A shipborne type wave height meter for oceangoing vessels, using microwave doppler radar

AbstractA wave-height meter using a simple microwave Doppler

radar, = 10 mW in power and10.525 GHzin frequency, isproposed so that we can measure oceanic waves effectively while the ship is steaming.

It was first applied to the measurement of the variation of water level

generated in a wave tank, which suggested that it is adequately applicable to the measurement of oceanic waves.Afield test was carried out off the

cape of Nojimazaki by installing the Doppler radar5m above the sea level at the bow of the ship. The result agreed reasonably well with that

measured simultaneously by the ultrasonic wave-height meter installed at

the same position. Another test is running successfully on a larger ship with the wave-height meter installed at 9 m above the sea level. The

significant wave height measured by the present meter is being compared

with that observed visually by the navigation officers. I. INTRODUCTION

THE

_{height, direction, and period, is significant not only for}INFORMATION about oceanic waves, such as wave

navigation of ships but also for works on fishing boats and rigs in the ocean. So far subjective data have been obtained through visual observation of the navigation officers on oceangoing vessels. Meanwhile, several instruments have been developed

and applied [l]-[3] in order to obtain reliable data. But the

devices are so large and expensive that they have not come to be used widely except for special research in limited areas and

periods. For practical purpose, shipborne-type wave-height

meters are to be the most useful

for replacing visual

observations. Such meters can consist of a transmitter and a receiver of microwave or sonic wave [4], [5], installed at the

bow of the ship, by which temporally varying range to the

surface of the water is measured while the ship is steaming. The measurable range is limited, however, in the meter with ultrasonic wave due to the low propagation ability.

There is another type of wave-height meter [6] of the

shipborne type that measures hydraulic pressure with gauges attached to the side of the ship below the water surface. But it is effectively used only when the ships are drifting, and not suitable for ships which are under way.

It has been reported recently [71 that the analysis of the scattered wave of a marine radar by the sea waves supplies useful information. Although the preliminary experiment

shows its possibility, further research seems to be needed for practical use.

Our wave-height meter, proposed here, adopts as a sensora

Manuscript received August24, ¡984;revised January9, 1985.This work was supported by T.S.K. Corporation, Yokohama 220, Japan, and Denshi Kougyou Ltd., Mitaka 181,Japan.

The authors are with the Department of Navigation Sciences and Engineer-ing.TokyoUniversityofMercantile Marine, Etchujima, Kotoku. Tokyo135,

Japan.

0364-9059/85/0400-O I 38$0 I .00 © 1985 IEEE

t.ab. y. ScIepsbouwkun

Tprhnìsche..,.flO4W-1-Deth

138 _{IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. OE.1O. NO. 2, APRIL 1985}

A Shipborne-Type Wave-Height Meter for Oceangoing

Vessels, Using Microwave Doppler Radar

AMO YASUDA, SUSUMU KUWASHIMA, AND YASUBUMI KANAl

small, inexpensive, and simply structured microwave Doppler

radar.

It was first tested by means of measuring waves

generated in a wave tank. Then a field test was carried out in which the wave record was analytically compared with that measured simultaneously by an ultrasonic wave-height meter. We are continuing another test on a larger ship, comparing the

significant wave-height records given by the simple analog

circuit of the meter with those observed visually by navigation

officers. It seems to be more useful for the officers in the

bridge of the ship to get the significant wave height rather than

the variation of the sea level at every moment. A circuit to

obtain the significant wave height is also proposed. II.THE STRUCTURE OF THE MICROWAVE

WAVE-HEIGHT METER [81

The present wave-height meter is composed of two compo-nents, a sensor with a microwave Dopplerradar and a signal

processor with a display panel, installed at the bow and the

bridge, respectively.

A. The Sensor

A microwave Doppler radar is used for the sensor. It is made of a short waveguide with a Gunrt oscillator and two detector diodes, mounted Xg/8 (with Xg the microwave

wavelength in a waveguide) apart from each other. Being so small, light, simple in structure, and cheap, it is widely used as the sensor of automatic doors, crime prevention devices, and so on.

