I SAXON 1I (SAR AND X-BAND OCEAN NONUNEARITIESbTIC

SCIENCE PLAN EETSEP 06 19

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REPORT DOCUMENTATION PAGE BEFORE COMPLETING FORM1. REPORT NUMBER 2. GOVT ACCESSION NO. 3. RECiPIENT'S CATALOG NUMBER

4. TITLE (amid Subtitle) S. TYPE OF REPORT & PERIOD COVERED

SAXON I SAXON Science Plan

Science Plan 1988/1990

1988/1990 S. PERFORMING ORG. REPORT NUMBER

7. AUTHOR,) 6. CONTRACT OR GRANT NUMBER(.a)

Omar H. Shemdin and Les McCormick N01---00with SAXON Experiment Team

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT. PROJECT. TASK

AREA & WORK UNIT NUMBERS

Ocean Research and Engineering 0810-Physical Oceanography255 South Marengo Ave. 0803-Dynamic OceanographyPagadenp. California 91101 170Q-Rndnr npi-prron

I1. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

Office of Naval Research August 1988

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II. SUPPLEMENTARY NOTES

It. KEY WORDS (Continue on roers* aide If necessary and Identify by block number)

Synthetic Aperture Radar* Radar Backscatter, Surface Waves,Capillary Waves, Internal Waves, Ship Wakes.,'-

20. ABSTRACT (Continue an revere side I necee.wy and Identity by block number)

This is a follow-on to the TOWARD experiment which was excecutedin 1984-1986 tb 'rovide experimental verification of the mechanismsresponsible for AD imaging of the ocean surface. The TOWARDexperiment provided a data set that is adequate for resolving the most

important issues related to SAR imaging of surface waves at L-Band.

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The mechanisms involved in SAR imaging at X-Band are farless understood (C-Band is in near proximity of X-Band). The SAXONexperiment, although similar in scope to TOWARD, addresses thenonlinear hydrodynamics associated with the very shortsurface waves (0.5 - 5 cm in length), and radar backscatter fromthe ocean surface at X-Band and higher frequencies.

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II

SAXON I

(SAR X-BAND AND OCEAN NONLINEARITIES)

(1988/1990)

SCIENCE PLAN

NTIS C t&I

DlCU TABUrianiio inced F1Just iIca t on

ByDistribution I

Availab!!;1y Corde.

DistPrepared by:

0. H. SHEMDIN F--Principal Investigator

L. D. Mc CORMICKTest Director

Ocean Research and Engineering255 South Marengo Avenue

Pasadena, California 91101

August 1988

(Cover: Circular Wave Generator, also Knownas "Bobbing Buoy", is Operated in SAXON)IU

ACKNOWLEDGEMENT

This Science Plan is based on a "Straw Man" planpresented by Dr. 0. H. Shemdin at a meeting of investigatorsheld in La Jolla, California on 16-17 July 1987.Contributions were provided by the attendees throughdiscussion and submission of work statements and viewgraphs.A second planning meeting was held 25-26 February 1988 inPasadena, California for review of the loss of the OCNRTower in a major storm. Careful consideration was given tothe Chesapeake Light Tower, located outside of theChesapeake Bay entrance on the East Coast, as the alternatesite for the SAXON I experiment. The experiment issponsored by the Office of Naval Research under the jointsponsorship of the SAR Program with Commander T.S. Nelson asProgram Manager and Mr. H. Dolezalek as Technical Monitor ofthe ocean SAR effort, and the Marine Microlayer AcceleratedResearch Initiative (ARI) Program with Dr. F. Herr asProgram Manager.

ii

2ZECUTIVBUAMRY

Synthetic Aperture Radar (SAR) provides high resolutionimaging of the ocean surface independent of sunlight andunder most weather conditions. SAR images can be processedto yield useful information on directional properties ofsurface waves, internal waves, location of boundarycurrents, wind speed and ship wakes. The spaceborne wide-swath high-resolution imaging capability in global orbitallows detection of these processes in both meso- and globalscales.

To transform the SAR into a useful tool, and tounderstand the limits of its validity, it is necessary tounderstand the process by which an ocean surface istransformed to the image or another form of output signal.To this end all significant elements of the transformatiinprocess, including extraneous influences, must be eitherquantitatively known or be measured. For Synthetic ApertureRadar, as an instrument to image or provide a measure ofthe ocean surface (and after the TOWARD, SARSEX and LEWEXExperiments), the still unknown elements are mostly in thearea of surface and subsurface hydrodynamics, and in certainaspects of radar backscatter from the ocean surface. Themain objective of future experiments, of which SAXON I isone,is to measure these elements.

The TOWARD experiment was executed in 1984-1986 toprovide experimental verification of mechanisms responsiblefor SAR imaging of the ocean surface. This experimentprovided a data set that is adequate for resolving the mostimportant issues related to SAR imaging of surface waves atL-Band. The data set also provided insights on issuesrelated to SAR imaging of internal waves, imaging of theocean surface with X-Band radars and the influence of themicrolayer.

The mechanisms involved in SAR imaging at X-Band arefar less understood (C-Band is in near proximity of X-Band).Here, an experiment similar in scope to TOWARD is presentedto understand the hydrodynamics and radar backscatterprocesses associated with the very short surface waves (0.5- 5 cm in length). For this purpose a stable platform ismandatory for generating credible hydrodynamic (directionalwave height and slope spectra) and radar backscatter data(obtained at fixed incidence angles). The insights gainedin TOWARD at L-Band, in determining the transfer functionfrom a radar backscatter map to a SAR scene, are potentiallyapplicable to X-Band. That is, the SAR imaging modelderived for L-Band can be extended in frequency to X-Band.However, the primary issues at X-Band relate to thehydrodynamics of the very short waves which are stronglynonlinear. This is clearly indicated in the ProbabilityDensity Functions (PDF) obtained with the laser slope

iii

sensor. The radar backscatter measurements at X-Band and

higher frequencies also show different characteristics, suchas frequency of spikes, compared to those at L-Band.

A basic X-Band experiment, similar in scope to TOWARD,is planned here as a follow-on to TOWARD. The experiment isgiven the acronym SAXON I to stress SAR X-Band and OceanNonlinear aspects of SAR imaging of the ocean surface. TheChesapeake Tower located on the East Coast, offshore of theChesapeake Bay entrance is selected as the stable platformfor deployment of near surface sensors. This site isselected following an intensive review of available towers,to replace the NOSC Tower, subsequently named the OCNRTower, after its destruction in a major storm on 18 January1988. The Chesapeake Tower site is a most representativealternative to the abandoned site. The plan calls for thefollowing suite of measurements:

1. Directional properties of short surface waves byin-situ capillary wave sensors, stereophotographyand Stillwell-type photography. A scanning-beamlaser-optical sensor is used as the in-situsensor.

2. Radar backscatter measurements from the oceansurface in the range 5-95 GHz. Single and multi-frequency radars are used in parallel to providefrequency coverage.

3. RAR and SAR images are obtained simultaneouslyover the instrumented areas. It is planned thatcoverage be provided by 1) NRL RP-3A, and 2) NADCRP-3A to include L- C-, and X-Band SAR and X-BandRAR.

