Eddy tracking with AIS.

Eddy tracking with AIS.

Is it possible to improve locations of ocean eddies with AIS?

Graduation thesis

Name: M.G. van der Neut.

Date: 01-July-2016

Revision: 3.1

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Title : Eddy tracking with AIS.

Revision : 3.1

Date : 01-July-2016

Status : Final

Auteur(s) : M.G. van der Neut (HERMESS)

Approved by :Dr. C.J. Calkoen (HERMESS)

Revision Status Date Commentary

1 Concept 02-01-2016 Concept.

1.1 Setup thesis 03-01-2016 Internal changes/ monthly back up

1.2 In progress 04-01-2016 Internal changes/ monthly back up

1.3 In progress 05-01-2016 Internal changes/ monthly back up

1.4 Raw concept 05-15-2016 Internal changes

2.1 Setup for MIWB 05-16-2016 Major Internal changes

2.2 First to MIWB 06-01-2016 Send to MIWB

2.3 Final concept 06-02-2016 Internal changes.

2.4 In progress 06-13-2016 Corrections from MIWB

3.1 Final 07-01-2016 Final Graduation Thesis

© Copyright HERMESS 2016

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Preface.

I (Minne Gijsbertus van der Neut) hereby present my Bachelor of Engineering final research thesis with

the title: “Eddy tracking with AIS”. For my bachelor Ocean Technology.

First of all I want to thank Mr. G.J. Wensink from HERMESS, for giving me the opportunity to conduct

my research at HERMESS and being my personal adviser during the four months of research. Without

his personal advices and support I would not have been able to successfully complete this research.

Dr. C. Calkoen from HERMESS is thanked for his outstanding advice, knowledge and for being my

mentor during my graduation. I learned how to conduct a research and the connection between your

research and the connection to companies. His information will help me to be a better hydrograph in

the future.

Ir. R.E. van Ree from the Maritime Instituut Willem Barentsz is thanked for his advice and for being my

mentor during my graduation and for his support and during the past four years, I would not be so

successful during my education without his outstanding support and teaching skills.

Last but not least thanks to all the colleagues that supported me the last four months. They helped

me to feel comfortable and happy at the office, were always willing to answer questions and helped

out with complex issues if needed.

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Abstract.

Ship’s Automatic Identification System (AIS) signals are primarily broadcasted to improve the safety of marine traffic. These signals, collected and stored at many ground stations along the coast, also make it possible to track the voyages of individual ships and to create time series of their position, speed, and heading. Variations in speed and course in these time series can be related to the MetOcean conditions that are encountered during the voyage, especially to the surface currents. This enables the assessment of currents from an analysis of these variations, making ship’s AIS data a potentially valuable addition to the buoys, drifters, and satellite altimeter measurements that are presently used to collect ocean current data. This study investigates the potential of AIS data to track large eddies and to improve the quality of ocean circulation models that forecast these eddies. In a case study an eddy was selected off the coast of S.E. Africa and AIS data from four ships that sailed through the eddy within a time frame of 9 hours were collected, analysed, and compared with the forecasts from the MyOcean current model. The current vector component along the ship’s route is assessed from speed variations. For one ship sailing through the centre of the eddy, the assessed AIS along ship currents agree well with the model value. The other three ships sailed closer to the shore through the peripheral part of the eddy. The along ship current assessments from these ships were internally consistent but significantly higher than the model values. The cross current vector components, perpendicular to the ship’s route, were assessed from variations in the course. The interpretation of assessments is less straightforward, probably because of interfering course corrections by the ship’s captain. Finally, it was investigated if a simple shifting and scaling operation on the model results could improve the consistency with all available AIS current assessments, per ship and for all ships together. For this purpose a cost function approach was used to obtain optimum shift and scaling parameters. The results show that a clear consistency improvement can be obtained per ship, but the effect of the combined comparison is less striking. A tentative explanation for this is that closer to the coast the interaction with the strong coastal currents affects the shape of the eddy so that a simple nudging operation is not sufficient to cover the complex current patterns there. To resolve these patterns better and produce more accurate forecasts the ocean circulation models need more local current measurements. This study shows that AIS data can be a valuable complementary source.

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Table of contents.

Preface. ----------------------------------------------------------------------------------------------------------- 3

Abstract. --------------------------------------------------------------------------------------------------------- 4

List of Figures. ------------------------------------------------------------------------------------------------- 6

List of Tables --------------------------------------------------------------------------------------------------- 7

List of Equations ---------------------------------------------------------------------------------------------- 7

List of Abbreviations. ---------------------------------------------------------------------------------------- 8

1. Introduction. ---------------------------------------------------------------------------------------------- 9

2. Data. -------------------------------------------------------------------------------------------------------- 12

2.1. Ocean models. ------------------------------------------------------------------------------------ 12

2.2. AIS data. --------------------------------------------------------------------------------------------- 13

2.2.1. Functioning of AIS. ------------------------------------------------------------------------ 13

2.2.2. Limitations of AIS. ------------------------------------------------------------------------- 14

2.3. Satellite AIS. --------------------------------------------------------------------------------------- 15

2.3.1. Present limitation of Satellite AIS. ---------------------------------------------------- 15

2.3.2. Satellite AIS in the future. ---------------------------------------------------------------- 16

2.4. Altimetry data. ------------------------------------------------------------------------------------- 16

2.5. Buoys and drifters. ------------------------------------------------------------------------------ 18

3. Approach. ------------------------------------------------------------------------------------------------- 19

3.1. Along ship currents. ----------------------------------------------------------------------------- 19

3.2. Cross currents. ----------------------------------------------------------------------------------- 22

3.3. Integrated comparison. ------------------------------------------------------------------------- 23

4. Case study. ----------------------------------------------------------------------------------------------- 25

5. Results ----------------------------------------------------------------------------------------------------- 30

5.1. Along currents. ----------------------------------------------------------------------------------- 30

5.2. Cross currents. ----------------------------------------------------------------------------------- 34

5.3. Integrated comparison. ------------------------------------------------------------------------- 35

6. Conclusions---------------------------------------------------------------------------------------------- 38

7. Outlook ---------------------------------------------------------------------------------------------------- 39

Bibliography --------------------------------------------------------------------------------------------------- 40

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List of Figures.