It emits microwaves of 10 mW in power and 10.525 GHz

in frequency vertically downward through a 19-dB electro-magetic horn attached to it as shown in Fig. 1. The emitted wave is reflected back to the radar by the fluctuating sea

surface, which causes the Doppler shift in its frequency. The

wave reentering through the horn is mixed with the wave

directly emitted from the Gunn oscillator. The ac components of the output voltage e1, e2 of two detector diodes D1, D2 are given, respectively, by 1 4ir

eix cos

j Z-4

and

e2 cos

j Z_iF--

747

7 2

where Z is the value of distance from the Gunn oscillator to the

sea surface, iF is a constant phase shift depending on distance

between Gunn oscillator Z1 and D1, X0 is the wavelength of the

(I)

e, i

)

cn Cab ej e2 Gunn 05C D' 02

Hor\

Window

Fig. 1. Schematic diagram of the sensor.

microwave in the free space, and it is assumed that the power of the reentrant wave is small enough.

Equations (1) and (2) show that e1 and e2 vary one cycle with every X0/2 variation of Z, e1 leads e2 by 7r/2 when Z increases

(i.e., the sea surface falls), and e1 lags e2 by i/2 when Z

decreases (i.e., the sea surface rises). Thus the temporal

variation of the sea surface can be obtained at the signal

processor by subtracting or adding the number of oscillations of e1 according to whether e1 leads or lags e2. e1 and e2 are transmitted to the signal processor through a long cable after being amplified.

The sensor, consisting of a microwave Doppler radar, a horn, and a preamplifier, is placed in a strong cylindrical container made of steel with polyethylene windows for the emission and reentry of the microwave. It is installed at the

bow df the ship.

B. The Signal Processor

The block diagram of the signal processor is shown in Fig.

2. Fig. 3(a) and (b) shows the waveforms of e1 and e2,

respectively. The phase discriminator detects which signal

leads, e, or e2. At the same time, it generates a pulse at the up terminal, as shown in Fig. 3(c), corresponding to every

one-quarter cycle of e1, when e1 lags e2. Therefore the resolution of

the measurement is X01 8, which corresponds to 3.6 mm in the

present experiment. When e leads e2, it generates the same

pulse, as showp in Fig. 3(d), at the down terminal. The output

voltage, shown in Fig. 3(e), of the integrator is the relative

sea-level variation, defined as relative encounter wave here including up and down motions of the sensor, because the ship itself pitches in a wavy sea. The distance of the up and down

motions of the bow is deduced by double integrating the acceleration, which is measured by an accelerometer and

mounted in a pendulum-type gimbal [9] installed just behind the bow. Thus the real sea-level variation i(t), defined as the

encountered wave, is given by the output voltage of the

differential amplifier. The word "encounter" is used here to suggest that the time scale of the sea-level variation is still

relative to the ship's speed.

(a)

e, from the sensor

SIGNIFICANT WAVE HEIGHT CIRCUIT DISPLAY PANEL e2

ALA

wvv

wvv

A

u

Il Ill

I

II II

I I I down e2 encounter wave

Fig. 2. Block diagram of the signal processor.

cm

DISPLACEMENT 3.6

TIME

'-Fig. 3. Voltage waveforms at (a) e,, (b) e1, (c) up terminal. (d) down

terminal, and (e) output of differential integrator, in Fig. 2. The significant wave height is deduced by processing the signal q(1) and displayed on the digital display panel.

III. THE SIGNIFICANT WAVE-HEIGHT CIRCUIT [8]

The wave height is the difference between a maximum and a

minimum of (t) adjacent to each other. The significant wave height H3 is defined as the mean of the highest one third of the wave heights measured. As the variation of the wave height follows a Rayleigh distribution [10], H is also defined as

H3=4.004 flrrns where DIFFERENTIAL AMPLIFIER rel a t i ve encounter wave displacement of the bow TIME ti 11 1111 11111111

UDA et a/. SHIPBORNE WAVE-HEIGHT METER FOR OCEAN VESSELS 139

flrnis =

i rO

2(t) dt.

As j(t) varies randomly and follows a Gaussian distribution,

s-,,,, is related with averaged s(t), positive value of s7(t), i,

51rms'fl+

(5) where

lo

+ =

(t) dt.

