subject: An Assessment Of Personal Locator Beaon Technology Use In The Marine Environment [print this page] ANALOGUE PERSONAL LOCATOR BEACONS ANALOGUE PERSONAL LOCATOR BEACONS
Background
Analogue distress beacons were first developed by the military in the 1960s as a way to locate pilots who had been forced to abandon their aircraft.
In the last 40 years, analogue distress beacons have evolved into several variants with the two most common being i) analogue EPIRB for vessels; and ii) analogue Personal Locator Beacons (analogue PLB) for individual seafarers.
The Cospas-Sarsat analogue EPIRB units transmit on two international distress frequencies and are based on the satellite compatible Maritime EPIRB specification.
In the mid 2000s, this analogue variant was superseded by the Digital 406 EPIRB which offers considerable advantages over the older type including the ability to uniquely identify individual units, provide GPS positioning and reduce the chance of false alarms.
As a result, 121.5/243 MHz processing by Cospas-Sarsat ceased on 1 February 2009 and according to the Australian Maritime Safety Authority (AMSA), it will be illegal to use an analogue 121.5 MHz distress beacon for any purpose after 1 February 2010 . However, Cospas-Sarsat 406 MHz beacons using this frequency as a secondary homing signal are unaffected.
The second variant, the analogue PLB, is designed for close proximity tracking and is a subset of the EPIRB types. It transmits on a single distress frequency (121.5 MHz) in the international VHF aircraft band.
The most common analogue PLB has limited functionality in that it transmits a very simple warble tone usually with no other information included. This tone can be picked up by dedicated receivers and aircraft band radios only. In order to locate one of these beacons, specialised (and expensive) equipment must be used that can determine the direction of the signal.
When signals are detected at medium to long range distances, only personnel trained in the operation of the equipment will be able extrapolate a usable bearing on the casualty.
The analogue beacons do not give a position; on picking up the warble tone, the receiving party must work out where the 121.5 MHz signal is coming from, then travel cautiously towards the signal source, continuously checking position and bearing in relation to the signal strength until the casualty is found.
Using the UKCS oil and gas fields as an example, three common types of analogue PLBs are used by surface and helicopter operators in the North Sea.
McMurdo Guardian wrist watch Compact enough to be worn on the wrist but as a result of its small size, has very limited power and range (3mW and 50 to 1000 meters). Will only work when the watch is manually armed and held above sea level (casualty must be conscious, uninjured and reasonably fit).
SeaMarshall PLB8 Worn around the neck or embedded into a lifejacket. Requires manual arming, will operate without any user intervention but has a low to moderate transmission range.
Rhotheta RT-B77 Strapped to lifejacket. Must have its folded antenna manually deployed to get maximum range. Good power output and long range when the antenna is deployed (casualty must be conscious and able). To aid identification, transmits a unique identification number. Unique ID can only be decoded by Rhotheta proprietary receivers.
Advantages common to all analogue PLBs
'Can be picked up by aircraft band radios
'Basic technology, low to moderate cost
'Some products are compact in size with wrist watch units available.
'Can be configured for automatic operation (water sensing)
Disadvantages
'No position information is transmitted
'All devices require specialised receiving equipment seldom found outside SAR agency rescue assets
In addition, field testing has identified the following issues with this technology:
'Cannot lock on individual signal if multiple beacons/casualties
All types of analogue PLBs transmit continuously on the same frequency (121.5MHz) which makes locating a single unit very difficult when more than one unit is activated in a small area.
For example, in a helicopter ditching where many analogue PLB equipped survivors are transmitting at the same time and visibility is bad, most standard direction finding (DF) equipment cannot get a lock on a single unit making recovery very difficult. In addition, once a casualty has been recovered the attached beacon must be shut down so that any remaining beacons in the vicinity can be identified and located. It has also been found that the use of analogue PLBs has disabled the more powerful and highly specified aircraft ELT.
