International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 1

Number 2

June 2007

145

Mental Workload of the VTS Operators by

Utilising Heart Rate

S. Kum

Kobe University, Graduate School of Science and Technology, Kobe, Japan

M. Furusho

Kobe University, Graduate School of Maritime Sciences, Kobe, Japan

O. Duru & T. Satir

Istanbul Technical University, Istanbul, Turkey

ABSTRACT: This study clarifies the mental workload of VTS Operator; by understanding their

characteristics during carrying out their task, with a physiological index. The objective is to determine VTS

Operators’ mind stress that might trigger any human error based on their mental workload during their

watches. For this purpose, Heart Rate Monitor (HRM) is utilized as physiological index. The VTS Operators

fitted the HRM and all of them have experience as a Master Mariner. During the all experiments, their heart

rates and behaviours were recorded on the Event Record Form based on the time scale. After getting the heart

rate variability, it is matched with the events, and then Operators’ behaviour is understood as the mental

workload due to such kinds of events. Furthermore, these workloads include the Operators’ mind stress and

their decisions under these circumstances. This study provides the fundamental information for understanding

the VTS Operators’ characteristics.

1 INTRODUCTION

This study is one of authors’ systematic studies for

understanding the behaviours of seafarers. And, the

investigation of heart rates is found a suitable

physiological index for evaluating mental tasks.

1.1 Heart Rates and Heart Rate Variability (HRV)

A healthy person’s heart will pump between 60 and

140 times per one minute, depending on whether the

individual is under exertion or at rest. Beating of the

heart is caused by rhythmic contraction of the atria

and ventricles under the control of Autonomic

Nervous System, ANS, (Malik 1996). ANS has two

components; Sympathetic Nervous System (SNS)

and Parasympathetic Nervous System (PNS).

Heart rate is a term used to describe the frequency

of the cardiac cycle. It is calculated as the number of

contractions (heart beats) of the heart in one minute

and expressed as "beats per minute (bpm)”.

Heart Rate Variability (HRV) is the variation in

times between successive heartbeats (i.e. R-R

intervals that is a term from the peak of R wave to

the next peak in microseconds). R-R interval always

fluctuates to reflect the physical and the mental

conditions. The frequency components of R-R

interval reflect Low Frequency, LF, (0.04 - 0.15 Hz)

and High Frequency, HF, (0.15 - 0.40 Hz). This

index is able to evaluate both SNS and PNS activity

at the same time. The LF provides a quantitative

index of the sympathetic and parasympathetic

activities controlling the heart rate, while HF

provides an index of the parasympathetic tone

(Ishibashi & Yasukouchi 1999).

The relationship between R-R interval and heart

rates (bpm) is when the interval time of R peaks

increases conversely heart beats decrease in minute.

It means that the fast heart pulsates, the smallest R-

R intervals. HRV analysis is based on measuring

variability in heart rate; specifically, variability in

intervals between R-R. Power spectral analysis

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(obtaining HF and LF) on HRV is useful to assess

the mental workload (Sloan et al. 1995).

HF relates to parasympathetic activity (such as

controlled respiration, cold stimulation of the face,

and rotational stimuli), and LF relates to both

parasympathetic and sympathetic activities such as

tilt, standing, mental stress, and moderate exercise

(Malik 1996). Some studies suggest that LF is a

quantitative marker of sympathetic modulations;

other studies view LF as reflecting both sympathetic

activity and parasympathetic activity. Consequently,

the ratio of LF/HF is considered to reflect the

sympathetic modulations. That’s why this ratio can

be considered to the SNS index for evaluating

mental workload as defined by Kobayashi & Senda

(1998).

Before carrying out experimental studies, authors

carried out pre-study for recording heart rates of

VTS Operators (VTS-Os) while sitting operation

desk (console table) when they are doing nothing

and when they arrange display of the operating

system before taking the watch. These two

conditions compared and the results can be

summarized as; two samples (resting and arranging

display at the console table) compared with ANOVA

analysis in heart beats and also R-R intervals. The

analysis results show that two samples data are

different (F statistics is significant at the confidential

level, p < 0.05). The physical actions of operator are

at the minimum level due to always sitting. It can be

assumed that this difference mainly occurs for the

effort of performing task as mental behaviours. This

information is useful for considering the results that

would be mainly affected by the fluctuation of heart

rates caused by sympathetic activities as mental

workload (until the respirations doesn’t influence for

the contra verse situation such as instantaneous

emotion).

The relation between HR and HRV is found from

the pre-study as; recording R-R intervals and

predicted R-R intervals (that estimated by heart

beats, using by average values of R-R for every beat

times) are the same pattern, (the correlation is 0.99

significantly (p < 0.01)). And also, two samples

belong to same population (F (0.0005), p: 0.98 >

0.05).

