First, the satellite constellation consists of four operational satellites operating 22,300 miles from earth. In addition, there are an adequate number of spares to insure system integrity. The constellation is arranged to allow the satellites to maintain a geostationary orbit relative to the equator. This enables coverage of the majority of the planet with the exception of the extreme North and South Poles.
A satellite control and operation centers are required to maintain the constellation. It is the responsibility of the control center to maintain satellite attitude and orbit. Adjustment is made to compensate for the effects of gravity, which could cause the satellite to drift from its orbital position plus on board power levels are monitored by the control center. Anytime the earth is eclipsing the satellite from sunlight a battery backup system takes the place of the solar power generating system. Four Telemetry, Tracking and Command stations are operated to provide the commands generated by the control center to the satellite. These stations are strategically located in Italy, China and two within the United States. The Operations center is responsible for management of communication traffic levels and frequency assignments to insure high quality service.
Ground Earth Stations
Ground Earth Stations are the second component in the INMARSAT system and include a ten meter diameter dish antenna as well as C-band (4-6 GHz) Radio Frequency controls and signaling equipment. This serves as the interface between the satellite and local Telecommunication Company.
Radio sets called Aircraft Earth Stations are installed in aircraft to permit two way communications with a satellite at L-band frequencies (1.5-1.6 GHz) and will usually interface with other aircraft communication systems.
In short, the aircraft would initiate communications through a specific satellite depending on the aircraft's geographic position to a selected ground station where the call would be routed through local telecommunication channels to essentially any telephone on earth (or above). A call could also be initiated from the ground to the aircraft providing its region of operation is known so a specific satellite can be used to make the connection.
Recently a satellite constellation called Iridium has been activated. This system uses 66 satellites in Low Earth Orbit (LEO) and claims to have close to 100 percent ground coverage. Utilizing low orbits enables less powerful transmitters to be used plus the usual time lag encountered in transmissions is cut in half.
Heavenly bodies have provided travelers throughout the ages with a means of navigation. By employing tools such as a sextant our ancestors could sight a celestial body and determine its precise angle to the horizon. A star map would then be used to determine the position of the navigating vessel. An accurate chronometer was another valuable tool as time is a key factor used to plot positions.
Around 1970, the United States Navy developed an orbiting satellite called TRANSIT to assist in the navigation of nuclear submarines. The principle was simple; the satellite would continuously transmit radio signals and as it moved closer to a receiver the frequency would increase and as it moved further away a frequency decrease would be noticed. Within a few years, a system of global radio navigation based on satellites was envisioned and the NAVSTAR Global Positioning System (GPS) was born.
NAVSTAR's 24-Satellite Constellation
The space segment consists of a 24-satellite constellation. In addition to being equipped with radio receivers and transmitters the satellites carry an atomic clock to be used for distance calculations and although the main mission is position determination various GPS satellites carry a Nuclear Detonation Detection System. Should an atomic blast occur, an electromagnetic pulse sensor would record it and through internal computations the exact time and location would be pinpointed and reported.
Full Operational Capability (FOC) requires 24 satellites to provide 100 percent coverage of the earth with three of these satellites in a reserve capacity. GPS satellites orbit at an altitude of 10,900 miles in six planes with three to four satellites in each plane. This constellation employs a semi-synchronous orbit, which means there is a four-minute per day difference between a satellites orbit time and the rotation of the earth. Positioning of these orbiting bodies is such that a minimum of five is normally visible by a user anywhere on earth at any given time.
GPS III is currently in development and on schedule for launch in 2014
Navigation performance and the growth of GPS
A look at analog and digital communications.
Reach Out and Touch Someone By Jim Sparks February 1999 Online at 40,000 ft. Making a telephone call has been for years a very simple task. With new technology, telephone lines now...