INMARSAT Systems
One of the major problems with communications at sea was the problem of atmospheric interference, static crashes, fading of signals, unreliable communications on certain frequencies, that would interfere with emergency communications. The IMO (International Maritime Organization) realizing this as a major problem looked to the major global satellite communications system of the INMARSA T Organization to the answer.
INMARSAT acronym for INternational MARitime SATellite, an organization of member countries established in 1979. When INMARSAT first started its operation it leased capacity on existing satellites. In the initial operations were just concerned with the maritime station. Soon they expanded in the aeronautical and land users. The system soon grew and INMARSAT then launched their own satellites. Maritime now uses about half of the operation of INMARSAT.
INMARSAT divided the world into four regions: Atlantic Ocean Region East; Atlantic Ocean Region West; Indian Ocean Region and Pacific Ocean Region. There are satellites for each region and some spares to assist each region. The satellites are in geostationary orbit at about 22,000 miles in the sky. The chart shows the location and some of the spares for each ocean region. The satellite has a footprint of about one third of the earth with some overlap. Each satellite provides coverage based on a maximum line of sight distance of 72 degrees from it's Geographical Position (GP).
This means that the polar regions are not covered by the INMARSAT system
There are on the surface of the earth about 34 Land Earth Station
(LES) which provide the link between the ship and the terrestrial telecommunications networks. We sometimes call these station Coastal Earth Station (CES). In the United States commercial stations have the intertie to land line communications. In the U.S.A COMSAT Mobile· Communications is one provider while lOB Mobile Communications, lately purchased by a Canadian firm call
STRATOS Communications and they operate the LESs.
The next satellite is the INMARSAT-B and is a digital version of the INMARSAT A. The use of digital technology uses less bandwidth and power and thus lowers cost of communications. The antennas are some what smaller than the "A" and so installation costs are lower.
INMARSAT-B supports direct-dial telephone, fax, telex and High Speed Data. It is assumed that INMARSAT B will replace the A. INMARSAT B does not support EGC for the reception of MSI. If your ship has only the "A" then there can be a EGC receiver as a stand alone unit or you could use the INMARSAT-C to receive MSI. The "B" has a single button to alert the shore station of a distress alert, but must input the position data.
INMARSAT-C is a digital only system and does not support voice and is a "store and forward" type of system, Le., it is a "one-way" type of transmission. The ship will transmit a message and then the shore station will transmit the response of the sending ship station. TELEX type messages can be sent but not in "real-time" mode. Faxes can be sent by the ship to the LES and they in turn will retransmit it on the land line system.
An advantage of the "C" is the small size and small omni directional antenna. No power hungry gyros to move the antenna following the satellite, a power saving. The antenna is a small diameter device and is fixed on the mast. The latest "C" has a built in GPS and can include the position when sending out an distress alert call.
In the above the "Distress" button will generate a signal to the Rescue Coordination Center. And to the rescuers.
Another satellite system is the INMARSAT-M system. The use of this system is mainly by Mobile units using digital telephone system as well as slow speed fax. This system is one that uses little bandwidth and low power and costs are lower than A.
