Introduction to Data Communications
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iv. Satellite

Satellites are transponders that are set in a geostationary orbit directly over the equator. A transponder is a unit that receives on one frequency and retransmits on another. The geostationary orbit is 36,000 km from the Earth's surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earths rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out to space.

The uplink is the transmitter of data to the satellite. The downlink is the receiver of data. Uplinks and downlinks are also called Earth stations due to be located on the Earth. The footprint is the "shadow" that the satellite can transmit to. The shadow being the area that can receive the satellite's transmitted signal.

v. Iridium Telecom System

The Iridium telecom system was an ambitious satellite project that was to be the largest private aerospace project. It was planned to be a mobile telecom system to compete with cellular phones. It relies on satellites in Lower Earth Orbit (LEO). The satellites were to orbit at an altitude of 900 - 10,000 km and are a polar non-stationary orbit. They were planning on using 77 satellites to provide 100% coverage of the Earth at any moment. 77 is the atomic number of Iridium. The plan was that the user's handset was to require less power and would be cheaper than cellular phones.

Unfortunately, it took so long to design and launch the satellites that cell phone technology surpassed the Iridium project in both size and power requirements. The Iridium phones ended up to be big and bulky and were very expensive compared to cell phones.

They launched 66 satellites during 1998 and were hoping to have 1.5 million subscribers by end of the decade. Unfortunately they found that the cell phone market had captured the majority of the world and they were left with expensive and large mobile phone systems that were only practical for those areas without cell phone coverage.

The Iridium project became financially unstable and went bankrupt in 1999. In 2001, there was talk of crashing the satellites back to earth because it was costing in the order of $1 million per day to keep them up. The original company was purchased by a consortium of private investors under Iridium Satellite LLC and the service re-established in 2001.

Introduction to Data Communications
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Copyright Eugene Blanchard Jan 1998, January 2007