There is very few study related to HR in the

maritime field, and they mainly consider to the

navigators’ heart rate variability during on board and

in the simulator experiments. These studies (Kum et

all. 2004, Kobayashi & Senda 1998, Murai et all.

2004a and Murai et all. 2004b) obviously show that

there were remarkable effects for the navigators’

heart rates during the task performing. On the other

hand, there is not found such kind of study for the

VTS-Os. That’s why heart rate variability has

enthusiastically been employed in physiological

assessment of respective working environmental

factors during the executing task process of VTS-Os

in this paper.

1.2 Mental Workload – MWL

Workload is defined as the physical and/or mental

requirements associated with a task or combination

of tasks (Gudipati & Pennathur, unpubl.). Workload

refers to the operators have limited capacity that

actually required to perform a particular task; it is

the interaction between the operator and assigned

task (Gopher & Donchin 1986).

Workload is classified as Physical Workload

(PWL) and Mental Workload (MWL). PWL is the

measurable portion of physical resources expended

when performing a given task (Gudipati &

Pennathur, unpubl.). MWL is defined as the level of

processing capacity while performing the task or the

difference between the capacity to affect the usable

real performance and human-information processing

system (Eggemeier & Wilson 1991). The common

idea for the MWL is directly proportional with the

difference between existing sources and required

sources by the task.

Under these explanations, we can consider three

kinds of loading for performing a task. Firstly, the

operator capacity is over to the required

performance. This situation tends to the operator

bored and then also tends to make mistakes.

Secondly, the capacity is equal to the required

performance that is the best fitting of the

employment not only for human resources but also

for the efficient performance of the task. Finally,

when the operator capacity is not enough to perform

a task due to overlapping task items; this situation

tends to the operator has stress. And, if the last

situation ordinarily continues, occupational stress

will occur automatically and it makes the operator

has not satisfied his/her job. That’s why; the

acceptable workload can be determined as the level

of workload not to impede the operator to manage

the system safely and effectively (Jung & Jung

2001).

The concept of mental workload has become an

important issue for all kinds of industry after 1960’s.

The main reason of that is the computer and it

becomes indispensable component of the life.

Therefore, there is a rapid increase for the publishing

of papers related to the MWL. On the other hand, the

quantity of research related to MWL in maritime

field is so restricted. Authors hope that this study

could take an interest to consider not only shore side

staff in maritime sector but also for personnel on

board and in shipping companies. It is assumed that

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the factors to cause MWL and its affects can be

analyzed by the common measures (such as NASA

TLX, SWAT and so on) for more understanding of

maritime human factors. Because when we

investigate the studies in the maritime field,

regretfully saying that there is not any study found

related to measurement to MWL in these common

measures such as utilized in the air and land

navigation field, just a few studies consider to MWL

of navigators by only using the heart rate variability

as mentioned by section 1.1. Authors also submitted

a questionnaire to the VTS-Os for determining the

MWL factors and applied NASA-TLX for measure

their MWL as the forward stage of this study.

In this study, the mental task of VTS-O is defined

as shown in Figure 1. VTS-O obtains cognitive

information from the VTS System and performs the

task under his skill based on the regulations. That’s

why the “mental workload” is the difference

between human information process and the

operator’s capacity which affects to the actual

performances that can be expended by operator (for

covering the required performance). Under this

excessive mental workload, operators may exhibit

delayed information processing, or even not respond

at all to incoming information, because the amount

of information surpasses their capacity to process it

(Ryu & Myung 2005).

It is mentioned in all studies (the common sense)

of the air and land navigation (except of maritime

navigation due to not found any related study) that

the factors to cause MWL is not clarified. On the

other hand, some studies show that nature of work,

training and age (Duru et al. 2005), physical

conditions of working environment (Leplat 1993),

the structure of task such as task aim, performing in

the most proper way and operator’s task perception

(Hart & Staveland 1988) have affect on MWL.

According to ISO 10075:1991 (Ergonomic

principles related to mental workload), the

fundamental factors of MWL are; task, work

equipment, physical work environment and social

work environment.

Fig. 1. Mental task of VTS-O

2 EXPERIMENTAL STUDY

Turkish Straits VTS (TSVTS) consists of Istanbul

VTS and Çanakkale VTS by covering area of the

Istanbul Strait, the Marmara Sea and Çanakkale

Strait (total length of the area is 164 nm). This

experimental study is carried out at the Istanbul VTS

Center. Istanbul VTS has 4 sectors; (North to South)

Sector Türkeli is the north entrance of the Istanbul

Strait (between the northern limit of the Istanbul

VTS and the line joining Fil-Çali Point), Sector

Kavak covers the area between the line of Fil-Çali

Point and the Fatih Sultan Mehmet Bridge, Sector

Kandilli’s southern limit is the line of Haydarpasa

Breakwater and Sector Kadiköy is the southern limit

of the Istanbul VTS.

