International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 1
Number 2
June 2007
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.
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
(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
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
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 >
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
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
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
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
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
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
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