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.

By Ian Poole


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