Figure 1 Thermohaline circulation on the world.. .......................................................................... 9

Figure 2 satellite AIS .................................................................................................................. 15

Figure 3 AIS satellite network. .................................................................................................... 16

Figure 4 Altimetry Satellite. ........................................................................................................ 17

Figure 5 previous AIS track with calm water log speed calculation. .............................................. 20

Figure 6 along track current explanation. ................................................................................... 21

Figure 7 cross track current influences. ....................................................................................... 22

Figure 8 deflection situation. ...................................................................................................... 23

Figure 9 Area South-Africa. ........................................................................................................ 25

Figure 10 Area Panama .............................................................................................................. 25

Figure 11 Area Florida ................................................................................................................ 25

Figure 12 current system South-Africa. ....................................................................................... 25

Figure 13 Myocean current model. ............................................................................................. 26

Figure 14 Hycom current model. ................................................................................................. 26

Figure 15 NOAA altimetry measurements. .................................................................................. 27

Figure 16 eddy currents Myocean ............................................................................................... 28

Figure 17 eddy currents HYCOM. ................................................................................................ 28

Figure 18 Altimetry data eddy. ................................................................................................... 28

Figure 19 positions of the vessels through the eddy. .................................................................... 29

Figure 20 first vessel trough eddy. .............................................................................................. 31

Figure 21 second vessel trough eddy. .......................................................................................... 31

Figure 22 third vessel trough eddy. ............................................................................................. 31

Figure 23 fourth vessel trough eddy. ........................................................................................... 31

Figure 24 calculated current with reversed values vessel 4 .......................................................... 33

Figure 25 predicted current out of MyOcean model. .................................................................... 33

Figure 26 calculated u component. ............................................................................................. 33

Figure 27 calculated v component. ............................................................................................. 33

Figure 28 predicted position changes with model values. ............................................................ 34

Figure 29 calculated position changes out of AIS data. ................................................................ 34

Figure 30 XTE system overview. .................................................................................................. 34

Figure 31 calculated current speed relative to their position. ....................................................... 35

Figure 32 current prediction with Myocean data background in a long track direction. ................ 35

Figure 33 current calculation with original quiver arrows. ........................................................... 35

Figure 34 current ship 3 corrected for speed changes. ................................................................. 35

Figure 35 Cost function improvement ship 1 ................................................................................ 36




Figure 39 drilling platforms in the Gulf of Mexico. ....................................................................... 39

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List of Tables

Table 1 Satellite measurement sensors. ...................................................................................... 12

Table 2 Four vessels through the eddy. ....................................................................................... 28

Table 3 cost function results. ...................................................................................................... 37

List of Equations

1 Vertical current calculation. .................................................................................................... 12

2 Horizontal current calculation. ................................................................................................. 13

3 Altimetry current calculation. ................................................................................................. 17

4 Coriolis force. ......................................................................................................................... 17

5 SOG. ....................................................................................................................................... 19

6 along track current from the model. ........................................................................................ 19

7 calm water log speed. ............................................................................................................. 19

8 along track current from AIS. .................................................................................................. 20

9 A calculation. ......................................................................................................................... 22

10 B calculation. ........................................................................................................................ 22

11 latc calculation. .................................................................................................................... 22

12 lonc calculation. ................................................................................................................... 22

13 dlat. ...................................................................................................................................... 22

14 dlon. .................................................................................................................................... 22

15 deflection. ............................................................................................................................ 22

16 across track current. ............................................................................................................. 23

17 current model. ...................................................................................................................... 23

18 cost function. ........................................................................................................................ 24

19 cost function with translation parameters .............................................................................. 24

20 Valongtrack. ......................................................................................................................... 24

21 Ualongtrack. ......................................................................................................................... 24

22 Vcrossstrack. ........................................................................................................................ 24

23 Ucrosstrack. ......................................................................................................................... 24

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List of Abbreviations.

AIS Automatic Identification System. cwls Calm water log speed. DP Dynamic positioning DSP Spectrum De-Collision Processing ECDIS Electronic Chart Display Information System ESA European Space Agency GNSS Global Navigation Satellite System. HYCOM Hybrid Coordinate Ocean Model km Kilometres kn Knot kph Kilometres per hour. LEO Low Earth Orbits. MIWB Maritiem Instituut Willem Barentsz. NASA National Aeronautics and Space Administration NM Nautical mile. OSCAR Ocean surface current analyses-real time. SAR Synthetic Aperture Radar SLR Satellite Laser Ranging SOG Speed over ground. SSH Sea surface height. SST Sea surface temperature. TDMA Time Division Multiple Access. VHF Very High Frequency. XTE Cross track error

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1. Introduction.

The oceans and seas are always moving. These movements, known as currents, can be noticeable and

unnoticeable to marine traffic but are always driven by a combination of variables such as: wind,

temperature, salinity difference and Coriolis effect.

The thermohaline circulation (Figure 1), with a length of 10.000 km is it one of the most important

oceanic currents of our planet. This current starts in the Arctic Ocean where the water will cool down

by the Arctic temperatures. As the salt in this water will not freeze the salinity will increase.

Subsequently the water layer will become heavier and sinks to the bottom. Warmer surface water

replaces the sinking water. The deep cold water mass moves south to Antarctica where it will further

decrease in temperate but increase in salinity. This makes that the current will be recharged in

Antarctica again.

The current is then bent to the left due to the Coriolis effect. Here it increases in temperature and will

flow back to the Arctic Ocean were everything will start again. The heat transportation dynamics of

the current are important for our climate on earth.

Figure 1 Thermohaline circulation on the world.1.

Under certain conditions, such as in the warmer upper regions of the thermohaline circulation, an

oceanic current will generate eddies; large rotating swirls of water that extend to great depth where

warm water is trapped by a ring of cold water or vice versa. The lifetime of an eddy depends on

temperature difference between the temperature of the core of the ring and the surrounding water.

1 Ahlenius, H. (n.d.). Thermohaline circulation [Illustration]. Retrieved from http://www.britannica.com/science/thermohaline-circulation

10

If this difference is big the eddy will have a longer lifetime. Eddies can exist 1 month up to year and

have a diameter up to 200 km. There are also some smaller eddies called mesoscale eddies. This eddies

can have an diameter up to 100 km and have a life time of up to 1 month (ESA, 2014). The direction

of the rotating eddy depends on the heat of the trapped water mass in the centre of the eddy, a cold

core eddy is rotating counter clockwise in the northern hemisphere and clockwise on the southern

hemisphere. For warm core eddies it is the other way around so clockwise in the northern part and

counter clockwise in the southern part.

Various models have been developed to describe, calculate and forecast currents and eddies. These

models require constantly updated measurements to keep the results reliable. Direct measurements

of currents are made by buoys and drifters, but these are scarce. Satellites measure water level

variations, from which currents can be assessed, but cannot measure currents directly.

Ships might form a valuable additional source of current measurements, the AIS systems on board of

ships make it possible to collect time series of the position, speed and direction. As these parameters

are directly affected by the local currents an analysis of the time series makes it possible to assess the

currents, turning all ships into “moving buoys”. Of course these currents assessments are less accurate

than what buoys produce, but there are many more ships than buoys, and the data is freely available.

The aim of this research is to investigate the potential of the analysis of AIS data to improve the quality

of ocean current models, in particular the localisation of eddies. There is particular reason for choosing

for eddies because they clearly stand out from the surrounding currents, and show strong variations

in speed. This is necessary to investigate if it is possible to determine currents from AIS data.

This research tries to give answer to the questions:

1. Can the absolute current strength be determined from AIS data?

2. Is it possible to determine the Location and size of an eddy from AIS data?

The expected outcome of this research is that AIS can supply ocean models with current data. This is

useful for several purposes. The first one is to increase safety during offshore operation around

platforms. There are always a lot of activities around oil rigs and platforms. All oil tankers, suppliers or

other kind of ships that are used for offshore operations are sailing with DP. DP is a system that tries

to hold the ship as close as possible to his predetermined position. The DP system is a database with

all kind of input such as wind meters, heading differences, position differences and current buoys. All

this data will be used to hold a vessel as steady as possible on position. The system is made to

anticipate on strong incoming wind and current boundaries. The main limitation of the system is that

it only measures the currents and wind close to the vessel, so that a high current boundary traveling

towards the platform will be noticed in the last moment. This can give a major safety issue. This high

current boundary can be created by eddies, and therefore it will be a great improvement to offshore

operations if eddies can be identified and tracked earlier on the ocean.

