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Knowledge Is Key
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With the evident success of mobile phone systems it was only a
matter of time until the development of satellite phone systems. The
need for this crucial communication tool emerged from the need for
maximum coverage in areas of the world that were not at that time
accessible to terrestrial mobile phone systems. As a result satellite
based phone systems were conceived and have been set up. The first
three systems were Iridium, Globalstar, Inmarsat and Iridium.
Satellites for these services were launched in the mid to late 1990s.
Although the satellite phone systems have been proven technically,
satellite phone technology has not taken off as originally conceived.
The take up of mobile phone systems was more rapid than originally
expected and their coverage is greater. Nevertheless satellite phone
systems are in use and provide essential communications in several
applications.
When devising a satellite phone system there are a number of
technical challenges that need to be addressed. The path length
between the earth and the satellite introduces significant losses, much
greater than those encountered with terrestrial systems. It is for this
reason that most of the systems use low Earth orbiting satellite
systems. Geostationary satellites are usually considered too high and
result in much greater levels of path loss. Additionally the fact that
the satellites are moving (in most systems) means that signals are
Doppler shifted, and the technology needs to take account of this
fact. With the satellites in a low Earth orbit and moving across the
sky each satellite will be in view for a certain amount of time. It is
therefore necessary even for a stationary phone to be able to
handover from one satellite to another.
Phones used for satellites are often larger in size than those used for
terrestrial applications. The antenna is often larger ensure the
required level of efficiency. A further challenge for satellite phones
arises from the backhaul communications and protocol exchanges.
Any mobile phone requires to quickly communicate with the network
to enable calls to be set up, controlled and finished. In view of the
altitudes of the satellites the round trip delay from the mobile to the
satellite and back to the earth station are too long to enable rapid
communications and exchanges to take place. As a result, much of
the intelligence of the system has to be placed within the satellite so
that the required protocol exchanges can take place rapidly.
The four major systems - Iridium, Globalstar, Inmarsat and Thuraya,
adopt different approaches in many areas of the technology that is
being used. The Iridium satellite phone system uses a total of 66
satellites orbiting in a low Earth orbit with an altitude of around 485
miles. This gives an orbiting time of around 100 minutes. Further
satellites are placed in orbit to enable them to quickly replace any
possible failure. The satellites are in polar orbits, i.e. orbiting from
pole to pole, in what is termed as a 'Walker Star' configuration. For
most instances the satellites in the adjacent orbit are orbiting in the
same direction, and the satellites in the next orbit are orbiting in the
opposite direction. This causes some problems in terms of handover
and communication between these satellites.
The satellites are able to communicate with both the ground stations
as well as with neighbouring satellites, using inter-satellite links. This
communication abilities support the handovers as satellites pass over
and out of range. No handovers are made across a seam because of
the fact that satellites are contra-rotating, meaning that Doppler shifts
are too large and handovers would need to be made too quickly.
The Iridium system uses four Earth stations. To provide the additional
required paths, space-based backhaul routes are used to send the
phone call packets through space to one of the downlinks known as
feeder links. These downlinks are required because onlly a few
satellites are within view of an Earth station at any given time. These
downlinks are primarily used for calls that need to be routed into the
terrestrial phone system. For satellite phone to satellite phone calls,
it is possible to route the traffic directly through space with no downlink.
Although the satellite to satellite routing and the associated downlinks
add a considerable degree of complexity to the Iridium system, it does
enable fewer earth stations to be required, and it also enables full
global coverage to be provided.
The Globalstar satellite phone system adopts a slightly different
approach to that used by Iridium, considerably reducing complexity
and hence the setup and running costs. The system uses a total of 48
satellites in orbits having an inclination of 52 degrees and 1414
kilometres or 878 miles above the Earth's surface. With this orbit it
takes just less than two hours for an orbit. Connectivity is provided
only by the network of ground stations. Unlike Iridium, Globalstar does
not support inter-satellite linking. This also means that coverage is not
worldwide as there are areas where there are no ground stations.
These areas are generally over the oceans and in remote areas of the
globe where it is not possible or practicable to set up ground stations.
The air interface used for Globastar uses CDMA to provide access for
multiple users. Additionally handsets use elements of GSM cell phone
system to provide greater functionality than the functionality offered
by cdmaOne / CDMA2000. One of the key elements is the use of SIM
cards. Numbers for Globastar phones use the North American '1'
prefix for the country code.
The Thuraya satellite system is different from the other systems,
since it's only regional. Its operations are based in the United Arab
Emirates and it provides coverage for areas within Africa, Europe the
Middle East. To achieve its tasks this system is utilising a single
geostationary satellite and it provides service through a network of
various providers. Thuraya handsets cannot be used with other
networks. They feature dual mode operation, operating not only via
a satellite, but they can also connect to GSM 900 networks.
Satellite phone sales are pretty high despite the fact that they have
not developed in the way they were anticipated to be. Nevertheless,
they are expected to grow exponentially over the coming years.
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