The SAXON I experiment is to address the followingtasks (consistent with many key recommendations inthe SAR Working Group Report, 1986):

1. Determine the modulation of centimeter waves bylong surface waves and by internal waves.Included in this determination is the influence ofthe intermediate surface waves (0.5-10m inlength).

2. Validate existing hydrodynamic models (e.g.Hughes, 1978) on modulation of centimeter waves bylong surface waves and by internal waves.

3. Determine the modulation of radar backscatter fromthe ocean surface induced by surface and/orinternal waves propagating through the radarfootprint. The emphasis is on radar frequenciesin the range 10-95 GHz.

iv

4. Test radar backscatter models by using short7surface waves as input, and comparing the output

with measurements.

5. Test validity of the TOWARD SAR imaging theory(validated at L-Band) for X-Band and higherfrequencies.

6. Specific experiments are appended to compareaccuracy and resolution of optical and passivemicrowave techniques in detecting modulations onthe ocean surface induced by natural or man-madeprocesses.

7. Applied experiments are appended, such as oceansurface effects in the wake of ships, providedthat they amplify the basic objectives statedabove.

The present plan indicates participation by twenty-six(26) independent research organizations. These are detailedin Section IV under Participation. The SAXON I experimentis executed from the Chesapeake Light Tower (CLT) inSeptember 1988, with data analysis and evaluation of resultsto ext&Ad into 1990.

TAkBLE OF CONTENTS

PAGE

EXECUTIVE SUMMARY..................................1iii

I. INTRODUCTION......................................... 1

II. SCIENTIFIC OBJECTIVES................................ 6

III. THE FIELD SITE....................................... 12

IV. PARTICIPATION........................................ 21

V. FIELD MEASUREMENTS.................................. 24

VI. FIELD SITE INSTRUMENT MOUNTINGS.................... 43

VII. EXPERIMENT COORDINATION ANDMEASUREMENT STRATEGY................................ 44

VIII. DATA MANAGEMENT...................................... 49

IX. DATA ANALYSIS........................................ 51

REFERENCES........................................... 54

APPENDIX A (DATA ON OTHER AIRCRAFT)................ 56

APPENDIX B (SAXON I DIRECTORY)..................... 71

vi

LIST OF FIGURES

FIGURE PAGE

1. Location of Chesapeake Light Tower, DepthContours in Meters. The Solid LinesSpecify Nominal Flight Track Over theTower ......... ...................... 14

2. Recent Photograph of Chesapeake Light Toweras Seen from the South ..... .............. 15

3. Side View of Chesapeake Light Tower as Seenfrom the North Showing Elevation Levels. TheBottom is Now at -15m ...... ............... .16

4. Catwalk Surrounding Chesapeake Tower at 5mLevel Above MLW ....... .................. .17

5. View of U.S. Coast Guard Helicopter Landingon the Helicopter Deck ..... .............. 18

6.. Surface Climatology in the Chesapeake TowerVicinity as Provided by U.S.Coast Guard Windand Wave Summary Report No. CC-D-05-84 for theMonths of July-September ..... ............. 20

7. Surface Climatology in the Chesapeake TowerVicinity as Provided by U.S.Coast Guard Windand Wave Summary Report No. CC-D-05-84 for theMonths of October-December .... ............ 21

8. Wave Follower System in Operation with theWave-Slope Sensor ..... ................ 26

9. Sketch of Laser Beam Acceptance Cones . ...... 27

10. Schematic of Stereo-Geometry. The Cameras Showthe T-Bar in Their Field of View for Calibra-tion ......... ....................... 29

11. Capacitance Wave Probe Spar Buoy ... ......... .. 33

12. Flight Patterns for SAR and RAR .. ......... . 37

13. Schematic of a Typical Ship Pass ... ......... .42

14. View of Instruments Mounted, as Seen LookingDownward at Base of Tower, -6m Level. 1. WaveSlope Sensor-(Not Shown), 2. Thermistor Arrays,3. Pressure Sensors, 4. Doppler Current MeterLocated Approximately 50m South East of TowerBase ......... ....................... 44

vii

mW,-

15. View of Instruments as Seen Looking from WestElevation. 1. Wave Slope Sensor, 2. ThermistorArrays, 3. Pressure Sensors, 4. Doppler CurrentMeter, 5(a). Wave Follower in Storage Position,(b) Wave Follower in Operating Position, 6. WindDrag and Short Wave Slopes, 7. Stillwell Photo-graphy, 8. Surface Tension, 9. StereophotographyT-Bar, 10. Surface Tension, 11. (Not Shown), 12. C-to W- Band Multi-Band Frequency Radar . ...... 46

16. RPI Surface Tension Measurement System . ...... .47

17. View of Instruments as Seen Looking Downward atCatwalk Level. 1. Wave Slope Sensor, 2. Thermis-tor Arrays, 3-4. (Not Shown), 5. Wave Follower,6. Wind Drag and Short Wave Slopes, 7. (NotShown), 8. Surface Tension, 9(b). Stereophoto-graphy T-Bar, 10. Surface Tension, 11(c). WaveGage Array ........ .................... 49

18. View of Instruments Mounted as Seen from SouthElevation. l.Wave Slope Sensor, 2. ThermistorArrays, 3. Pressure Sensors, 4. Doppler CurrentMeter, 5(a). (Not Shown), (b) Wave Follower inOperating Position, 6. Wind Drag and Short WaveSlopes, 7. Stillwell Photography, 8. SurfaceTension, 9(a). Stereophotography, (b) Stereo-Photography T-Bar, 10. Surface Tension, 11(a).Ku - Band Radar, 11(b). Ku-Band Radar and 11(c).Wave Gage, 12. C- to W-Band Multi-FrequencyRadar, 13. C-, ka and G-Band Radar .. ........ .50

19 13. C-, Ka - and G-Band Radar Instruments Mountedas Seen Looking Down from the Helicopter Deck Level.52

20. U.S. Coast Guard 25m Cutter Point Huron. One ofThree Cutters will be made Available for Trans-portation to the Tower on a NoninterferenceBasis ........ ...................... 55

21. Rubber Raft Used by 25m Cutter to On/Off LoadPersonnel to and from the Chesapeake LightTower. The 12m Cutters Can On/Off Load at thisTower Landing ....... .................. 56

22. Organization Plan for SAXON I Experiment ...... .. 57

23. SAXON I Schedule of Events .... ............ 58

24. Data Management Center for the SAXON IExperiment ........ ................... 60

viii

LIST OF TABLES

TABLE PAGE

1. Radar Frequencies and Wavelengths Proposedfor SAXON I, and Investigated in PreviousExperiments ...... ................. 8

2. Aircraft Participating in SAXON I ... ...... 10

3. Matrix of SAXON I Science Objectives vs. thePlanned Data Sets ...... .............. 11

4. System Characteristics of C- and X-Band RAROn Board NRL RP-3A Aircraft ... ......... .36

5. System Characteristics of L-, C-, and X-BandSAR System on Board NACD RP-3A Aircraft . . . 39

6. System Characteristics of the Ku Band WaveSpectrometer on Board the NRL RP-3A Aircraft 41

7a. Investigators and Corresponding Areas ofAnalysis ....... .................. 62

7b. Investigators and Corresponding Areas ofAnalysis (Continued) .... ............ 63

ix

I. INTRODUCTION

SAR imaging of the ocean surface promises to be animportant tool in ocean remote sensing. The long range goalof the proposed research is to understand SAR imaging insufficient detail so that airborne and/or spaceborne SARscan become reliable tools for oceanic research, and foroperational use.

