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Underwater staffTape

Figure 1 Conventional underwater positionmeasurement

Development of Precision Underwater Positioning System

Hideki YoshidaShimonoseki Research and Engineering Office Port and Airport

Kyushyu Regional Development Bureau, Ministry of Land, Infrastructure and Transport2-29-1, Higashi Yamato Town, Shimonoseki City, Yamaguchi Prefecture, JapanE-mail mailbox-k89be@pa.qsr.mlit.go.jp

Takayuki MoriOki Electric Co., Ltd.

681-1 Ozuwa, Numazu-City, Shizuoka 410-0873, JapanE-mail : mori184@oki.com

Abstract - The Shimonoseki Research and Engineering

Office for Port and Airport, Kyushu Regional Development

Bureau, Ministry of Land, Infrastructure and Transport in

cooperation with the Oki Electric Co. Ltd., has developed the

advanced underwater positioning system.

A measurement of position x, y and z of underwater

structure is needed to settle the structure in water, and to

manage the construction in port area. A quality inspection of

structures at port such as pier by the Remote of Vehicle

(ROV) is also needed to grasp its exact 3 dimensional position

in water for manipulating it.

Presently, the above mentioned underwater measurement

work, particularly required high accuracy, is needed many

workers, surveyors work on land for measuring the scale of

underwater-staff and divers work in water for supporting the

underwater-staff, and is met highly risks of incident.

Especially, in case of the offshore working, longer distancefrom land and deeper depth of water are cause of difficulty of

its implementation.

To cope with these issues, we have developed the system

and have verified its effectiveness by demonstration on the

sea. The system consists of 2 measurement technologies, the

Global Positioning System (GPS) is utilized measuring on

water and supersonic wave with ROV is for probing under

water. Using the system can be readily available to high

accuracy measurement without divers, even offshore

working.

This paper presents outline of the system and the result of

the demonstration.

I. Introduction

When installing structural objects below the surface ofthe water or managing execution of port/harborconstruction works, it is necessary to measure x, y and zcoordinates of the position of the structures to obtain theirthree dimensional position.

Conventionally, when performing the underwaterpositioning works, especially with a high degree ofaccuracy, the method of using an underwater diver(s) hasbeen adopted, where many work forces are required, suchas a surveyor on the land, an underwater diver that holdsthe underwater staff, and work boats that support the

measurement (Refer to Figure 1). Especially, whenperforming the works offshore, the operations becomemore difficult with increasing distance of the site from the

land.To deal with these issues, we have developed a

positioning system that combines different measurementtechnologies including GPS (Global Positioning System)for the measurement above the water surface andultrasonic system for the measurement under the watersurface. The use of this new system makes it possible toperform precision measurements offshore, and acombination of the system with an ROV (RemotelyOperated Vehicle) allows the measurement without the useof a diver.

II. Principle of measurement

This system adopts various methods to measure thecoordinates of underwater positions simply and precisely.

A. RTK-GPSThe Real Time Kinematic Global Positioning System

(RTK-GPS) is a positioning method that uses two GPS

receivers to measure the coordinates of a mobile stationmore precisely. The system allows to suppress thepositioning error to several centimeters by introducing the

UT07+SSC 07, Tokyo, Japan, 17-20 April 2007.1-4244-1208-0/07/$25.00 c 2007 IEEE.

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phase difference of a carrier wave from a satellite betweenthe two stations (the phase difference is the phase angledifference in degrees between two waves) into thecalculation.

Since the RTK-GPS requires the phase information, itis necessary to perform a process called initialization atthe base station and mobile stations to resolve theambiguity equivalent to integral multiple of the wavelength(the carrier wave from the satellite is approximately 19 cm)beforehand. The initialization is performed basically withthe base and mobile stations in their resting states, however,it can be done while the stations are moving if they canreceive the waves from five (5) or more satellites at thesame time. This function is called On The Fly (OTF).

B. How to calculate coordinates of measurement pointsAs shown in Figure 2, in the space coordinate system,

positional coordinates of an object point can be obtainedby numerical calculation if the coordinates of threeobservation points and slant ranges from the three points to

the object point are known. That is, the positionalcoordinates of an object point are those of an intersectionpoint of three spheres of which centers are the threeobservation points and radii are the slant ranges. Theprocess for introduction of the calculation formulas isdescribed below.

When the space coordinates of three points A, B and Care known and spheres of which radii are the distancesfrom the points to an object point M are drawn, the

relationship between the radii and the coordinates arerepresented by the following formulas.

