A system for using radar to record wave direction

Ay HOE 307

TR 79-1

A System for Using Radar to Record
Wave Direction

by \ DOCUMENT }
M.G. Mattie and D.L. Harris )

TECHNICAL REPORT NO. 79-1
SEPTEMBER 1979

Approved for public release;
distribution unlimited.

U.S. ARMY, CORPS OF ENGINEERS
COASTAL ENGINEERING
228 RESEARCH CENTER
OO Kingman Building
Wes Fort Belvoir, Va. 22060
yw: 7H

Reprint or republication of any of this material shall give appropriate
credit to the U.S. Army Coastal Engineering Research Center.

Limited free distribution within the United States of single copies of
this publication has been made by this Center. Additional copies are
available from:

National Technical Information Service
ATTN: Operations Division

5285 Port Royal Road

Springfield, Virginia 22161

Contents of this report are not to be used for advertising,
publication, or promotional purposes. Citation of trade names does not
constitute an official endorsement or approval of the use of such
commercial products.

The findings in this report are not to be construed as an official
Department of the Army position unless so designated by other
authorized documents.

NOUN OL

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REPORT DOCUMENTATION PAGE BEFORE COMPLETING FORM
1. REPORT NUMBER 2. GOVT ACCESSION NO 3. RECIPIENT'S CATALOG NUMBER
TR 79-1

4. TITLE (and Subtitle) 5. TYPE OF REPORT & PERIOD COVERED

A SYSTEM FOR USING RADAR TO RECORD WAVE DIRECTION

Technical Report

6.

PERFORMING ORG. REPORT NUMBER

7. AUTHOR(a) 8. CONTRACT OR GRANT NUMBER(a)

M.G. Mattie and D.L. Harris

10. PROGRAM ELEMENT, PROJECT, TASK
AREA & WORK UNIT NUMBERS

9. PERFORMING ORGANIZATION NAME AND ADDRESS
Department of the Army
Coastal Engineering Research Center (CERRE-CO)
Kingman Building, Fort Belvoir, Virginia 22060

A31462

12. REPORT DATE
September 1979

13. NUMBER OF PAGES
50

15. SECURITY CLASS. (of thie report)

11. CONTROLLING OFFICE NAME AND ADDRESS
Department of the Army

Coastal Engineering Research Center
Kingman Building, Fort Belvoir, Virginia 22060
14. MONITORING AGENCY NAME & ADDRESS(if different from Controlling Office)

UNCLASSIFIED

DECL ASSIFICATION/ DOWNGRADING
SCHEDULE

15a.

16. DISTRIBUTION STATEMENT (of thie Report)

Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT (of the abstract entered in Block 20, if different from Report)

18. SUPPLEMENTARY NOTES

19. KEY WORDS (Continue on reverse aide if necessary and identify by block number)

Aerial photos Radar imagery Wavelength Wave trains
Radar Wave direction Wave refraction Waves

20. ABSTRACT (Continue on reverse side if neceasary and identify by block number)
A radar system that provides images of waves in the coastal zone to obtain
wave direction information is described. The heart of the system is an X-band
marine radar which operates at 9,375 megahertz with a 3-centimeter wavelength
and a 0.05-microsecond pulse width. The records are accumulated by photograph-
ing the plan position indicator (PPI) scope.
The system is designed for unattended operation and includes an automation

unit that activates the radar at preset times, usually at 2-hour intervals. A
continued

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sequence of one to nine photos is obtained on each of four ranges after which
the system turns off until the next scheduled observation.

Wave images show single and multiple long-crested wave trains permitting
observation of wave refraction and shoaling. The information content in the
radar images is similar to that in aerial photos. However, radar images can
be collected at night, during overcast, and in light rain conditions. Radar
imagery is more limited than aircraft imagery in geographical coverage and in
the quality of the imagery.

Good agreement was found between estimates of wave direction and length
obtained with radar and those obtained with other observational techniques.

2

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PREFACE

This report is published to provide coastal engineers with the
results of a project to develop a radar device that will automatically
collect wave direction information at coastal sites. The work was
carried out under the coastal hydraulics research program of the U.S.
Army Coastal Engineering Research Center (CERC).

This report was prepared by Mr. Michael G. Mattie, Physicist, and
Dr. D. Lee Harris, Chief, Coastal Oceanography Branch, CERC.

The development of the CERC radar system was accomplished through
the advice and support of many people and several organizations. Par-
ticular recognition is given to CWO L. Brown of the U.S. Coast Guard
Research and Development Center; C. McKinney, Pacific Missile Test
Center, who assembled the CERC system; C. Gable who monitored the radar
system assembly; and J. Dayton who provided photographic services.
Thanks are also extended to Dr. O. Shemdin of the Jet Propulsion
Laboratory for providing the West Coast Experiment data.

Comments on this publication are invited.

Approved for publication in accordance with Public Law 166, 79th
Congress, approved 31 July 1945, as supplemented by Public Law 172,
88th Congress, approved 7 November 1963.

ED E. BISHOP
Colonel, Corps of Engineers
Commander and Director

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CONTENTS

CONVERSION FACTORS, U.S. CUSTOMARY TO METRIC (SI).
INTRODUCTION .
A SYSTEM FOR RECORDING WAVE DIRECTION BY RADAR .

RADAR RESULTS. :
1. Images of Single aud | Murer pire Wave ieatine
2. Possible Measurements with Radar.
3. Methods for Analysis of Radar Photos.
4. Measurement Errors. LA EGIESA, |e)

THEORY OF RADAR IMAGING.
1. Radar Resolution. a i
CPeSicakeerninceMechandsmSmrnn) rem men leemnr
3. Role of Wind in Scattering.
4, Scattering from Wave Crests

VALIDATION .
1. Comparison mareh Reman ipheres .
2. Comparison with One-Dimensional Wave Saacian.

OTHER RADAR DEVICES AND DIRECTION MEASURING SYSTEMS.
1. Radar Devices i :
2. Determination of Wave Dissection Anon an 3 serpey7 of Sensors.

DISCUSSION .
1. Additional Capabilities ae CERC Rader r System.
2. Future Plans. o bo) 0 6 Q

SUMMARY .
LITERATURE CITED .

TABLE
Comparison of information from CERC radar images with that
obtained with other direction measuring devices

FIGURES
CERC radar image of waves in the Mission Beach area, San Diego,
California, taken 29 March 1977 .

Aerial photo of the Mission Beach area, San Diego, California,
taken 29 March 1977 by a NASA U-2 aircraft in Rte ee of the
West Coast Experiment siya ecacit a hes .

CERC radar system as deployed at Mission Beach, San Diego,
California, during the West Coast Experiment in March 1977.

Page

37

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18

19

20

CONTENTS

FIGURES--Continued

Page

CERG wadaxz sysitemyPPLsandiBollexdcamenrall sy ey uae caney a ne
PPI photo of a single wave train imaged by the Decca radar... 14
PPI photo of a single wave train imaged by the CERC radar. ... 14
Two wave trains as seen by the CERC radar at Channel Islands

Harbor, Californias, ef) a ohio ole gas: modus didcoiihs musk <4 Sao ee
Three wave trains on a PPI photo taken at Channel Islands

Harbor, (Cala fornadays iy us aii Oe Ue cetaan aay en lne ponies nsell: on tea rr TES
Radar wave image taken 12 January 1976 at Cape Cod,

Masisachusetts;) Rangel 1s) 926 umetersigc ea cpieh i leit ou, ee omusn monn nn
Radar wave image taken 12 January 1976 at Cape Cod,

Massachusetts. Range is 1.39 kilometers. .......... . M17
Radar wave image taken 12 January 1976 at Cape Cod,

Massachusetts. Range is 2.78 kilometers. . . 18
Radar wave image taken 12 January 1976 at Cape Cod,

Massachusetts. Range is 5.56 kilometers. ........... 18
Radar wave image from Channel Islands Harbor, California,

29 Aprv dl TOLD ox eyheoine Ao eh yak epee ee ep cai hee Saale po
Radar wave image taken 5.5 seconds later of same area shown