.1T H3 is deduced from (3)-(6) 113=10.0 +.

from the accelerometer DOUBLE INTEGRATOR up DIFFERENTIAL INTEGRATOR PHASE DISCRIMINATOR own

The significant wave-height circuit proposed here is com-posed of a rectifier, an averager, and a multiplier, as shown in Fig. 4, where Va

K(t), Vb = Ki+(t), V =

and

= Kif. (K: constant). The output voltage of the averager V is given by

V= Ç

o

V,, exp - dt.

(t)

CR-r

CR

This equation is the average of the voltage from the past to the present with a weight of exp[t/(CR)] (t < O). It is coincident

with (6) when 77(t) is a stationary process and CR is long

enough compared to the time constant of the variation of i(t).

Fig. 5 shows the result of a computer simulation, which

proves that the present circuit works accurately enough. j(t),

which has a Pierson-Moskowitz spectrum [11], varies in a

stationary manner for up to 25 min and then becomes

nonstationary, as shown by a wavy line around the horizontal zero line in the figure. The solid curve is the H deduced rigorously from (t), drawn in the figure, by (3) and (4) with T

= 6 min. The curve with dots and H3 with daggers are

those deduced by (7) and (8) with CR = 6 and 3 mm,

respectively. The curve is very close to the curve H5.3,

which is in principle to be obtained approximately by the present Circuit whose time constant CR is half of the time

period of the rigorous calculation.

IV. EXPERIMENTAL RESULTS AND DISCUSSIONS

A. Wave-Tank Test

The wave-height meter, introduced here, was tested first by measuring various plane waves generated in a wave tank. Fig.

(8)

Fig. 4. Schematic diagram of the significant wave-height Circuit.

10 20 30

TIME (minute)

Fig. 5. Temporal variation of (t) and its significant wave heights deduced rigorously H,, by (3) and (4) with T = 6 mm, and approximately H3 and H by (7) and (8) with CR = 3 and 6 mm, respectively, corresponding to the output of the circuit in Fig. 4.

Fig. 6 Records of water- evel variations of a wave of 0.5 Hz in frequency and 8.5 cm in height by the present wave-height meter (upper trace) with the sensor hung 2 m above the water level and by the capacity-type water-level meter (lower trace) obtained in a wave tank.

6 is an example of the experimental records. The upper trace shows water-level variation, produced by a wave of 0.5 Hz in

frequency and 8.5 cm in height, measured by the present

wave-height meter. The sensor was hung from the ceiling, 2 m above the water level. The lower trace is that measured as a reference by the capacity-type water-level meter immersed in the tank at the same distance from the wave-generating board. The vertical scales are 2.0 and 2.5 cm/div, respectively. They

are recorded with various time scales, 10, 1, and 3s/div, in order to show the waveform as precisely as possible. They

T -T '

I

11111011 r iliflifli

-12

₅₌

EEE!!EEE!EI!II5

Á

IllhIIÎiIIl,ij

J_j

.

iIIIIiIIj

S...

I:+

.Ii:îi

140 IEEE JOURNAL OF OCEANIC ENGINEERING. VOL. OE.IO, NO. 2. APRIL 1985

RECTIFIER A VE RAGER MULTIPLIER

ASUDA et al.: SHIPRORNE WAVE-HEIGHT METER FOR OCEAN VESSELS

agree perfectly with each other, although the measurement procedure in the upper trace was rather sophisticated. The

lower trace is shifted to the right by about onc-halfdivision in reference to the upper one, due to the pen arrangement of the recorder.

The ratio of h ' to h was deduced for Various waves and

altitudes of the sensor in the tests. h ' is here a wave height deduced by averaging displacement between the maxima and the minima adjacent to each other in the upper trace and h is that in the lower trace.

