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 and other distress beacons. 
Click here to download a white paper examining MSLD and other types of Personal Locator Beacons
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 (MOB) situations, the vessel from which the person has fallen (the parent vessel) is closest to the incident and is therefore in the best position to rescue the 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.
There are several types of MSLDs available today, each using different technology. Watch this short animation which explains the key differences between MSLD and other types of Personal Locator Beacons.
The key points for each type of MSLD i.e. beacons which are NOT monitored by satellite are listed below; -
VHF DSC- Automatically transmits a man overboard VHF DSC distress alert to the parent vessel and other vessels or aircraft in range - Transmissions include GPS position and unique MMSI identity - Transmissions sent via Digital Selective Calling (DSC) and in a digitized voice on channel 16 (depending on regulatory restrictions regarding transmissions in some parts of the world) - Regular transmissions update the MOB casualty's GPS postion to enable in-water tracking - Water-activated or manually operated - Standard VHF marine radios will receive distress alerts when in range (VHF line-of-sight) |
121.5 MHz- Self-managed alert and rescue system comprising personal alerting unit and dedicated receiver on board the vessel - Rescue assets can only track an MOB if direction finding (DF) equipment is installed - Alerting unit transmits a continuous tone on 121.5 MHz, which causes the parent vessel's onboard receiver to alarm. A DF system is then used to home in on the person in the water - The COSPAS-SARSAT satellite system ceased to monitor 121.5MHz on 1 February 2009, but SAR authorities carry DF equipment and can track the signal if the parent vessel requests their help |
Proximity/polling- Crew on board the vessel wear a small transceiver that continuously sends a signal via a wireless sensor network of routers to a central control unit - Falling overboard causes this signal to disconnect from the network, resulting in an alarm on board - If the system is wired to the ship's chart plotter, the system will log a waypoint at the place where the signal was lost to mark the man overboard fall point, enabling ship's crew to track-back and locate the casualty - This closed-loop system only alerts the vessel that the MOB fell from |
Responding to man overboard VHF DSC distress alerts - advice for radio operators
Port authorities, offshore oil and gas companies and commercial marine operators are using VHF DSC Maritime Survivor Locating Devices such as the Crewsafe V100 for fast location and self-rescue in a man overboard event.
This personal radio distress beacon operates differently to Personal Locator Beacons which transmit on 406 MHz to land-based Rescue Coordination Centres.
This document answers questions about the device and recommends the action to take if you receive a man overboard VHF DSC distress alert.