The TOWARD Experiment provided a comprehensive data setthat was required for verification of theoretical conceptson SAR imaging of the ocean surface. In the formulation ofthe experiment it was recognized that three distinctdisciplines have to be addressed: (a) hydrodynamics, (b)radar backscatter and (c) SAR image processing.

A significant achievement from TOWARD is thedetermination that none of the then available theories onSAR imaging of long surface waves could explain all the SARobservations satisfactorily. Work is presently in progressto amend existing models. In addition, the followingspecific results were reported:

1. The probability density functions of wave slopesindicated Gaussian distribution for wavefrequencies less than 2.5 Hz and non-Gaussiandistribution for wave frequencies greater than 7.5Hz. These distributions suggest that the longerocean waves are weakly coupled, as expected, butthat the very short surface waves are stronglycoupled through possibly strong nonlinearinteraction.

2. The measured wave slope spectra from the laserslope sensor and stereophotography in TOWARDindicated lower intensity levels compared topredictions by Pierson and Stacy (1973) andDonelan and Pierson (1985).

3. The radar backscatter results suggested that seaspikes do not contribute significantly to theaverage backscattered power at L-Band. Sea spikeoccurrence increased with radar frequency and windspeed.

4. The cross-wind radar backscatter results suggestedthat cross-wind or cross-wave modulation doesoccur, and constitutes a mechanism for SAR imagingof azimuthally traveling waves. The data alsoindicated that the radar backscatter spectrum hada small peak near the dominant frequency of long

1I

waves, indicating a significant role for thevelocity bunching mechanism.

5. SAR focusing results for azimuthally travelingwaves indicated that waves have the highestcontrast at a focus setting that is one order ofmagnitude greater than the wave orbital velocity.There is theoretical evidence that the range 0.5-1.0 times the phase velocity is the relevantmagnitude.

The TOWARD results provide definitive conclusionsregarding the physics of SAR imaging of the ocean surface.Indeed, because the TOWARD data sets provide the mostcomplete surface-truth measurements in support of SAR today,it is anticipated that these data sets will be used for anumber of specific scientific investigations related to SARin the future. It is noted that the TOWARD data sets weredirected at solving the L-Band SAR problem. A relativelysmall number of X-Band measurements were obtained at thesame time to improve the basis for planning of futureinvestigations on SAR imaging at higher frequencies such asSAXON I.

The achievement derived from understanding SAR imagingof surface waves at L-Band has an importance that transcendsthe immediate application of SAR detection of surface waves(although this application is important in itself). The mostsignificant outcome of understanding SAR imaging mechanismsis the development of a theory for imaging the ocean surfacethat is consistent with observations. Understanding SARimaging of surface waves at L-Band is a critical first steptowards understanding SAR imaging of other ocean surfacesignatures. Hence, it is a natural follow-on from TOWARD toconsider SAR imaging of internal waves, boundary currentsand ship wakes, in various sea states and with multi-frequency radars.

SAR imaging of internal waves in the SARSEX experimentdemonstrated that the observations at L-Band could beexplained, to factors of 2-5, by the Bragg-resonantmechanism of the modulated surface waves. But theobservations at X-Band could not be explained by Bragg only.The influence of intermediate surface waves was invoked, andshown to be crucial for explaining the observations. InSARSEX, it was implicitly assumed that simulated radarbackscatter from the ocean surface is equivalent tovariations in the SAR image intensity. This assumption isreasonable in low sea states, but not so in the higher seastates. In the latter situation, the orbital velocity fieldassociated with surface waves induces a distortion in themapping of the ocean surface to the SAR scene. The SAR

2

resolution is also degraded by surface motion effects, sothat certain length scales on the ocean are eliminated inthe SAR scene. The influence of surface waves on imaginginternal waves is not well understood at present.

A limitation of the TOWARD experiment stems from thefact that the sea states encountered were low, and highlydirected (due to the directional windows imposed by theChannel Islands). While the limitations allowed carefulscrutiny of the SAR imaging theories, the need remains fortesting such theories in higher sea states. To a limitedextent, this need has been addressed in the LEWEX experimentwhere useful data sets have been acquired.

In summary, it can be stated that significant insightshave been gained from TOWARD and SARSEX. Yet, gaps remainin understanding the SAR imaging processes. These arestated below:

1. Hydrodynamic interactions of very short waves(0.5-5 cm long) are strongly nonlinear andpresently not understood.

2. Radar backscatter from short surface waves isinfluenced by the entire spectrum of surface waveswith wave numbers smaller than the Bragg wave-length. The implied consequence is thatmodulation levels of long waves can increase withradar frequency. There is need for precisemeasurements and simulations, in an input-outputsense, to demonstrate this potentially seriousimplication.

3. Radar backscatter measurements from the oceansurface exhibit spikes that increase with radarfrequency and sea state. The available radarbackscatter models have not explained theseobservations satisfactorily. Additionalmechanisms due to specular, wedge and breakingwave effects must be considered.

4. SAR imaging of surface waves at X-Band and higherfrequencies requires validation. There is alsoneed for testing SAR imaging theories in high seastates.

5. There is a need for understanding the influence ofsea state on imaging internal waves.

An important environmental factor in microwave remotesensing that is given emphasis under the Marine Microlayerprogram is the physio-chemical properties of the air-water

3

interface. This interface (marine microlayer) is differentfrom the bulk water below and the atmosphere above it. Itis a region where viscous forces are preeminent, wherebiogenic organic compounds form films, inducing wide rangingphysio-chemical changes, and where the majority of oceanremote sensors image the sea surface. It is in this regionthat occurrences on molecular and micrometer scalesinfluence air-sea coupling and interact with surface waveenergetics which in turn modify the electromagnetic signalreflected or emanating from the sea surface. Thus, thetechnical issues which arise naturally from the availableenvironmental data are:

1. What is the dynamic equation-of-state of theocean's surface? How does the visco-elasticnature of the sea-surface change with time?

2. Is the surfactant concentration at a given placeand time near a 2-D transition?

3. How do films originate? What is the residencetime of surfactants at the surface of the ocean?

4. Is the ocean's interface a habitat formicroorganisms? How do unique adaptations revealbiological and chemical succession at theinterface?

5. What is the chemical composition of surfactantfilms? How does it vary oceanographically?

6. How do films affect interscale energy transfer ofsurface capillary and capillary-gravity waves?

The scientific objectives for SAXON I are carefullyselected to include the above issues and are consistent withrecommendations of the SAR Working Group report (1986).They are detailed in Section II.

The experimental approach for achieving the above goalsis highlighted below:

1. Use a stable platform so that measurements ofshortwaves and radar backscatter can be free ofplatform motion.

2. Provide simultaneous measurements of short surfacewaves, long surface waves, internal waves andrelevant environmental parameters.

3. Provide for platform-based coherent and calibratedradar measurements in the frequency range 5-95 GHz

4

(corresponding to radar wavelengths of 0.3-6.0cm). The aim is to relate these toamplitudes and slopes of surface waves so that:(a) the effect of the surface wave modulations onboth SAR and RAR can be established, and (b) aradar confirmation of the modulation of shortwaves by long waves can be provided.