In Figure Se ee co ys Mi te Ber ier ecet rainy lel Sk oes deh tos sunc’es erneeel ee a
Radar wave image of same area taken 5.5 seconds after Figure

14 and,1l)seconds) after, Figureil3)< o) 2-00) ee eee LS
Radar wave image from Cape Cod, Massachusetts, taken at 0026,

V2) SeaM Vary 19/7 Gy pa. y5s cee le ty jem ert Vass S vi eSlngce ody dead erstiojes teye) ti an oa)
Radar wave image from Cape Cod, Massachusetts, taken at 0500,

12 Fanuary; 1976 cd ee kj hose tambo eis) eye aan OTe ae a a 0)
Radar wave image from'Cape Cod, Massachusetts, taken at 0924,

12) Sanuary UOMO 2g) i ce. eb Mclivey et Wied faaerie Wiel el Geib Geniyicl Wee eA een ee eB
Radar wave image from Cape Cod, Massachusetts, taken at 1342,

12 January V9 76s cy Fe) es i ele) el lee nee a a ae ee ante
Radar wave image of Torrey Pines, San Diego, California,

takene2 March: WQi77 oy. prcumpene ts comedy coimeshh ue R om Keane Ritei ea areas der a eI

21

22
23

24
25

26

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29

30

31

32

33

34

35

36

37

CONTENTS
FIGURES--Continued

Radar wave image from Cape Cod, Massachusetts, taken
8 January 1976.

Radar film analysis device .

Radar image from Channel Islands Harbor, California, showing
measurement of wave direction and length.

Schematic showing Bragg scattering . .

Radar image taken with the CERC radar at Channel Islands
Harbor, California. er meiiey Miay rahe dane es

Radar wave image taken at Channel Islands Harbor 2 hours
after the photo in Figure 25.

Composite of o° data for a "medium" sea.

Schematic showing how for low radar antenna elevations larger
waves, at times, shadow the following wave crests and limit
effective range of instrument for imagery .

Schematic showing how high radar antenna elevations give a
radar return from troughs of waves, decreasing the contrast

in the imagery.

Schematic showing the restriction on line-of-sight by
curvature of the Earth.

Schematic showing the illumination of wave crests by radar
at 4.6 meters elevation...

Aerial photo taken by NASA U-2 aircraft over Mission Beach,
_San Diego, California, 14 March 1977.

SAR image of Mission Beach at San Diego, California, 28
March 1977.

Two-dimensional Fourier transform of SAR image shown in
Figure 33 .

CERC radar image at Mission Beach, 28 March 1977 .

Scatter plot of radar measurements versus pressure gage
measurements of wavelength.

Scatter plot of radar measurements versus pressure gage
measurements of period.

Page
212.
24

24
I

29

29

30

31

31

33

34

36

39

40

41

42

44

CONVERSION FACTORS, U.S. CUSTOMARY TO METRIC (SI)
UNITS OF MEASUREMENT

U.S. customary units of measurement used in this report can be converted
to metric (SI) units as follows:

Multiply by To obtain

———$—<—$<—$—————— eee eee nnn a ad
inches 25.4 millimeters
2.54 centimeters
square inches 6.452 square centimeters
cubic inches 16.39 cubic centimeters
feet 30.48 centimeters
0.3048 meters
square feet 0.0929 square meters
cubic feet 0.0283 cubic meters
yards 0.9144 meters
square yards 0.836 square meters
cubic yards 0.7646 cubic meters
miles 1.6093 kilometers
square miles 259.0 hectares
knots 1.852 kilometers per hour
acres 0.4047 hectares
foot-pounds 1.3558 newton meters
millibars 1.0197 x 1073 kilograms per square centimeter
ounces 28.35 grams
pounds 453.6 grams
0.4536 kilograms
ton, long 1.0160 metric tons
ton, short 0.9072 metric tons
degrees (angle) 0.01745 radians
Fahrenheit degrees 5/9 Celsius degrees or Kelvins!

ee
Ifo obtain Celsius (C) temperature readings from Fahrenheit (F) readings,
use formula: C = (5/9) (F -32).

To obtain Kelvin (K) readings, use formula: K = (5/9) (F -32) + 273.15.

A SYSTEM FOR USING RADAR TO RECORD WAVE DIRECTION

by
M.G. Mattte and D.L. Harris

I. INTRODUCTION

The Coastal Engineering Research Center (CERC) and the Beach Erosion
Board (BEB) (predecessor to CERC) have collected wave data along the U.S.
coasts for nearly 30 years. Although a wealth of statistics on wave
height and period and thousands of wave spectra have been obtained, few
measurements of wave direction have been made. Information on wave direc-
tion is needed for the estimation of longshore sand transport, harbor
design, and the solution of other coastal engineering problems. Many
techniques for recording wave direction have been proposed and tested
but none have been entirely satisfactory.

This report discusses a technique for recording wave direction, based
on the use of imaging radar. Ijima, Takahashi, and Sasaki (1964) and
Wright (1965) were probably among the first to report the use of radar
for imaging ocean waves. Oudshoorn (1960), Wills and Beaumont (1971),
Evmenov, et al. (1973), and others have reported wave images obtained
with radars similar to those used in the experiments described in this
report. However, it is not known whether any of these authors attempted
to develop imaging radar as an operational tool for collecting wave
information.

Radar may be used to image the prominent wave crests in a wave field
(when conditions are suitable) by photographing the display scope (see
Fig. 1). Figure 2 is an aerial photo of the same area shown in Figure 1.
In general, radar images show many of the same characteristics as aerial
photos and several distinct wave trains can often be identified. Although
radar images are not as clear as aerial photos, they have the distinct
advantage of being obtainable at night and during storms; an expensive
platform (aircraft) is also unnecessary. Interpretation of the radar
image is often as simple as that of a good aerial photo. The interpre-
tation of aerial photos has been examined by McClenan and Harris (1975).

The CERC radar system is described in Section II as a wave data col-
lection device. Samples of radar wave imagery and a data analysis system
are presented in Section III. The essentials of radar theory and opera-
tion, needed for an understanding of the engineering characteristics of
a wave data collection system, are discussed in Section IV. Section V
compares the data on wave direction and length obtained by radar with
that obtained by other methods.

Other procedures for using radar in the collection of wave data,
including several proposed procedures for obtaining these data without
radar are presented in Section VI. Section VII discusses additional
capabilities of radar and future plans for its use. A summary high-
lighting the advantages and disadvantages of obtaining wave information
with an X-band shore radar is presented in Section VIII.

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II. A SYSTEM FOR RECORDING WAVE DIRECTION BY RADAR

The CERC radar system has been assembled to automatically obtain wave
images, and includes a Raytheon 1020/9XR Mariners Pathfinder X-band radar
mounted in a van to provide mobility. A rotating antenna is mounted on
the roof of the van (Fig. 3), or on a 2-meter (6.56 feet) mast at the end
of the trailer for additional elevation. This radar has a pulse width of
0.05 microsecond (50 nanoseconds) with a range resolution of 10 to 20
meters (32.8 to 65.6 feet). The 2.74-meter (9 feet) slotted-array an-
tenna gives a horizontal beam width of 0.9° at 3 decibels and has a ro-
tation rate of 33 revolutions per minute. Pulses of electromagnetic
energy with a nominal wavelength of 3 centimeters (nominal frequency
1019 hertz) are beamed over the water. A part of this energy is scat-
tered back to the antenna by a process explained in Section IV. The
back-scattered energy is displayed on a 10-inch-diameter cathode-ray
tube (CRT) called a 'plan position indicator" (PPI) in the form of light
and dark patches which parallel the wave crests. Data are recorded by
photographing the CRT using a Bolex 16-millimeter H-16 reflex camera.

The PPI and camera are shown in Figure 4.

A CRT with a fast-decay phosphor is used. The standard CRT uses a
medium- to slow-decay phosphor which retains the target for easy viewing.
The fast phosphor is used for data collection to obtain sharp images with
the time-lapse photography.