The second purpose is that with a better understanding of ocean currents the understanding of

transportation of ocean nutrition’s also increases. If the understanding about ocean transport is

better, it will improve the understanding about ocean animal populations and global warming.

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The research is done as graduation thesis for the Bachelor Ocean Technology at the Maritime Institute

Willem Barentsz. The Maritime Institute providing programs for masters and chief engineers 'all ships',

as well as hydrographic surveyor. Hydrography literally means describing the waters. The work

consists out of making observations, measurements and soundings of all aspects in the water and on

the seabed. So a research over improving ocean currents models with the use of measurements is a

perfect graduation project. This project can only be done with the expertise and help of HERMESS.

HERMESS is developing and providing innovative solutions on environmental issues to the offshore,

coastal and harbour sector. Close links with research institutes, universities and being backed up by

the maritime industry ensures that the latest techniques and services are available for clients.

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2. Data.

There are 4 main data sources used during this research. This chapter describes the purpose of the

data, and how they are calculated or measured. How they are used in the researched is documented

in chapter 3.

Ocean models.

Ocean circulation models numerically map ocean currents in a 4d solution (latitude, longitude, depth

and time). Currents are forced by several components such as. Gravitation, Coriolis force, sun

intensity, wind stress, pressure differences, salinity and friction with internal layers or bottom.

Currents cannot be measured out of space. Therefor the current has to be calculated. The calculations

are based on measurements of wind, waves, temperature, pressure, salinity and Sea surface height

(SSH). This parameters can be measured with different kind of satellites.

Scatterometer Wind speed and direction.

Altimeter SSH, wind speed and wave height.

Microwave radiometer Salinity and sea surface temperature.

Infrared radiometer Sea surface temperature.

Spectroradiometer Sea surface temperature. Table 1 Satellite measurement sensors2.

Al these parameters will be given in the model and are used to calculate the current. These calculations

are complex and based on several assumptions and physical laws. Most assumptions are based on an

isopycnic surfaces. An isopycnic surfaces is a surface with constant density. This assumption makes it

easier to model wind and gravity currents. But the surfaces is not always and everywhere in an

isopycnic state. This can generate errors in the model. There are several different ocean circulation

models available. They are all using the same kind of measurements as input. But the model settings

such as resolution, depth layers and time steps are different. This is caused by the different

interpretations of the assumptions. This leads to differences in current speeds most of the time. The

wind and waves are generally corresponding between the models. This is because they have been

measured by the various satellite sensors and modulated out of different variables. Models are

calculating the ocean current at different depth layers up to 2000 meters. These layers are not that

interesting for this research because vessels will only be influenced by the top layer of the ocean and

bottom layer of the wind.

Eddies are modulated separately in most of the models. This is because it is assumed that there exists

a steady uniform current, and a uniform thickness gradient. This current has to be geostrophic and

have to be aligned with the direction of constant layer thickness. With this assumption the current is

calculated with equation 1 and 2 (Cushman-Roisin & Beckers, 2011).

−𝑓�̅� =

1

ρ0∗

𝜕�̅�

𝜕�̅� 1

2 Copernicus. (n.d.). Satellites. Retrieved from http://marine.copernicus.eu/web/40-satellites.php

13

+𝑓�̅� = −1

ρ0∗

𝜕�̅�

𝜕�̅� 2

𝑓 = f-plane approximation where the Coriolis parameter is constant. p = pressure

ρ0 = density 𝑢 = horizontal current

𝑣 = vertical current These are simplified equation and explanation about Ocean current models. For a complete detailed explanation about fluid dynamics and Ocean models go to: Cushman-Roisin, B., & Beckers, J.-M. (2011). introduction to Geophysical fluid dynamics (2nd ed.). Oxford, United Kingdom: Elsevier.

Wallcraft, A. J., Metzeger, E. J., & Carrol, S. N. (2009). Software Design Description for the HYbrid Coordinate Ocean Model (HYCOM) (Version 2.2). Retrieved from http://hycom.org/attachments/063_metzger1-2009.pdf

AIS data.

The Automatic Identification System (AIS) is developed for communications between ships to avoid

collisions. This solution was not only successful for the communication between the vessel on sea, but

also for harbours and vessel traffic service. Since 2004 the international maritime organisation (IMO)

has made the use of AIS systems compulsory aboard of ships larger than 300 gross tonnages on

international voyages, cargo ships larger than 500 gross tonnages, and all passenger ships irrespective

of size(International Maritime Organization, 2016).

2.2.1. Functioning of AIS.

The AIS system can interoperate over the VHF frequency 161,975MHZ and 162,025MHZ and is based

on a special transmission scheme called self-organizing time division multiple access. This scheme

requires each AIS system to communicate within a given slot. AIS systems will pre-announce the time

slot to be used and will not use slots that are utilised by other AIS systems in order to prevent multiple

vessels sending in the same slot. Each frequency band can create 2,250 slots every minute. In the rare

event of slots being overloaded, the system will preference the nearest vessels and will drop-out

vessels further away. The range of the VHF band depends on the height of the antenna above the

surface but commonly ranges between 10-20 NM. Ground stations receive the signal on the shore.

The antennas of these stations are normally placed high above the ground in order to increase the

range to 40-60 NM (Marine Traffic, n.d.).

The AIS system transmits organised data-packages, consisting out of data that is subjected to changes

and data that is not subjected to changes. Data that will change is retrieved from sensors on-board

such as a GNSS solution for the position and a Doppler log for the speed over ground. Data that is not

subjected to changes are installed as a default option in the system, such as: Call sign, IMO ship

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identification number and dimensions of the vessel. The AIS system also sends destination, number of

passengers and type of cargo. This data has to be typed in by the 1e officer/captain.

The interval at which AIS transmits depends on the kind of data. Every 2 to 10 seconds the following

items are send (United States coast guard, 2011):

MMSI number - unique referenceable identification

Navigation status (as defined by the COLREGS - not only are "at anchor" and "under way using

engine" currently defined, but "not under command" is also currently defined)

Rate of turn - right or left, 0 to 720 degrees per minute (input from rate-of-turn indicator)

Speed over ground - 1/10 knot resolution from 0 to 102 knots

Position accuracy - differential GPS or other and an indication if Receiver Autonomous

Integrity Monitoring (RAIM) processing is being used

Longitude - to 1/10000 minute and Latitude - to 1/10000 minute

Course over ground - relative to true north to 1/10th degree

True Heading - 0 to 359 degrees derived from gyro input.

Time stamp - The universal time to nearest second that this information was generated

The following information is sent every 6 minutes.

MMSI number - same unique identification used above, links the data above to described

vessel

IMO number - unique referenceable identification (related to ship's construction)

Radio call sign - international call sign assigned to vessel, often used on voice radio

Name - Name of ship, 20 characters are provided

Type of ship/cargo - there is a table of available possibilities.