An extract from the Oil and Gas UK Helicopter Task Group update "" Reintroduction of PLBs on helicopter flights (3 June 2009)
"Following the ditching of an offshore helicopter in the UK sector in February 2009, investigations by the UK Air Accidents Investigation Branch (AAIB) found that interference from the personal locator beacons - or PLBs - worn by passengers had effectively switched off the "smart" long range rescue beacons that were fitted to the life rafts. This could have impacted the effectiveness of the search and rescue operation. The smart technology fitted to the life raft beacons is designed to shut the beacon down if it detects another beacon signal within a certain radius. This is supposed to ensure that the aircraft can easily be homed in on by Search and Rescue (SAR) aircraft by having only one high-powered aircraft beacon transmitting at a time. However, in the ETAP ditching, the lower powered passenger PLBs (non-smart) were detected by the smart beacons, which caused all of the aircraft and life raft beacons to power down.
As a result, the UK Civil Aviation Authority (CAA) instructed offshore helicopter operators within the UK to stop carrying personal beacons in 'stand by' mode because of the risk of them being accidentally switched on, interfering with the aircraft's safety systems and shutting down its long-range beacons."
Note: Newly developed Doppler-based DF equipment is far better at managing multiple casualties. Unfortunately, the price for airborne Doppler DF equipment is approximately US$30,000 per unit.
'Multi path fading and reflections in open water
Finding a person in the water is a very difficult task. The rescuer must locate an object the size of a lifejacket with only the casualty"s head above water level. Even in the calmest sea conditions, the survivor"s head will be below average wave height making visual contact very difficult even when the rescue boat is as close as 20 metres away.
Just like rays of light, VHF radio waves travel in straight lines, as a result, a beacon at sea level will have its radio signals reflected in different directions by the surrounding wave motion. Although this also affects the Digital PLB (see below), this phenomenon can easily fool direction finding equipment through total loss of signal and a wide variation in perceived direction.
Add the complication of multiple targets and the task of the rescuers becomes near impossible with potentially fatal consequences.
'No unique identification
Most analogue PLBs on the market do not support unique identification. This means that in the event of a false alarm, it is very difficult to determine which beacon has been activated, where it is located and who owns it.
"
DIGITAL 406 PERSONAL LOCATOR BEACONS
Background
The digital 406 PLB is the latest generation of satellite compatible personal locator beacons.
They are built to an international specification and are a subset of the SOLAS compliant EPIRBs carried by ships. The compromises made for the PLB are in the areas of flotation, battery capacity and environmental endurance.
Upon activation, the digital 406 PLB will transmit a burst of digital data incorporating information about the identity of the unit, its country of registration and possibly the beacon"s GPS location. Cospas-Sarsat satellites receive this information and relay it back to an earth station. The earth station then relays the data to the appropriate Maritime Rescue Coordination Centre which will subsequently alert all shipping in the area and local Search and Rescue (SAR) agencies. If the emergency is verified, SAR assets are dispatched to investigate the incident.
Advantages
'Uses a dedicated satellite network to receive the 406 alert from anywhere in the world. It is not reliant on being received by a boat, aircraft or coastal station.
'406 is a well defined standard with many choices of product in the market place.
'Ability to provide an approximate location. (5km radius down to 120m radius if fitted with a GPS)
'Each beacon is uniquely identified and information is (should be) stored on a central database.
"
Disadvantages
'No automatic activation. IMO standards prohibit the use of water activation for 406 PLB devices. In addition, no 406 PLBs (as at publication date) have been identified that can be embedded into a lifejacket for automatic operation.
'Low accuracy of position. The location accuracy of a standard (non-GPS equipped) 406 PLB is stated to be +/- 5,000 metres. Some 406 PLBs are equipped with GPS receivers and can provide more accurate location information. Unfortunately the Cospas-Sarsat system cannot transmit the last decimal place of the position data so absolute accuracy is degraded to +/- 125 metres, which is not good enough for finding a casualty in the water quickly. For this reason, all EPIRBS and a lot of 406 PLBs also have a 121.5 MHz homing beacon and have to rely on dedicated direction finding receivers to complete the rescue.
