GSM network which is responsible for many of our mobile calls is that a unit can easily stray out of coverage. This mobile phone now cannot contact the Base and you have no coverage. Luckily this was a problem also for Karen Bertiaer , the wife of a Motorola Executive. While on a holiday in the Caribbean in 1985 she was frustrated at not being able to make a phone call to the U.S. She encouraged her husband to try to convince Motorola of the need for a global wireless network powered by a constellation of satellites orbiting the earth.
Motorola believed in the idea and decided to sponsor a consortium of telecoms operators and industrial companies. This consortium was called Iridium as they planned to launch 77 low earth orbit (LEO) satellites and Iridium is the 77th element in the periodic table. Iridium scaled back their plans and decided to use a network of 66 satellites, fortunately the kept the name Iridium as the 66th element is Dysprosium which is Latin for “difficulty of access”!
2. Installing the Iridium Network:
In October 1995, 10 years after the initial idea, Iridium was finally ready to send some satellites into space. The first satellite flight bus was delivered by Lockheed Martin to the Motorola Sat COM facilities in October 1995. By 1997 Iridium had 47 satellites into low earth orbit. Their plan of 66 satellites was completed in 1998 and they now had a fully functioning network. There are also 6 spare satellites in orbit in case of failure. Boeing now operates this constellation, they launch up to 5 new satellites with each launch.
3. The Satellites:
These satellites were placed into LEO at only 485 miles above the surface of the earth in a polar orbit. This is unlike geosynchronous satellites which hover above the equator at an altitude of 35,786 km . They travel in six orbital planes inclined at 86.4 degrees with an orbital period of 100 minutes, 28 seconds (a speed of 16,832 Mph). This constellation has coverage of the entire globe (5.9 Million square miles).
Each plane has 11 satellites operating as nodes in the network. Every area of the globe is covered by at least 1 satellite at a time.
There are many benefits to the Iridium customers of using the LEO satellites
· Minute transmission delays
· Lower transmit power needed
· Smaller antennae needed
· Smaller units
· Improved battery life
4. Hardware – Technical Data :
The satellites used by Iridium are approximately 8 meters wide and can receive power from 2 solar panels if required. They weigh 689kg (1,500 lb). Each Satellite has 48 spot beams and each spot beam is 30 miles in diameter. They have a link margin of 16db and an average life-span of 7-9 years .
The iridium satellites can communicate with each other. Communication can be achieved between satellites via routing through Earth Stations can communicate directly with each other. The frequencies used for satellite cross connections are in the Ka band (23.18 -23.38 GHz). For telephone and messaging they operate on the L-band (1616 -1626.5 MHz).
They need to exchange info when one satellite is about to disappear over the local horizon of a user. They will hand-over the user to another satellite, this action is transparent to the user (as in GSM). FDMA and TDMA are used to multiplex the voice and data signals on an Iridium telephone. Satellite cross communication is used to manage the hand-over. This is faster than routing through the earth as it reduces the delays and takes some load off the earth station. Many users require fax and data transmission, 2.4 kbps can be achieved with Iridium.
Gateways are used to connect the network to the public switched telephone network. This is the technology that allows a satellite phone to connect to any landline or other non-Iridium phone in the world. A Siemens GSM-D900 is the type of switch used in the ground network.
5. Packets:
Within the Digital Iridium Network short fixed length packets are used to route your call. A packet contains a destination address, a source address and a payload. The payload is made up of your encoded voice (or data). The node (handset, satellite or gateway etc.) which has your packet tries to select the path with the lowest delay, it works almost like TCP as multiple packets can travel multiple routes and arrive at the destination at different times and will be re-ordered.
6. Accessing a Satellite:
Although the satellite is relatively low, it is still too high to be able to receive it with much strength inside a building. Typical Iridium handsets transmit approx. 0.5 – 0.6 watts. This is not enough to reliably reach the satellite. You are promised world wide access to the satellite constellation however this is predicted on line of sight access. There are many articles available on the web reviewing the Iridium network .
A summary of these informs us that the phones are best operated when you have a clear view of the horizon all around you or are on top of a building etc. However Iridium have made Dual Access phones which will give you access to the existing GSM network when it is available. This should improve your signal in populated areas or inside buildings etc.
7. The Phones:
The Motorola 9500 series contain many different modes on the handsets. You have both terrestrial and satellite components; it is possible with these multi-mode phones to connect to satellite, GSM networks or AMPS . When this phone looses connection to the terrestrial network it can switch automatically switch over to the Iridium Satellite Network.
8. Phone Data:
A Motorola 9520 is approx. 25cm in height, 9cm width and 4cm deep. It weighs 594g (1.3lb). It operates at a frequency of 1616 -1625.5 MHz and uses TDMAFDMA. It has a link margin of 12.5 db and its average power output is 0.57 watts.
9. Call Cost:
A Motorola 9505 costs around $1500 and calls are priced at $1.50 per minute although you have a one time activation fee of $50. Iridium – Iridium calls are very reasonably priced at only $0.50 per minute although phoning an airplane Iridium unit from a handset costs $10 a minute.
10. Conclusion
I believe that at the present time Iridium is a very extravagant gadget. Anyone who lives in a city or travelling to a foreign country probably doesn’t need this as even a hotel telephone (regardless of the high call prices) must be cheaper than a $1500 phone and up to $10 per minute phone calls.
However 2/3 of the world is covered by water and for maritime and air usage this could be an invaluable tool. The guarantee of being covered by a satellite at any time and always within easy contact of the rest of the world makes this a desireable product for people in these fields.
Oil and Gas exploration, Explorers in the desert & wilderness, mountain climbers, expeditions to the poles of the earth and rescue missions must be a safer prospect nowadays. Dual Mobiles (IridiumGSM) will also be a major factor to the uptake of these units. As more units are sold the prices should continue to
fall and soon the prices may be within reach of the average person.
This article was originally written by DONEGALVOLVO |
Copyright Open Source Institute, 2006 