Dimensions of ship - to nearest meter

Location on ship where reference point for position reports is located

Type of position fixing device - various options from differential GPS to undefined

Draught of ship - 1/10 meter to 25.5 meters [note "air-draught" is not provided]

Destination - 20 characters are provided

Estimated time of Arrival at destination - month, day, hour, and minute in UTC

2.2.2. Limitations of AIS.

The limitation of the system is that it can be switched off. This happens normally in areas where pirates

are active. Pirates can track the vessels with AIS what gives them a benefit when entering a vessel.

This limitation makes that a captain cannot sail on the AIS system only, and still has to use the radar

and bearing measurement system. The limitation for the research purpose is that if the system is off

we will not have AIS data in that area. And the other limitation is that ground station can only collect

AIS data up to a range of about 60nm out of the coast.

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Satellite AIS.

Satellite AIS is an extension of the normal used VHF AIS. Satellite AIS is an AIS solution that will not

affect the VHF AIS. It works with a LEO satellite system that orbits over the north- and south-pole. The

satellites have a speed of around 7 km/s, which means that one rotation around the earth takes 100

minutes. The orbit of the satellites is fixed while the earth rotates underneath the satellites. The

satellite orbit is around 850 km above the earth surface and has a footprint of around 5,000 km2

(ExactEarth Ltd,2015). It can be that big, because the satellites do not need a high-resolution beam for

measurements. The satellite receives the AIS data off each vessel inside its footprint. The satellite will

store the received data until it is in range of a ground station. If the satellite passes the ground station

it will send the data to the ground station. This ground station will decode the data and forward it to

the authorized users. The biggest problem that had to be overcome was the use of the AIS slots, the

slots are designed in such a way that every vessel can send the AIS message and will not interrupt

someone else. But the satellite has such a big footprint that it will see many AIS cells at the same time,

resulting in collision of the massages. ExactEarth Ltd, (2015) developed a solution for this problem and

is called Spectrum De-Collision Processing (SDP), with this system the satellite will not only receive the

strongest signal in one specific slot but will store all of the signal in that slot.

When the satellite passes a ground station it will send all the raw data that is received. But the data is

now one big file, requiring special computed software that tries to identify individual ships by

differences in frequency and strength of the received data.

Figure 2 satellite AIS3.

2.3.1. Present limitation of Satellite AIS.

The main limitation of the present system is that there is a long delay between the first received

package and the second received package. This because there are not that many AIS satellites, so if

there is only one satellite it can take up to 12 hours before it will receive the AIS data from the same

vessel. At the moment there are eight operational satellites in the space what makes that the update

rate of the AIS data is around 6 hours. The average speed of a container ship is 20 knots so in 6 hours

the vessel has sailed 222 km.

3 Esa. (2014, September 09). SAT-AIS artists impression [Illustration]. Retrieved from

http://www.esa.int/spaceinimages/Images/2014/07/SAT-AIS_artists_impression

http://www.esa.int/ESA_Multimedia/Images/2014/07/SAT-AIS_artists_impression

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2.3.2. Satellite AIS in the future.

Several companies already developed solutions with a few satellites that track the vessels and

deliver the AIS data to the authorized users with a delay of maximum 6 hours. But since June 2015

the companies’ exactEarth and HARRIS (HARRIS, 2015) works together to launch a satellite network

consisting of 66 operational satellites and 6 spare satellites (Figure 3). The network will not only

cover the complete earth constantly, but the satellites can communicate as a network so there will

be a real-time download and upload link with the ground stations. This makes that the AIS Data will

be available within a minute. The expectation is that all the satellites should be launched before

2018 and the complete system should be operational before 2020.

Figure 3 AIS satellite network4.

Altimetry data.

Satellite altimetry is used to measure wind speed, wave heights and sea surface height. Altimetry base

satellites are measuring this with a microwave pulse of around 13.5 Ghz. The satellite will send the

pulse to earth where the pulse will reflect on the ocean and return to the satellite, at the moment that

the pulse was send an internal clock started which will stop when the signal returns. The time

measured is called the two-way travel time. From the travel time, together with position and height

of the satellite the sea surface height can be derived.

4 ExactEarth. (2016). The Next Generation Satellite AIS Constellation Provides Real-Time Global Ship Tracking [Illustration]. Retrieved from

http://www.exactearth.com/technology/exactview-rt-powered-by-harris

17

Figure 4 Altimetry Satellite5.

The wave heights and wind speed can be measured out of this same signal. The roughness of the sea

is affected by the wind, which gives a relation between the wind speed and the roughness, and the

roughness of the sea can be measured out of the strength of the returned signal. Because if the sea is

rough the signal will reflect to more multiple directions so the signal will lose strength. The wave height

can be calculated out of the distortion of the mean shape of the return pulse, the earlier returned

signal from the wave crests and the retarded return of the wave troughs leads to a broadening of the

signal what is directly related to the significant wave height (Komen et al., 1994).

To improve the accuracy of satellite altimeter measurements several corrections (such as orbit error

and delay) are applied during processing. The sea surface height can be measured with an accuracy of

± 5 cm (NASA, n.d.). Altimetry data will be used in the Ocean models but is also handy to determine

large geostrophic currents. A current is geostrophic when there is a balance between pressure

gradient and the Coriolis force. And the current can be calculated with equation 3. The limitation of

the system is that it will take around 10 days before the satellite will measure the same position on

earth again.

𝑐𝑢𝑟𝑟𝑒𝑛𝑡 =

𝑔

𝑓

𝜕ℎ

𝜕𝑥

3

g = gravitational acceleration (m/s). 𝜕ℎ

𝜕𝑥 = pressure gradient with h sea level height (m) and x distance (m).

𝑓 = Coriolis parameter (s).

The Coriolis force is subjected to the latitude. And can be calculated with equation 4.

𝑓 = 2 ∗ ∗ sin ( 𝜑) 4

= angular velocity (s).

𝜑= Latitude (degrees).

5 ESA. (2015, November 18). Basic Principle [Illustration]. Retrieved from Basic Principle /en-us/articles/203990918--What-

is-the-typical- range-of-the-AIS-

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Buoys and drifters.

Buoys and drifters are the only instruments supplying ocean models with real measured current data,

instead of the calculated and derived data from previous stated sources. One kind of buoys that are

used are the horizon buoys. These buoys are deployed on several tactical places and are tracking the

current speed and direction using an installed GPS system. These tactical places are locations

downstream so the buoys will track the complete current from the beginning. The buoys send the data

to a Low Earth Orbit satellite, which sends received data to the nearest ground station. This method

has proven its ability to find eddies in the Gulf of Mexico but gives only a local solution. As the buoys

float on the current and can drift off far away, retrieving them for a next deployment can be expensive.

Often buoys are damaged or destroyed by vessels or rocks and no longer fit for service. The cost of

one buoy is around 1800 dollar (NOAA AOML,2011)., making it an expensive method.

Argos floats is a system that used drifters that are floating over the global ocean, this reduces the cost

of retrieving them and are a good data input for ocean models. There are 3600 free-drifting floats

active in the ocean. But the ocean is still not completely covered, and the density fluctuates in time.

This makes that models have a varying accuracy. Because if a drifter is close to a specific area the

current in the model will be more accurate, but if the distance become higher the accuracy is going

down. The limitation of the floats is that they will be destroyed as well. Coming year 800 floats have

to be deployed to maintain the 3600 floats network (Argo,n.d.).

