|The Inmarsat Services: [A], [B], [C], [E], [Mini-M]|
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|The Inmarsat satellite system is concerned with two kinds
of communication as far as maritime stations are concerned - the
transmission of emergency traffic, such as distress, urgency and safety
messages, to and from vessels at sea, and for routine communications,
also in both directions.
The Inmarsat system of global satellite communication was proposed by the International Maritime Organisation as a way of overcoming congestion and atmospheric interference. The developing satellite technology of the 1970s made it become a reality and the system became commercially operational in 1982.
When Immarsat first started operations it leased capacity on existing satellites. Its initial operations were exclusively concerned with maritime stations, but the system soon expanded as aeronautical and land users realised the benefits of the system and Inmarsat began to launch its own satellites. At the present time, maritime stations still generate about two thirds of the system's traffic.
The world according to Inmarsat is divided into four regions:
Atlantic Ocean Region East
Indian Ocean Region
Atlantic Ocean Region West
Pacific Ocean Region
There is a satellite, and usually at least one spare, in geostationary orbit at a height of about 36,000 km over each region. The present orbital locations of the satellites are:
Each satellite can 'see' about one third of the Earth, so there is some overlap between adjacent regions. From a Mobile Earth Station, in our case the vessel wishing to communicate, the satellite being used must be at an elevation of at least 5 degrees above the horizon for there to be reliable communications. Since the four satellites are in orbit over the equator, this means that the very high latitudes of the polar regions cannot be covered. The area of coverage is effectively from about 70º N to about 70º S.
On the ground there are 34 Land Earth Stations, which provide the link between the satellites and the terrestrial telecommunications network. Not all of the LESs can provide all the services so a suitable LES, which is within the footprint of the satellite being worked, must be chosen. The LESs are sometimes called Coast Earth Stations and they are the satellite equivalent of the HF or VHF Coast Stations. However, since there is no necessity for a satellite earth station to be on the coast, and since one third of the mobile traffic generated on the satellite system is from non-maritime mobile stations, Land Earth Station is probably the better term to use.
Each LES is assigned a two-digit code number. If that LES can access more than one satellite, then the same code number is used through each satellite that the LES can access. Some LESs in Europe can actually access three satellites, the only one beyond their reach is the Pacific Ocean Region. For any satellite that the LES cannot access, that particular code number could well be allocated to another LES on that satellite.
For example, 01 through AOR-W, or through AOR-E is Southbury LES in the eastern USA, but 01 through the POR is Santa Paula on the west coast of the USA. 02 via AOR-W is Goonhilly in the UK, but on the POR or IOR satellite it is Perth in Australia. So care must be taken that the antenna is pointing at the correct satellite, or in the case of Inmarsat-C, that you are logged onto the correct satellite before the code for the LES is entered.
In each Ocean Region there is a Network Co-ordination Station. Each NCS continuously monitors the flow of traffic through its satellite and ensures that calls are set up correctly. The NCS is permanently connected to all the LESs that are working through that particular satellite, and it monitors them all to ensure that they are functioning correctly. The NCS allocates the channel to be used by the mobile station and by the LES for each and every call. All the NCSs are in turn monitored and controlled by the Network Control Centre. The NCC is at the Inmarsat Headquarters in London and it is permanently connected to all the NCSs.
Finally, there is the Satellite Control Centre, also located in London, which is responsible for looking after the satellites themselves. The SCC is linked to a series of tracking stations around the world and it monitors the orbit of each satellite, adjusting it as necessary to keep the satellite exactly where it is supposed to be.
In 1990, Inmarsat launched the first of its own second-generation satellites and by April 1992, each region had a Series 2 satellite. These satellites were built by a consortium headed by the Space and Communications Division of British Aerospace. The Series 2 satellites were designed with an expected ten-year life. Each weighed some 1,300 kilos at launch and has 1,200 watts of available power - more than twice the power of the original satellites.
These satellites in turn are being replaced by Series 3 satellites - the first of which was launched in 1996, and by now, the fourth and last should be in position. These are a lot more powerful again - eight times the power of the Series 2, and some twenty times as powerful as the originals. In addition to the global beam, each of these new satellites has five spot-beams. These are focused onto specific areas of the earth, just as a searchlight might be. This not only focuses the energy into smaller areas, so allowing operation at much lower energy levels, but it also allows the same frequency to be used at the same time in different areas but through the same satellite. This increases the scope for greater numbers of calls to be carried at any given moment. This spot-beam technology, with its lower energy requirement, has opened the door to the possibility of a global portable phone system.
