These issues apply mainly to GMDSS Sea Area A3 Coast Radio Stations.

 Radiotelephone services

A GMDSS A3 station designed strictly in accordance with GMDSS requirements needs only 2 transmitters – one for DSC and one for Radiotelephone (R/T).  No R/T watchkeeping is required, and no scheduled R/T Maritime Safety Information (MSI – weather and navigation warning) broadcasts are needed – broadcasts for A3 Sea Areas are sent by the Inmarsat EGC system. 

However, the reality is that Coast Radio Stations are required to provide services for all types of vessel – large and small, SOLAS and non-SOLAS.

Despite the best efforts of the IMO, small vessels have not really embraced the GMDSS.  Market penetration of MF/HF DSC into the recreational market remains very low.  The majority of small craft venturing offshore use basic R/T radios on MF/HF. 

So, if Coast Radio Stations are to provide services for small craft, it stands to reason that watchkeeping must be maintained on the GMDSS R/T distress and safety channels. 

There are distress and safety channels in each of the marine radio bands from 4-16 MHz (and of course, on 2 MHz). 

Given the size of most A3 Sea Areas, frequency diversity is needed to provide complete coverage.  Therefore watch needs to be kept on all of the distress and safety channels during daylight hours.  At night, the 16 and the 12 MHz watches can be dispensed with. 

The recommended watchkeeping configuration is: 

Day:                4125, 6215, 8291, 12290 and 16420 kHz

Night:             4125, 6215 and 8291 kHz

Number of transmitters

Small vessels do not normally carry an Inmarsat C system to receive MSI.  The only method they have to receive the broadcasts is R/T.

Accordingly, voice MSI broadcasts need to be conducted at regularly scheduled times.   

If R/T watchkeeping is being provided from 4-16 MHz, it is only logical that MSI broadcasts for small ships also use frequency diversity. 

Simultaneous transmissions need to be made on working channels in the 4, 6, 8 and 12 MHz bands during daylight hours, and the 4, 6 and 8 MHz bands in the evening.

This requires the use of 4 transmitters.

So, in summary, an A3 GMDSS station needs a minimum of 5 transmitters – 1 dedicated to DSC, and four for R/T.

A simplified transmitter configuration drawing is shown below.  The audio and keying control system allows selection of any or all transmitters for MSI broadcasts.

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