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3. Approach.

In this study AIS data (ship’s location, speed and direction along a track) are compared with current

vectors on a 4D (latitude, longitude, depth and time) grid resulting from ocean models. To be able to

make this comparison the data must be transformed to be of the same kind, either in speed and

position or, surface currents. The most straightforward way is to simulate a ship’s speed and position

from the model data, including wind and waves. An inversion of this simulation leads to an assessment

of the surface currents. A distinction is made between along ship currents by analysing speed

variations and cross ship currents by analysing course deflections.

Along ship currents.

The basic assumption is that wind, waves and currents causes speed variations of a vessel. The simple

equation to calculate the speed over ground is.

𝑠𝑜𝑔 = 𝑐𝑤𝑙𝑠 + 𝐾𝑤 ∗ 𝐷𝑤 + 𝐾𝑔 ∗ 𝐷𝑔 + 𝐶𝑎𝑙𝑜𝑛𝑔 5

Cwls= calm water log speed, the expected speed in absence of wind, waves and currents in m/s

Dw= wind drag based on effective along wind speed in m/s.

Dg=wave drag based on effective along wave speed in m/s.

Calong = current strength along in m/s and can be calculated with equation 6.

Kw and Kg are strength parameters that depend on the vessel type. This because wind and waves will

have more influence on a vessel with a high air draft than vessel with a lower air draft. Dr. Charles

Calkoen has obtained an approximation for these values from a regression analysis of many AIS tracks.

𝐶𝑎𝑙𝑜𝑛𝑔 = sin(𝑐𝑜𝑔) ∗ 𝑈𝑐 + cos(𝑐𝑜𝑔) ∗ 𝑉𝑐 6

Cog= course over ground of the ship in degrees.

Uc= model current component to wards East in m/s.

Vc= model current component towards North in m/s.

If the cwls is known Equations 5 can be used. But this is normally not the case, the sog is known

because it is saved in the AIS message and if the sog is known the cwls is calculated with equation 7.

𝑐𝑤𝑙𝑠 = 𝑠𝑜𝑔 − 𝐾𝑤 ∗ 𝐷𝑤 + 𝐾𝑔 ∗ 𝐷𝑔 + 𝐶𝑎𝑙𝑜𝑛𝑔 7

The assumption is made that when the variables wind, waves and current are correct and the captain

did not change the engine power, the cwls should be consistent. This assumption has been tested by

Dr. Charles Calkoen. Figure 5 shows the result of this assumption in the channel between France and

England.

20

Figure 5 previous AIS track with calm water log speed calculation.

The figure is showing the AIS speed, which is the speed measured with the speed log on-board of the

vessel. The SOG synth data is the speed over ground calculated from the GNSS position of the vessel

using robust filters. And the cwls is the calm water log speed calculated with equation 7.

The figure above shows two sections where the calculated cwls is constant in. a good approximation

of the wind waves and currents are supporting the validity of the assumption. The strong changes in

cwls, marked by the blue circles, are attributed to ship manoeuvres where the captain changed the

engine power. Other variations in cwls indicate an incorrect wind, waves or current prediction. This

leads to that the cwls method can be used to detect when the ocean model is correct and when it is

not, assuming that the captain did not change the speed.

From the three possible incorrect predictions the current will have the most influences on the ship:

the change in speed and or position are mainly caused by the currents and to a lesser extent by the

wind or waves. This assumption can be made because the wind and waves are measured and

modulated making their accuracy consistent higher than the accuracy of the current.

The along track current can be calculated with this assumption and equation 8. The cwls has to be

estimated what is a bit harder. The estimation is based on an area where the vessel had a consisted

speed for longer time without any influences of current. This area should not be too far away of the

expected eddy so the assumption can be made that the ships calm water log speed was equal to the

cwls in area where the eddy is.

Alongtrack current = (𝑒𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑐𝑤𝑙𝑠 − 𝑠𝑜𝑔 − 𝐾𝑤 ∗ 𝐷𝑤 + 𝐾𝑔 ∗ 𝐷𝑔) ∗ −1 8 With the estimated cwls in m/s

21

The calculated along track current can show the boundary of an eddy. But it will only be possible if a

vessel is sailing at the boundary of the eddy. Because the highest along track current will be in the

boundary of the eddy such as in Figure 6. In this situation the vessel will not have any cross track

current, but will have a high along track current. The position of the high along track current is

directly the boundary of the eddy. This is only applicable if the ship is sailing with a course of 0, 90,

180 or 270 degrees. And can be used if the location of the eddy is known. The absolute current

strength can only be known on the edge of the eddy and there is nothing to say about the current

speed profile inside the eddy or the location of the centre. The speed profile can only be known if

the vessel is sailing a bit more to the centre of the eddy. The boundary of the eddy can still be

determinate but it is not corresponding with the location where the vessel gets his highest along

track current anymore, because the vessel will encounter cross track current. The along track current

consistency can be checked with the use of other parallel sailing ships. The current appears to be

correct if the other sailing ships confirm each other.

Figure 6 along track current explanation.

22

Cross currents.

Figure 7 cross track current influences.

The second step is to calculate the cross track current. The assumption is that the ship will follow the

current and the captain will not correct the course. So a ship with a predetermined heading will sail

through the ocean. The moment that the ship will enter the eddy it will deflect from his original

course. The predicted deflection out of the model will be the same if the predicted current out of

the model is right. This deflection can be calculated by multiplying the model current strength with

the travel time, what will give the deflection in meters.

The distance between the true AIS position and a straight track representing the estimated

unperturbed track will be calculated to determine the deflection from the AIS data and is done with

equation 15.

𝑠 ∗ 𝑙𝑎𝑡 − 𝑐 ∗ 𝑙𝑜𝑛 = 𝐴 = 𝑠 ∗ 𝑙𝑎𝑡0 − 𝑐 ∗ 𝑙𝑜𝑛0 9

With s=sin(smooth-cog)

c=cos(smooth-cog)

Perpendicular line through (𝑙𝑎𝑡𝑖, 𝑙𝑜𝑛𝑖):

𝑐 ∗ 𝑙𝑎𝑡 + 𝑠 ∗ 𝑙𝑜𝑛 = 𝐵 = 𝑐 ∗ 𝑙𝑎𝑡𝑖 + 𝑠 ∗ 𝑙𝑜𝑛𝑖 10

Intersection point:

