Guest Blog: How Satellites Work, by Philip J. Reed

How Satellites Work

--Philip J Reed on behalf of Exede, a satellite internet provider

While most people are aware of satellites, very few understand just how important they are to modern society. In addition, beyond a vague concept of some sort of spaceship, very few people actually understand what satellites are and how they work. Satellites are the most common form of spaceship, but they neither carry passengers nor travel far away into space. Satellites carry important communications or observational equipment, and they stay in an orbit around the Earth.

What Satellites Do

According to the Union of Concerned Scientists (UCS), 999 operational satellites are now in orbit around the Earth. However, estimates of the total number of satellites in orbit are closer to 3,000. Most of these satellites are owned by corporations, and they serve in establishing communications networks. Satellite communications cover nearly the entire planet, and some parts are covered by hundreds of networks.

Satellites are very versatile pieces of equipment, and they can support all of the different types of communications required in today’s world, including the following:

• Video and voice
• Cellular telephone and data
• Multimedia
• News and entertainment
• Broadband data
• Business transactions

Satellite Orbits

Satellites are little more than signal relays that are in space above the Earth. They receive and rebroadcast analog or digital signals at specified radio frequencies. Satellites are launched into space with rockets, and they carry navigational equipment that keeps them at the proper altitude, moving at the proper speed and in the proper direction. Satellites are programmed to orbit the Earth from one of three orbital positions:

• Low Earth Orbit (LEO) – This orbit is at an altitude of 300 to 1200 miles above the surface of the Earth. Satellites using this orbit must travel very fast to avoid being pulled in by Earth’s gravity.
• Medium Earth Orbit (MEO) – This orbit is at an altitude of 5,000 to 12,500 miles above the surface of the Earth. Satellites in this orbit follow an elliptical path, and they are primarily used with ground stations located near the poles.
• Geosynchronous Orbit (GEO) – Most satellites used today are in geosynchronous orbit. The altitude of this orbit ranges from 22,223 to 22,282 miles above the surface of the Earth. Satellites in GEO travel at about 1.91 miles per second, which keeps them in the same position relative to the Earth as it rotates. The primary advantage of GEO is that satellite dishes and other ground antennas can be locked into a position to send and receive the strongest possible signals.

Onboard Satellite Systems

Digital and analog signals are sent to satellites from ground stations, and the signals are rebroadcast by satellites to other ground stations that may be located thousands of miles away from the origin station. The signals are processed on the satellite through electronic devices called transponders. Each satellite holds from 24 to 72 transponders, and each transponder can process 155 million bits of data per second. At this speed, even the most complex signals can be quickly relayed through satellites. In addition to the transponders, satellites are comprised of the following other systems:

• Solar energy collectors
• Propulsion and altitude control
• Fuel, batteries and power
• Signal amplifiers and filters

Transmitting and Receiving Signals

All satellites in GEO are assigned an orbital location, which is a reference to a satellite’s position in space relative to the longitude of the Earth. A satellite’s location affects the area on the surface of the Earth where signals can be sent to or received from the satellite. This area is known as the satellite's footprint.

When data is transmitted to and from satellites, it is done through C-band, Ku-band or Ka-band radio frequencies. The combination of the frequency, power level and geographic area of a signal is called a beam. Beams can be global, semi-global or targeted to a specific area.

Satellites use different beams and ground networks for different uses. For instance, TV networks use a simplex transmission system, which is a one-way transmission from a ground station to the satellite and from the satellite to several receiving stations. Other systems, such as star networks and mesh networks allow for two-way communications with multiple ground stations.