4. Obtain SAR and RAR digital images of the oceansurface with airborne radars at various azimuthalangles, height to velocity ratios, andenvironmental conditions; the emphasis is on 10GHz radars.

5. Incorporate specialized systems for measuringsurface tension, decay rate of short waves(Bobbing Buoy) and near surface wind speed.

6. The Chesapeake Light Tower is a stable platform,free from influence of land atmospheric influence.It is used in SAXON I to provide a host ofspecialized experiments.

5

I. SCIENTIFIC OBJECTIVES

The specific scientific objectives of SAXON I are:

1. Investigate the Hydroya _. Modulation of Shor_GrAvityand_. C p a__Waves 10.3-50cm) by LongSurface Waves (50-500m) and Internal Waves,Included in this Itnves igation is the Role ofTiterediate Waves (0.5-50m).

There is now substantial evidence that short gravitywaves 30 cm in length are better behaved hydrodynamicallycompared to 3.0 cm long waves. Hsiao and Shemdin (19s3)report on this difference based on their MARSEN results.Hwang and Shemdin (1988) comp to a similar conclusion basedon Probability Density Functions (PDF) derived from theTOWARD laser slope data.

SAR images from the SARSEX experiment confirm thatinternal wave modulation levels at X-Band are similar inmagnitude to those at L-Band. These observations can beexplained by Bragg scatter modulations at L-Band but not soat X-Band. Two different analyses have been advanced toexplain the X-Band observations. Both utilize multi-scaleinteractions. These are:

(a) X-Band Bragg waves are modulated hydrodynamicallyby intermediate scale surface waves which are in turnmodulated hydrodynamically by the internal waves. Thisanalysis is advanced by Watson (1986) and is primarilyhydrodynamical in its approach.

(b) Internal waves modulate intermediate surface waveshydrodynamically. These modulated intermediate waves tiltthe X-Band Bragg waves. The approach invokes a combinationof radar backscatter and hydrodynamics. This analysis isdiscussed by Holiday (1986). The method is shown by Jang(1986) to be similar to a three-scale approach introduced in1975 to explain the OWEX observations.

Considering the many assumptions invoked in both of theabove approaches, it is seen desirable that a careful set ofobservations, involving short surface waves and radarbackscatter, be acquired simultaneously to provideindependent hydrodynamic and radar backscatter verificationsto the SARSEX observations. Such measurements are alsorequired for validating the hydrodynamic forcing functions,such as introduced by Hughes (1978), among others.

6

I j ,, j ; 14 %

2. Determine Exprimentally. iuFnc Tower-- 4ed~m, RadarBackscatter ModUlations Induced by

Internal Waves and Lonq Surface Waves, with

Emhasip__ Radar Freguencies in the Range 10-35

The available evidence suggests that radar backscatter

modulations, induced by internal waves, decrease with

increasing radar frequency from P-Band to C-Band, then

increase toward X-Band, presumably because of theintermediate surface wave effects discussed above.Considering that radar resolution and the number ofindependent samples increase with radar frequency, it can bedetermined that higher signal-to-noise ratios can beachieved at higher radar frequencies. The above suggeststhat radar frequencies higher than X-Band may yield largermodulations and higher signal-to-noise ratios compared to X-Band.

The OWEX data set covered the frequency range P- to Ka-Bands, as shown in Table 1. The TOWARD experiment coveredthe frequency range L- to Ku-Bands. It is planned here thatthe range L- to G-Bands be covered in SAXON I, with emphasison X- to Ka-Bands. These measurements are crucial fordetermining the modulations induced by internal and/orsurface waves at X-Band and higher frequencies. Bothplatform-based and aircraft deployed radar systems are to beutilized for this purpose.

The OWEX data set emphasized near grazing angles forthe frequency range P- to K -Bands. In SAXON I, it isplanned that measurements be otained at incidence angles inthe range 20O-600, at frequencies similar to OWEX andextending upward to G-Band. The planned radar backscatterand in-situ short surface wave measurements allow testing ofavailable radar backscatter models by using the in-situ dataas input (into the simulation algorithms) and comparing theoutput with radar backscatter measurements. Themeasurements obtained in OWEX did not allow such validation.New mechanisms involving wedge and breaking wave effects,which were recognized after OWEX, will be explored indetail.

3. Test SAR Imaging Theories at X-Band and HigherFreauencies

The TOWARD data set has allowed resolution of longstanding differences between the available SAR imagingtheories. The theories that are being validated with the

7

_ I 47:] _ -'

---- I1--, . ~... , -ICIJil u

III

Table 1. Radar Frequencies and Wave Lengths Proposed forSAXON, and Investigated in Previous Experiments.

ks = k, sin0

Surface Radar Incidence AngleXr

Xs = 2- (Valid for Bragg)2 sine

0 = 0 ° Iks -- 0

0 = 900 Xs -X,/2 0

0=300 Xs -\ -"S

Band Freq

G 215 GHz 0.14 cm

W 100 GHz 0.3 cm

S Ka 35 GHz 0.9 cm

A!- Ku 15 GHz 2.0 cmX TO 0 X 0 10 GHz 3.0 cmN W C 5GHz 6.0 cm

A ER S X 3 GHz 10.0 cm

L 1 GHz 30.0 cmP 0.5 GHz 60.0 cm

VHF 0.1 GHz = 3.0 m

100 MHz

HF 10 MHz 30.0 m

MF 2 MHz 150.0 m

8

TOWARD data will be further tested at X-Band and higherfrequencies in SAXON I. It is proposed that equivalentresolution SAR and RAR images be obtained simultaneously inSAXON I over the instrumented site to enable suchvalidation. As in TOWARD, surface observations will be usedas input in the SAR simulations. The outputs will becompared with modulations derived from the actual images.The goal is to determine the skill of the SAR algorithms indetermining the modulation levels and in estimating thedegradation of SAR resolution. The simultaneous SAR/RARimages will be instrumental in achieving this objective. IThe aircraft participating in SAXON I are shown in Table 2.

4. Investigate the Influence of the Microlaver onBoth Active and Passive Remote Sensina Techniues i

The basic objective of the Marine Microlayer program ofONR is to understand the biological, chemical and physicalprocesses controlling those properties of the ocean'ssurface responsible for textural variability imaged bymicrowave, visible and IR remote sensors. A two-prongedeffort will be pursued as part of the Marine Microlayerprogram working in conjunction with the Ocean SAR program.In addition to the surface tension measurements taken at theTower, a research vessel will operate in other coastalareas, e.g. off the U.S. West Coast.

In summary, four primary objectives are identified forthe SAXON I experiment. These objectives are listed intabular form, in relation to the data sets that areacquired, in Table 3. Here, the definition of subsurfacehydrodynamics is shown separately because of its underlyingimportance in meeting the stated objectives. Table 3 showsthat certain data sets are essential for all the SAXON Iobjectives.

It is emphasized here that the objectives stated aboveare intimately tied to the central goal of understanding themechanisms responsible for SAR imaging of the ocean surface.Such understanding is prerequisite to establishing where andhow the SAR can be used as an oceanographic research tool orfor operational goals. The first of the above objectivesallows understanding of the hydrodynamics associated withthe ocean surface microstructure which the radar detects. j

I

9!