'Delayed response. A 406 PLB alert message transitions through many processes before the SAR agency is notified, which may result in a sometimes significant delay between the PLB activation and a SAR response.
For a vessel in distress, 2 to 4 hours may be tolerable but for a man in the water this delay is usually fatal.
'Proprietary signalling. The digital signal from a 406 PLB or EPIRB is only a fraction of a second long and can usually only be received by the satellite network. It is possible to purchase specialised equipment that is capable of receiving 406 transmissions. However this equipment is rare and expensive.
In a man overboard situation, the crew aboard the man overboard"s vessel may be unaware that the event has occurred. Even when notified, the boat cannot receive the 406 PLB signals, so the 406 satellite component of the PLB is of no benefit to the crew for locating the casualty.
All compliant 406 PLBs also transmit a low power 121.5 MHz homing signal that can be received from close range with specialised direction finding equipment (as discussed previously).
'Physically larger products. There is a lot of technology built into a 406 PLB and as a result, these devices are usually larger than the other two types of device in the PLB category. This may result in the product being difficult to wear for long periods or integrate into lifejackets. However, as technology advances are made, it is anticipated that units will reduce in size.
"
DIGITAL VHF PERSONAL LOCATOR BEACONS
Background
A digital VHF (Very High Frequency) PLB is a small, lightweight and waterproof transmitter that can be worn by people in danger of falling (or being ditched) into water.
When activated by a switch or contact with water, it performs three functions:
1.It activates the GPS receiver and begins to calculate its position;
2.It transmits a spoken word emergency message to all VHF radios within range:
3.It starts transmitting short bursts of SMS type digital data to all Digital Selective Calling (DSC) marine radios within range.
Contained in this SMS type data packet are three pieces of information:
'The type of emergency (man overboard);
'The unique serial number of the device;
'The location of the beacon (accurate to about 10 to 30 metres)
All ships within approximately 8 nm radius will receive the DSC distress alert, automatically causing their DSC marine radio to alarm loudly. The position coordinates of the casualty are displayed on the radio screen.
The close proximity of these ships to the location of the incident means they are able to offer immediate assistance and/or confirm the emergency.
Although the range of the DSC transmission to other surface vessels is relatively short, aircraft can receive these transmissions at ranges in excess of 20 nm . This allows a SAR aircraft to fly directly to the rescue zone at high speed and plan the rescue en route thus reducing the exposure time in the water for the casualties. These factors combined increase the likelihood of more people being recovered alive.
Advantages
'The precise location of the casualty is transmitted and is accurate to approx. 10 metre radius (10x better than 406 PLBs).
'DSC message duration is 1.6 seconds every 5 minutes allowing the device to transmit at high power levels for a small battery size.
'Ability to individually identify and locate many simultaneous casualties in a crash zone. (Can appear as multiple targets on a moving map display.)
'Will not interfere with other EPIRBs or ELTs.
'Transmits the distress and location information using the international VHF marine radio band, part of GMDSS. (DSC radio receiving equipment can be purchased for as little as US$300).
'Supports automatic water activation. Flexible design means that the Digital VHF PLB can be integrated into a variety of products (lifejackets etc.) resulting in completely automatic operation.
Disadvantages
'Device must be in range of a DSC radio for the transmission to be received.
'Complex construction when compared to the simple 121.5 MHz transmitters. However, not as complex as a 406 PLB.
'Higher cost when compared to the simple 121.5 MHz transmitters. However, comparable cost to many 406 PLBs.
'Limited choice of products available in the market (as at publication date)."
FREQUENTLY ASKED QUESTIONS
QWhat if 20 casualties hit the water at the same time? Will the Digital VHF PLB signals jam up the emergency VHF DSC channel?
