COMMUNICATIONS SATELLITES
Most communications satellites are in GEO. A single geostationary satellite can cover as much as 40 percent of the earth’s surface; so, in theory, three such satellites can provide global coverage. To ensure accurate and strong coverage of a specific region, continent or country, the transponders are often “shaped” to focus transmission and increase signal strength for a service area.
A satellite’s job in the communications network is to serve as a repeater. That is, it receives a signal from one location and rebroadcasts it so another station can receive the signal. Reception and retransmission are accomplished by a transponder. A single transponder on a geostationary satellite is capable of handling approximately 5,000 simultaneous voice or data channels. A typical satellite has 32 transponders.
Transponders each work on a specific radio frequency wavelength, or “band.” Satellite communications work on three primary bands: C, Ku and Ka. C was the first band used and, as a longer wavelength, requires a larger antenna. Ku is the band used by most current VSAT systems. Ka is a new band allocation that isn’t yet in wide use. Of the three, it has the smallest wavelength and can use the smallest antenna.
Because of attenuation and business competition, there are far more than three GEO satellites. Satellites of similar frequency can be as close as 3 degrees apart without causing interference. Since there are 360 degrees in a circle, that means 120 satellites of a specific frequency can be placed in GEO orbits.
The combination of individual transponder volumes and the number of transponders in orbit means today’s communication satellites are an ideal medium for transmitting and receiving almost any kind of content, from simple data to the most complex and bandwidth-intensive video, audio and data content.
A communications satellite is a specialized wireless receiver/transmitter — receiving radio waves from one location and transmitting them to another (also known as a “bent pipe”) — that is launched by a rocket and placed in orbit around the earth. Today, there are hundreds of commercial satellites in operation around the world. Those satellites are used for such diverse purposes as wide-area network communications, weather forecasting, television broadcasting, amateur radio communications, Internet access and the Global Positioning System.
Satellites have many important uses, not just communications. Most modern weather reports rely on satellite information. Global Positioning systems work because of a linked set of satellites. Scientific studies of our planet, the atmosphere and the universe all rely on satellites.
ORBITS
There are three areas for satellite orbits:
GEO: Geostationary Earth Orbit
MEO: Medium Earth Orbit
LEO: Low Earth Orbit
GEO satellites orbit the earth directly over the equator, approximately 35 400 km (22 000 miles) up. At that altitude, one complete trip (orbit) around the earth takes 24 hours. Thus, the satellite remains over the same spot on the surface of the earth (geo) at all times, and stays fixed in the sky (stationary) from any point on the surface from which it can be “seen.”
MEO is defined simply as the area between LEO and GEO. The primary satellite systems there are the GPS (Global Positioning System) satellite constellations.
LEO is between 200 and 1400 km above the earth. Satellites in LEO rapidly circle the earth and are typically in range of one location for only 90 minutes. Their main advantage is how close they are, providing shorter delays for faster communications. However, for consistent communications they require a constellation of satellites so that communications can be maintained as one satellite moves out of range and another moves within range of the ground station. LEO satellites are less expensive to build, typically less powerful, and have a shorter average life span.