A test was made in order to obtain h '1h in reference to h/L (with L the wavelength) which gave the maximum gradient of

the water surface for sinusoidal waves. The altitude of the sensor was fixed at 3 m above the water level. The result showed that h '¡h starts to decrease when h/L exceeds 0.04 and reaches 0.9 at h/L = 0.05. The standard deviation of h'

also starts to increase when h/L exceeds 0.04. This means that a measured value of the wave height is underestimated for a

wave with a steeper surface than h/L

0.05. It can be

explained as follows. When the water surface, where the

microwave is reflected, has a gradient, the re-entrant power

decreases due to the directional dependency of the horn

antenna in transmitting and receiving microwave. Thus the

signals corresponding to e1 and e2 in Fig. 3 are too small to

surmount the noise around the maximum gradient of the

surface when hIL exceeds 0.04.

Then we applied the other test to obtain h '1h in reference to the altitude of the sensor by generating stationary waves of 0.8 Hz in frequency and 10 cm in height. The result showed that

h'/h decreases sharply when the altitude exceeds 2.8 m,

which corresponds to 1.2 times the wavelength. This means that the present wave-height meter underestimates the wave height for waves shorter than the altitude of the sensor. The diameter of the footprint, on the fringe of which the irradiating power is half of that at the center, should be less than half of

the .wavelength to measure the wave height with a good spatial

resolution, as it is about half of the altitude of the sensor used

here.

These results seem to suggest that applicability of the present meter to the real sea is rathcr limited. But while the

wave surface in the tank is quite smooth, the real sea surface is usually composed of waves of different frequencies, heights, directions, and ripples generated by winds, which can enhance

the reflection cross section by means of Bragg effect. Thus

even though h/L of the dominant wave in the real sea is larger than 0.05, there can be enough re-entrant power to the sensor, and then the present wave-height meter is expected to work reasonably well, as the results of the field tests suggest. As for the waves shorter than the altitude of the sensor, they are not significant in considering the interaction with ships.

B. Field Test I

A field test was carried out in July of 1982 on a research ship (331 GT) off the cape of Nojimazaki. The sensor was

installed at the bow, 5 m above the sea level. A transmitter and a receiver of a conventional ultrasonic wave-height meter [5J

were installed at both sides of the sensor and the data were

Fig 7. Relative encounter wave records measured simu taneously by

ultrasonic wave-height meter (upper trace) and the present wave-height meter (lower trace).

loo

E 10

>-I

(n

z

LU

a

1

LE

I

u

LU û-0.1

1982. 7.28

- sonic wave

xxx microwave

X X

xx

)Qt >occ 141 0.2 0.4 0.6 0.8 1.0 FREQUENCY (Hz)

Fig. 8. Power spectral-density functions deduced from the wave records in Fig. 7 by a maximum entropy method.

used as a reference for those by the present wave-height meter. The ship was steaming with her normal speed.

Fig. 7 shows the records of the temporal variation of the relative encounter wave measured simultaneously by the

ultrasonic wave-height meter (upper trace) and the present one (lower trace). The lower trace is shifted to the right by about one division in reference to the upper one. Although the fine structure of the two records is different from each other, the variations of the wave heights are similar.

Fig. 8 shows the power spectral-density function deduced from the records in Fig. 7 by a maximum entropy method. The solid line shows the values obtained by the ultrasonic

height meter and the crosses of those by the present

wave-.-

r.rr .,fl,r,

I- - -. - P111

9.2 72B

iiIIIIiiI;iiiM;i;;;

:Il

II I.

. J II ih1!ItIItII

I ILUIIUII

uhII

II II

igflI g

uiuuului

UHhlIllLIo

I!ilJjiI!

lUll

cm

Il

I IJ1I sonic Os

-lIIIUlI

I

IIIIIIIIIII -

INIIIII

,iiI,J ii I1ilfluJ!Ii

I. -i il: 111

Fig. 9. The sensor installed at the bow in the field test II.

SIGNIFICANT WAVE HEIGHT (e)

1 2 3 4 5 o ° o o 't. o' .

O...

t o :

,1'

o o r:

_'I.'

/

₁

o

0

:-'

b.

NOON POSITION 00'aO' S 173°47' E 04'04' N 167'37' E 08'30' N 16225' E 13O9' N 157'24' E 17'38' Pl 1522l'E 22'12' N 147'30' E 26'32 N 142°02' E 29'34' N 14O36' E TOKYO

Fig. 10. Comparison between the measured and visually observed signifi-cant wave heights. Dots are plotted from the recorded data by the present wave-height meter. Circles show the wave heights observed visually by the officers.