Table 2. Aircraft Participating in SAXON I.

SAR Mode RAR Mode

Aircraft L C X C X

NRL RP-3A

NADC RP-3A

10

Table 3. Matrix of SAXON Science Objectives vs. the PlannedData Sets.

Define Investi- Test Test Investi-Sub- gate Radar SAR/RAR gate

Objectives surface Hydro- Back- Imaging Micro-Hydro- dynamics scatter Theories layer

dynamics of Very Modula- at High Influ-Data Short tions Fre- encesSets Waves quencies

1 Laser Slope Sensor

System-Short Waves

2 Stereo Photograph

3 Surface Tension

4 Directional Spectrumof Long Waves

5 Meterulogical Data

6 Thermistor Chains

7 Subsurface & Near-surface Current

8 Multi-FrequencyTower Radars

9 C-Band SAR & RAR

10 X-Band SAR & RAR

1ll I

11llllm mll ll~lll lmlm l lllllllll

The second objective investigates the link between the oceansurface and radar backscatter. The third objective allowsdetermination of the effect of surface motion on the SARimaging process, and the last provides needed information onhow the surface microstructure can be enhanced or destroyedby natural surface films, which is frequently inferred to be

the cause of certain strong modulations in SAR images.

S12

III. THE FIELD SITE

A location off the U.S. East Coast, 22 km offshore CapeHenry, Virginia, known as the U.S. Coast Guard ChesapeakeLight Tower (CLT), is selected as the field sight for SAXONI, as shown in Figure 1. The following attributes are notedfor the site:

1. It is the most suitable tower known as analternative to the originally proposed field site,the OCNR Tower, San Diego, which was destroyedbeyond repair in a major storm on 18 January,1988.

2. The Chesapeake Light Tower is a rigid platformoperated by the U.S. Coast Guard and is madeavailable to SAXON I at no cost to ONR.

3. The Tower is located in 15m water depth, alocation that is otherwise representative of openocean conditions.

4. The site is free from wave refraction from nearbyisland effects.

5. Internal waves are generally present from Aprilthrough October.

6. Swells propagate from the open ocean. Co-existence of swell and internal waves is adesirable environmental condition for addressingthe SAXON I objectives.

The Chesapeake Light Tower is located 14 miles offshoreof Cape Henry at 36 55' N and 750 43' W. A recentphotograph is shown in Figure 2. A side view of the Tower,as seen from the North, is shown in Figure 3. Platformelevations are marked in relation to the MLW level. A metalgrating catwalk, shown in Figure 4, surrounds the Tower atthe 5m level and provides a significant convenience forservicing many of the near surface instruments. The maindeck has a grated platform at the 17m level. The quarterdeck is at the 22m level and provides facilities for datagathering, computer systems and individual quarters. The26m level is a helicopter deck, as shown in Figure 5.Helicopters can be used for transporting personnel andequipment to and from the tower. Facilities allow up to 20scientists to operate during the day and remain on boardduring the night.

13

BATMOXATLANTIC CITY -

390 DELAWARE

ANNAPOLIS CP A

-k

38''

61

SLITTLE370 CRE CPE (~CHESAPEAKE

CEKHENRYLIHTOE

NORFOLK

36' -~

76' 75. 740

Fiue1. Location of Chesapeake LgtTwr etContours in Meters. The Solid Lines SpecifyI

Nominal Flight Tracks over the Tower.

14r

Figure 2. Recent Photograph of Chesapeake Light Tower asSeen from the South.

15

33M Light

Platform

-26M -- Helicopter

DeckS22M -- Quarter

L Deck

5M -- Cat WalkBoat Dock- "

-/ I

,/ ' [OM, MLW

-__-12M -- MLW

Figure 3. Side View of the Chesapeake Light Tower as Seenfrom the North Showing Elevation Levels. TheBottom is Now at - 15m.

16

Figure 4. Catwalk Surrounding Chesapeake Tower at 5m LevelAbove MLW.

17

Figure 5. View of U.S. Coast Guard Helicopter Landing on

the Helicopter Deck.

I_ 11

18 I

!

The environmental conditions in the Chesapeake regionare summarized in a May 1984 report entitled "Wind and WaveSummaries for Selected U.S. Coast Guard Operating Areas"prepared by the U.S. Department of Commerce. Wind speeds,direction and frequency of occurrence for the months of Julythrough December are extracted and shown in Figures 6 and 7.This represents information gathered from a location 30 kmEast of the tower site. During the month of September,winds are shown to be predominantly from the southwestranging from 11 to 15 knots (5.7 to 7.8 m/s). Maximum windsof 42 knots (21.6 m/s) are a seldom occurrence. similarconditions are reflective for the month of October with somevariance of increased overall wind speeds, particularlyabove the 11-15 knot (5.7-7.7 m/s) range. During Septemberto October, the wind direction is from direction west southwest (WSW). Surface currents for August-September are 0.6knots (0.3 m/s) from the north. The tidal range for thistime period is 1.6m.

19

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21

IV. PARTICIPATION

The potential organizations, investigators andassociated activities planned for the field operations phaseof the experiment are shown below:

organigation Investigator Investigation

APL/JHU B. Gotwols StillwellR. Sterner/ PhotographyR. Chapman

Case Western J. A. Mann/ Surface TensionUniversity J. Sanders

E. Bock Surface TensionAnalysis

DTI P. Jang/ Internal Wave DataS. Borchardt Analysis

ERIM A. Milman Passive MicrowaveRadiometer

GSFC/NASA T. Liu Internal Wave DataAnalysis

ISA 0. Lee Internal WaveMeasurements

JPL R. Martin/ Wave FollowerD. Hoff/ InstrumentR. Steinbacher Design and

Operation

La Jolla F. Henyey/ Hydrodynamic and SARInstitute J. Wright Modeling

R. Daschen

M.S. Longuet-Higgins NonlinearHydrodynamicsTheory

MIT K. Melville/ Breaking Waves andA. Jessup Backscatter Looking

at Nadir

22

MPL K. Watson Hydrodynamic

Modulation TheoryP. Hanson/ Tower SupportS. Beck/P. Jordan

NADC A. Ochadlick/ L, C-, and X-BandH. Kritikos/ SARJ. Lee

NORDA P. Smith Surface Waves BuoyNOSC J. Rohr Acoustic

MeasurementsNPGS K. Davidson Wind Stress

MeasurementsE. Thornton/ Doppler AcousticT. Stanton Measurements

NRL G. Geernaert Atmospheric Turbu-lence, Drag andStabilityMeasurements

W. Keller/ C-Band RARP. Richardson MeasurementsW. Plant C- and Ku-Band

D. Schuler Tower Radar

W. Barger Surface FilmObservations andCharacterizationsG. Sandlin/ Passive RadiometerJ. Hollinger/ (SSM/I)A. Rose Measurements

ORE P. Hwang/ Short Surface WaveC. Palm Slope Measurementsand Analysis

D. Hayt SAR/RAR DataProcessing

D. Kasilingam SAR/RAR Theory andSimulation

M. Tran Stereophotography

23

ORINCON W. Garrett Measurement,

Analysis, andModification ofSurface Slicks andSurface Compressi-bility

RPI J. Korenowski/ Surface TensionB. Asher Measurements

SIO R. Guza/ Directional LongM. Clifton Wave Measurements,Ambient TowerMeasurementsUCSD R. Daschen