To adjust the radar to obtain wave images, the signal gain is in-
creased for the weak sea clutter return to appear on the CRT. The scope
intensity is kept low so the CRT is not saturated. Rain clutter and sea
clutter controls are turned to a low position. The rain clutter control
is used in the "just on" position to limit some of the strong return near
the center of the scope which tends to saturate that part of the CRT.
This gives better images in the surf zone while not affecting the return
from the more distant wave areas.

The radius of the region displayed may be varied by discrete steps
from 0.695 to 44.4 kilometers (0.375 to 24 nautical miles). The opti-
mum radius or "range" for wave imaging varies with the ambient wave
conditions—shorter ranges are best for shorter waves.

The CERC system is designed for unattended operation; an automation
system turns on the radar at a periodic time interval that can be set
from 1 to 9 hours. After a suitable warmup time (about 10 minutes) the
radar signal is fed to the antenna as it scans the sea, and a sequence of
1 to 9 photos is obtained at each of four ranges, 0.695, 1.39, 2.78, and
5.56 kilometers (0.375, 0.75, 1.5, and 3 nautical miles).

The automation unit controls the opening and closing of the camera
shutter and the sector of the sweep which is photographed. The system
is then turned off until the next scheduled observation. Photos are
taken at several ranges because the optimum range to obtain information
on wave direction or length depends on the wavelength and height of the

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Figure 3. CERC radar system as deployed at Mission
Beach, San Diego, California, during the
West Coast Experiment in March 1977.

Figure 4, CERC radar system PPI
and Bolex camera.

2.

ambient waves. Multiple photos will permit the resolution of any 180°
ambiguity about wave direction and will minimize the chance that the
random selection of observation times will give misleading data. The
system can be operated for prolonged periods with a constant range if
desired. In the photographic mode the camera shutter is opened when the
radar antenna sweep crosses from land to sea and remains open until the
entire sea has been scanned. The shutter then closes and the film is
advanced one frame. The duration of a 360° radar sweep is 1.82 seconds.
Additional variation of parameters can be programed into the system if
experience shows this to be desirable. Power to the radar antenna is
shut off when the antenna is not facing the sea.

The unit has a clock which provides a time signal in hundredths of a
second to the light emitting diodes (LED) on the PPI that document each
photo. With normal operation, one roll of 16-millimeter film will last
about 1 week. The developed film is analyzed in the CERC laboratory with
the aid of a viewing device described in Section III.

IIIT. RADAR RESULTS

Radar images of waves presented in this report were collected using
two different radars. A short sequence of images was collected with a
Decca radar during an experiment in January 1976 at Nauset Beach on Cape
Cod, Massachusetts, to test the concept of using radar to provide wave
direction information. The Decca radar was part of a vessel tracking
system on loan from the U.S. Coast Guard Research and Development Center
in Groton, Connecticut. All other radar images were obtained with the
CERC radar system. For several months the CERC system regularly collected
wave images in an automated mode at Channel Islands Harbor, California, in
support of a sediment transport study. An additional set of images was
collected at San Diego, California, with the CERC radar during 22 Febru-
ary to 31 March 1977 in support of the West Coast Experiment organized
through the Jet Propulsion Laboratory (JPL), Pasadena, California. The
Decca radar images clearly showed single and multiple wave trains, and
led to the construction of the present CERC radar system.

This section shows the type of images available from the two radars
and highlights some special features.

1. Images of Single and Multiple Wave Trains.

Figure 5 shows a PPI photo of a single wave train imaged by the Decca
radar during the Cape Cod test. The wave train is from the east with a
wavelength of 90 meters (295 feet) near the outer limit of the image.

One disadvantage of the Decca radar is that when the gain is turned high
enough to see the waves near the edge of the scope at 1.39 kilometers
(0.75 nautical mile) from the radar, the surf area is saturated and the
waves in the surf zone cannot be distinguished due to the bright return.
The Raytheon radar in the CERC system handles this problem much more
Satisfactorily. Figure 6 is a photo of the PPI scope taken by the CERC
radar in California at the 1.39-kilometer range, and shows one primary

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wave train. Waves in the surf zone are usually clearer with the CERC
unit. Reproduction has unavoidably reduced the definition of the radar
photos for this report.

Radar can also image multiple wave train conditions. Figure 7 from
the CERC radar in-California shows two wave trains (a swell from the
southwest and a second wave train from west-northwest). Three wave
trains can be seen in Figure 8 (also from the CERC radar). Single or
multiple wave trains were seen in 80 to 90 percent of the PPI photos
taken with the two radar systems; surf zone waves were seen in all the
photos.

In the sequence of radar images in Figures 9 to 12, the radar return
for the same location and sea conditions is shown for four different
radar ranges. These images were all taken within a 2-hour period dur-
ing the Cape Cod test. Good return was seen even on the 5.6-kilometer
(3 nautical miles) range with waves obvious to at least 3.7 kilometers
(2 nautical miles) from the radar.

2. Possible Measurements with Radar.

Since the CERC system takes an image of the wave field every radar
sweep and the time between sweeps is recorded with LED's, the wave speed
can be measured by noting the distance traveled by a particular wave be-
tween two frames. Figures 13, 14, and 15 show how this measurement of
wave speed is determined. The figures show the radar return for identi-
cal conditions and settings, except that each image was taken 5.5 seconds
later than the preceding one. In comparing these figures, the distance
traveled by a particular wave can be measured (marked by arrows). A
rough estimate of period can also be made since both Figures 13 and 15
show a wave just reaching the breakwater. The times on the photos indi-
cate that the wave period is approximately 11 seconds.

By viewing a series of radar images taken over a longer time period,
the evolution of the wave field can be documented (see Figs. 16 to 19
which were taken at approximately 4-hour intervals, and show that the
wavelengths increase with time).

Refraction phenomena as imaged in radar photos can be compared with
that predicted by refraction theory. Figures 20 and 21 show two cases
of refraction as seen in radar images. Figure 20 taken at Torrey Pines,
San Diego, California, shows strong bending of wave crests in the left
of the image due to an underwater feature (Scripps Canyon). Figure 21
shows the bending of a wave train which occurs when deepwater wave crests
approach the shore at an angle of approximately 45°. A short wave train
due to a local wind from the northeast and a longer swell from the south-
east also appear in the figure. Refraction is shown by arrows drawn per-
pendicular to the waves on each wave train at two different distances
from shore. If the bathymetry is known, and the waves are not too high,
it may also be possible to separate refraction due to the bottom from
that due to currents.

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22

Nearshore currents might be measured by tracking float-mounted
corner reflectors on a sequence of radar images.

3. Methods for Analysis of Radar Photos.

Because the CERC radar system was designed primarily for obtaining
wave direction statistics, a method of analysis is needed to permit rapid
and accurate interpretation of the direction of wave propagation from a
large set of radar photos. An additional complication is that a single
radar picture often shows several wave trains, each of which has a di-
rection and a wavelength to be determined. An effective and convenient
method of analysis is to scan the film with a device similar to that
shown in Figure 22. The operator can control a 16-millimeter projec-
tor while viewing the images, in cinema mode or individually, on a rear
projection screen. A rule with a crossbar assembly that fits over the
screen can be lined perpendicular to the direction of wave propagation
in an area of the image to read the relative angle from a protractor.

A true direction can be derived by referencing a known azimuth available
from charts. The shore direction or some landmarks shown in the radar
photo can be used to define a true azimuth. Any 180° ambiguities can be
resolved by observing two successive frames to note the direction the
wave crests are propagating. The distance between wave crests can be
measured with the rule. Figure 23 shows a radar image with annotated
directions and wavelengths. The measurement of the wave direction rela-
tive to the shore is shown for two wave trains, where 09, and 69 are
the angles between the wave train propagation direction and the shore-
line. The measurement of the wavelength for each wave train i}, and d2
is also shown.

