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MARITIME SURVIVOR LOCATING DEVICES (MSLD)

In the event that a person finds themself in the water, having gone overboard, washed off deck or forced to abandon ship, there exists a number of locating and alerting devices which aid notification of the emergency and locating of the person in distress.

Each device has a range of features which utilise differing systems in their operation.

Emergency Position Indicating Radio Beacon (EPIRB)

EPIRBs were originally designed to operate on the aeronautical frequencies of 121.5 and 243 MHz only and were used by downed pilots to summon help from SAR aircraft. Tracking (homing) was carried out by suitably equipped aircraft.

With the advent of the COSPAS-SARSAT satellite EPIRB detection system in the 1980s, EPIRB use spread to the maritime and land mobile sectors, and as technology has developed, so too has the size and portability of such devices, culminating in a range of personal EPIRBs being available.

Modern EPIRBs now operate simultaneously on the 121.5/243 MHz aviation channels and also the 406 MHz COSPAS-SARSAT channel, hence the term 406 EPIRB. The EPIRB system is designed to alert shore authorities, often over considerable distances, not ship-to-ship alerting. This involves an inherent time delay.

The COSPAS-SARSAT system uses a series of satellites to provide world-wide detection accuracy of around 3 nautical miles for older beacons, and around 100 m for new, GPS-enabled EPIRBs. Final location of the beacon is undertaken by homing on the 121.5 or the 243 MHz channels.

In addition to the 406 MHz channel, the COSPAS-SARSAT system currently operates on the 121.5 and 243 MHz channels. However, from 1 February 2009, the system will cease to monitor 121.5 MHz, because of the high number of false alerts and interference. This will relegate 121.5 MHz to short range homing only.

The EPIRB system has saved many thousands of lives over the years, however its principal shortcoming is that it relies on a remote shore infrastructure to process and then re-broadcast the alert message to ships in the vicinity of the person in distress. Depending on the location of the EPIRB, this could involve delays in the order of hours.

Personal Locator Beacon (PLB)

A PLB is a miniaturised 406 EPIRB, designed to be attached to the wearer's clothing or lifejacket. PLBs are used by hikers, pilots and seafarers.

VHF Position Indicating Radio Beacon (VPIRB)

The acronym VPIRB is a combination of the terms VHF (radio) and EPIRB. A VPIRB is essentially an EPIRB operating in the marine VHF radio band rather than the COSPAS-SARSAT 406 MHz channel, utilising DSC technology to broadcast a distress signal to all VHF DSC radio-equipped vessels in the vicinity, including the parent vessel.

A VPIRB comprises a small DSC beacon, which is attached to the user's clothes or lifejacket. If the user falls overboard, the beacon activates and sends a distress message via VHF DSC. Reception of the distress message from the beacon will sound an alarm on all VHF DSC radios within range.

If fitted with a GPS receiver, a VPIRB can also automatically transmit its position as part of the distress alert message, which will also be received on all VHF DSC radios in range.

This is very useful if recovery could be delayed by sea conditions and/or weather.

For a VPIRB to be effective, the parent vessel must be be fitted with a VHF DSC radio. VHF DSC equipment was originally designed for the commercial, merchant ship market and was expensive, and complex to operate. However, manufacturers are now also producing VHF DSC equipment aimed specifically at the recreational marine market, offering simple to use equipment at a more affordable price (typically under US$500).

All merchant ships are fitted with VHF DSC systems, and the low price of recreational craft systems is driving penetration into that market. If the parent vessel is operating in an area covered by VHF DSC Coast Radio Stations, the beacon distress message may also be received by shore authorities.

However, there are still many recreational craft not fitted with VHF DSC, so for this reason, the VPIRB also provides alerts to those vessels fitted with only a basic radiotelephone marine VHF radio by means of a synthesised voice MAYDAY message transmitted on VHF channel 16.

The VHF DSC and VHF voice messages are repeated at 5 minute intervals for the first 30 minutes, updating the GPS position of the person in the water with each transmission, then the repetition period will change to ten minutes. This will continue until the device is switched off, or the batteries expire.

At present there is only one VPIRB model available, the Mobilarm V100 VPIRB.

MAN OVERBOARD SYSTEMS

In most man overboard (MOB) situations, the crew aboard the vessel from which the MOB came, the parent vessel, are in the best position to render immediate assistance to the person in the water. This is particularly important in high latitudes, where hypothermia can render a person in the water unconscious within minutes.

Man overboard systems are intended as a short range beacon, specifically designed to alert the parent vessel that they have lost one of their crew overboard. Alerting of other ships and/or shore authorities is considered a bonus.

There are different systems currently in use for man overboard systems:

Proximity/polling type system

Proximity/polling type man overboard systems, such as the Mobilarm man overboard system, require each person onboard the vessel to wear a small pendant that continuously transmits 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 - radio waves cannot transmit through water - 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.

Proximity/polling man overboard systems are very successful and popular; however they are a closed loop arrangement, which alert only the parent vessel.

121.5 MHz homing systems

121.5 MHz homing systems use small beacons operating on the aviation EPIRB frequency of 121.5 MHz, in conjunction with a receiver/direction finding (DF) system on the vessel.

If a person falls overboard, the beacon is manually activated or may automatically activate to transmit on 121.5 MHz, which causes the boat's receiver to alarm. The DF system is then used to home in on the person in the water. Ships do not normally carry 121.5MHz receivers, so a dedicated receiving/direction finding base station must be installed on the vessel for this man overboard system to work.

Furthermore, 121.5 MHz homing systems are not intended to be used with the COSPAS-SARSAT system because of the beacons' low power output, and moreover, COSPAS-SARSAT will cease to monitor 121.5 MHz at the end of 2008.

This effectively makes 121.5 homing-type MOB systems closed loop arrangements.