IN MARSAT is an organization composed of a number of countries (some 64 as of May 1991)
dedicated to improving maritime communications via the use of satellites. INMARSAT provides a satellite communications system which makes available to ships a full range of distress alerting and other communications capabilities including voice, telex, data and telefacsimile.

The COSPAS-SARSAT System is really a one way only communications system, from the EPIRB via the satellite to the rescuers.

The INMARSAT System allows two way communications to/from earth stations, ship stations, etc. via voice, telex, data transmission or fax. In other words the EPIRBs transmission and nothing else is passed via COSPAS- SARSAT. All other satellite traffic is via INMARSAT.

The 121.5 MHz units, designed for detection by over flying aircraft, have been in production for several years. Their transmitting frequency is that of the International Aeronautical Emergency Frequency. The 406 MHz units are more sophisticated because of the inclusion of identification coding capability. The COSPAS-SARSAT System provides two coverage modes, real-time and global. The 121.5 MHz operates only in real-time, therefore the short range homing capability. The 406 MHz operates in both modes. To quote IMO's GMDSS publication "once the satellite receives the 406 MHz beacon signals, the Doppler shift is measured and the beacon digital data recovered from the beacon signal. This information is time-tagged, formatted as digital data, and transferred to the repeater downlink for real-time transmission to any LUT [a LUT is a Local User Terminal] in view." With the 406 MHz unit the information is also stored for sending to other LUTS, thus the global aspect The 121.5 MHz units are only real-time, the satellite relays the signal directly to the ground. If both LUT and EPIRB are in view of the satellite, the signal can be received.

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How does the system (COSPAS-SARSAT, INMARSAT, RCCS and SAR units) know whose EPIRB it is? In other words the old blinker light query "what ship? where bound?" still prevails.

Each 406 MHz EPIRB transmits a digital message, coded into its memory, which provides distress information to SAR authorities for more rapid and efficient rescue than was previously possible. This information includes an MID (see below) and either a Ship Station Identifier, a Ship Radio Call Sign or a Serial Number, to specifically identify the ship in distress.

The use of EPIRBs is controlled by national administrations and several coding schemes or
protocols have been developed. These add flexibility to satisfy unique requirements for various applications. Coding schemes or protocols include:

	Maritime/Location
	Maritime User
	Radio Call Sign User
	Serialized User

Currently, national authorities such as Vanuatu specify which protocol or protocols can be used along with any optional or national use fields. If your vessel does not fly the Vanuatu flag, contact the appropriate authority to determine which protocol is to be used. Vanuatu is coding in accordance with the Maritime User Protocol which coding is consistent with that for Digital Selective Calling (DSC) used in an emergency.

First let's look at the decimal system:

There have been a number of proposals put forward on the subject of EPIRB codes. An inherently simple and logical system has been derived with the understanding that eventually all the other systems will be discontinued. Yet this system, no matter how well thought out was not without a few hitches.

Vanuatu has been engaged in quite extensive correspondence with INMARSAT and COSPAR-SARSAT Secretariats. Two main areas of concern were identified and one corrected. The first is that the format of INMARSAT Mobile Numbers (IMNs) did not always conform to the Maritime Mobile Service Identity (MMS*) as used for search and resuce purposes and DSC. INMARSAT has since issued instructions which correct this problem.

The other area of concern is the difference between the numbering systems of the INMARSAT A and C Stations. For technical reasons the coding of the INMARSAT A Station cannot conform to the MMSI.

Vanuatu has used, and will continue to use, unique Marine Mobile Service Identity (MMSI) recommended by the ITU for ship earth station identities for:

	Coding the float free satellite EPIRBs
	DSC number
	As the basis for numbering INMARSAT Standard C stations, ie. the INMARSAT Mobile Number (IMN).

The interested reader is referenced to the diagram below where it will be seen that the MMSI is formed from the country's Maritime Identification Digits (MID) and the Ship Station Identity (SSI). The three squares of the SSI represent digits (between 1 and 999) allocated by Vanuatu to identify each ship on the register.

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The IMN is formed by inserting a "4" in front of the MMSI and dropping the final zero to retain 9 digits. The penultimate remaining zero must not be retained and is, for Vanuatu, changed to a "1" or "2" to represent the station number.

The use of the MID and SSI in the Maritime User Protocol for EPIRB coding adopted by Vanuatu is also indicated. All EPIRB coding systems include the MID in the protected field. Therefore, on change of registry recoding by the manufacturer will be necessary. The alternative codings for 84-85 show the provisions which have been made to indicate the type of homing device fitted to enable a searching vessel to "pin-point" the position of the EPIRB. The Search and Rescue organization thus knows the ship's identity and the position of the EPIRB (within a few miles) and the type of equipment to use when locating the vessel.

This is an introduction to the intricacies of EPIRB and SATC. If you wish more technical detail, look for our coming Technical Bulletin.