The newer satellites can also support a special type of EPIRB called the Inmarsat-E type. With the EPIRB service there are now six Inmarsat services which may be of interest to the mariner. Each service has its own merits and disadvantages. For compulsory GMDSS vessels, not all of the systems can be GMDSS certified. Let's look at each system in turn.
This is the old original Analogue system, which began operation in 1982. Although A is now classed as rather old technology, new stations are still joining the system, and there are now some 22,000 mobile stations of various sorts operating Inmarsat A.
Inmarsat-A can support two-way telephone, telex, fax, E-mail and with the High Speed Data option, it can support data transmission up to 64 kilobits per second. In addition to supporting simultaneous two-way data transmission, the HSD option can be used for the transmission of still pictures and compressed video pictures or for video conference calls. Inmarsat-A can support Enhanced Group Calling - the reception of Maritime Safety Information which can be sent to 'All Stations', or to a group of vessels, or to vessels in a specific region. The ability to receive EGC is a pre-requisite for GMDSS certification. Some Inmarsat-A terminals will require the fitting of an external EGC receiver for reception of these MSIs.
The mobile station communicates with the satellite on frequencies in the L-band - 1.5 to 1.6 GHz (remember, that is 1,600 MHz, or 1,600,000 kHz). The satellite in turn communicates with the LES on frequencies in the C- band - 4 to 6 GHz.
There are two principle drawbacks to the Inmarsat-A system - it is the most expensive of all of them in terms of call charges, and because it uses high bandwidth and relatively low power, it needs a large parabolic antenna. On board a vessel at sea, this antenna must be kept constantly pointed at the satellite regardless of how the vessel moves. This requires gyroscopes, motors and considerable power. A typical marine Inmarsat-A antenna is housed in a dome up to one and a half metres in diameter, and weighs around a hundred kilos.
Inmarsat-B is the newer digital version of the analogue Inmarsat-A. (A for Analogue and B for Binary!) The digital technology makes better use of the bandwidth and the satellite power which results in lower call charges, although the initial equipment cost is higher than for A.
It is possible for larger vessels to be fitted with an extra large, high-gain antenna. This can operate with much lower satellite power and so call charges using this system are about half the normal B charges. However, even the normal B antenna is large, virtually identical to the dome of an A system, so the extra large antenna can really only be fitted to big commercial ships where space and weight are not a consideration. Inmarsat-B supports direct-dial telephone, fax and telex calls, and with the High Speed Data link, simultaneous two-way high speed data can be transferred, including pictures and video.
It is the intention for Inmarsat-B to supersede A, but it is likely that both systems will coexist for a number of years. At the present time B does not support Enhanced Group Calling for the reception of Maritime Safety Information, and so it cannot be GMDSS approved for compulsory vessels. A compulsory GMDSS vessel equipped with Inmarsat-B could either fit a stand-alone EGC receiver or an Inmarsat-C system for reception of the MSIs.
Inmarsat-C supports only data transmission, not voice. It does allow Enhanced Group Calling, and it is the system of choice for compliance with the GMDSS. Data is transferred at a rate of 600 bits/second and call harges are based on the number of bits of data transferred. Inmarsat-C terminals are suitable for almost any size of vessel. The power requirement is minimal, the unit itself is normally a small 'black box' which is interfaced with a computer, and best of all, the antenna is a very small fixed antenna - there are no big and power hungry gyros, just a little, plastic, fixed antenna, similar to that used by GPS navigation receivers.
There are several different services which Inmarsat-C offers:
Two way messaging
The normal method of communication on Inmarsat-C is by store-and-forward telex. Messages up to 32 kilobytes - probably about 6,000 words, can be sent in either direction. Messages from the mobile station are transmitted via the satellite to an appropriate Land Earth Station. The data is sent in packets to the LES where it is reassembled into the complete message. The message is then sent to the addressee by the national or international telecommunications network. It can be delivered to a fax, telex or E-mail address, or failing any of these, the message can be printed and be sent by mail. Messages sent in the reverse direction can be sent to an individual vessel, or simultaneously to a group of vessels.