4. Measurement Errors.

A measurement of the propagation directions for the principal wave
trains in the CERC system can be obtained with an accuracy on the order
of 8°. Uncertainties in the direction measurement are due to (a) radar
angular resolution of about 1°, (b) resolution on the protractor of 0.5°,
(c) errors in lining up the ruler perpendicular to the wave crests and in
determining a reference angle (estimated to total 5°), and (d) motion of
the waves during the time the radar sweeps the sea surface (an error of
I tO 2)

Errors in measuring wavelength due to resolution of the rule would
be least significant in measurements at shorter ranges, such as the 1.39-
kilometer (0.75 nautical mile) range. Errors of from 5 to 10 percent in
measuring wavelength are encountered at this range.

IV. THEORY OF RADAR IMAGING
Objects are detected with radar by illuminating the scene with short
bursts of electromagnetic energy of a discrete frequency. A part of the

radiated energy may be returned to the transmitting antenna by specular
reflection (mirrorlike reflection) or by back-scattering (discussed later

23

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24

in this section). The distance to the object is determined by the time
between the emission of an energy pulse and its return to the transmitter.
The direction of the object is determined by varying the orientation of a
narrow beam antenna. Images are constructed by scanning the scene and
displaying the results on the PPI scope. The sweep of the PPI is syn-
chronized with the antenna rotation so that a return received by the
antenna is recorded on the PPI at the appropriate azimuth. The object

is imaged as an illuminated spot on the PPI at a distance from the center
of the PPI, proportional to the distance of the object from the radar.

In the CERC system, the antenna rotates at constant speed around a ver-
tical axis. Thus, the site of the antenna appears near the center of

the PPI scope.

1. Radar Resolution.

The resolution of the radar along a radial line is determined by the
pulse length. For an object to be detected, a pulse of energy must go
out to the object and return to the transmitter; i.e., to detect an
object at a distance r, the radiation must cover a distance of 2r.
Thus, the distance is obtained as

2S (5) ct (1)

where c is the speed of electromagnetic radiation, and t the time
interval between the emission of an energy pulse and the return of the
same pulse. The pulse, however, has finite duration, +t. Equation (1)
should then be written

r+ 6r= (3) c(t + T) (2)

where or is the uncertainty in measuring r, or the resolution of the
radar system in a radial direction The nominal resolution is shown to be

CE

SRS ovetiet (3)
Resolution in a radial direction is improved by using small values of tT.
Reducing the pulse length also reduces the power of the returned signal,
and extremely short pulses may also be associated with a reduction in the
reliability of electronic components. A pulse length of 50 nanoseconds
(50 x 10°29 seconds) used in the CERC radar system for imaging waves is
the shortest pulse width available in commercial marine radars. This
type of radar is used in the automatic wave direction system because of
its reliability and lower cost. Some special purpose and research radars
use shorter pulses which may eventually be used in marine radars. The
CERC system has a nominal resolution of

6r

(3) 3 x 108 meters per second x 50 x 107? seconds

6r

7.5 meters.

ZS)

Directional resolution is determined by the ratio of the effective
aperture of the antenna (a measure of the effective length presented by
the antenna to the incident wave) to the wavelength of the radar waves.
The CERC system has a nominal horizontal angular resolution of about
0.9° or 0.0157 radians. Equipment now available (after the CERC system
was purchased) will permit an improvement to 0.6°.

The resolution normal to the radius is given by

6c = ré60

0.0157r . (4)

Thus, the nominal resolution along a radial line and normal to it is
equal at a range of 477 meters (1,565 feet). The angular resolution
is inferior to the radial resolution at greater distances.

The angular resolution in a vertical direction (23° for the CERC
System) is not critical, and little effort has been expended in opti-
mizing this.

The radar in the CERC wave imaging system transmits 3,600 pulses per
second with a pulse length of 50 nanoseconds. Since the antenna rotates
at 33 revolutions per minute, about 18 pulses are emitted for each 1°
rotation of the antenna. Near the center of the scope, at least, the
smallest element of the phosphor will sustain an angle of more than 1°.
Thus, each spot will display an average of many radial scans. The theo-
retical optimum radial resolution is also degraded due to distortions of
the pulse on scattering and slight misadjustments to radar circuitry.
When all factors are considered, the manufacturer claims a resolution of
10 to 20 meters in the radial direction and 0.9° in an azimuthal direction.
Therefore, the shortest wave likely to be detected with the system in deep
water can be expected to have a period of 3.6 seconds. The shortest detect
able periods in shallow water will be even longer.

Radars which use pulse lengths much longer than 50 nanoseconds and
angular resolutions of less than 0.9° are unlikely to be satisfactory
for imaging the wave field.

Die Scattering Mechanisms.

Radar is scattered from the ocean by two mechanisms. The first is
specular reflection, where the microwave radiation is reflected by a
facet or a surface that is perpendicular to the radar beam. Radar al-
timeters and other radars that look straight down at the sea obtain re-
turn signals via this mechanism. A return can be seen when viewing the
surf zone with a radar at grazing angles; facets caused by the breaking
waves present surfaces that are perpendicular to the radar beam. Waves

26

outside the surf zone can also be viewed with the antenna at grazing
angles; a second mechanism, Bragg scattering, is responsible for the
return. In Bragg scattering the ocean waves appear to the microwave
radiation as a scattering lattice. Scattered radiation from successive
ocean wave crests will reinforce constructively at the radar antenna, if
the difference in the path lengths from the radar to each of two wave
crests is an integral number of radar wavelengths. Figure 24 illustrates

Water Wave L=I.S5cm

Figure 24. Schematic showing Bragg scattering. Wave-
length of radar is ip, wavelength of waves
is hy, and degression angle is 6.

this constructive interference in Bragg scattering where the antenna to
scatterer distance is large compared to i,,, the ocean wavelength. Let
Ay be the radar wavelength and © the grazing angle which is small.
The difference in the round-trip distance between a signal return from
the first crest and that from the second crest is 2x. The condition
for constructive reinforcement is then

Wes 2x

n = integer (positive and nonzero) . (5)

But
<I y Cos 8 (6)

thus, the Bragg relationship is
Ws = Bh, Cos Os (7)

The strongest return is usually for the first-order Bragg scattering
when n equals 1. For most shore-based or ship radars, the grazing

2

angle is small and the first-order Bragg scattering is due to ocean waves
with lengths on the order of one-half the radar wavelength. For X-band
radar, which has a A, of about 3 centimeters, the Bragg scattering is
from the capillary and short gravity ocean waves.

3. Role of Wind in Scattering.

At times no waves appear on the radar images, although waves are
known to be present because they can be observed in the surf zone due
to radar return from facets and spray as specular reflection. A case
where no waves are seen outside the surf zone is shown in Figure 25.
The figure clearly shows a wide surf zone which was caused by swells
from two directions. Outside the surf zone, no waves are visible in
Figure 25, because since the winds were calm or low, little capillary
wave development was present to provide back-scatter radar return.
Figure 26 shows the same image taken about 2 hours after Figure 25 when
the winds had freshened. The offshore swell is now clearly visible be-
cause of the radar return from the well-developed capillary waves modu-
lated by the swell. In general, field tests have indicated that at least
a 5-mile-per-hour wind is necessary for the radar to image waves outside
the surf zone.

4. Scattering from Wave Crests.

When an X-band imaging radar is used at a shore location, a return
is seen from the surf zone due to specular reflection and scatter from
small droplets and spray facets perpendicular to the radar beam. A
return from outside the surf zone is caused by Bragg scattering, where
a strong return is obtained from along the ocean wave crests. Several
factors contribute to this phenomenon.

The slope of the sea surface is one factor. The strength of the
radar return, measured in terms of radar cross section, 9, is a func-
tion of the local grazing angle. The radar cross section, o, isa
measure of the ratio of the power density scattered toward the receiver
to the power density incident on a target. Figure 27 shows a cross sec-
tion per unit area o° plotted versus grazing angle (o° is in decibels;
16Gb 5 O° B'S: iO 1og,, o°). In the figure, the graph for an X-band
horizontally polarized signal shows an increase in return with increased
grazing angle. Thus, a better return would be expected from the upper
part of the forward face of gravity waves, where the local grazing angle
between the water surface and a radial line from the radar antenna is
largest.