142 _{IEEE JOURNAL OF OCEANIC ENGINEERING. VOL. 0E-10, NO. 2. APRIL 1985}

height meter. They are very close to each other at around the frequency of the peak spectral density. Although the discrep-ancy is found at the higher frequencies, the spectral density is so small there compared to the peak values that the measure-ment by the present meter is accurate enough for practical use.

C. Field Test II

The other field test is being carried out in a larger ship (5875

GT). The sensor is installed at the bow, 9 m above the sea

level, as shown in Fig. 9. There is no reference instrument to

measure wave height on the ship. The altitude makes it

impracticable to adopt the ultrasonic wave-height meter used in Section IV-B. On the other hand, the maximum measurable

range of the wave-height meter is estimated to be 30 m by

measuring the displacement of a small copper plate (25 X 50 cm2) on land. The measured significant wave height, by the

circuit in Fig. 4, is recorded on a rolled paper of

a Yt

recorder. The recorded data are compared with those observed visually several times a day by the navigation officers.

Fig. 10 shows an example of the comparison between the recorded and the observed data for the significant wave height.

The dots represent the recorded data obtained every i h.

The circles are those observed visually by the officers in turn. They were collected from February 26 to March 5, 1984 on a voyage in the Pacific Ocean from American Samoa to Tokyo.

The daily position of the ship at noon is indicated in the right- DATE TIME hand side of the figure.

26 Feb. '84

The data, obtained by visual observation, scatter closely 12

around those measured when the wave heights are low. This

seems to suggest that the present wave-height meter is reliable 27 Feb. '84 O

enough so long as the sea is fairly calm. On the other hand, when the wave grows, they scatter more widely and are lower

than those measured. But it does not necessarily imply that the 28 Feb. 84 meter is unreliable. The visual observation is not reliable ina

rigorous sense, as it

depends greatly on the

ability of

individuals. Besides, the estimation of the wave height is not 29 Feb. 84 easy on a ship which is steaming in a large swell with a long

period, especially on dark nights.

The officers' estimates indicate that the measured valuesare

reasonable, although the visually observed data

_{are not}

sufficient to allow us to conclude that the present wave-height meter is definitely reliable and applicable. We are continuing this test on the same ship in order to confirm the applicability

more accurately and at the same time

to bring about

improvements in the reliability and usefulness of the

instru-ment.

V. CONCLUSIONS

The test of the wave-height meter with a simple microwave Doppler radar seems to suggest that the meter is on the whole

reliable for the practical purposes of measuring both the temporal variation of the sea level and the significant wave height.

Considering that the microwave Doppler radar and the

signal-processing circuit are quite simple and inexpensive and that the size of the sensor is so small compared with that of the conventional wave-height meter that the protection from direct

wave impact is easier, it is expected that the present

wave- 12-O 12 O 12-O 12 O 12 O 12 O 12 O 12 I Ma,-. 84 2 Mar. '84 3 Mar. '84 4 Mar. 84 5 Mar. 84

height meter will be used widely on the merchant ships to

obtain accurate wave data.

Visually observed wave data are being gathered at the

moment to compile a chart of ocean conditions. The

introduc-tion of the instrument is helpful not only for the accurate

drafting of the chart but also for the reduction of the load of the officers. It is also useful for research on the wave effects upon ships and/or their cargo.

ACKNOWLEDGMENT

The authors appreciate Prof. N. Iwata and Prof. H. Ando of Tokyo University of Mercantile Marine for their useful

suggestions and continuous encouragement. Prof. M. Ishizuka is also appreciated for his help in preparing the manuscript.

The Captains and other staffs of "Shioji-maru" of Tokyo University of Mercantile Marine and "Taisei-maru" of the Institute for Sea Training of Japan are appreciated for their

cooperation on the field tests I and II, respectively. We also

thank Dr. N. Hogben of National Maritime Institute Ltd.,

London, for his critical reading of the manuscript.

