New Man Overboard Beacon standard using DSC and AIS
The US based Radio Technical Commission for Maritime Services (RTCM) has published a new standard for Man Overboard (MoB) beacons using DSC and/or AIS.
People don't normally fall overboard on nice sunny days....
The standard requires MoB beacons using DSC and sending "open loop" messages (i.e. standard all ship distress calls) to be fitted with a GPS receiver and a DSC channel 70 receiver.
The GPS receiver automatically inserts a position in the DSC (and AIS) call, and the DSC receiver allows the beacon to receive DSC acknowledge messages from shore stations and other ships. If an acknowledgment message is received, an indicator light on the beacon turns on - so you know that people are looking for you.
When first activated, the beacon sends a "closed loop" DSC alert - this is a DSC distress relay call (with nature of distress "man overboard"), addressed to a particular MMSI (normally the parent vessel).
Of course, being a distress priority call, it rings the alarm on the receiving radio. As the call is addressed only to a specific station (or a group), other ships will not receive the call.
This is based on the rationale that the vessel you have just fallen off is in the best position to help you.
As soon as the GPS locks up and provides a fix, a second closed loop alert is sent with position.
If an acknowledgment message is not received after 5 minutes (i.e. you haven't been found by the vessel you fell off due to sea state/darkness/fog), the beacon will then switch to "open loop" mode, by transmitting a standard all ships DSC distress alert with nature of distress "man overboard" and GPS position automatically inserted.
This will, of course, alert all DSC equipped ships and shore stations in range (normally about 2 nm).
If an acknowledgement message is still not received, the beacon will switch to a duty cycle of an alert every 5 minutes for 30 minutes. If an acknowledgement message is not received after 30 mins, the duty cycle changes to a message every 10 minutes until either an acknowledgment message is received, or the batteries are exhausted.
The AIS transmitter will send the standard Message 1 "Position report" with the Navigational Status set to 14 and Message 14 "Safety related broadcast message" with the text "MOB ACTIVE".
This will display on a ship's AIS display as a cross/circle symbol (the same as for an AIS SART).
The ID number for a DSC and/or AIS MoB beacon is:
xx = manufacturer ID1 01 to 99; and
yyyy = the sequence number 0000 to 9999.
Beacons with DSC and AIS will use a common ID number for both transmitters.