Maritime Survivor Locating Devices (MSLD)
Different systems exist that do not use satellites to transmit an emergency signal. These are known as Maritime Survivor Locating Devices (MSLD) and are a good term to describe most man overboard systems.
It is widely acknowledged that the probability of survival in a man overboard incident is directly related to the length of time in the water before recovery, particularly in cold waters (such as the North Sea) where hypothermia can render a person in the water unconscious within minutes. In most man overboard situations, the vessel from which the person has fallen is closest to the incident and is therefore in the best position to rescue a MOB casualty.
Generally, a man overboard system or MSLD is a short range signalling beacon, which is specifically designed to alert the vessel that a crewman has gone overboard. If the beacon also alerts other vessels and/or SAR authorities, then this is considered a bonus.
Proximity/polling type system
Mobilarm man overboard systems are proximity/polling type MSLD. Each person on board the vessel wears a small transmitter that continuously sends a signal via a closed wireless network to a central console on the boat.
If a person falls into the water, the signal is broken, causing the system to raise an alarm. The GPS coordinates of the man overboard event are also recorded to a chart plotter to provide track back information, enabling the boat to recover the casualty.
This closed loop arrangement is a successful and popular system that only alerts the vessel that the MOB fell from.
121.5 MHz homing systems
Another MSLD variant uses small beacons operating on the aviation EPIRB frequency of 121.5 MHz, in conjunction with a receiver/direction finding (DF) system carried onboard the vessel.
If a person falls overboard, they must manually activate the beacon (although some models may automatically activate) to transmit on 121.5 MHz, which causes the vessel's onboard receiver to alarm. The DF system is then used to home on the person in the water.
121.5 MHz homing type systems must be used in conjunction with a dedicated onboard receiver/DF, as vessels do not normally carry 121.5 MHz receivers.
The COSPAS-SARSAT satellite system will cease to monitor 121.5MHz by 1 February 2009 which effectively makes 121.5 homing MOB systems closed loop arrangements.