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1 INTRODUCTION

One of the inherent features of the maritime industry

is that it often operates in adverse conditions that

include heavy weather, extreme state of the sea and

wind, work at night and in poor visibility, at height or

on slippery surface. If we add other risks, e.g.

insufficient experience, hardworking environment,

watch length, inadequate equipment, inadequate

supervision and other potential risks, it is clear that

undesirable events such as injuries and MOB

situations are likely to arise. Furthermore,

additional risk stems from the fact that, in maritime

shipping, seafarers are often far away from the shore

and cannot be assisted timely in case of accident.

Therefore, considerable efforts are made to prevent

incidents on board and, if they do occur, to report

them to the shore-based services as soon as possible in

order to launch adequate assistance / rescue

operations. Modern crews perform safety drills and

establish safety procedures regularly, safety

equipment is mandatory. Yet, all these measures are

insufficient in MOB (Man Over Board) situations as

the equipment itself does not provide timely alerts [4].

One of the measures for increasing the safety of

on-board staff is the application of the personal

locator transmitter, which makes part of the broad

family of MSLD (Maritime Survivor Locating

Devices) devices. The purpose of these personal

locators is alerting and/or reporting the position of the

persons who accidentally fall over board and are

unable to return to the vessel or offshore structure by

themselves. Since the chances of survival in the sea

are directly related to the time spent in the water [8],

it is necessary to detect the MOB situation and launch

the search and rescue (SAR) operation as soon as

possible. In the event the crew fail to respond timely,

additional MOB situation requirements include [9]:

− Ensure the way of reporting the MOB situation to

the maritime rescue coordination centres and other

vessels in the vicinity,

Application of Radio Beacons in SAR Operations

M. Bakota, Z. Lušić & D. Pušić

University of Split

, Split, Croatia

ABSTRACT: This research features an overview of the available PLB technologies, their advantages,

shortcomings and areas of their optimum application. A test of the locator transmitter emitting both 406 MHz

AIS and 121.5 MHz signals was performed with a focus on tracking the homing 121.5 MHz signal. The

efficiency of the homing signal was examined by using two separate radio locating systems. One of them

comprised multi-purpose and widely available components and programs, while the other was a specialised

radio beacon system with dedicated components. In addition to the results, their analysis and evaluation of

efficiency, the paper discusses the applicability of the available PLB technologies and provides guidelines for

adequate selection of the PLB devices and position indicating radio equipment.

http://www.transnav.eu

the

International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 14

Number 3

September 2020

DOI:

10.12716/1001.14.03.13

622

− Ensure the means of defining the MOB position

without compromising the GMDSS (Global

Maritime Distress and Safety System),

− Ensure the way of updating the MOB position

without compromising the GMDSS.

While the use of PLBs (Personal Locator Beacon) is

mandatory in all offshore helicopter transport

operations in most European countries [15], their use

on offshore structures and vessels under way remains

optional.

PLB devices apply a number of technologies since

these devices are not used just in maritime industry.

The use of PLBs is mandatory in air transport (ELT

(Emergency Locator Transmitter) is a basic locating

beacon designed specifically for use on general

aviation aircraft; in certain situations, PLB and EPIRB

devices may be installed as well) and their use is

encouraged in a range of activities on land, e.g.

mountaineering, expeditions, and the like. Today,

PLB devices are widely available and they greatly

vary in price and terms of service (subscription,

prepayment), and in cost of activating the

search/rescue service (free or not). Moreover, there are

variations in consequences in case of false alarms

(potentially sparing the beacon’s owner from

significant false alert fines). Finally, there are various

degrees of individualisation, i.e. the beacon may or

may not be registered to a specific person, and the

very devices are quite different regarding the search

precision, response time, range, reliability, etc.

The purpose of this research was to establish the

efficiency of tracking the locator’s homing signal

transmitted on 121.5 MHz. The procedure included

two separate antenna systems featuring adequate

receivers and programs for processing the 121.5 MHz

signal. The distance from the PLB was measured to

establish the reception range and the quality of the

signal that allowed to pinpoint the beacon’s location.

