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 were launched and 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.
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 sizethan 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.
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 neighboring 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 that are 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 only 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.
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 fulfill more specialised roles and as a result of this the sales are still strong.
For more information about Satellite Commuications Contact AA Radio