Reader ‘Green Arc’ sent in this short primer on satellite communications basics based on my last post on monitoring Chinese Satellite Guidance systems coming from IntelSat General. Good stuff. -NCS
Satellites are relay stations in space for the transmission of voice, video and data communications. They are ideally suited to meet the global communications requirements of military, government and commercial organizations because they provide economical, scalable and highly reliable transmission services that easily reach multiple sites over vast geographic areas. Transmissions via satellite communications systems can bypass the existing ground-based infrastructure, which is often limited and unreliable in many parts of the world.
Satellite communications involves four steps:
- An uplink Earth station or other ground equipment transmits the desired signal to the satellite
- The satellite amplifies the incoming signal and changes the frequency
- The satellite transmits the signal back to Earth
- The ground equipment receives the signal
Satellites are built using sophisticated electronic and mechanical components that must withstand the vibrations of a rocket launch and then operate in the environment of space – without maintenance – for periods of 15 years or more.
A satellite consists of the spacecraft bus (which is the primary spacecraft structure containing power, temperature control and directional thrusters) and the communications payload (which receives, amplifies and retransmits the signals over a designated geographic area).
Two critical considerations in spacecraft design are power and coverage. A satellite contains multiple channels, called transponders, that provide bandwidth and power over designated radio frequencies. The transponder’s bandwidth and power dictate how much information can be transmitted through the transponder and how big the ground equipment must be to receive the signal. In addition, the satellite’s antennas direct the signal over a specific geographic area.
Commercial satellite communications services are grouped into three general categories:
- Fixed Satellite Services (FSS), which use ground equipment at set locations to receive and transmit satellite signals. FSS satellites support the majority of our domestic and international services, from international Internet connectivity to private business networks.
- Mobile satellite services (MSS), which use a variety of transportable receiver and transmitter equipment to provide communication services for land mobile, maritime and aeronautical customers
- Broadcast satellite services (BSS), which offer high transmission power for reception using very small ground equipment. BSS is best known for direct-to-consumer television and broadband applications such as DIRECTV.
Commercial satellite services primarily use three radio frequency bands:
- C-band, which provides lower transmission power over wide geographic areas and generally requires larger ground equipment for reception.
- Ku-band, which offers higher transmission power over smaller geographic areas and can be received with smaller ground equipment.
- L-band, which is used for mobile applications, such as maritime and aeronautical communications, employing a variety of ground equipment.
In addition, satellite operators are now developing applications over the Ka-band frequency bands, which will facilitate high transmissions speed and significant information transfer with the use of small ground equipment.
GEO versus LEO
Most of the satellites that Intelsat General uses for its customer services are located in geostationary orbit.
The concept of geostationary satellite communications systems is generally credited to the futurist Arthur C. Clarke. Mr. Clarke wrote an article in 1945 stating that communications signals could be transmitted to and from Earth by a relay station launched into orbit at a distance of about 22,300 miles (36,000 kilometers) above the Earth’s equator. From that altitude, the satellite would travel at the same rotational rate as Earth and would appear to remain fixed over a location on the ground below, thereby providing a stationary platform for the continuous relay of communications signals.
In addition to geostationary spacecraft, a few commercial satellite communications systems operate from low Earth orbits (typically several hundred miles above Earth). The lower orbit significantly reduces the delay that is created as the signal travels between Earth and the satellite. This approach is particularly advantageous for global mobile telephone services in which signal delays during two-way communications can be disruptive and confusing.
Unlike geostationary satellites, low Earth orbit satellites do not remain in a fixed position in the sky relative to Earth. As a result, the satellite must have the capability to hand off the signal to another satellite or a local ground-based gateway once it passes beyond direct view.