Satellites
From Sputnik to SATCOM
By Jim Sparks
June 1999
It all began on October 4, 1957 with a crescendo of sound and light
as Sputnik 1 rocketed into earth's orbit. A steel ball, weighing 184 pounds with a 23-inch diameter, and carrying a radio transmitter, was to be a watershed in modern history. Much of today's meteorology, communications, surveillance, and navigation are dependent upon a network of 4,000 satellites circling our planet.
Prior to 1956, capabilities for transatlantic voice communications were limited to radiotelephones and were often limited by atmospheric conditions. When the first cables were strung across the ocean floor in 1956, it soon became apparent they could not handle the rapidly increasing volume of calls. At that time, scientists decided to look skyward for a solution. The age of airborne telecommunications began in the early 1960's with the launch of TELSTAR 1 and has been advancing ever since. In 1962, the United States formed COMSAT (Communication Satellite Corporation) with the intent of developing a worldwide stellar communications network. Two years later, this turned into a global endeavor with eleven nations joining to form INTELSAT (International Telecommunications Satellite Organization). Today more than 130 nations participate.
Early Communications Satellites
Early communications satellites did not operate in a geosynchronous orbit (that is they did not remain above one specific place on earth), which would result in periodic drifting out of the range of ground stations. For example, the early TELSTAR satellite could only communicate with ground stations on an average of four hours per day.
The Hughes Aircraft Company working with NASA developed SYNCOM a high altitude geostationary satellite in 1963. This new technology enabled 24-hour communications with stations on the ground. Several other SYNCOM-type devices have been launched since the mid-1980's using the space shuttle as a delivery tool. The early INTELSAT network included 20 satellites that circled the earth in a geosynchronous orbit, meaning each satellite will remain over a particular point on the earth's surface and could receive and transmit signals from relay stations on the ground. Each satellite contained a receiver and an amplified transmitter, which were used to hand off messages. These "early bird" devices were used for about three and a half years and appeared as metal cylinders just over 2-feet wide and 1 1/2-feet high, encased in solar cells, and could handle 240 telephone lines or one television channel at any given time.
INTELSAT 4 revolutionized the industry in the early 1970's with a physical dominance over all previous models. Standing a towering 16 feet in height with antennae extended it was the only device of its day capable of carrying color television signals along with 6,000 telephone channels.
In the 1980's, the "spinning drum" technology of earlier years was cast aside and new models with outcast wings of solar panels were deployed. The extra power generated was employed to enable 15,000 telephone calls at any given time. Thirteen of these satellites were delivered from 1980 to 1989. The more recent INTELSAT 7 can handle up to 22,500 telephone calls and three television channels at any given time and are expected to last between ten and fifteen years in orbit.
With the high degree of mobility required in today's economy a service was required to provide communications with land vehicles, ships and aircraft. INMARSAT is an international organization whose purpose is to establish, operate and maintain a worldwide satellite network that complies with standards developed by the International Civil Aviation Organization (ICAO). Service includes two-way voice, fax and data for aircraft operating in most areas around the globe.
SATCOM
The INMARSAT satellite communications (SATCOM) system consists of three
components:
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 receivers will vary drastically depending on application. A typical handheld unit can display position in terms of latitude and longitude as well as provide both speed and direction computations. In most aircraft applications, position information is interfaced with a navigation database. This is frequently accomplished by accommodating GPS as part of the Flight Management System (FMS).
Rate X Time = Distance
The principle of GPS operation is based on time. Each satellite is scheduled to transmit two "L" band radio signals complete with a digital
code that includes exact time of transmission. If the clock in the receiver is exactly synchronized with the satellite transmitter, an exact computation can be made to determine the distance between the two. This computation is based on the radio signal travelling at 186,000 miles per second and the ability of the receiver to know exactly when the signal was transmitted by the satellite and exactly when the signal was received.
Each satellite that is in range of the receiver will supply a radio transmission on a scheduled basis. The receiver then performs multiple computations and determines an exact distance to each satellite. Using the equation Rate X Time = Distance, and knowing the relative position of each satellite, the receiver can then process the data and come up with a very exact position based on the World Geodetic System map datum absolute earth's coordinates in degrees of latitude and longitude.
Only a few centuries have passed since communication was accomplished using smoke signals produced from fire and green wood. What will the future hold?
