Satellites Know-How


Why 'Geostationary' Satellites?

Communications SatelliteThis refers to satellites orbiting at a height of some 36,000 kilometers that remain in the same position over the earth - hence the term geostationary. Over a third of the earth's surface is within view of each geostationary satellite, so only three such satellites, equally spaced over the equator, are needed to give almost complete global coverage (the exception being the extreme Polar regions).
Inmarsat has four satellites to handle higher traffic volume in the Atlantic region.

Ground control earth stations monitor the position of each satellite and check that they are keeping to their predefined positions. This is known as 'keeping station' and means that they stay within a box about 40 miles square.

If a satellite shows signs of drifting out of position it receives an instruction to move back to the correct position using its on-board motors. One of the factors that limits the service life of a satellite is the amount of fuel it uses up keeping station.
What Happens When a Satellite Breaks Down?

 Some of the larger satellite networks already have an 'in-orbit' spare in case a satellite fails. The spare can be then be moved into position in a matter of days, plugging the gap in the network.

How Do Satellites Actually Work?

 Radio signals sent from an earth station are received by the satellite, amplified and processed, and transmitted back to earth. The electrical power for the transponders that carry out this task is derived from massive solar cell 'wings'.

The introduction of larger and more powerful rocket launcher systems has enabled heavier satellites to be placed in orbit. Hence satellites can now be equipped with a large number of transponders, typically twenty or more, operating in different frequency bands and dedicated to different applications.

Transponders vary both in terms of power and operating channels. Differing amounts of power are required depending on the application. For example, direct TV broadcasting uses higher power transmitters as the residential receiver equipment is relatively simple. Often a dish as small as a 45 cm is used to receive the satellite signal. At the other end of the spectrum KPN for instance uses 34 metre dishes to receive international traffic, which are more efficient in capturing even weak signals.

What Is a 'Footprint'?

 The footprint of a satellite is the area on earth that is covered by the signal of the satellite. The footprint of a geostationary satellite can theoretically be as large as one third of the globe (global beam). Most modern satellites use several spot beams. Spot beam technology makes it possible to concentrate the satellites signals on several smaller areas within the global beam. Apart from a more efficient use of frequencies, spot beams are also used to concentrate the satellite signals to areas where traffic is expected to be generated. Thanks to spot beam technology a stronger signal can be received on earth, which makes it possible to reduce the size of the antennas of satellite terminals. The development of the Inmarsat mini-M telephone became possible with the use of spot beam technology.

Low orbiting satellites have much smaller footprints. The lower the satellite's orbit, the smaller its footprint. The footprint of the low polar orbiting satellites of the Iridium network can be as small as a (circular) area of a few hundred miles. Thus more than 60 satellites are necessary to achieve full global coverage.

How Fast Do Satellite Signals Travel?

 With satellites in orbit at 36,000 kilometers, the round trip from the ground to the satellite and back takes the signal roughly half a second. While this is undetectable in the case of broadcasting or data transmission, it is evident in real-time applications such as speech or videoconferencing.

The delay between question and answer for instance can be quite noticeable in news reports where a journalist in the field is being interviewed by an anchorman in a studio.

But then of course these reports wouldn't happen at all without satellite links.

What Is a 'Low-Orbit' Satellite?

 The only way of reducing this signal delay to an imperceptible level is to reduce the orbital height and therefore lessen the signal round trip distance. This can be done with Low Earth Orbit (LEO) satellites. These are typically located at a height of between 650 and 1,600 kilometers.
Consequently, the signal's round trip takes less than 100 milliseconds and is imperceptible in conversation. However, in contrast to geostationary a constellation of 50 or more satellites is needed to cover the globe.

The low altitude also means that less powerful transmitters are required both on the satellite and on any ground-based equipment. This makes it possible for such networks to support hand-held telephone handsets.

In recent years there has been a great deal of interest shown in LEO networks with Iridium already in operation and others, such as Skybridge, at the advanced planning stage.

The absence of any noticeable delay in speech transmission makes LEO satellites an attractive proposition for real time applications but the system is still very much in its infancy.

Does '99% Coverage!' Refer to 'People' or 'Area'?

 These new networks face major competition from GSM cellular phone operators who offer customers the ability to use of the same phone in many different countries.

However, even in developed countries, there are frequently areas that are beyond the reach of terrestrial cellular networks. When a cellular operator talks about 98% coverage, they're usually talking about population coverage and not geographical area! Satellite operators on the other hand are usually talking about geographical coverage.

How Are Satellites Connected to Networks

 The transmission of signals between satellites and the terrestrial networks is mostly handled by a number of Land Earth Stations (LES). Land Earth Stations use large dish-antennas of (sometimes) up to 32 metres in diameter. The use of these large antennas reduces the required transmission power of the satellite to earth link, so more power can be used on the link from the satellite to the mobile terminals.



Separator

Back to Home Page

Separator