REFERENCES

J. Darbyshire. "Wave measurements with a radar altimeter over the

Irish sea,"Deep Sea Res., vol. 17, pp. 893-901, 1970.

A. Jam, "Determination of ocean wave heights from synthetic aperture

radar imagery," Appl. Phys., voI. 13, pp. 371-382, 1977.

131 G. P. de Loor and P. Hoogerboom, 'Radar backseatter measurements

from platform Noordwijk in the North Sea," IEEE J. Ocean Eng., vol. 0E-7, no. 1, pp. 15-20, Jan. 1982.

K. Taira and A. Terada, "On the response characteristics of a sonic wave gauge,"J. Oceanog. Soc. Japan, vol.25, pp. 299-306, 1969.

S. Kuwashima, "Measurement of the encounter wave using ship-borne ultrasonic wave meter,"J. Japan Inst. Navigation, vol. 64, pp. 87-95, Jan. 1981 (in Japanese).

M. J. Tucker. "A ship borne wave recorder," Trans. Inst. Naval Architect, London, no. 98, pp. 236-346, 1956.

T. moue and M. Kato, "A method of wave observation and analysis using a shipborne navigation radar," J. Japan Inst. Navigation, vol.

67, pp. 127-135, Aug. 1982 (in Japanese).

Yasuda, Y. Kanai, and S. Kuwashima, "A wave height meter for ocean going vessels using a simple microwave radar," J. Japan Inst. Navigation, vol. 66, pp. 3 l-38. 1982 (in Japanese).

S. Kuwashima, A. Yasuda, and Y. Kanai, "A characteristics of gimbals on a shipAn effect on the installed accelerometer," J.

ItA C! al.: SlIlIlIORNI \VAVIIIId(I Il MIIIR IOI (X1AN VISSII.S 143

J. Japan Inst. Navigation. voI. 69, pp. 159-170, Sept. 1983 (in

Japanese).

¡lOI M. S. Longuet-Iliggines, ''On the statistical distribution of the heights of sea waves," J. Marine Res., vol. 4, no. 3, pp. 245-266. 1952.

[Il]

W. J. Pierson and L. Moskowitz, "A proposed spectral form for fully

developed wind seas based on the similarity theory of S. A. Ki-taigorodski," J, Geophys. Res., vol. 69, no. 24, pp. 5181-5190.

1964.

*

Akio Yasuda was born in 1943 in Nagoya, Japan. He received the B.S. degree in electrical engineer-ing from Nagoya Institute of Technology. Nagoya, Japan, in 1966 and the M.S. and Dr.Eng. degrees from Nagoya University.

He is currently an Associate Professor with the Department of Navigation Sciences and

Engineer-ing at Tokyo University of Mercantile Marine, Tokyo. Japan. His research interests are in the oceanic engineering and plasma diagnostics by

lasers.

Dr. Yasuda is a member of the Japan Institute of Navigation, the Japan Society of Plasma Science and Nuclear Fusion Research, the Physical Society of Japan, the Institute of Electronics and Communication Engineers of Japan.

the Institute of Electrical Engineers of Japan. and the Laser Society of Japan.

*

Susumu Kuwashima was born in 1943 in Tokyo, Japan. He received the B.S. degree in navigation

sciences from Tokyo University of Mercantile

Marine, Tokyo. Japan.

He is currently an Associaie Professor with the Department of Navigation Sciences and Engineer-ing at Tokyo University of Mercantile Marine. His research interests are in the field of ocean-wave and ship-weather routing.

Mr. Kuwashima is a member of the Japan

Institute of Navigation, the Society of Naval Archi-tects of Japan, and the Oceanographical Society of Japan.

*

Yasubumi Kanai was born in Saitama, Japan, on February 5, 1943. He received the B.S. degree in

electrical engeering from Tokyo University of Electric Machine, Tokyo. Japan. He obtained the licence of the first-class radio operator in 1976.

He is currently a Teaching and Technical

Assist-ant and a Radio Officer at Tokyo University of

Mercantile Marine, Tokyo, Japan.

Mr. Kanai is a member of the Japan Institute of Navigation.