Point A( ) ( ) ( ) 21

21

21

21 rzzyyxx =++ (1)

Point B( ) ( ) ( ) 22

22

22

22 rzzyyxx =++ (2)

Point C( ) ( ) ( ) 23

23

23

23 rzzyyxx =++ (3)

Then, the coordinates of the object point (x, y and z)are given as follows from the formulas (1), (2) and (3).

11 += zx(4)

22 += zy (5)

( ) ( ) ( )( )1

1

22

21

21

22222

21

21221112211 1

21

++

++++++=

rzzzz

(6)

Then, x and y are obtained by substituting (6) for theformulas (4) and (5) as described below.

( ) ( ) ( )( )12

221

21

22222

21

21221112211

11

11

21

+

++

++++++=

rzzzx

( ) ( ) ( )( )22

221

21

22222

21

21221112211

21

11

21

+

++

++++++=

rzzz

y

Where;

( ) ( ){ }222121212122222212

1rrzyxzyx +++++=

( ) ( ){ }232121212123232322

1rrzyxzyx +++++=

( )( ) ( )( )( )( ) ( )( )13121312

311221131

xxyyyyxx

zzyyzzyy

=

( )( ) ( )( )( )( ) ( )( )13121312

211331122

xxyyyyxx

zzxxzzxx

=

( ) ( )( )( ) ( )( )13121312

2121131

xxyyyyxx

yyyy

=

( ) ( )( )( ) ( )( )13121312

1132122

xxyyyyxx

xxxx

=

111 x=

122 y=

D. Synchronization of dataSince the time the receiving transducers receive

acoustic signal from the transmitting transducer and thetime of GPS positional data are not synchronized with eachother, the results of calculation of positions of an objectpoint become incorrect when the measurement isperformed on the boat that is rolling and/or pitching andthe received data are used as they are. To correct the data,the system synchronizes the time of the slant range dataobtained from the acoustic signal with the positional datareceived from GPS.

x

y

z

r1 r2 r3

A(x1,y1,z1)

B(x2,y2,z2)

C(x3,y3,z3)

M (x, y, z)

Figure 2 A, B, C and M space coordinate diagram

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III. Outline of the system

The system that has been developed consists of asurface positioning section, an underwater positioningsection, and an onboard processing section, of whichschematic view is shown in Figure 4.

The surface positioning section adopts the RTK-GPSsystem that allows precision measurement of positions onthe sea.

The underwater positioning section adopts a methodthat minimizes the positional error by using four sets ofultrasonic receiving transducers that are placedimmediately below the precision RTK-GPS positioningsystem, making this system more practicable.

A measurement process that moves the ultrasonictransmitting transducer to a measurement point iscompleted within 10 to 20 seconds. The measured positioncan be confirmed as needed on the LCD display thatshows the positional information.

The onboard processing section is used mainly fordisplaying positions of GPS receiving antennas and theultrasonic transmitting transducer, setting destinations, andoutputting the measurement results.

Acoustic signal received by transducer

Signal processed by processor

GPS1 received signal

The system calculates the position at the time the acoustic signal is received based on the GPS

positional data obtained before and after the receiving of an acoustic signal by interpolation.

Since the position of GPS (receiving transducer) at the time of receiving of the acoustic wave is

obtained, the position is corrected.

Figure 3 Calculation of coordinates in the new system

Attitude of sensor frame (side view)

GPS2 received signal

GPS3 received signal

GPS4 received signal

Figure 4 Schematic view of the simplified underwaterpositioning system

Onboard processing section

GPS receiver (mobile station)

Multiplexer

GPS receivingprocessor

Information

processor

Acoustic signal

processor

Wirelessantenna

Wireless

antenna GPS antenna

(reference

station)

GPS receiver

(reference station)

GPS antenna

(mobile station)

receiving transducer

LCD display

transmitting transducer

Surface processing section

Underwater processing section

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. Targets of development and technical issues

Taking into consideration the utilization of this systemfor management of execution of port and harbor works,the targets of development were established as shown inTable 1.

Table 1Targets of developmentItem Development target Remarks

Measurement accuracy Within 5 cm in X, Y and Z directions Wave height : 1 m or lower

Depth of water Down to approximately 30 meters

Dimensions of system Installable on the small boat

Maneuverability Single operator

The following matters were examined for adoption ofthis system.

