621

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.

623

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

__________________________________________________________________________________________________

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

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

__________________________________________________________________________________________________

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

__________________________________________________________________________________________________

Source: [16]

Table 2. Specification of PLB M100 transmitter

__________________________________________________________________________________________________

Homing Transmission

__________________________________________________________________________________________________

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

__________________________________________________________________________________________________

Source: [17]

624

Table 3. Technical specification of the antenna Hyperlog 7060

__________________________________________________________________________________________________

Design Active Logper

__________________________________________________________________________________________________

Frequency range 700MHz-6GHz (down to 120MHz with limited directivity)

Preamp Noise „linear“ increasing, 100MHz: 3,5dB, 3GHz: 4dB, 6GHz: 4,5dB

Preamp Gain (Typ.) „linear" falloff, 1MHz: 40dB; 3GHz: 37,5dB; 6GHz: 35dB

Nominal Impedance 50 Ohm

WSVR (typ.) < 1:2

Gain (typ.) 45 dBi

Calibration points 533 (10 MHz steps)

RF-connection SMA (f) or N (see optional adapter)

__________________________________________________________________________________________________

Source: [18]

Table 4. Technical specification of the receiver Spectran HF 6065

__________________________________________________________________________________________________

Rf frequency range: 10Mhz to 6Ghz

__________________________________________________________________________________________________

Danl displayed average noise level -135dBm(1Hz)

Max. Power at Rf input: +10dBm

Lowest sample time: 10mS

Resolution (rbw): 3kHz to 50MHz

Units: dBm, dBµV, V/m, A/m, W/m² (dBµV/m, W/cm² etc. via PC software)

Detectors: RMS, Min/Max

Demodulator: AM, FM

Input: 50 Ohm SMA RF-input (f)

Accuracy: +/- 2dB (typ.)

Interface: USB 2.0/1.1

__________________________________________________________________________________________________

Source: [18]

The other system included the antenna (Table 5),

USB DVB-T+FM+DAB 820T2 receiver and SDR

program. The antennas of both systems were placed

at the height of 4.6 m. The antenna polarisation was

not altered during the test. The antennas used in both

systems are yagi directional antennas.

Table 5. Technical specification of the antenna (system B)

_______________________________________________

Indoor TV antenna

_______________________________________________

Frequency range vhf:87.5-230mhz uhf: 470-862mhz

Preamp noise 5db

Preamp gain (typ.) vhf30db/uhf36db

Nominal impedance 75 ohm

_______________________________________________

Minimum requirements for the 121.5 MHz

maritime radio beacon system are shown in Table 6.

Table 6. Basic features of the maritime 121.5 MHz radio

beacon systems

_______________________________________________

Sensitivity (i.e. correct operation at minimum wanted

signal)

Minimum wanted signal = 10 DBΜV/M

_______________________________________________

Directivity (minimum resolution and accuracy)

compass safe distance test

±5 DEGREES

IEC 60945

_______________________________________________

Source: [9]

The device was attached to the life jacket, activated

manually and moved away from the vessel,

measuring its distance in a controlled way (Figure 1).

Figure 1. PLB M100 transmitter attached to the life jacket

The results of the signal reception are presented in

Figures 2 to 13. Each figure shows the simultaneous

reception of the signals by systems A and B at the

same distance from the transmitter. As the 121.5 MHz

signal is also a homing signal, the reception of the

signal was tested by the antennas directly focusing to

the transmitter (Figure a) and at the antenna deviation

of 45o from the source of the signal (Figure b).

A (a) A (b)

B (a) B (b)

Figure 2. Reception of the systems A and B at 55 m distance.

625

A (a) A (b)

B (a) B (b)

Figure 3. Reception of the systems A and B at 105 m

distance

A (a) A (b)

B (a) B (b)

Figure 4. Reception of the systems A and B at 155 m

distance.

Figures 2 – 4 show the reception of the signal at

distances up to 155 m. Figures 2 and 3 show a clear

and well isolated signals in both systems, while

reception interference starts to appear in both systems

at distances above 155 m (Figure 4).

A (a) A (b)

B (a) B (b)

Figure 5. Reception of the systems A and B at 205 m

distance

A (a) A (b)

B (a) B (b)

Figure 6. Reception of the systems A and B at 250 m

distance

A (a) A (b)

B (a) B (b)

Figure 7. Reception of the systems A and B at 310 m

distance

Figures 5 – 7 show the signal reception at distances

from 155 to 310 meters. At 205 m, the signal is still

clearly visible in both systems without any need of

additional receiver adjustment. Because the signal

reception gets remarkably weaker as the distance

increases, the signal becomes successfully isolated

and visible through adequate adjustment of the span

function in system A (Figures 6 A(a) and 6 A(b)). The

system B’s reception is remarkably weaker (Figure 6

B(a)), especially with the antenna deviated by 45o

(Figure 6 B(b)). The distance of 310 meters represents

the maximum range at which system B recognises the

signal but the reception quality is the same at both

antenna positions, and the signal’s display and

quality have no practical radio locating value any

more (Figures 7 B(a) and 7 B(b)). In system A, a higher

level of interference is obvious, but the signal is

clearly isolated (Figures 7 A(a) and 7 A(b)).