𝑙𝑎𝑡𝑐 = 𝑠 ∗ 𝐴 + 𝑐 ∗ 𝐵 = 𝑠2 ∗ 𝑙𝑎𝑡0 − 𝑠 ∗ 𝑐 ∗ 𝑙𝑜𝑛0 + 𝑐2 ∗ 𝑙𝑎𝑡𝑖 + 𝑠 ∗ 𝑐 ∗ 𝑙𝑜𝑛𝑖 11

𝑙𝑜𝑛𝑐 = −𝑠𝑐 ∗ 𝑙𝑎𝑡0 + 𝑐2 ∗ 𝑙𝑜𝑛0 + 𝑠𝑐 ∗ 𝑙𝑎𝑡𝑖 + 𝑠2 ∗ 𝑙𝑜𝑛𝑖 12

𝑑𝑙𝑎𝑡 = 𝑙𝑎𝑡𝑖 − 𝑙𝑎𝑡𝑐 = 𝑠2 ∗ (𝑙𝑎𝑡𝑖 − 𝑙𝑎𝑡0) − 𝑠 ∗ 𝑐 ∗ (𝑙𝑜𝑛𝑖 − 𝑙𝑜𝑛0) 13

𝑑𝑙𝑜𝑛 = 𝑙𝑜𝑛𝑖 − 𝑙𝑜𝑛𝑐 = −𝑐2 ∗ (𝑙𝑜𝑛𝑖 − 𝑙𝑜𝑛0) − 𝑠 ∗ 𝑐 ∗ (𝑙𝑎𝑡𝑖 − 𝑙𝑎𝑡0) 14

𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 = √cos(𝑙𝑎𝑡𝑖)2 ∗ 𝑑𝑙𝑜𝑛2 + 𝑑𝑙𝑎𝑡2 ∗ 111120 15

With smooth and cog in degrees, latitude and longitude in degrees. The deflection is calculated in

meters.

23

Figure 8 deflection situation.

Equation 16 is used to calculate the estimated cross track current.

𝑎𝑐𝑟𝑜𝑠𝑠 𝑡𝑟𝑎𝑐𝑘 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 = −sogi ∗ sin(cog − smooth) 16

sog= speed over ground m/s.

cog= course over ground degrees.

smooth= predicted cog without current influences.

This method can give a solution to determine the centre of the eddy, because if a ship is sailing

through the middle of an eddy it will only be affected by the cross track current. The position where

the cross track current will reverse is also the centre of the eddy.

Integrated comparison.

The two calculated currents can be compared with the predicted current out of the model. This can

be done for the along track current, by plotting the predicted current in the same direction as the

ships. Equation 17 will be used to transform the model data to a single vector what is negative for a

current opposite to the direction of the vessel and positive for a current in the same direction as the

vessel.

𝐶𝑚𝑜𝑑𝑒𝑙 = sin(𝑐𝑜𝑔) ∗ 𝑈𝑐 + cos(𝑐𝑜𝑔) ∗ 𝑉𝑐 17

With the Uc and Vc in m/s and the cog in degrees.

The calculated current can be compared with the model current. With a cost function approach such

as equation 18 the agreement can be quantified.

24

Cost = ∑ (𝐶𝑎𝑖𝑠,𝑖(𝑝𝑜𝑠) − 𝐶𝑚𝑜𝑑𝑒𝑙,𝑖(𝑝𝑜𝑠))2 18

With 𝐶𝑎𝑖𝑠,𝑖(𝑝𝑜𝑠) AIS current value for each ship position in m/s.

𝐶𝑚𝑜𝑑𝑒𝑙,𝑖(𝑝𝑜𝑠) is the model current value for each ship position in m/s.

This cost function approach is used to test the potential of AIS data to improve the model results.

The basic assumption is that the model eddy has the right shape but may be shifted or scaled. This is

described by four parameters: a shift in longitude (dlon), shift in latitude (dlat), a strength scaling

factor adjustment and an offset adjustment. This leads to the following cost function:

Cost = ∑(𝐶𝑎𝑖𝑠,𝑖(𝑝𝑜𝑠) − 𝑓𝑎𝑐𝑡𝑜𝑟 ∗ (𝐶𝑚𝑜𝑑𝑒𝑙,𝑖(𝑙𝑜𝑛 + 𝑑𝑙𝑜𝑛, 𝑙𝑎𝑡 + 𝑑𝑙𝑎𝑡) + 𝑜𝑓𝑓𝑠𝑒𝑡))2

19

The parameter values that minimize the cost function provide the best agreement between model

results and AIS measurements.

The size of the shift is first determined by hand, this can be done by plotting the calculated current

over the MyOcean to detect the first raw correction. Thereafter a minimisation process of MATLAB is

used to determine the shifts, what will give the lowest cost function value. If the cost function will

give a too high value means that the assumption about the model is incorrect and a more detailed

eddy parameterization equation has to be made. The combination of the along track and the cross

track current gives a total current model. What is calculated with equations 20 till 23.

𝑉𝑎𝑙𝑜𝑛𝑔𝑡𝑟𝑎𝑐𝑘 = 𝑐𝑜𝑠(𝑐𝑜𝑔) ∗ 𝑎𝑙𝑜𝑛𝑔 𝑡𝑟𝑎𝑐𝑘 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 20

𝑈𝑎𝑙𝑜𝑛𝑔𝑡𝑟𝑎𝑐𝑘 = 𝑠𝑖𝑛(𝑐𝑜𝑔) ∗ 𝑎𝑙𝑜𝑛𝑔 𝑡𝑟𝑎𝑐𝑘 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 21

𝑉𝑐𝑟𝑜𝑠𝑠𝑡𝑟𝑎𝑐𝑘 = 𝑐𝑜𝑠(𝑐𝑜𝑔 − 90) ∗ 𝑐𝑟𝑜𝑠𝑠 𝑡𝑟𝑎𝑐𝑘 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 22

𝑈𝑐𝑟𝑜𝑠𝑠𝑡𝑟𝑎𝑐𝑘 = sin(𝑐𝑜𝑔 − 90) ∗ 𝑐𝑟𝑜𝑠𝑠 𝑡𝑟𝑎𝑐𝑘 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 23

With the cog in degrees. And Along and cross track current in m/s.

25

4. Case study.

The first step is to select a location that are complying with some specifications. There are two main

requirements for the locations. The first requirement is that eddies have to occur in the location. The

second requirements is that there have to be enough AIS data available in that area. With the use of

previous studies (NASA ECCO2, n.d.), initially three locations are selected were eddies are known to

occur and sufficient marine activity. These areas are off the coast of South-Africa (Figure 9), between

Panama and Curacao (Figure 10) and around Florida (Figure 11).

Figure 9 Area South-Africa.

Figure 10 Area Panama.

Figure 11 Area Florida.

Figure 12 current system South-Africa6.

Unfortunately the density of ground stations around Florida as well as Panama proved to be

insufficient, causing a shortage of data that is needed for a proper analysis.

Therefore only the area in the east of South-Africa was selected for the research. The eddies

experienced around South-Africa originate out of the Aguilhas current such as showed in Figure 12.

Figure 13 to Figure 15 are showing the current profiles in this specific area predicted by the various

current models. The three models are not completely the same. The first one is the Myocean ocean

model. This model has a resolution of 1/12 of a degree and compute for every 2 hours wind, waves

and currents. The HYCOM ocean model has the same resolution but only calculates once a day at

midnight the wind, waves and currents. The third model is the NOAA Oscar data and is based on the

altimetry data, this model is showing a 5-day mean-current calculated from altimetry data and has a

resolution of 1/3 of a degree.

6 Edens. (n.d.). Ocean currents in South Africa. [Illustration]. Retrieved from http://www.pbs.org/edens/namib/earth2.htm

26

Figure 13 Myocean current model.

Figure 14 Hycom current model.

27

Figure 15 NOAA altimetry measurements7.

All models are showing different mean currents but are showing different eddies. This disagreement

is directly showing the problem with the models and is attributed to a lack of measurements. The use

of ships as ‘’moving buoys” could reduce this problem.