Radar and Hydro-

dynamic theoryU. Delaware D. Lyzenga SAR Theory

U Kansas R. Moore/ 5, 10, 15, 35 andJ. West/ 95 GHz Tower RadarP. Gogineni/ Measurements

J. HoltzmanU Mass C. Swift/

5, 35 and 200 GHzR. McIntosh/ Tower RadarI. Popstefanija Measurements

U Texas A&M A. Fung Radar Backscatter

Arlington ModelingU.S. Coast D. Paskausky/

Tower SupportGuard J. Abbott/P. Pallo/C. Parks/J. Brown/J. Armquest

USGS S. Wu Stereo-Processing

24

V. FIELD MEASUREMENTS

The experimental resources pzoposed for SAXON I areoutlined below:

A. Chesapeake LiQht Tower

The Chesapeake Light Tower is a stable platform located22 km offshore of Cape Henry. The wave follower mechanismis mounted on the south side of the tower structure. Anendless cable system, shown in Figure 8, allows theinstrument frame to be moved vertically so it maintains aconstant submergence depth as it rides on the long oceanwaves (Shemdin, 1966 and 1980, and Hsiao and Shemdin, 1983).The vertical motion of the instrument frame is controlled bya wave gage which provides the signal to the servo-motor.

The following measurements are incorporated at thetower site:

1. Directional Spectra of Short Surface Waves

Short surface waves are measured by three independenttechniques. The first uses a laser-optical system thatmeasures directional wave properties. The second techniqueutilizes stereophotography to measure directly the two-dimensional wave number spectra of short surface waves. Thethird uses the familiar Stillwell photography which is knownto be dependent on sky-brightness. These techniques aredescribed briefly below.

a) Laser-Optical System - The ocean-going wave slopeinstrument used in TOWARD is modified so that space-timevariabilities of surface waves can be measured. Figure 9shows a sketch of the laser beam acceptance cone of theoptical receiver portion of the wave slope instrument fortwo different maximum wave slope measurement capabilities.The 25.49 degree acceptance cone is associated with a 45degree wave slope; the 7.12 degree acceptance cone isassociated with a 20 degree wave slope. It is seen herethat a portion of the optical receiver is unused for seastate conditions which produce smaller maximum wave slopes.The wave slope instrument is designed to be insensitive towhere the laser beam enters the optical receiver; it simplymeasures the laser beam deflection angle, regardless of thebeam's entry location. The unused portion of the opticalreceiver acceptance cone is therefore available for spatialscanning purposes.

25

To Tower

Servo Drive TElectric Motor 7

10m

(C) Wave FollowerInstrument Frame MSLError Gauge

EM-Current Meter -./ l 0m--2.5m -

1000# Tension

Cable

To Tower

sC-- EM-Current Meter (b)Endless Cable .Pressure Sensor (a)Assembly g

Figure 8. Wave Follower System in Operation with the WaveSlope Sensor.

II

26 1

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Ej 0C)~

mJ E )

CU a)

Lcci C.)< QC\JQ

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aIO./

r-4

27

b) Stereophotoqraphy - For short surface wavesstereophotography has an advantage over Stillwell typephotographic analysis in that the directional spectraof the ocean surface are not sensitive to skybrightness variation or sun angle. The directionalspectra obtained by stereophotography are an order ofmagnitude more accurate and can extend to a higherrange of wave numbers where the amplitudes aretypically very small. This requirement is consistentwith the need to resolve Bragg waves of order 1.0 cm inorder to compare with 35 GHz radar measurements.Stereophotographic analysis from the TOWARD experimenthas clearly demonstrated the feasibility of thistechnique. A schematic of typical tower stereo- Jgeometry is shown in Figure 10. An automated high-speed stereo-system is used in SAXON I.

c) Stillwell Photography - Although stereo-photography has some distinct advantages, the Stillwelltechnique is faster, more economical, and ofconsiderable importance; it provides a means forcomparison with the stereo-derived wave number spectra.The Stillwell photography consists of the use of one ormore video cameras that have high or low resolutions.This technique is sunlight dependent, and the mountinglocation of the cameras is crucial in that the sun mustbe behind the cameras. Clouds are a problem in thatthey cause changes in the brightness of the images.Clear weather is preferred. The cameras must bepositioned as high above the surface of the ocean aspossible, usually 8-9 meters with 10-11 meters mostdesirable. The cameras are operated in the lookdirection of SAR. The information is recorded on videotape, and analyzed subsequently in the laboratory.

2. Directional Spectra of Long Surface Waves

The measuring system consists of an array of pressuresensors mounted on the bottom and on the tower frame.Although high resolution directional (± 100) wave heightspectra are desired in support of radar observations thesystem to be deployed in SAXON I will have directionalresolution of order + 250. Only waves with frequencies lessthan 0.3 Hz will be measured with this array. The object isto obtain frequency and directional spectra for comparison Iwith radar images. The measurements also allow thedetermination of the nonlinear distortions in the wavedispersion relationship.

II

28 I

7mBase

- Width

h-surface Area Covered -

Depends on Lenses Used

./\.

Camera '1 7m N

/ I/ . IT-Bar

/~ ~Sd iew -, rotVe

Figure 10. Schematic of Stereo-Geometry. The CamerasShow the T-Bar in Their Field of View forCalibration.

29

3. Directional Spectra of Internal Waves

Internal wave measurements are obtained at the towerwith an array of thermistor chains. Four 28-elementthermistor chain arrays are deployed to insure measurementof space-time variability of internal waves. These aresuspended by strain members positioned at each leg of thetower. Each thermistor chain will have 28 thermistorscalibrated according to pre-determined specifications.

4. Microlayer

In collaboration with ONR's Microlayer AcceleratedResearch Initiative program, several techniques formeasuring surface tension, compressibility and othermicrolayer properties are deployed to obtain theseproperties simultaneously with other surface measurements.One of these techniques is deployed on board the wavefollowing C-frame so that microlayer properties and shortwave slopes can be obtained simultaneously in the proximityof the same ocean surface patch. Other microlayertechniques are deployed at a different location on thetower.

5. Ambient Measurements

Measurements of the wind stress (via both sonic anddissipation systems on the tower, and dissipation techniqueon a nearby buoy), wind speed and direction, atmosphericstratification (heat flux), air and water temperatures, waveheight, tide level, near surface currents (using Doppleracoustic sensors), and surface tension are obtained. Allthese quantities are routinely recorded during the intensivemeasurement periods of SAXON I. The data will bedistributed to all investigators as background informationto support the analyses.

6. Tower Mounted Radar Systems

Two multi-frequency and one single frequency radarsystems are deployed on the tower, as detailed below: I

a) University of Kansas Multi-Frequency (C- to W-Band) Radar - These are broadband FM tracking radarscatterometers. One system covers 5, 10, and 15 GHzfrequencies, and two other systems operate at 35 and 94 GHz.Use of excess sweep bandwidth allows averaging moreindependent samples for each measurement than wouldotherwise be possible, thereby reducing the degradation ofmeasurement precision by fading. Separate antennas are usedfor vertical and horizontal polarizations at the three lower

330 I

frequencies. Beamwidths of the antennas range from 2.5 degto 6 deg. The antennas are mounted together so that all maybe pointed at the same spot on the surface. These systemstrack the slant range by automatically adjusting the sweepduration, which is measured. This allows measurement of thesurface elevation at the centroid of the beam, therebypermitting direct computation of the cross-spectrum of thewave height and radar signal strength for studies of themodulation of the radar signal and the ripples by the longwaves.