2 TYPES OF TECHNOLOGIES APPLIED IN

PERSONAL LOCATOR BEACONS

The requirements of PLB devices in the modern

maritime industry are primarily based on the

requirements of the offshore industries that regulate

the PLBs in transfers from ship to ship and from

helicopter/vessel to offshore structure, depending on

the conditions and risks involved in these operations.

For instance, the PLB must be attachable to the

lifejacket, must be serviced once a year [8], activation

should be automatic and the beacons should be

visible on AIS receivers. Besides the signal reception

by the AIS, and ability to operate in 121.5 MHz, there

are a number of technologies designed to receive and

forward the PLB signal.

It is important to underline that this research deals

with the available technologies in maritime

environment, with no reference to their performance

on land or in air transport. Most of the available PLB

devices combine two or more technologies.

2.1 Epirb

An emergency position-indicating radio beacon

(EPIRB) buoy is a mandatory part of the vessel’s LSA

(Life Saving Appliances) equipment, which is

automatically activated in the event of maritime

accidents and is used in emergencies to locate vessels

in distress and in need of immediate SAR operation.

The system emits the 406 – 406.1 MHz signal that is

detected by satellites operated by COSPAS-SARSAT

(Cosmicheskaya Systyema Poiska Avariynyich Sudov

– Search And Rescue Satellite Aided Tracking.),

rescue services [5]. The signal contains the distress

code, owner’s identification code, and location

identification code for SAR assistance, based on the

Doppler frequency shift or GNSS (Global Navigation

Satellite System) coordinates, along with a low-power

homing beacon that transmits on 121.5 MHz (Radio

direction finding tone), allowing SAR forces to home

in on the distress beacon once the 406 MHz satellite

system has provided the necessary position

information [10, 11, 20]. When one of the COSPAS-

SARSAT satellites detects a beacon, the detection is

passed to one of the program's earth Mission Control

Centres (MCC), where the detected location and

beacon details are used to determine which Rescue

Coordination Centre (RCC) to pass the alert to. The

RCC investigates the beacon alert (45–60 min on

average) [21], and launches the SAR operation. The

system has global coverage and the position accuracy

varies from 2–5 km (without GNSS signal) to 100 m

(with GNSS signal) [11]. The average price of these

PLB devices is around 300 US dollars [14]. The device

has to be registered [6]. Due to the obvious

advantages of 406 MHz beacons and the significant

disadvantages to the older 121.5 MHz beacons, the

International COSPAS-SARSAT Program stopped

monitoring of 121.5/243 MHz analogue signals [1, 7,

10, 16]. However, the 121.5 MHz signal is still used

for close-in direction finding by SAR parties.

2.2 VHF DSC (Digital Selective Calling)

This system transmits alerts on VHF 70 Ch. Although

the GNSS position is shown, the bearing and distance

from the MOB/devices are not defined. This PLB

transmits the distress signal ('Mayday') until it

receives acknowledgment. The signal transmission

can be performed in two ways: in a closed loop,

where the PLB must contain the registered MMSI

(Maritime Mobile Service Identity) number of the

mother vessel (otherwise the vessels in the vicinity

will not receive any signal) and in an open loop,

where the signal is emitted to all vessels, without the

need of programming the mother MMSI number [8].

The priority is given to the open loop transmitting as

the system can be set to alert the mother ship only for

the first 5-10 minutes and then to switch to open loop

option, alerting all vessels or a group of vessels using

the MMSI format [12]. As most of the received DCS

messages are false distress signals and secondary

maritime information, it is very likely that the PLB

signal transmitted via VHF DSC will remain

unnoticed. The signal range varies from 15 NM (other

vessels) to 150 NM (air-borne search resources) [21].

However, the system enables the signal reception by

the mother vessel and the vessels in the vicinity, thus

allowing a timely response to the MOB situation.

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2.3 SEND (Satellite Emergency Notification Device)

Although originally designed for the use on land, this

system can be used by maritime structures as well.