626

A (a) A (b)

Figure 8. Reception of the system A at 400 m distance.

A (a) A (b)

Figure 9. Reception of the system A at 510 m distance.

A (a) A (b)

Figure 10. Reception of the system A at 650 m distance

Figures 8 – 10 show the signal reception at

distances from 310 to 650 meters. The signal was

received only by system A. Higher levels of

interference and oscillations in signal consistency can

be noticed. Furthermore, there was a temporary signal

loss at 510 m and at the antenna deviation of 45o

(Figure 9 A(b)). Upon adjusting the receiver and

increasing its sensitivity, the signal was successfully

recovered and isolated at 650 m distance, in both

antenna positions (Figure 10).

A (a) A (b)

Figure 11. Reception of the system A at 760 m distance

A (a) A (b)

Figure 12. Reception of the system A at 870 m distance

A (a) A (b)

Figure 13. Reception of the system A at 960 m distance

Figures 11 – 13 show the reception of the system at

distances over 650 m. At the distance of 760 m, a

further reduction in signal/interference ratio can be

noticed; this is particularly obvious at 870 m, when

receiving the signal by the antenna deviated by 45°

from the source (Figure 12 A(b)). Upon further

bandwidth reduction of frequency range to 200 kHz

from the central frequency, the signal was detected at

the distance of 960 m, at both antenna directions, and

was good enough to be useful (Figure 13).

At initial distance points, the signals were clearly

isolated and could be used to home in on the source

of transmission. There is no observable difference, in

any of the two systems, in the signal reception

between the antenna directly focused and the antenna

deviated by 45°. As the distance becomes larger, the

received signal becomes weaker, and the band width

of the observed frequency is changed by altering the

span value. As the targeted frequency is known, the

span of system A is set within 1 MHz and less as the

transmitter moves away and increases the distance.

System B continues to operate within the initial

parameters. The difference in reception between

systems A and B becomes significant, as shown in

Figures 6 and 7. System A clearly isolates the signal

intensity in the range of –88.8 to –91.2 dBm, whether

the antenna is directly focused or deviated. At the

same time, system B with a directly focused antenna

manages to detect the signal up to 250 m distance,

whereas the signal received by a deviated antenna

becomes remarkably weak. The greatest distance at

which system B was able to detect the signal was 310

metres.

As the transmitter moves away, system A requires

the adjustment of sweeptime period to increase the

time, i.e. resolution and accuracy of scanning the

selected frequency bandwidth, because an excessive

scanning interval may result in poorer signal

detection, given the position and movement of the

transmitter at sea. Parameter VBW (Video band

Width) is used to reduce the present clutter, but it

must be taken into consideration that an excessive

reduction of the VBW parameter reduces the targeted

signal as well. Therefore, as the transmitter drifts

away, the smallest usable value is set. Moreover, it is

desirable to activate the preamp function in the menu

“internal attenuator”, thus increasing the receiver

sensitivity by 15 dbi. This procedure was also applied

while testing at greater distances. The alteration and

combined adjustment of the above parameters were

aimed at isolating the signal. The results are

presented in Figures 10 – 12. It can be noted that the

signal is clearly isolated in both antenna positions, its

minimum value amounting to over – 58.4 dBm. The

greatest effective distance of the signal reception was

about 960 metres; the quality of the detected signal

627

allowed focusing towards the source of the

transmission.

4 DISCUSSION

Although the testing did not achieve the average

values, as presented in Table 1, the results indicate

that there are considerable differences in performance

between the ordinary antenna system (system B) and

the professional system for signal reception and

processing (system A). The most important

differences lie in the receiver and the associated signal

processing software. As the power of the emitted

signal is 50 mW, the ability to adjust the parameters of

the signal reception and the receiver sensitivity are

the crucial features.

Professional equipment in SAR vessels should

include an omnidirectional polarisation antenna for

receiving the emergency signals that are often weak

and transmitted in poor weather conditions. Although

the sensitivity of such an antenna is usually lower, it

would ensure detection and tracking the transmitter

upon entering the transmission area, as the 121.5

MHz signal primarily acts as a homing signal. On the

other hand, it is obvious that the AIS signal with 1W

output is the primary source of the emergency alarms.

