Low Earth orbiting satellites (LEOs) are often deployed in satellite constellations, because the coverage area provided by a single LEO satellite only covers a small area that moves as the satellite travels at the high angular velocity needed to maintain its orbit. Many LEO satellites are needed to maintain continuous coverage over an area. This contrasts with geostationary satellites, where a single satellite, moving at the same angular velocity as the rotation of the Earth's surface, provides permanent coverage over a large area.

Low Earth Orbit (LEO) refers to a satellite which orbits the earth at altitudes between (very roughly) 200 miles and 930 miles.

Low Earth Orbit satellites must travel very quickly to resist the pull of gravity — approximately 17,000 miles per hour. Because of this, Lowe Earth Orbit satellies can orbit the planet in as little as 90 minutes.

Low Earth Orbit satellite systems require several dozen satellites to provide coverage of the entire planet.

Low Earth Orbit satellites typically operate in polar orbits.

Low Earth Orbit satellites are used for applications where a short Round Trip Time (RTT) is very important, such as Mobile Satellite Services (MSS).

Low Earth Orbit satellites have a typical service life expectancy of five to seven years.

The Cell Patterns

            The satellites had the capability of projecting 37 spot beams on the earth. The spot beams formed a series of overlapping, hexagonal patterns that would be continuous. The center spot beam was surrounded by three outer rings of equally sized beams. The rings worked outward from the center beam in rings of 6, 12, and 18 spot beams. A spot beam was 372 nautical miles in diameter, and when combined they covered a circular area of approximately 2,200 nautical mile diameter. The average time a satellite was visible to a subscriber was approximately nine minutes.

Modulation Techniques

            The modulation process and multiple access capabilities of Iridium were modeled after the conventional terrestrial cellular networks, and particularly after the international GSM standard. Combining frequency and time division multiple access, the system also used a data or decoder voice and digital modulation technique (that is, QPSK, MSK, and so on). Each subscriber unit operated in a burst mode transmission by using a single carrier. The bursts were controlled to appear at the precise time necessary to be properly integrated into the TDMA frame.

The Switching Equipment

Each gateway housed the necessary switching equipment to interface between the communications payload in the Ka band and the voice/data channels from the PSTN. The switching systems performed the following functions:

·         Transferred common channel signaling information from the PSTN to the RF portion of the Iridium network.

·         Transferred line and address signaling information from the PSTN to establish circuit−switched calls.

·         Supplied in−band tones and announcements to PSTN users calling onto the Iridium network with necessary progress tones and conditions.

·         Digitally−switched PCM signals between channels derived from the terminal channels to the PSTN and provided channels to support the necessary in−band signaling capability for call control and progress.

Interconnecting to the PSTN

            Voice connections were designed to be fully compatible with applicable ANSI T1 standards for the United States and the CCITT G and Q Recommendations (International T1/E1 standards) for digital transmission systems using either SS7 or R1 signaling. The data channel specifications were compatible with the OSI standards and with CCITT V and X series recommendations.


http://www.tech-faq.com/low-earth-orbit.html
http://en.wikipedia.org/wiki/Satellite_constellation
Broadband telecommunications handbook / Regis J. "Bud" Bates. — 2nd ed.