FOA Guide

 

Satellite Communications

Satellite connections are more complicated, especially for the Internet. And, of course, like anything based on launching stuff into space, satellites are expensive.


There are two types of satellites, geostationary satellites and low Earth orbit (LEO) satellites and they are very different in operation.



Geostationary Satellites
Geostationary or geosynchronous satellites are very large satellites placed in orbit at ~
36,000 km (22,000 miles) above the equator. At this altitude they maintain position above a single point on the Earth because their orbital period is the same as the rotation of the Earth. They can communicate continuously with devices within the coverage of the satellite’s antennas. Because these satellites are large, they can support many users at reasonable Internet connection speeds.


Telstar 1962  NASA and AT&T Telstar, 1962, Bell Labs photo


The first geostationary satellites like Telstar shown above were used for telephone conversations between continents instead of using undersea cables which proved problematic for wired communications, with limited bandwidth and reliability. Later satellites were adapted to service the Internet.



Geosynchronous satellite


Geostationary satellites can cover large areas of the Earth with many users so they are big satellites - the size of a bus or truck - with many antennas. They have a network of earth stations connected to the Internet or private networks on fiber optics that provide uplinks and downlinks to the Internet for the devices connected to the satellite. Users must have their own antenna and electronics for their uplink/downlink to the satellite.


Users of geostationary satellites must have an antenna that is pointed directly at the satellite, similar to those used in satellite TV. However, these antennas must be bidirectional, capable of sending signals up to the satellite as well as receiving signals from the satellite. Here is an example of an antenna for a VIASAT link.


VIASAT antenna



All satellites suffer from limited bandwidth; bandwidth requires radio frequency spectrum which is limited and power, also limited. The other downside of synchronous satellites is latency. It takes about 250 milliseconds for the signal to get from Earth to the satellite and return, which made voice conversations with the delay annoying. That delay also makes the latency extremely high in Internet terms. Latency is also a problem for Internet users because a click on a link on a browser means it takes about a second for response. as the signal goes to the satellite, the satellite processes the request and sends it down to a data center on the ground, uploads the requested data and then downloads it to the user, with each cycle taking about 125ms. In Internet time, that is a long time. That latency makes some applications like videoconferencing difficult.




Low Earth Orbit (LEO) satellites
Low Earth Orbit (LEO) satellites are small satellites that orbit at around 550 km (340 miles) above the earth and zoom over an Earth station at high speeds. These small satellites have limited bandwidth so they support smaller numbers of users. But because they are low altitude, their antennas cover a limited area on the ground and that area is moving at a very high speed. That also means that a large number of LEO satellites are required to provide wide coverage. Most current and proposed systems involve thousands of small satellites to provide adequate coverage.

Because the satellites pass overhead quickly,  the earth stations must track them continuously. The user must be passed off from satellite to satellite continuously to provide constant Internet service.

LEO Satellite

Since it is difficult to have sufficient numbers of ground stations for wide coverage, especially coverage in rural areas, most LEO satellite networks use inter-satellite connections (even using lasers for higher bandwidth than radio) to relay signals to end users remote from ground stations, essentially creating a matrix network among the satellites.

LEO matrix



These satellites require a complicated Earth-based communications network involving many ground stations (connected on fiber of course) to connect to the satellites and they even use satellite to satellite links (using lasers in space in one instance) to connect to the Internet. The complication of the LEO network means multiple links add latency, but since the uplink to the satellite is only about 4 milliseconds, the total latency is much less that geostationary satellites and perhaps even comparable to the ground based Internet with it's multiple switching options connecting users to data centers.

antenna

Users must have their own antenna and electronics for their uplink/downlink to the satellite, but it is more complicated than with geostationary satellites because it must track fast moving satellites and jump from satellite to satellite as they pass overhead. The antenna shown is a phased-array antenna that tracks the satellite electronically instead of mechanically.


Advantages/Disadvantages
Satellites are an excellent option for Internet access in remote or rural areas. It is even possible to use some of these services on boats or vehicles as well as buildings or homes. The disadvantage, compared to FTTH or HFC DOCIS cable systems is cost and bandwidth. Both these offer services up to Gigabit speed, while satellites are much slower and generally will have usage limits which can affect video streaming and gaming.
The advantages for remote communications makes satellites popular for government and military communications also.







More on the applications of fiber optics.

 

 

 

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