The user part features personal transmitters assisting

in locating a person via a satellite signal that is not

part of the SARSAT system. Some designs of this

device allow a two-channel radio-communication,

message sending, navigation assistance, etc. The

devices has to be registered, the service implies a

monthly subscription or other ways of payment, as

the system uses the satellites engaged in commercial

systems, e.g. Globalstar or Iridium Satellite LCC [13].

2.4 AIS

The mandatory AIS system may be used for receiving

PLB signals within the reach of the VHF transmission.

It is considered as best in MOB locating, provided that

the AIS devices displays the MOB symbol accurately

and triggers the MOB alarm, otherwise the PLB AIS

signal may remain unnoticed. Depending on the very

PLB device and the antenna direction, the range

varies from 8 NM (AIS receivers on vessels) to 75 NM

(AIS receivers on air-borne search resources) [8, 21].

The system is most efficient when a MOB situation is

handled with the mother vessel, since the AIS receiver

cannot re-transmit the MOB message [20].

2.5 121.5 MHz PLB. AIS.

The 121.5 MHz is originally an analogue aviation

band distress frequency that can be used by PLB

devices independently or in combination with any of

the above-described technologies. If used

independently by PLB equipment, the signal can be

received across 2/3 of the global surface [11]. The

system does not indicate the GNSS position and the

beacon position is determined by the signal’s

direction and intensity. The devices does not have to

be registered and is used anonymously [2], which has

resulted in a large number of false alarms and

unnecessary SAR operations (around 2% of the

received signals referred to real accidents) [11].

Moreover, a 121.5 MHz signal may be triggered by

cash machines (ATM), video walls, large screens at

playing fields and the like, and the interference from

other electronic and electrical systems is common. The

frequency is often routinely monitored by commercial

aircraft, but has not been monitored routinely by sea-

going vessels and the necessary response may fail.

Another downside is that the devices are subject to

national legislations and are not present worldwide.

For instance, the use of PLBs transmitting only

analogue signals in maritime environment is banned

in Japan, Korea and Malta, and is limited in Spain,

Poland, Australia, Canada, Germany, etc. [3, 19]. The

results of previous testing of the range and efficiency

of these devices are shown in Table 1.

3 FINDINGS

In early November 2019, tests were carried out on the

training-research vessel “Naše more” in Brački Kanal

(Brač Channel between the Island of Brač and City of

Split on the mainland). The weather conditions

included the state of the sea 2–3, WSW wind 3m/s,

waves 0.5–1.25 m. A PLB model M100 made by Ocean

Signal company was used for testing. In addition to

the analogue 121.5 MHz, the device transmits AIS

signal as well. Table 2 presents technical specification

of the PLB transmitter.

The testing was performed through simultaneous

reception of signals by two separate systems. The first

system included the antenna HYPER LOG 7060 and

receiver SPECTRAN HF 6065, with their respective

specifications in Tables 3 and 4. The signals were

processed by a dedicated program MSC Realtime

Spectrum Analyzer Software, designed by the same

manufacturer.

Table 1. Average detection range of AIS and 121.5 MHz devices

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System frequency Typical surface detection range Typical detection range Detectable by low earth

by ship/(antenna height) by aircraft/(altitude) orbiting satellite

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156.525 MHz (Annex 2) 1-5 NM (2-9 km)/(10 m) 20-30 NM (37-56 km)/(2,000 ft) No

121.5 MHz (Annex 4) 8 NM (14.8 km) /(2 m) 40-70 NM/(30,000 ft) No

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Source: [16]

Table 2. Specification of PLB M100 transmitter

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Homing Transmission

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Transmit Power 50 mW

Frequency 121.5 MHz

Modulation AM, Sweep tone

AIS transmission

Transmit Power (EIRP) 1 Watt

Frequency 161.975 / 162.025 MHz +- 500 Hz

Baud rate 9600 baud

Synchronisation UTC

Messages Message 1 (Position), Message 14 (MOB status)

Repetition interval 8 messages/minute; Message 14 sent twice every 4 minutes

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Source: [17]