A vector slope gage is used for the first time duringSAXON I. This system uses three 35-GHz ranging radars withnarrow beams pointed to different spots within the widerbeam of the lower-frequency system. The heights of thethree spots can be used to determine orthogonal componentsof the local surface slope within the wider radar beam.This permits determination of the true slope modulation ofthe radar signal so that slope and hydrodynamic modulationscan be separated.

A video camera is used to monitor the scatterometerbeam location to assist in identifying causes of sea spikes.

b) University of Massachusetts Radar Systems - Thissystem has three radars that are used on the tower. Theyare C-, Ka- and G-Bands with frequencies corresponding to 5,35, and 215 GHz, respectively. The beam widths of the Ka-and the G-Band system is operated in FMCW mode to measureradar cross-section, but is now modified to obtain, inaddition, Doppler information. The University ofMassachusetts data acquisition system is able to handle datafrom all three radar systems, simultaneously.

c) NRL K -Band Radar System -Two K - Band (14 GHz)coherent Doppl er scatterometers witg dual linearpolarizations are used on the tower. One is mounted at avertical incidence on a 7.5 m horizontal boom along with aThorn IR wave height sensor. This scatterometer has a 5degree two-way half-power beamwidth. The secondscatterometer has a 2.52 degree two way, half powerbeamwidth. It is mounted at a 45 degree incidence anglealong with a video camera.The azimuthal angles depend onwind and wave directions.

.3

31

!i

A wave gage array with one decimeter spacing is locatednear the illuminated area.

d) NRL Passive Microwave Radiometers - NRLparticipates in SAXON I with their SSM/I - simulatorradiometers tuned to frequencies 19GHz, 22GHz, 37GHz and85.5GHz (the SSM/I passive microwave imager is deployedaboard the DMSP satellite). The radiometers are operatedduring the periods of DMSP overflights. Also the NRLradiometers are operated in parallel with the NPGS windstress sensors to determine the correlation between the twotechniques in measuring surface wind stress. Particularemphasis is devoted to the passage of meteorological fronts.Additional emphasis is given to the detection of surfacefilms, and sea surface variation induced by ship wakes.

B. In-Situ Measurements in Proximity of Chesapeake LiQhtTower

The in-situ measurements in proximity of the ChesapeakeLight Tower include the following:

1. Capacitance Wave Probe Spar Buoy

A schematic of the buoy is shown in Figure 11. The sparbuoy is 9 cm in diameter and 6m in length. The suite ofinstruments mounted on the spar include an array of sixcapacitance surface elevatation probes, a compass, anaccelerometer, and an anemometer. The buoy is equipped witha vane which generally moves down-wind, keeping the array ofwires up wind of the buoy. The compass provides an estimateof the wind direction. Data is logged on board the buoy ina Sea Data digitizing data logger. The resonant frequencyof the spar (in vertical motion) is approximatly 10 seconds.The buoy is designed to "follow" swell with period 10seconds or longer. The accelerometer is included to recordthe omni-directional height of the low frequency swellcomponent.

The array of capacitance wires is spaced at distancesappropriate to the segment of the wave number spectrum forwhich an estimate of the directional spectrum is desired.The array is scanned at 32 Hz, and pre-filtered at 15 Hz toavoid aliasing. For SAXON I, the sensor wire is replaced bya resin-coated piano wire with the working section being 170 Icm long.

III

32 I

I"

Anemometerr-Analog Electronics

cmWn

Array

Vane

Battery

6 m Pack Digital RecorderBuoyanc Compass

Buoyancy TAccelerometer

a

b

>ca

a = 7.20 cm bb = 11.55 cm /

Damper c = 8.77 cm /a

Ballast Plan View of Array

Figure 11. Capacitance Wave Probe Spar Buoy.

33

The mode of operation is to deploy the buoy from a shipfor a period of from 25 to 30 minutes, using the ambientcurrent or wind to advect it through the area of interest.The buoy is then brought back aboard the deploying vessel, Iand the 1 megabyte memory dumped to the shipboard computer

while the batteries are re-charged. This process takesabout 25 minutes after which the buoy is re-deployed.

The data provides estimates of frequency directionalspectra and wave slope statistics as a function of the buoyposition averaged over the appropriate time interval. Slope Istatistics are obtained by producing a time-dependentestimate of the wave slope, spatially averaged over the areasubtended by the area of the array. This is accomplished byfitting the best flat facet through the elevation data. Theslope distribution obtained in this way, from data collectedat Nantucket Shoals, shows an extremely directional natureto the distribution, for wavelengths of 15 cm or greater.

Directional spectra are derived from a least squaresfit of the cross spectra of the wire data with a model, Iusing an expansion of the wave field in circular harmonics.

2. Wind Stress Buoy - This buoy is deployed off theChesapeake Light Tower to evaluate tower measured stressresults, and to relate the momentum flux co-spectra (sonicanemometer) to the direction of the surface stress vectorunder different vector wind and wave conditions. Themeasurements are designed to correlate surface layer windvelocity and wind stress with scatterometer and SARsignatures.

3. Acoustic Measurement - The effect of monomolecularfilms on acoustic measurements will be determined. Althoughmeasurements of the damping of ocean surface waves bynonmolecular films, using wave staffs and microwave radar,is not new, those measurements combined with acousticmeasurements are. Besides allowing for a generally much Ibetter correlation between ocean roughness and ambientnoise, there is an opportunity to determine the following:

a. The threshold of ocean surface roughness when thefilm's effects become first noticeable. Previousmeasurements at very low sea states indicate that the filmdoes not have an effect.

b. At higher sea states, it is observed thatdifferent films have nonidentical effects on the higher Ifrequency ambient noise. An effort will be made todetermine if this is associated with similar effects onbubbles. 3

I34

FI

I I I

C. The acoustic effects of the slick are unsteady. Atthe lower sea states, the film's effect can persist for 1-2hours; it is not clear if this is a consequence of the filmdrifting past the hydrophone or of the film breaking up. Anattempt at such evaluation will be made.

d. In the event of extremely high sea state, it is ofinterest to determine if the films can significantly reducewhite capping.

Two sonobuoys are to be deployed near the towerapproximately one or two kilometers apart. Ambient noiselevels are recorded at the tower and comparedsimultaneously. One of the sonobuoys is placed within afootprint where surface roughness measurements are obtainedbefore and after the film is deployed. In-zitu measurementsof wind speed, current and sound velocity profiles arecollected simultaneosly with the acoustic measurements.

C. Airborne Measurement Systems

The airborne radar systems that are participating inSAXON I are shown in Table 2. Here, both systems arebriefly discussed and the radar characteristics given.