TSVTS is operated in 24 hours; there are two

watching system as day shift and night shift. The

shifts have 2 hours watches. There are; two VTS-Os

(while one of them operates the system the other is

standby), one Assistant Supervisor and one VTS

Supervisor, VTS-S, (who is an appropriately

qualified VTS-O carrying out supervisory duties on

behalf of the VTS Authority) in every watch. Additi-

onally, some of the VTS-S (who engaged for daily

administrative works other than interaction with the

vessels), Data Input Operator (who engaged to

input the vessels sailing plans to the system) and

other staffs are involved day shifts.

The number of VTS-Os is 48 (including VTS-S)

at the Istanbul VTS and 30 operators at the

Çanakkale VTS. All of the Turkish VTS-Os has sea

experience (their average sea experience is 13.6

years) as a Ship Master (5.5 years is the average

level of their experience as actual Ship Master).

Their average age is 33 years and all of them have

Bachelor’s degree from a maritime faculty.

2.1 The Profile of VTS-Os

There are 48 persons employed for the Vessel

Traffic Services at the Istanbul VTS Center, 32 of

them are actually engaged for operating the system

to communicate with the ships [two operators for

every sectors at the each (A, B, C and D) shift].

Eight of 32 VTS-Os have fitted HRM under

different conditions for experimental studies. There

are not any specific criteria to choose these VTS-Os,

they were chosen randomly. But authors mainly want

to investigate different environmental situation in the

VTS. That’s why; four of experimental study was

particularly carried out simultaneously for the four

sectors. It was quite difficult to carry such kind of

experiment during the actual task execution not only

for the authors but also for the VTS Authority for

giving any permission. In this concept, during the

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permission time to carry out these experiments,

authors made as much as experiments for

understanding of the harmonized environments.

And, finally 8 VTS-Os were analysed from total of

32 VTS-Os by the ratio of 25%.

Table 1 shows the profile of these operators.

VTS-Os have 6.8 years experience as Ship Master

and 2.5 years as VTS-O (in average). Majority of

operators doesn’t smoke and they are healthy. Their

average weight is 91.5 kg, average height is 178.3

cm and average age is 41 years.

Table 1. Profile of VTS-Os

2.2 Conditions of Experimental Study

The experimental studies are carried out in different

conditions such as; the flow of traffic direction,

watch keeping times, weather conditions and sectors

in VTS area. Authors carried out totally 8

experimental studies at the Istanbul VTS Center as

shown in Table 2. Four of them are carried out

simultaneously for determining the interactions

among the Operators by covering all sectors in the

VTS area. The others are carried out randomly.

Normally, the traffic situation in the Istanbul

Strait is two ways traffic based on the Traffic

Separation Schemes. But at that time, the flow of

traffic was just one way due to the operation that

carried at the southern entrance of the Strait for

underground railway system. That’s why during our

experimental study, the traffic in the Strait was just

one way, it is important to indicate this information

due to considering workload of VTS-Os’. Their

workload level was under the normal case because

the capacity of the system was not routine. As the

mentioned by VTS-Os, they don’t feel that it is

really as difficulty as comparing the normal traffic

situation in the Strait, so it is observed that they are

in quiet behaviours (especially VTS-O in the middle

sectors for just observing to passage of one way

traffic). One of the experiment was carried out when

the traffic direction was changed due to estimating

the VTS-O’s MWL in more congested traffic cases,

and also authors mainly focused the south/north

entrance of the Strait depends on the traffic

direction. The environmental conditions in the Strait

are fine.

Table 2. Outline of the experimental studies

* Traffic direction was change to South to North.

2.3 Recording Data

The Heart Rate Monitor (HRM) which is equipment

that consists of a wrist watch, a chest belt (strap) and

software produced by POLAR Co. Ltd. is utilised in

this study. The type of this model is S 810i which

can measure not only heart beats (bpm based on time

sequence as 5 sec., 15 sec. and so on) but also R-R

intervals.

Before executing the experimental studies,

authors needed to prepare equipments (such as

HRM, voice data recorders and camera) and

documents (such as Information Data Sheet for

recording ships’ data (e.g. ship’s name, type, length,

cargo, pilot on board or not and so on) that

interrelated to VTS-Os’ communication and Event

Record Form for recording their all behaviours and

actions as shown in Figure 2) for reliable keeping

records.

Fig. 2. Event Record Form for VTS-Os

2.4 Analysing Data and MWL Assessment

In the analysis, the software (Polar Precision

Performance SW Version 4.03.044) is used for

getting the characteristics of heart rate and frequency

components. And for the further analysis, the