A second factor is that the antenna is mounted at such a low eleva-
tion that most wave troughs are shadowed by the previous crests but high
enough to ensure that a high wave would not shadow any following wave
crests. The optimum antenna elevation would be between 10 to 20 meters
(33 to 66 feet). With the antenna below 10 meters, the grazing angle is

28

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220MHz (HOR. POL.)
50MHz (HOR.POL.)

Rs eae SE pee alee eS
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GRAZING ANGLE (deqg:ees)

Figure 27. Composite of o° data fora
"medium'' sea (Skolnik, 1970).

so small that many waves which are more than 1.5 kilometers (4,920 feet)
from the radar, will not be seen because of shadowing by nearer waves
(see Fig. 28), and also because of the small radar cross section at small
grazing angle. At elevations of about 20 meters, the radar begins to see
some return from wave troughs which makes the lines of the wave crests
harder to distinguish (Fig. 29).

Another factor is due to the modulation of the radar-scattering cap-
illary waves by the longer gravity waves and swell. This modulation,
due to hydrodynamic interactions, results in a concentration of capillary
wave energy near the wave crests giving increased capillary amplitude and
stronger radar return there. Phillips' (1966) description on the strain-
ing of small, short waves by the larger waves is one of the hydrodynamic
mechanisms contributing to this concentration. The effect of modulation
of capillary waves on radar return has been investigated by Yeshchenko
and Lande (1972), Keller and Wright (1975, 1976), Plant, Keller, and
Wright (1978), Reece (1978), and Wright (1978).

The rotating display sweep registers an image of the sea scatterers
from the wave crests on the PPI display which in turn can be recorded by
time-lapse photography. The waves usually exhibit a long-crested charac-
ter (also appears in aerial photos) and appear as light and dark strips
across the PPI scope. It is often assumed that the light areas repre-
sent the front face or crest of the waves, and that the dark strips repre-
sent the backface or trough. The correctness of this intrepretation is

30

Shadowed Area

Figure 28. Schematic showing how for low radar antenna elevations
larger waves, at times, shadow the following wave crests
and limit effective range of instrument for imagery.

Figure 29. Schematic showing how high radar antenna eleva-
tions give a radar return from troughs of waves,
decreasing the contrast in the imagery.

3]

uncertain; however, it is not essential to a determination of wave direc-
tion or wavelength. The direction of wave propagation can be determined
from the orientation of the lines of scatterers.

The distance at which waves will give a return detectable on the
radar depends on several factors, including the antenna elevation, the
steepness of the long gravity waves, and the atmospheric conditions.
With commercial radars such as that used in the CERC system, the maximum
range for wave imaging is restricted by the parameters of the radar unit
to 5.28 kilometers (3 miles). The pulse width of 50 nanoseconds needed
for wave resolution is available only for ranges of 5.28 kilometers or
less. A minor change to the radar would allow use of this shorter pulse
for the longer ranges. However, the radar power would then become the
limiting factor for the ranges longer than 5.28 kilometers. Unless the
transmitted power could be increased, the return may be too weak to give
wave images.

If radar radiation is assumed to travel in straight lines, then the
range would also be expected to be restricted by the curvature of the
Earth as shown in Figure 30. If d = 5.28 kilometers, then a minimum
radar height, h, is found by

(rx + h)? = x2 + d?
r2 + 2hr +h? = r2 + d2 (8)
h(2r + h) = d?
but
r>>h
d2
h = 3 9
= (9)
let
r = 4,000 miles (6,439 kilometers)
then

h = ( 9 ) 5,280 feet
8,000

Sr
ir

6 feet (1.8 meters)

Since radar waves follow a straight line only in a medium of constant
index of refraction, this method is not quite correct. In the atmosphere,
the gradients of humidity and density cause the radar beam to bend. Typi-
cally in a standard atmosphere, radar radiation starting out parallel to
the Earth will travel along an arc with a radius approximately equal to
4/3 the radius of the Earth. The actual radius varies with the vertical
gradient of temperature and humidity in the atmosphere. The radius is
least when temperature increases and humidity decreases rapidly with

32

Figure 30. Schematic showing the restriction on line-of-sight
by curvature of the Earth. h = height of radar,
r = Earth's radius, and d = range by line-of-sight.

increasing height. The distance to the horizon for a radar experiencing
this refraction can be given as (Skolnik, 1962, pp. 506-509):

d = v2kah (10)
where a is the radius of the Earth. The term Ka is then an effective
radius of the Earth. For k = 4/3, equation (10) can be conveniently

written where d is measured in statute miles and h in feet as

d(statute miles) = v2h(feet) sti
h(feet) = {eletaens miles Te

or

where d is the distance to the horizon, and h the height of the radar;
for d = 5.28 kilometers (3 miles), then h = 1.4 meters (4.5 feet).

For most radar installations, this antenna elevation is easily ob-
tained. However, further restrictions on the useful distance that radar
can view waves are imposed because the radar scatterers ride on the for-
ward face of the longer gravity waves. Thus, with an antenna at the
minimum required height as determined above, waves would not be seen out
to 5.28 kilometers because of shadowing of the radar scatterers by nearby
gravity waves. An example of the radar antenna at an elevation of 4.6
meters (15 feet) is shown in Figure 31 where two waves only 1.6 kilometers
(1 mile) from the radar with 1.5 meters (5 feet) in height are inspected.
Geometry shows that only the top 23 centimeters (0.75 foot) of the second
wave is seen by the radar. Unless strong winds were present, the return
from this small area of the wave would likely give a weak radar return.
Return from following waves farther from the radar would give an even
weaker return; therefore, the effective range for this installation is
about 1.6 kilometers.

S)S)

i ie HH SG Ae
Sa SEPT Et «Ole KM yore aes ac ee 100 m —+

Figure 31. Schematic showing the illumination of wave
crests by radar at 4.6 meters elevation.

Several other complicating factors make exact calculations of maximum
range difficult in every situation. These include the decrease of radar
cross section with incidence angle (see Fig. 26), the unknown phase of
the radar scatters with respect to the long waves, variation in tempera-
ture and humidity in the air, and the variation in height of the long
waves. Thus, the choice of the optimum antenna elevation must be made
empirically with consideration of the wave height to be expected, the
features to be investigated, and the sites available.

V. VALIDATION

Wave data from imaging radar should be compared with wave data from
other devices. At least two types of comparative data may be used:

(a) Other types of wave imagery, including aerial photog-
raphy and aircraft or satellite radar such as synthetic aper-
ture radar (SAR) and side-looking radar (SLAR).

(b) Data from conventional wave gages or arrays of conven-
tional wave gages.

All imaging processes present a nearly instantaneous, two-dimensional
view of the water surface. For aerial photography, the duration of an
observation is a few milliseconds; for scanning radar (used in these
experiments), successive parts of the image are formed at later times,
and the duration of data collection is about 1 to 2 seconds. In all
imaging processes, ripples are required on the water surface to reveal
the presence of longer waves.

With aerial photography, the principal process for obtaining infor-
mation about the water surface is specular (mirrorlike) reflection; for
surface-based radar, the principal process is Bragg scattering.

34

Most wave gages record the time history of some property of the wave
at a fixed point, or a very small area, over a finite time interval,
usually about 20 to 30 minutes. When an array of gages is used to deter-
mine wave direction, it is necessary to postulate an analytic function
which describes the geometry of the sea surface and to determine the
coefficients in this function from gage measurements.

Measurements from a single gage generally represent some type of
time average. When arrays are used, space-averaging based on discrete
points is also involved.

Both types of data yield information on waves, but the information
revealed by different. procedures is not identical because each observa-
tion technique concentrates on one or two facets of wave behavior and
neglects others. Exact agreement is not to be expected. Nevertheless,
the comparisons discussed in this section reveal a significant level of
agreement.