Several eddies have been tested for the purpose of this research. The main limitation is that the eddy

will deform close to the shore, because shore current, land boundary and depth deform the

characteristic circular shape, making them a lot harder to assimilate. This deformation is a lot harder

to assimilated and is not in the scope for this research. On the other hand if the eddy will occur far

from the shore it will be a nice uniform eddy. But the AIS ground station will not have the range the

receive AIS data. So it is not easy to find an eddy that is not influences by the surrounding, and with

enough data available. After monitoring data, freely collected from marinetraffic.com, of about 160

ships one eddy was chosen. The eddy is ad the boundary of the AIS range but it is expected to be quite

uniform.

Big differences between the MyOcean model (Figure 16), the HYCOM model (Figure 17) and Oscar

altimetry data (Figure 18) are shown when plotting the eddy. The figures presenting the same location

but where MyOcean and Oscar are showing an eddy, HYCOM is showing nothing at all. Between

Myocean and Oscar is also a big difference because in the Myocean data is the eddy rotating clockwise,

but in the Oscar data it is rotating counter clockwise. Altimetry data is one of the most important

inputs of the model what makes it strange that they are not supporting each other. These difference

are nevertheless supporting the idea that models are not always reliable and can be improved.

7 NOAA OSCAR. (n.d.). Ocean Surface Current Analyses. Retrieved from

http://www.oscar.noaa.gov/datadisplay/oscar_latlon.php?pagetype=nonjava

28

Figure 16 eddy currents Myocean.

Figure 17 eddy currents HYCOM.

Figure 18 Altimetry data eddy8.

For this case study four vessels (Table 2) were found that where sailing through the eddy, within a

time frame of less than 9 hours.

Ship Name MMSI Course (degrees)

Speed (m/s)

Time Colour

1 DRAFTSLAYER 241283000 246 5.5 11:15 Green

2 CAPEJACARANDA 355471000 246 6.3 10:45 bleu

3 ROSCO POPLAR 477743200 247 5.6 12:00 black

4 MARIA MARIA 210813000 66 5.6 06:15 red Table 2 Four vessels through the eddy.

The four vessels are all bulk carriers and are around the same size. This makes that the influences of

wind, waves and currents will be similar subsequently reduces the error. Figure 19 is showing the

positions of the vessels relative to each other and the eddy. Ship 4 was sailing towards the north-east,

the others towards the south-west.

8 NOAA OSCAR. (n.d.). Ocean Surface Current Analyses. Retrieved from

http://www.oscar.noaa.gov/datadisplay/oscar_latlon.php?pagetype=nonjava

29

Figure 19 positions of the vessels through the eddy.

30

5. Results

In this chapter the results of the case study described in chapter 4 are shown. All results are calculated

with the calculation described in chapter 3.

Along currents.

The Figure 20 to Figure 23 present the ship’s speed as given in the AIS data (black dots), the filtered

speeds (green dots), and the calculated calm water log speeds (red dots) which are corrected for the

influence of wind, waves and currents using model results. A constant value of the cwls would

indicate a good agreement between the observed ship’s speed variations and the applied model

results. This is clearly not the case.

Variations in the cwls can be cause by ship’s manoeuvres or by applying corrections using modelled

MetOcean conditions that deviate from the actual conditions. As the corrections for wind and waves

are minor and these models are fairly reliable, the variations in the cwls are mainly attributed to

deviations in the current.

Power adjustments by the captain are not stored in the AIS message, so the only way to know if the

power of the engine is changed, is to compare speed changes of different ships. Figure 20 and Figure

21 are showing two different vessels, with different speed but with a same pattern in the speed

change. This is a first indication that the vessel did not change the power of the engine, because it

would be quite a coincidence if two vessels would reduce the engine power around the same

location and by the same amount.

The fourth vessel (Figure 23) supports this statement even more. The fourth vessel is sailing from

west to east, in the opposite direction of the other two vessels experiencing an increase in speed

instead of a decrease. If it is busy sailing route, or something else happened what forces captains to

reduce their speed all the ships will do so. So also the vessels that are sailing in the opposite

direction. In this case the vessel is showing an increase what supported the statement that the

speed deviations is caused by the current. In all of the three vessels is the calculated cwls speed

varying what also indicates that the used model current is incorrect.

The third vessel (Figure 22) is sailing further to the south and is also showing a varying cwls. But this

pattern is showing around 31.3 east a quick increase and decrease of speed. This change is not

supported by any other vessel and is probably caused by the captain instead of the current.

31

Figure 20 first vessel trough eddy.

Figure 21 second vessel trough eddy.

Figure 22 third vessel trough eddy.

Figure 23 fourth vessel trough eddy.

In the figures above AIS speed is the given speed of the vessel measured with the speed log. SOG synth

is the calculated speed out of the GNSS position. And cwls is the calculated calm water log speed.

32

Vessels one, two and four are showing the same trend in speed changes. Three vessel are

statistically seen not enough to say that the vessels did not changes their engine power. However

with a lack of ships that are going through the eddy is it a good first indication that current changes

can be seen in AIS data. Therefore the assumptions are made that the vessels had a constant speed

and the speed differences that can be seen are caused by the current.

With these assumptions the along track current can be calculated and are shown in Figure 24. They

are calculated by the method described in chapter 3.1 (equation 8) with the predicted cwls being:

5.7, 6.47 and 4.36. Ship 4 was sailing in the opposite direction so all his values are reversed to make

it comparable. The current calculation is showing a corresponding speed pattern but the maximums

are shifted. There are two mean possible explanations for. First is the time differences between the

vessels. The vessels are not sailing at the same time through the same position, so the eddy may

have moved, leading to a shift in the AIS current. A time range was chosen to reduce the time shift.

The time difference between the maximum current out of vessel 1 and 4 is five hours. The shift in

between the two is around 10 km. NASA found in several studies that an eddy could move with a

maximum of 0.9 km per hour (NASA, n.d.). So in 5 hours it could only be moved 4.5 km instead of 10

km.

The second possibility of the shift depends on the location of the vessel compared the centrum of

the eddy and the vessel course. The time that the vessel is under influences of an along track current

is shorter close to the centre, than it is far from the centre. So that can explain the shift between

vessel 1 and 2. The shift between 1 and 4 can be explained with the heading difference. The

calculated current is still relative to the vessel. So a vessel with a different heading can have his

highest along track current on a different position. The current will split in u and v components to

correct for the heading. Where u is the horizontal current negative to the east and v the vertical

current negative to the north (Figure 26 and Figure 27). The values in this two figures cannot be used

yet because the cross current has also a u and a v component.

The calculated current along track can be compared with the along track current out of the models

as shown in Figure 25. There are some similarities between the two. Vessel two is showing a same

kind of pattern as the model current however. The strength is not similar. This was expected with

the interpretation of the cwls. Additionally the absolute AIS current can have an offset because it is

calculated with a predicted cwls. If this cwls was not predicted properly the closest two vessel should

not give the same maximum current speed. But in this case the vessels one and four are the closest

to each other and only shows a difference in current maximum of 0.06 m/s.

33

Figure 24 calculated current with reversed values vessel 4.