If the transmitter does not feature a GNSS module the

AIS message does not contain the coordinates, it is

necessary to combine the AIS signal to locate the

source of the transmission accurately and the 121.5

MHz homing signal to pinpoint the transmitter.

In SAR resources, especially in air SAR, more

effective would be the systems with directional

antennas. Although these antennas have the angularly

directed area of reception, the sea surface that is

covered is large enough to allow systematic search

operations. As a rule, such systems are more sensitive

than omnidirectional systems, and their reception

capacities and quality of the received signals are

considerably higher. In the practical use, such

systems require greater crew engagement

Besides the adequate personal locator beacon, the

crucial factor in search operations is the type of the

maritime structure and the potential search area that

it covers. In case of fixed maritime objects, e.g. oil rig

or similar off-shore structure, the resources based on

the 121.5 MHz signal are able to efficiently perform a

search operation, provided that these structures are

fitted with the equipment for radio direction finding.

If, at the same time, an additional alert is emitted

(VHF, AIS), there is a greater chance of detecting the

MOB signal timely by the staff on the maritime

structure. An early activation of the transmitter

(manually or automatically) is essential. In case of the

ocean-going vessel, there is a realistic possibility that

the emergency signal could remain unnoticed. The

121.5 MHz frequency is not routinely monitored by

SAR centers, and if the transmitter does not emit an

additional emergency signal, the transmission is

likely to be undetected. Likewise, the AIS and/or VHF

signal may be sent automatically and remain

unnoticed because there are no other vessels within

the VHF range or due to oversight of the navigating

bridge staff to detect the signal and launch the timely

SAR action. The use of 121.5 MHz devices is much

more justified in case of the vessels engaged in coastal

navigation and the leisure craft. Coastal areas are

under constant surveillance by SAR and Vessel Traffic

Service (VTS) centers that routinely monitor AIS and

VHF frequencies, so that the mother vessel’s failure or

inability to render assistance is not crucial. The radio

direction finding equipment used by VTS and SAR

centers depends on the search resources they use and

on the potential search area. The direction finders

designed for the air SAR resources make use of the

devices that are capable of simultaneous multi-band

monitoring of the 121.5 MHz VHF, Ch 16 VHF, 243

MHz UHF and 406.025 MHz (COSPAS-SARSAT).

Direction finders on SAR vessels, depending on the

configuration, are also able to receive VHF air band:

118.800 – 124.000 MHz and VHF marine band: 156.000

– 162.025 MHz [22]. Again, it is desirable that the

device transmits an additional AIS and/or VHF signal.

The fact that the International COSPAS-SARSAT

Program stopped monitoring of 121.5 MHz does not

present a major drawback in this situation.

5 CONCLUSION

The testing of 121.5 MHz PLB devices revealed a

limited efficiency if the signal locating is performed

exclusively by the systems installed on vessels and if

only this frequency is monitored. The tested maritime

radio beacon systems clearly prove that the use of

professional equipment is justified. Although the

testing did not achieve the average nominal values,

the analysis of the results proves that the possibilities

of specialised equipment far outstrip the abilities and

usefulness of the universal ordinary components. The

results imply that radio locating with dedicated

professional equipment is more effective in MOB

situations, even at larger transmission distances from

the ones tested in this research, provided that AIS /

VHF signals are monitored at the same time. The

purpose of the 121.5 MHz signal is homing and

pinpointing the MOB location, while the emergency

alert is more efficiently performed by transmitting

AIS and/or VHF signals. However, their application

on ocean-going vessels is not recommended, given the

work conditions and organisation on board such

vessels. There is a realistic possibility that PLB alert

messages sent through AIS / VHF frequencies may

remain unnoticed and that the response of the crew to

the MOB situation may fail. Hence, the 121.5 MHz

signal may remain undetected by the SAR locating

resources as well. The termination of COSPAS-

SARSAT processing of 121.5 MHz signals reduces the

efficient application of PLB devices to coastal seas and

the areas that are home to off-shore structures. For

these reasons, the use of this type of devices in

maritime traffic is prohibited or limited by many

countries. The advantages of PLB equipment include

low costs and unregistered use. In the international

maritime trade, the most efficient type of these

devices are the PLB EPIRB devices whose signals are

monitored by COSPAS-SARSAT, while the area of

efficient application of PLB AIS / VHF equipment

remains in coastal navigation and fixed off-shore

structures.

628

ACKNOWLEDGEMENTS

Research activities presented in this paper were conducted

under the scientific research project "Use of a radio locating

in SAR operations-2675/2017" supported by the University

of Split-Faculty of Maritime Studies, Croatia.

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