The primary source of comparison data was the West Coast Experiment
(sponsored by National Aeronautics and Space Administration (NASA)) con-
ducted during February and March 1977 off the coast of southern California
(Shemdin, Inman, and Blue (1977)). The objective of the CERC participa-
tion in this experiment was to conduct a field shakedown test of the first
prototype CERC radar system, which had only recently been assembled, and
to validate as many CERC radar measurements as practical. Certain factors
prevented the CERC system from performing in an optimum manner. The
limited time for deployment and the available locations for operation
did not allow ideal antenna elevations. A subsequent discovery indicated
that the radar was not properly tuned and that misadjustment of pulse
width may have given a pulse as wide as 150 nanoseconds, three times the
desired value. This would have resulted in the radial resolution being
degraded from about 10 meters to between 30 or 40 meters (98 or 131 feet).
However, radar wave images were produced which provided wave direction
Measurements to compare with measurements from other instruments operating
simultaneously. These comparisons along with some information gathered
during an earlier experiment at Cape Cod are discussed below.

ihe Comparison with Aerial Photos.

During the West Coast Experiment in March 1977, several U-2 aerial
photo missions were flown off the California coast in the area of San
Diego. Simultaneously, radar images of the waves were obtained with the
CERC radar system located at Mission Beach, San Diego. Two cases exist
where photos (as shown in Fig. 2) could be compared with radar images.
In these cases, measurements were made of the directions of propagation
for long-crested wave trains detectable on the radar image and the U-2
photos. Figure 2 shows a part of the aerial photo taken 29 March; Figure
32 shows a photo taken 14 March. The results are shown in a Table. The
CERC radar images and the U-2 photos were both taken within a 10-minute
period on each of the two dates. Wave measurements on the imagery were
made either at the U.S. Navy Undersea Center (NUC) tower at a depth of

35

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18.3 meters (60 feet) or shoreward of the tower with wave direction cal-
culated at NUC by refraction theory (McClenan, 1975). The measurements
were made manually with a protractor and straight edge. The true bearing
of 275° from the NUC tower to the radar site was used as a reference. To
resolve the 180° ambiguity, several radar images were viewed which showed
that the waves were moving shoreward. For the U-2 photos, it was assumed
that the waves were propagating toward the shore.

In both cases, two wave trains were detected on the radar images, and
the directions for these wave trains agreed well with measurements from
the U-2 photos. However, in both cases, another very short wave train
appeared in the U-2 photo but was not detected by the radar. Perhaps
this was due to the misadjustment of the pulse width resulting in a
radar resolution that was too poor to image the short wave train. With
the radar optimally adjusted, it is expected to produce images of waves
with lengths on the order of 20 meters. Wave trains with lengths of 35
meters (115 feet) have been measured by a radar with the same design
resolution as the CERC radar; these results are discussed later in com-
paring pressure gage data with radar measurements at Cape Cod.

Wave images were also collected by JPL during the West Coast Experi-
ment using a SAR mounted in the NASA Convair 990 aircraft. On 28 March,
a SAR image of the waves was taken in the Mission Beach area at the same
time CERC radar images were obtained (see Fig. 33). Figure 34 is a two-
dimensional Fourier transform of the marked area of the SAR image. On
the transform, the predominant wave train shows as a bright radial band.
A close examination also shows a faint second band oriented 28° in a
counterclockwise direction from the predominant band, which corresponds
to a second wave train. These light lines or bands in the transform are
normal to wave crests. Wave direction and period for the SAR data were
taken from direction plots derived from the Fourier transform and sup-
plied by JPL. The CERC radar results for 1800 on 28 March are based on
an average of three images taken within +30 minutes of 1800 (photo of
the radar scope for this observation is shown in Fig. 35). The 285°
wave train appears on all three images. Each of the 261° and 242° trains
appears on only two of the three averaged images. The Table shows that
when the direction of the prominent wave trains of the SAR image are
refracted to the NUC tower (location of the CERC radar measurements) ,
there is good agreement in the wave direction measured by the two instru-
ments for these wave trains. However, a third train from 242° with a
period of 15 seconds appears on the CERC radar and not on the SAR. A
possible explanation is that three wave trains were present-the prominent
train from 285°, a second from 263°, and a third swell train from DER?
Such a swell, even if the wave height were low, could show up on the
CERC radar because of shadowing of wave troughs with the low antenna
elevation. It might not have appeared with the SAR because of the small
amount of modulation of the radar-scattering capillary waves resulting
from such a wave train's small wave steepness.

The Table shows that for 14 and 29 March the wave periods for the
long-wave trains measured from the CERC radar image do not agree well

38

Figure 33. SAR image of Mission Beach at San Diego, California,
28 March 1977. Outline shows approximate area used

for transform in Figure 34.

Figure 34.

Two-dimensional Fourier transform of SAR image
shown in Figure 33,

40

ket——— .75 nmi ————>|

Figure 35. CERC radar image at Mission
Beach, 28 March 1977.

with the periods measured from the U-2 photos. These long waves were
part of secondary wave trains which did not show up well on either the
aerial photos or the radar images. Thus, the wavelengths were difficult
to determine with the manual analysis scheme used. In general, a Fourier
transform of the photos should provide a more reliable wave period meas-
urement. A reliable wave period measurement from the radar images may be
obtained by recording the time for one wavelength to pass a fixed point.

2 Comparison with One-Dimensional Wave Spectra.

One-dimensional wave spectra from a pressure gage have also been used
in radar imagery evaluation. The Decca marine radar, with specifications
Similar to the CERC radar, was operated for 2 weeks at Cape Cod. Wave
images were obtained with the radar operating at 18 meters (60 feet)
above mean sea level (MSL). Simultaneously, a pressure gage was oper-
ated offshore of the radar location. The lengths of the visible wave
trains were measured on the radar photos at the same location as the
bottom-mounted pressure gage. From the spectrum obtained with the
pressure gage, a peak period was determined from which wavelength was
calculated using linear theory and depth. A scatter plot (Fig. 36) of
radar-measured wavelength versus that from the pressure gage shows a good
correlation. The radar-measured wavelengths are systematically shorter

4

Wavetength by Radar (ft —>)

400

Least Squares Fit =
Slope =0.81 Slope = 1.0
Intercept =15.4

350

300

250

200

150

100

50 100 150 200 250 300 350 400
Wavelength by Wave Gage (ft >)
Wave Gage vs. Radar (WAVELENGTH) Nouset I2, 13, and 14 Jan.1976

Figure 36. Scatter plot of radar measurements versus
pressure gage measurements of wavelength.

42

than those seen by the pressure gage. The precise position of the pres-
sure gage was not known. The error could be accounted for if the wave-
length measurements were made on the radar image at a location shoreward
of the actual pressure gage position. This hypothesis is supported by

a scatter plot (Fig. 37) of the wave period measured by the pressure
gage and the wave period obtained using the wavelength as measured by
CERC radar, the depth from charts, and Airy theory. This is the same
data set used in Figure 36, but for period no systematic error is seen.

An extensive comparison study of wave direction as determined by the
CERC radar, aerial photos, SAR imagery, and a pressure gage array is pre-
sented in Mattie, Evans, and Hsiao (in preparation, 1979).

VI. OTHER RADAR DEVICES AND DIRECTION MEASURING SYSTEMS

1. Radar Devices.

Radar is used in many different ways to obtain information on ocean
waves and winds. Radar altimeters used on aircraft and satellites can
provide information about wave height (Rufenach and Alpers, 1978). These
satellite altimeters measure significant wave height to an accuracy of
1 meter (3.28 feet) or 25 percent of the actual height over a range of
1 to 20 meters. Images of an ocean surface area can be obtained from
aircraft and satellites by the SAR (Elachi, 1978). In the SAR system,
the return along the flight path is combined in the data processing so
that the angular resolution is greatly improved, and an antenna is simu-
lated that is much longer than the one actually carried on the platform.
With this increased angular resolution, the satellite SAR's have a reso-
lution cell 25 by 25 meters (82 by 82 feet). Spatial wave and current
information can also be obtained with high frequency (HF) radar. A
typical radar of this type has a wavelength of 5 to 100 meters (16.4 to
328 feet), and usually requires an antenna array on the order of one to
five wavelengths long to form a fairly narrow azimuthal beam. Barrick
(1977) has developed a phase-difference technique to obtain narrow beams
with small HF antennas. Most HF units are doppler radars which measure
the speed of the radar scatters from the doppler shift of the radar sig-
nal. When HF radar is scattered from the sea surface, the first-order
doppler return can give a measure of surface currents. Lipa (1978) dis-
cusses the theory and verification from one experiment for a method to
obtain wave directional spectra from the information contained in the
higher order doppler return. The resolution provided by this system is
unlikely to be fine enough to meet the needs of coastal engineers for
nearshore measurements.