Figure 25 predicted current out of MyOcean model.

Figure 26 calculated u component.

Figure 27 calculated v component.

34

Cross currents.

Figure 28 and Figure 29 show the modelled and observed course detections as described in chapter

3.2 for the four ships. This result is showing a big difference between the predicted and observed

currents. Especially the size of the deflection is different. This can indicate the error in the model but

the vessel are also not corresponding exactly with each other. Such as vessel 4 is showing a smooth

pattern in deflection but vessel 1 and 2 are showing a steeper shaped graph. These graphs are

sometimes corresponding with each other but most of the time not. So this can indicate that captain

will correct for the deflection.

Figure 28 predicted position changes with model values.

Figure 29 calculated position changes out of AIS data.

The tentative explanation is that the absence of course corrections is not correct. And the

explanation for this can be the use of a course correction system. ECDIS is an electronic chart display

system where skipper can plan their sailing route. If the vessel will come outside the predefined

sector the system gives a warning, the skipper can decide if he will make a course correction or he

will leaves it and let the vessel follow the current.

Figure 30 XTE system overview.

35

This system is called XTE (cross track error) and the skipper can choose the boundaries of the alarm

(Figure 30). Normally in narrow sailing areas the boundaries will be tighter than on the ocean. This is

quite a nice system for the skippers but not for the eddy theory. The assumption that was made

about the captain that he will not change the course is incorrect. So the current can be calculated

but the cross track current estimation out of AIS will not be accurate and reliable.

Integrated comparison.

The comparison have to be made between the along track and the cross track current to improve the

model. Unfortunately, as stated in the previous paragraph, the cross track estimation is not a reliable

source yet making it impossible to combine both. Nevertheless the along track calculation can be used

to improve the model in the sailing direction on its own. This is done with equation 19. Each vessel is

compared with the model current in the sailing direction as shown in Figure 32.

Figure 31 calculated current speed relative to their position.

Figure 32 current prediction with Myocean data background in a long track direction.

Figure 33 current calculation with original quiver arrows.

Figure 34 current ship 3 corrected for speed changes.

Vessel 1 Vessel 4

Vessel 2

Vessel 3

36

In Figure 32 and Figure 33 can been seen that vessel 2 is quite a good match with the predicted

model current, and vessel three is matching on the left part but not on the right. The different

between the left and the right part of vessel 3 can be caused by the predicted speed correction as

mentioned earlier in chapter 5.1. This correction is tried to solve with the use of two cwls instead of

one. By splitting the track in to two, the current is calculated with two different predicted cwls

speeds (left side a cwls of 6.78 and right part 6.96).The impact of this fix on the result are impressive

compared with the model data (Figure 34).The size of the difference is calculated with the cost

function approach (equation 19), and the results are showed in Figure 35 till Figure 38.

Figure 35 Cost function improvement ship 1.




37

Vessel Basic C (m/s)

Fitted C (m/s)

Dlon (degrees)

Dlat (degrees)

Factor Offset (m/s)

1 49.14 7.32 0.28 0.16 2.99 0.27

2 10.98 3.63 0.039 0.10 1.32 0.23

3 0.20 0.06 0.01 0.10 0.75 0.05

4 46.90 12.26 0.43 0.25 1.26 -0.05

Table 3 cost function results.

These 4 cost functions are showing a quite clear trend (Table 3) between the two ships that are

sailing close to the middle of an eddy are more consistent with the model than the vessel that are

sailing closer to the edges of the eddy. The rest of the values do not really correspond with each

other. What reveals the complexity of an eddy and the assumption about the model that it can be

changed in the 4 parameters is not right. Because if that was the case all the valuables should be

around the same size. And the complete model could be adjust whit the parameters. Which

unfortunately is not the case.

38

Conclusions

Based on the results presented in chapter 5 we can conclude that:

The results from the two ocean models used in this study did not correspond with each other.

It is generally accepted that a lack of current measurements limits the accuracy of these

models.

The two vessels that are sailing nearest to each other (ship 1 and 4), although sailing in

opposite direction, are showing the same changes in current speed at a given position. This

demonstrates that AIS data is useful to detect current changes in the along track direction.

The cross track current results are showing that skippers are correcting the course more often

then initially assumed. This leads to an unreliable cross track current estimation. Nevertheless

the influences of cross currents can be derived from AIS data when course corrections are

monitored.

Ship 3 is showing, after the correcting for changes in power, an improved current calculation

that corresponds with the model current.

The improvement of ocean models in the along track direction is done with nudging the ocean

model with 4 parameters. However this solution is showing large improvements, if the

translation parameters are calculated for each ship separately. When the parameters are

calculated for the combined vessels still showing an improvement, but this is not as impressive

as the results for each ship. This means that the eddy strength and position of the model is

not completely right and have to be corrected with another nudging method.

The following answer can be given based on the above mentioned conclusions:

1. Can the absolute current strength be determined from AIS data?

When talking about the along track current strength in eddies we can say this is plausible. The first

indications are quite promising with vessels that corroborate each other. The statement cannot yet

be confirmed because there are not enough ships sailing through an eddy at the same time.

Nevertheless, the cases that have been investigated, the vessels were always corroborate each other.

The accuracy of the calculations still has to be determined. This requires further research.

Current strength in the cross track direction can be calculated but are more an approximation because

it is harder to determine the average course. The initial assumption that was made about the

expectation that the captain will barely correct for the course, is incorrect. The course corrections are

seen in all of the cases and interfering the current calculation. If the course correction can be

monitored and corrected for, it is possible to calculate the cross track current.

2. Is it possible to determine the location and size of an eddy from AIS data?

An eddy is more complicated than initially expected. An eddy cannot always been seen as a simple

circle. Therefor it is harder to localize eddies, but it is possible to detect across track influences in the

AIS data. Therefore it is plausible that the eddy location and size can be determined with AIS if course

correction can be monitored.

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6. Outlook

As mentioned in the conclusions extended research is necessary. Nevertheless this research has

already proven that AIS data can be utilised as source for current calculations. In this chapter some

recommendations are mentioned for further research.

As the collection of AIS data proved to be a struggle, due to the lack and range limitations of ground

stations, it is advised to switch to satellite AIS data in the future. When the AIS satellite network is

prepared ships can supply current measurements on global scale, in the meantime offshore platforms

can be used as extra data receiving point in order to build a local solution.

The amount of active vessels in the AIS range of an offshore platform was determinate with the use

of satellite data retrieved from vesseltracker (n.d.), such as the red circles in Figure 39. The centre of

the circles is the position of an active drilling platform. The range off an AIS receiver is around 100 km

which is therefore also the diameter of the red circles.

Figure 39 drilling platforms in the Gulf of Mexico.

In one area 28 ships are active every 48 hours. The big benefit of this method will be that the most of

the vessels that are sailing in that area are vessels that will work for, or supply, the platform. This

means that those vessel can be asked to keep a consistent speed and course what can improve the

current calculation. Therefore the recommendation is to make platform measurements available for

analysing.

Finally, the assessment of currents would benefit from more details in the AIS code such as the

moment that a course or speed correction is made. This data can be used to correct the calculations,

if this results will be compared with measured currents the accuracy of current strength can be

determined.

40

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Eddy tracking with AIS.

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