Polling and data reporting
Polling allows a shore station to interrogate a mobile station at any time. Such polling will trigger the automatic transmission of the required pre-programmed information. Such information could be the position of the vessel, its course and speed, or the read-out of any on-board sensors. Data reporting allows for the automatic transmission of short packets of information at axed pre-arranged intervals.
An Inmarsat-C terminal can easily be interfaced with a wide variety of navigation systems such as Decca, Loran or GPS. The derived position can be transmitted automatically at fixed intervals or on demand.
Marine Inmarsat-C terminals are equipped with a special facility, which when activated, automatically generates and transmits a priority distress message to the Rescue Co-ordination Centre. If the terminal is linked to a navigational system, the message will automatically include the position of the vessel.
Enhanced Group Calling
This is the feature of Inmarsat-C which makes it so valuable to the GMDSS. Using EGG, messages can be sent to groups of vessels, via SafetyNET or FleetNET.
SafetyNET provides an efficient and low cost method of sending Maritime Safety Information to vessels at sea. The messages can be restricted to individual vessels, or to vessels in a particular geographic area. This service is used by meteorological and hydrographic authorities for issuing forecasts or warnings, as well as by Coast Guard and Search and Rescue centres for disseminating MSIs, and for co-ordinating rescue in a distress situation.
FleetNET is a commercial service which allows the sending of messages to a virtually unlimited number of terminals simultaneously. This could be all the vessels in a particular race, or perhaps all the vessels belonging to one company. It could even be everybody who subscribes to a particular service, such as stock exchange reports or specialised weather analyses.
E is the Emergency system within Inmarsat. There are three LESs which are equipped to receive Inmarsat-E signals, from the four Ocean Regions.
These stations are at:
Niles Canyon, USA - Atlantic Ocean Region - West
Perth, Australia - Indian Ocean Region
Raisting, Germany - Atlantic Ocean Region - East
Niles Canyon and Perth share responsibility for Pacific Ocean Region.
When an Inmarsat-E EPIRB transmits a distress message to one of the satellites the message is instantly relayed to the LES, from where it is sent to the appropriate Rescue Co-ordination Centre who can then organise and co-ordinate the rescue.
The biggest advantage of this type of EPIRB is that since the satellites are geostationary they are always in view, so there is no waiting for a satellite to appear over the horizon. This means that distress messages are relayed almost instantaneously. The disadvantage is that since the satellites are geostationary, they cannot determine where the casualty is. The EPIRB itself must be combined with a GPS navigation receiver so that the position can be included as part of the message. This contributes not only to the bulk of the equipment but also to the cost.
The 'M' is for Mobile, not marine. By far the greatest number of M units are land mobile units. M does not support Enhanced Group Calling, so it is not approved for compulsory equipment for GMDSS compliance. However, marine M units usually incorporate a distress-alerting button and so are suitable for use at sea on non-compulsory GMDSS vessels, although MSIs will not be received.
As Inmarsat-M is a digital system, it uses minimal bandwidth and power, resulting in lower call charges than A. M offers good quality two-way voice communications, slow speed facsimile, and slow speed data (2.4 kbit/sec) transmission and reception. The equipment costs and call charges are higher than C, but M does allow voice traffic. At sea, a steerable parabolic antenna is required. This is similar in operation to the antenna used for A and B, but it is very much smaller, lighter and cheaper.
|Inmarsat Phone Mini-M|
Phone Mini-M is the result of the new spot-beam technology. Each of the series 3 satellites has five spot-beams, focused on areas of greatest use. These beams focus not only the energy being transmitted from the satellite, but also allow the very weak signals from the mobile stations to be received. This allows the use of truly portable telephones within the satellite system. The mini-ill phones are not unlike the ubiquitous cell-phones, but unlike the cell-phones, they offer close to global communications. The current units are about the size and weight of a laptop computer and use a flat-plate antenna.
The Mini-M can support two-way voice, fax and data transmission. The principal advantage is the extreme portability of the units. The disadvantage, as far as we are concerned, is that it does not support Enhanced Group Calling, and so cannot be GMDSS approved and since the beams are focused on areas of high traffic, considerable areas of the world's oceans are outside the beams. In these areas, the mini-M system cannot be used at all.
Also, the flat-plate antenna has to be pointed in the general direction of the satellite and on board a small vessel in a big sea, this is not always easy to achieve.
|INMARSAT MINI-M SERVICE PROVIDERS|