A variety of other specialized radars is available for making speci-
fic measurements in the ocean environment; e.g., a radar scatterometer
provides estimates of local wind velocities by measurement of the radar
cross section, because at some radar wavelengths, the cross section is a
function of windspeed. An inexpensive radar with a resolution fine enough
to obtain images of sea surface wave fields for ranges to include coastal
wave phenomena is needed. The CERC radar will meet this need for many
operational conditions.

43

Period by Radar (S—>)

Least Squares Fit
Slope = +0.87
Intercept=+0.86

6

Slope=0.87

7 8 9 10 Il 12
Period by Wave Gage (S —>-)

Wave Goge vs. Radar (PERIOD) Nauset 12, 13, and 14 Jan.1I976

Figure 37.

Scatter plot of radar measurements versus
pressure gage measurements of period.

44

2. Determination of Wave Direction from an Array of Sensors.

Information on wave direction is available from analyses of the rec-
ords obtained from an array of wave sensors. In essence, this procedure
is based on an assumption about the geometry of the water surface and the
relation between the wave height and the pressure, the velocity or both,
as a function of depth. The assumption may have a deterministic or prob-
abilistic form, but it must be expressed in analytic form so that coeffi-
cients obtained from the wave record analyses can be used to specify the
distribution of wave directions.

The simplest model assumes that the important waves are low, mono-
chromatic, unidirectional, long crested, and are traveling without change
of shape over a horizontal bottom. If these conditions hold and the phase
differences between three wave gages in a two-dimensional array can be
measured, a unique wave direction can be derived from the data. A unique
direction may also be determined from measurements of pressure and water
particle velocity along orthogonal horizontal axes or from measurements
of the wave height and surface slope along these axes.

It may also be assumed that the wave energy at a given frequency is
distributed over a range of directions according to a formal law and to
use wave observations from an array of sensors to determine the coeffi-
cients in the selected law. It is important to note that the determina-
tion of wave direction by analyzing the records from an array of sensors
is necessarily based on an assumption about the law which governs the
distribution of wave directions. If the assumption is precisely correct
and there are no instrument or analyses errors, the information on wave
direction will be correct. If the basic assumption is incorrect, the
results of the analyses may be misleading even when there are no instru-
mental or data processing errors.

Techniques for determining wave direction by analyzing the records
from an array of wave gages have been reported by Fujinawa (1974),
Panicker (1971, 1974), Esteva (1977), and others.

When an image of the wave field is analyzed, some trains of low waves
may be overlooked; however, if a wave is identified in the image of the
wave field, then that wave train must have been present on the ocean and
the direction determined from the image cannot be greatly in error.

VII. DISCUSSION

The CERC radar system was designed to automatically obtain images of
the wave field for measurement of wave direction and length. The system
also has the capability of determining wave speed, current speed and direc-
tion, information on refraction, and a potential of obtaining additional
information.

1. Additional Capabilities of CERC Radar System.

| There are two situations in which the CERC radar system can be used
to measure wave period. If the radar is in the manual operation mode,

45

an operator can leave the radar on a single range and view the return.
To measure the period, the operator would either select a stationary
radar scatterer, such as a tower or end of a pier shown in the radar
photo, or chose a point on the PPI and determine the time (with the aid
of the LED clock) for one wave (crest to crest) to pass the chosen point
on the scope. For better accuracy, the time is determined for the pass-
ing of 10 waves; the wave period would then be this time divided by 10.
From a radar picture, the wave period could be found by measuring the
wavelength at a location in the frame where the depth is known. Linear
wave theory could then be applied to give an estimate of the period.

Information on wave height may be obtainable with the CERC radar.
The strength of the radar return is a function of the amplitude of the
capillary waves and the local grazing angle. Since the capillary waves
are modulated by the longer gravity waves, it would then appear that the
modulation of the radar return signal should contain information on the
long-wave heights. A complicating factor is that the capillary wave
development is influenced by the local winds. A complete theoretical
model is not now available to relate radar cross section or radar return
Signal strength to wave height for radars such as the CERC radar. How-
ever, an empirical calibration of the radar return for wave height may
be possible.

2. Future Plans.

The main directions for further development are to increase resolu-
tion of the radar system and to develop automatic data reduction. The
CERC system has a 2.74-meter (9 feet) antenna array; however, a recently
available 3.66-meter (12 feet) array will give a slightly better angular
resolution of 0.6° instead of 0.9°. More importantly, it will give a
better signal-to-noise ratio.

A shorter pulse width to improve resolution and perhaps provide better
radar wave images may be possible with only minor adjustments to the radar.
One drawback in shortening the pulse width is that this lowers the average
power transmitted. Weak scatters imaged when using the 0.05-microsecond
pulse, may be invisible with the shorter pulse. Tests are needed to de-
termine the conditions when the shorter pulse would be useful.

An additional refinement is to modify the radar circuitry so that the
gain function more closely follows the decrease in radar return strength
with range to give a better image with a more even contrast across the
image and less saturation near the center of the PPI. This will also
give an image better suited to automatic analysis procedures.

If a large quantity of data is collected with these radar systems,
then automated methods are needed for analysis. Suggested methods in-
clude (a) Fourier transforms of the radar film using either optical or
digital methods, (b) analytical means for obtaining wave direction from
the direct radar return signal, and (c) a TV recording system in lieu of

46

the 16-millimeter film may permit improvement in the original settings
of some of the console controls and may facilitate automatic data anal-
ysis routines.

The CERC radar is being tested at the CERC Field Research Facility
(FRF) at Duck, North Carolina, where wave height spectra are available
from a number of other sensors (including a waverider buoy) within the
radar field of view. Windspeeds and wave spectra at FRF present an
opportunity to better define the minimum conditions to obtain wave
images. A study of the quality of wave images for various wave heights,
wave periods, and windspeeds is needed. Since the radar-scattering
capillary waves are enhanced due to the curvature of the gravity waves,
a stronger return would be expected from the steeper waves. Thus, the
minimum conditions for wave imagery would probably be a function of wave
height and wavelength. Antenna height also has an impact on whether par-
ticular wave conditions can be imaged. Although images have been ob-
tained for wave heights of 1 meter or greater for most conditions and
often for waves of smaller height, these future tests should provide
the quantities of data necessary to precisely establish limits on the
conditions suitable for radar wave imagery.

These tests at FRF will also present an opportunity to study the
relationship between radar return and wave height. An "A-scope'' display
will be inserted into the radar system to show the return along a partic-
ular azimuth where the display is similar to that of an oscilloscope with
the vertical axis representing the magnitude of the radar return and the
horizontal giving the range to the targets. In the CERC system the re-
turn along a particular azimuth will be gated to the A-scope, and will
likely be a type of storage oscilloscope. Radar return strength as
measured from the A-scope will be compared with the wave spectra for
a variety of sea states and wind conditions.

The CERC radar system has been used at Channel Islands Harbor,
California, in support of a sediment study and to obtain wave imagery
as part of the West Coast Experiment. This system has been moved to
Duck, North Carolina, to further develop the system's capabilities and
to support research projects at the FRF. The unit will also be available
to support CERC or U.S. Army Engineer Districts in projects involving
wave imagery.

VIII. SUMMARY

This report has shown that images of the most prominent waves on the
sea surface can often be obtained with the aid of commercially available
marine navigational radar. Images of the wave field are collected by
photographing the display scope of the radar. Records may be obtained
in an unattended mode by a programing device which activates the system
and collects a sequence of photos at fixed-time intervals. Wave direc-
tion at a fixed point or the prevailing wave direction for a designated
area may be determined by inspecting the photos.

