Satellite phones
- overview, summary, tutorial about the basics of satellite phones and
satellite phone systems including Iridium, Globalstar and Thuraya.
With the success of mobile phone systems it was believed that
satellite phone systems and satellite technology would be able to provide phone
access 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 three satellite systems were Iridium,
Globalstar and Iridium. Satellites for the three systems were launched and they
entered service 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 phones and
satellite phone systems are in use and provide essential communications in
several applications.
Satellite phone basics
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.
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 to provide
to ensure the required level of efficiency. This naturally impacts on the size
of the satellite phone.
A further challenge for satellite phones arises from what are
termed 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.
Satellite phone systems
Three of the major satellite phone systems that are in use
are Iridium, Globalstar and Thuraya. These satellite systems adopt different
approaches in many areas of the technology used.
Iridium The Iridium satellite 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 that fail. The satellites
are in polar orbits, i.e. orbiting from pole to pole, in what is termed a
'Walker Star' configuration. For most instances the satellites in the adjacent
orbit are orbiting in the same direction, there is what is termed one 'seam'
where the satellites in the next orbit are orbiting in the opposite direction.
This gives some problems in terms of handover and communication between these
satellites.
The satellites are able to communicate with the ground as
well as neighbouring satellites when they use inter-satellite links. This aids
the handovers as satellites pass over and out of range. No handovers are made
across a seam as the fact that satellites are contra-rotating would mean 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 paths thata re needed, space-based backhaul routes are used to send
the phone call packets through space to one of the downlinks known as feeder
links. These down links 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.
Globalstar The Globalstar satellite phone system
adopts a slightly different approach to that used by Iridium and this
considerably reduces the complexity and hence the set up 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, the Globalstar satellite phone system 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 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.
Thuraya The Thuraya satellite phone system is again
different to the other two systems. It is only regional, with operations based
in United Arab Emirates and it provides coverage for areas within Africa, Europe
the Middle East. To achieve this it utilises a single geostationary satellite
and it provides service through a network of service providers.
Thuraya handsets are cannot be used with other networks and
are therefore specialised. They feature dual mode operation, operating not only
via a satellite, but they can also connect to GSM 900 networks.
Summary
Satellite phones and satellite phone systems are used in a
number of applications and the satellite phone sales are still strong despite
the fact that they have not developed in the way that was originally
anticipated. With cellular technology becoming more widespread, and this
occurring more quickly than originally anticipated, the mass market that was
envisaged for satellite phones did not emerge. In many cases the satellite phone
systems and their owners have experienced financial difficulties, but today
these satellite phone systems fulfil more specialised roles and as a result of
this the sales are still strong.
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