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INMARSAT Communications

Standard satellite communications is conducted via INMARSAT, the International Maritime Satellite System based in London, England. The maritime satellite system has three major components: the space segment provided by INMARSAT, the Coast Earth Stations (CES) provided by INMARSAT signatories and SES (Ship earth station).

INMARSAT establishes equipment standards and operates the satellite (space) segment. Private companies operate the land (earth) segments that connect with local and international telecommunications networks. There are several INMARSAT systems. Each offers a range of services and capabilities.

INMARSAT-A is the original analog system offering telephone and telex services. INMARSAT-A will eventually be replaced by INMARSAT-B which is a digital system having more channels and a higher data transfer rate. INMARSAT-C is a digital telex system with EGC/SafetyNet capability, and INMARSAT-M is a compact digital system with voice and data capability but does not have telex. INMARSAT A, B and M are real-time systems (i.e. on-line direct contact). INMARSAT-C is a store and forward system (i.e. sent to a CES and then forwarded). The advantage INMARSAT-C is that it has EGC capability for reception of SafetyNet/MSI messages. Being able to receive SafetyNet satisfies requirements for receiving MSI in sea areas outside coastal NAVTEX coverage. NOTE: HF NBDP telex is used outside INMARSAT coverage.

The INMARSAT space segment consists of four satellites in geostationary orbit over the equator. Each satellite provides an area of coverage roughly from 70° N to 70° S and roughly 70° of longitude to either side of its meridian. Although there is overlap in coverage, particularly in the Atlantic, there is one satellite for each of the ocean regions. Vessels using INMARS A, В or M when first turning their SES on or when changing ocean regions may have to align their antenna to the satellite. This is usually done by specifying the satellite and entering an azimuth and elevation angle to the satellite. Some newer SATCOM use the nav-input to align the antenna automatically. Vessels using INMARSAT-C simply specify the satellite and lock-on is automatic (SAT-C antenna is omni directional). In either case, if you are not able to lock-on, it may be that the antenna is being shaded by the ship's structure or other obstruction. You may have to wait, change course or change satellites. If you do temporarily change satellites, remember to return to the correct satellite for your region.

Calls from shore to ship or ship to ship are made by dialing the ocean area code and the ship's (SES) ID number. NOTE: Ships with more than one SES terminal or having both INMARSAT-A and С will have more than one ID number.

INMARSAT traffic is handled according to its priority. Obviously distress is the highest priority. Distress traffic (PRIORITY 3) is automatically routed to an RCC. Priority codes are as follows:

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