47

Comparisons of estimates of wave direction and wavelength (based on
radar photos with estimates from other imagery, including aerial photos
and gage records) generally show good agreement.

Although the quality of radar imagery is not as good as that of aerial
photography, it has the advantage of being available at night and during
storms. Radar also permits images of the wave field to be collected in
the time-lapse mode and is less expensive than aerial photography. Radar
images have a distinct advantage over an array of wave gages as a source
of information on wave direction because the radar image provides visual
evidence of refraction (when present) and of the relation between wave
direction in the designated region and the surrounding area.

The radar imaging technology in determining wave direction has some
distinct restrictions to be recognized if this technique is to be prop-
erly explored.

The most fundamental limitation is the need to have ripples (at least
1.5 centimeters in length) coexisting with more prominent wave trains to
obtain sufficient signal return. However, a windspeed of 5 knots will
generally assure sufficient ripple formation. Thus, this condition is
usually satisfied in a growing wave field.

The resolution of commercial radars is not adequate to ensure detec-
tion of wavelengths less than about 25 meters (83 feet) corresponding to
periods of 4 seconds in deep water. However, slightly shorter waves may
be distinguished on occasion. This restriction can be reduced by re-
designing some components of the radar system.

The contrast between the appearance of the wave crest and wave
trough is greatest when the wave crest is high enough relative to the
radar antenna to shadow the following trough, but not high enough to
shadow the following crest. For most coastal locations an antenna
elevation between 10 and 20 meters is best, although a variable antenna
height could be more useful.

The optimum console settings vary with the ambient conditions of
wind and waves. Some allowance for this factor has been made in the
CERC system by photographing the display scope with a series of ranges
during each observation. A median level for the other console settings
to achieve the best imaging in most conditions has been determined.

Radar images generally indicate long-crested waves with character-
istics which change only slowly in time. A slight variability in inten-
sity and direction is apparent along each crest, but the general pattern
is usually stable.

This report has shown that high-resolution imaging radar is a useful
tool in the study of waves in the coastal zone, and that this approach is
more useful than other alternatives for some applications. Many years of
research and development are required before most of the questions on the
use of imaging radar in the study of waves can be answered.

48

LITERATURE CITED

BARRICK, D.E., EVANS, M.W., and WEBER, B.L., "Ocean Surface Currents
Mapped by Radar," Setence, Vol. 198, No. 4313, Oct. 1977, pp. 138-144.

ELACHI, C., ''Radar Imaging of the Ocean Surface," Boundary-Layer
Meteorology, Vol. 13, Nos. 1, 2, 3, and 4, Jan. 1978, pp. 165-179.

ESTEVA, DINORAH, C., ‘Evaluation of the Computation of Wave Direction
with Three-Gage Arrays,'' TP 77-7, U.S. Army, Corps of Engineers,
Coastal Engineering Research Center, Fort Belvoir, Va., July 1977.

EVMENOV, V.F., et al., ''Test of the Radar Method of Defining Ocean Wave
Elements,'' Flutd Mechantes—Soviet Research, Vol. 2, No. 5, Sept. 1973,
pp. 141-145.

FUJINAWA, J., "Measurements of Directional Spectrum of Wind Waves Using
an Array of Wave Detectors," Journal of the Oceanographical Society of
Japan, Tokyo, Japan, Vol. 30, No. 1, Feb. 1974, pp. 10-22.

IJIMA, T., TAKAHASHI, T., and SASAKI, H., "Application of Radars to Wave
Observations ,"' Proceedings of the 11th Conference on Coastal Engineering
tn Japan, Japan Society of Civil Engineers, 1964, pp. 81-88.

KELLER, W.C., and WRIGHT, J.W., "Microwave Scattering and the Straining
of Wind Generated Waves,"' Radto Sctence, Vol. 10, No. 2, Feb. 1975,
pp. 139-147.

KELLER, W.C., and WRIGHT, J.W., "Modulation of Microwave Backscatter by
Gravity Waves in a Wave Tank,"' NRL Report 7968, Naval Research Labora-
tory, Washington, D.C., Mar. 1976.

LIPA, B., ‘Inversion of Second-Order Radar Echoes From the Sea," Journal
of Geophystecal Research, Vol. 83, No. C2, Feb. 1978, pp. 959-962.

MATTIE, M.G., EVANS, D.C., and HSIAO, S.V., "SAR, Coastal Radar, NOSC
Array, and Aerial Photo Intercomparisons of Wave Direction Information
at Mission Beach, San Diego," (submitted to Journal of Geophystcal
Research) (in preparation, 1979).

McCLENAN, C.M., "Simplified Method for Estimating Refraction and Shoaling
Effects on Ocean Waves,'' TM-59, U.S. Army, Corps of Engineers, Coastal
Engineering Research Center, Fort Belvoir, Va., Nov. 1975.

McCLENAN, C.M., and HARRIS, D.L., "The Use of Aerial Photography in the
Study of Wave Characteristics in the Coastal Zone,'' TM-48, U.S. Army
Corps of Engineers, Coastal Engineering Research Center, Fort Belvoir,
Vides amin ll OWS.

OUDSHOORN, H.M., ''The Use of Radar in Hydrodynamic Surveying," Proceedings

of the Seventh Conference on Coastal Engineering, Council of Wave
Research, Berkeley, California, Vol. 1, 1960, pp. 59-76.

49

PANICKER, N.N., ''Determination of Directional Spectra of Ocean Waves from
Gage Arrays," Technical Report HEL 1-18, University of California,
Hydraulic Engineering Laboratory, Berkeley, Calif., Aug. 1971.

PANICKER, N.N., "Review of Techniques for Wave Spectra," Proceedings of
the International Sympostum on Ocean Wave Measurement and Analysts,
American Society of Civil Engineers, Vol. 1, 1974, pp. 669-688.

PHILLIPS, O.M., The Dynantcs of the Upper Ocean, Cambridge University
Press, London, 1966, pp. 56-63.

PLANT, W.J., KELLER, W.C., and WRIGHT, J.W., "Modulation of Coherent
Microwave Backscatter by Shoaling Waves," Journal of Geophystcal
Research, Vol. 83, No. C3, Mar. 1978, pp. 1347-1352.

REECE, A.M., "Modulation of Short Waves by Long Waves," Boundary-Layer
Meteorology, Vol. 13, Nos. 1, 2, 3, and 4, Jan. 1978, pp. 203-214.

RUFENACH, C.L., and ALPERS, W.R., "Measurement of Ocean Wave Heights
Using the Geos 3 Altimeter," Journal of Geophystcal Research, Vol. 83,
No. C10, Oct. 1978, pp. 5011-5018.

SHEMDIN, O.H., INMAN, D.L., and BLUE, J.E., "West Coast Experiment Test
Plan,"' Document No. 900-765, Jet Propulsion Laboratory, California
Institute of Technology, Pasadena, Calif., Jan. 1977.

SKOLNIK, M.I., Radar Systems, McGraw-Hill, New York, 1962.

SKOLNIK, M.I., Radar Handbook, McGraw-Hill, New York, 1970.

WILLS, T.G., and BEAUMONT, H., "Wave Direction Measurement Using Sea
Surveillance Radars,"' Technical Memorandum IR 118, Royal Aircraft

Establishment, Farnbough, Great Britain, Apr. 1971.

WRIGHT, F.F., "Wave Observation by Shipboard Radar," Ocean Scetence and
Ocean Engineering, Vol. 1, 1965, pp. 506-514.

WRIGHT, J.W., "Detection of Ocean Waves by Microwave Radar: The Modula-
tion of Short Gravity-Capillary Waves," Boundary-Layer Meteorology,
Vol. 13, Jan. 1978, pp. 87-105.

YESHCHENKO, S.D., and LANDE, B.SH. "Radar Image of the Surface of the

Sea," Radiotekhnika I Elektronika, No. 8, Moscow, U.S.S.R., Nov. 1972,
pp. 1590-1597.

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