Sensor frame is to be given rigid body to prevent

deterioration of measurement accuracy

The GPS receivers and ultrasonic receivingtransducers are distributed in four (4) points tominimize the measurement error and for efficientmeasurement.

The measurement error is to be suppressed bycorrecting the error of individual data for themovements of the system components caused by themovement of the water mass.

The ultrasonic transmitting transducer is to bedesigned so that it can be moved either by a diver or

an ROV.

. Verification of operations on actual sea area

The on-site examination on underwater positioning ofthe system was performed in the revetment construction

site on the artificial island off Shimonoseki, Japan, andthe results were compared with those obtained byconventional surveying (leveling) method. For theunderwater position measurement obtained by thissystem, the experimental results obtained by using theultrasonic transmitting transducer installed by a diver arecollectively named , and those obtained by using anROV that is equipped with the ultrasonic transmittingtransducer is collectively named.

Figure 5 shows the site where the experiment wasperformed. The sensor frame is shown in Figures 6-1 and6-2, surface positioning section is shown in Figures 8-1and 8-2, onboard processing section is shown in Figures6-1 and 6-2, underwater positioning section is shown in

Figure 9-1 and 9-2, and the ROV equipped with theultrasonic transmitting transducer is shown in Figure 10.

Figure 5 Experimental site (Revetment work site on artificial island, off Shimonoseki, Japan)

Caisson

(2)

Caisson

(1)

Reference point

(reserved)

Reference point

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Figure 6-1 Sensor frame (assembled)

Figure 8-1 Onboardprocessor (data

transmission antenna)

Figure 6-2 Sensor frame (on the sea)

Figure 8-2 Onboardprocessor (signal andinformation processor)

Figure 7-1 Surfacepositioning section (GPS

reference station)

Figure 9-1 Underwaterpositioning section

(ultrasonic transmittingtransducer)

Figure 9-2 Underwaterpositioning section (ultrasonictransmitting transducer, placed

on sea bottom)

Figure 10 ROV equipped with ultrasonictransmitting transducer

Figure 7-2 Surfacepositioning section (data

transmission antenna)

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The experimental results that were obtained at themeasurement points (No.18 and No.17) of caisson footingwhere the measurement accuracy can be made relatively

easily are shown in the following tables, where Table 2shows the experimental results and Table 3 shows theexperimental results .

Table 2Comparison of experimental results (PointNo.18)

Measurement Conventional method New system (diver)point

Item(m) (m) (m) (m) (m) (m)

Measurementvalue

-387.627 -34.561 -12.02 -387.601 -34.601 -12.01218

Absoluteerror

-0.026 0.04 -0.008

Table 3Comparison of experimental results (PointNo.17)

Measurement Conventional method New system (ROV)

pointItem

(m) (m) (m) (m) (m) (m)

Measurementvalue

-386.173 -51.298 -12.001 -386.181 -51.288 -11.977

17 Absoluteerror

0.008 -0.01 -0.024

From the Tables 2 and 3, we are able to verify that allthe measurement values X, Y and Z are within theaccuracy of 5cm.

With the experiment that used ROV equipped withthe ultrasonic transmitting transducer, we verified theeffectiveness of the system through the followingmatters.

No restriction on the diving period of time The measurement point can be confirmed on the

display. The ROV can be guided to the measurement point

while monitoring the movement on the screen.(Figure 11 shows an example of movement of ROV.)

. Discussion

According to the results of experiment of this systemon the actual sea area, we verified that it is effective forunderwater positioning with a high degree of accuracyand it can be operated easily. Based on these results, weare able to expect the effects as described below whenthis system in put into actual construction works.

For example, as for the number of persons required

for the measurement on the leveled surface of foundationriprap, conventional surveying method requires 8persons (3 surveyors, 2 for holding underwater staff, and

3 divers including assistant(s)), the new systemrequires 4 persons (1 surveyor and 3 divers includingassistant(s)), the new system with ROV requires 1person (1 surveyor).

Although assembling of the sensor frame spend sometime, the use of the system significantly reduces the totaltime of the process, such as reduction of themeasurement time and instant output of the measurementresults that eliminates manual data processing.

As a conclusion, practical application of this system

may promote the efficiency of the surveying work,reduce the cost and improve the safety and workability.

East-West(east on the right side of the screen)

Figure 11 Result of analysis [measurement point No.17: New system (ROV)]

ROV launched

Depthfroms

easurface(m)

No.17