ANo. A Digital VHF PLB transmits a precise position and identification message in 1.6 seconds. Even if the device transmitted every 5 minutes (Australian standard) this represents a 0.6% use of the emergency band leaving 99.4% free for other DSC devices in the same 8 mile radius.
"Results from field trials support theoretical data produced from the mathematical modelling and indicate that the chance of a V100 DPLB causing harmful interference to VHF GMDSS services is negligible."
For a full mathematical proof, see appendix A: "Assessment of the probability of one or more V100 digital Personal Location Beacons (transmitting VHF DSC only) causing interference with a ship"s GMDSS emergency and routine VHF DSC messaging capability."
QThe Digital VHF PLB has no in-built receiver which means that the emergency message it transmits cannot be acknowledged by the receiving party. Without acknowledgment, will the device set off the DSC radio alarms on all ships within range every 5 minutes?
AYes. Any person who finds themselves alone in the ocean is in serious trouble. Depending on the water temperature they may only have 20 minutes left to live and are fighting to survive.
Although a repetitive DSC alarm may be irritating to the radio operator of a ship within an 8 mile radius (the operator will have to press a button on the radio every 5 minutes), the digital VHF PLB transmits the precise GPS coordinates of the person in the water to help rescuers find them quickly and then deactivate the device. The coordinates sent in the distress alert are updated with each transmission. This may be the only chance the casualty has of alerting rescue authorities and indicating their location before they perish. Those receiving ships may be the only ones who will be able to help in time.
Another argument against enabling acknowledgement of the V100 emergency transmission is that a casualty in fast moving water (tidal or current) will quickly drift from the original fall point. If the signal has been switched off by a remote acknowledgement and the rescue cannot be actioned for 20 minutes, rescuers will have limited data to know where to perform the search. If the casualty realises that their beacon has been silenced they may be able to reactivate it manually, but if they have succumbed to the cold or injury, the rescuers have little hope of finding them and the person"s life is lost.
QIs the Digital VHF PLB like other beacon types and does it require special receiving equipment?
ANo. The Digital PLB uses VHF Digital Selective Calling (DSC) which is part of the Global Maritime Distress and Safety System (GMDSS) and one of the most open and widely adopted emergency communication systems ever rolled out on an international scale. Maritime law dictates that every commercial vessel must have a DSC radio and under the SOLAS convention (Safety of Life at Sea), every ship while at sea must maintain a continuous watch on the distress and safety frequencies. DSC radios are also fitted to many offshore platforms, SAR assets (including most Fast Rescue Craft) and aircraft involved in maritime search and rescue. (DSC-capable marine radios can be purchased for under US$300.)
VHF radio is the lowest common denominator for all maritime communications.
QWhy is a Digital VHF PLB needed "" are the current analogue PLBs not good enough?
ANo "" according to the latest analogue 121.5 MHz PLB trials.
Independent testing has concluded that the 121.5 MHz PLB technology is very lacking when it comes to finding a survivor in the water.
Tabled below is an extract from a report on the effectiveness of analogue PLBs after the ditching of a helicopter in December 2006 approximately 24 miles from the shoreline of Morecambe Bay, Lancashire, England. All seven lives were lost and as a result an investigation was commissioned into the effectiveness of the PLB carried by aircrew and passengers.
The full report, "New Personal Locator Beacon Options. Contractors HS&E Forum October 2008." can be found at: http://stepchangeinsafety.net/stepchange/News/StreamContentPart.aspx?ID=3404
An extract from Contractors HS&E Forum October 2008 follows:
NEW PERSONAL LOCATOR BEACON OPTIONS
Page 6: Points of Note - PLB
'The marine based rescue effort did not detect any PLB signals when at crash scene.
'R122 identified three signals via their DF at 0.5nm from site at 500 ft.
'Detectability of PLB signals severely attenuated when wrist or PLB is under water.
'Only two PLB recovered out of five.
'Lack of PLB signal hampered marine based recovery which was heavily reliant on aerial lighting from R122.