1. C- and X-Band RAR on Board NRL RP-3A

The Naval Research Laboratory has developed a realaperture radar (RAR) for the purpose of producing images ofbackscatter from the ocean surface in terms ofradiometrically correct normalized radar cross sections(NRCS). The system characteristics are given in Table 4.The system is flown on NRL's RP-3A aircraft and is tooperate at both X- and C-Bands. It is developed for theprimary purpose of co-flying with a synthetic aperture radar(SAR) in order to examine and understand differences inocean imagery between the two systems. The RAR recordsits data in a digital format directly on disc to facilitatethe production of images of absolutely-calibrated NRCSvalues. A coherent pulse-to-pulse integration isincorporated to improve signal to noise ratios, whilesubsequent averaging of detected signals producestatistically stable NRCS values. Presently plannedoperating procedures call for the production, not ofcontinuous imagery, but of imagery of a finite area(probably of dimensions 2 to 4 km by 1 to 10 km) followed bya gap while data are being stored.

Five RAR flights are scheduled during the SAXON Iexperiment. An eight-sided flight pattern, shown in Figure12, is to be flown in a manner identical to that flown bythe SAR aircraft (see 2, below). The RAR system is flown

35

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37

only at 915 m elevation. The pattern is rotated with

respect to the direction of surface waves or internal waves.

2. L-, C- and X-Band SAR on Board NADC RP-3A

A multifrequency-polarimetric SAR is constructed forremote sensing applications. There are many applications ofmultifrequency-polarimetric SARs. These use thepolarimetric channels to extract geophysical information aswell as to gain insight into the backscatter mechanism.This SAR operates at a range resolution of 2.4m and anazimuth resolution of 2.2m in the fine resolution mode, andat twice the range resolution in the course resolution mode.This new SAR is installed in an NADC RP-3A aircraft which ismodified for research and development use. The systemcharacteristics are shown in Table 5. Three frequencies areused, L, C, and X-Band. All of the elements of thepolarization matrix are collected at each of thefrequencies. The data is recorded in several forms. Highdensity digital tape (HDDT) is used as the primaryrecording medium. Data is recorded on photographic film forsubsequent processing and a real-time image formationprocessor records image data on heat-developed paper. Fourchannels of HDDT are available, each capable of rendering4096 range elements.

Five SAR flights are scheduled during the SAXON. Iexperiment. These are executed in an eight-sided patterncentered around the tower, as shown in Figure 12. The samepattern is flown at three different altitudes (1,525 , 3,049and 7,012 m), consecutively. The pattern is rotateddepending on swell or internal wave direction.

The flight duration consists of five minute runs, each3.0 km long, with headings 0, 45, 90, 135, 180, 225, 270,and 315 when the nominal swell or internal wave direction isfrom the East. The estimated flight time for one eight-sided pattern is 80 minutes. The estimated flight time overthe tower is four hours. Allowing for two hours in transitthe estimated mission duration is six hours.

The SAR system is equipped with four channels ofrecording, hence allowing simultaneous recordings of onefrequency at four polarizations, or three frequencies at onepolarization. The emphasis in SAXON I is on simultaneousmeasurements at different frequencies. Calibrated radar.measurements obtained with this system will be compared withequivalent RAR measurements under similar environmentalconditions. Both aircraft systems are calibrated usingcorner reflectors placed at the Patuxent River Naval AirStation.

I38

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~39

3. Ku-Band Wave Spectrometer On Board the NRL P-3

A three-Frequency Ku-Band scatterometer, designedto make directional measurements of ocean wave spectra isflown aboard the NRL RP-3A aircraft. This system is theimplementation of a new concept and is capable of measuringocean wave slope spectra with 15 degree azimuthalresolution. Data from the system recorded in a digitalformat directly on discs. Data processing is well developedand preliminary spectra become available just after eachflight. Measurements are (typically) made on ocean Iwavelengths between 5 and 200m. This radar issimultaneously used as a calibrated Ku-Band scatterometermeasuring aO Estimates of surface wind speed and directionare produced from this data. The characteristics of thisKu-Band system is shown in Table 6.

D. Ship Wakes

Consistent with the objectives of the SAXON Iexperiment, several ship passes are scheduled during theperiod 19 September to 14 October. A schematic of a typicalship pass is shown in Figure 13. Simultaneous SAR and RARimages of the ship wakes are scheduled during the variouspasses. Also, continuous backscatter and in-situ capillarywave measurements are scheduled during the periods when theturbulent and Kelvin wakes drift past the tower.

4II

II

40 !

Table 6. System Characteristics of the Ku Band WaveSpectrometer On Board the NRL RP-3A Aircraft.

Center Frequency (GHz) 14.0

Wavelength (cm) 2.01

Pulse Length (gs) 1-10

Peak Power (W) 10

Antenna Beamwidth (Deg.) 4.5 x 4.5

Nominal Altitude (km) 3.05

Nominal Ground Speed (m/s) 103

Polarization VV

Range of Incidence Angles(Degrees) 5-55

Number of IndependentAverages/Measurement 96

Along track extent ofmeasurements (km) 5.2

41

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2 42

VI. FIELD SITE INSTRUMENT MOUNTINGS

Pre-experiment site preparation is essential to theexperiment. This includes close coordination of instrumentplacement and the evaluation of each instrument location onthe tower to insure that proposed mounting specificationsare possible, and that each assembly of instruments canfunction properly once installed.

The handling of heavy equipment (such as the wavefollower, booms for instrument mounting, and two electricpower generators) is accomplished with the use of a mobilecrane. The crane weighs nearly 500 kg. It has a 7.5 tonlift capacity and a 6.1m hydraulic boom which can rotate 360degrees. The boom is equipped with a 61m cable reel tomaximize load reach. The crane is used for loading and offloading at the shore staging area. It is transported byhelicopter and placed on the tower helicopter deck forgeneral purpose use.

Sufficient power is provided to meet the experimentalrequirements of SAXON I investigations.A 40 kW generator isdevoted to running the tower facility. Two additional 50kW generators are provided to support the specific powerrequirements of the experiment. One 50 kW generator isdesignated to support the power needs of the wave follower.A second 50 kW generator provides power for the variousinstrument systems. Cabling is routed to the wave followerand to the other instrument systems, as required.

The following breakdown shows the placement of

independent instrument systems at each level of the tower:

A. Independent Systems

1. Subsurface Level

a) Thermistor Array - Four 28 element thermistorarrays are suspended by strain members from the cornersat each leg of the tower for the recording of internalwave activity. These are shown in Figure 14. Inaddition, a portable sparse array is to be fabricatedto serve as an "early warning" internal wave monitor.It is deployed in the up-wave direction (or to theeast) of the tower.

b) Pressure Sensors - Approximately 18 pressuresensors are placed at the -6m level positioneddiagonally across the tower from leg to leg, as shownin Figure 14. Additional sensors are placed just belowthe sea level, either beneath the tower or (<30m) to

43

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the east to reduce contamination of the wave field bythe tower structure. An additional square (2m x 2m)array of four pressure sensors is mounted 5m below thesurface on the tripod located at a distance of 50m fromthe south east corner of the tower.

c) Current Meter - A coherent Doppler acousticcurrent meter is mounted on a tripod, as shown inFigure 15, positioned below the mean water level, andwithin 50m off the south-east corner of the tower.

2. Surface Level

In-situ measurements of surface films areobtained in support of the SAXON I experiment. Duringsignificant experimental events the following areobtained:

(i) In-situ single point determinations of seasurface tension at the base of the tower.

(ii) Collection of occasional water samples forlater analysis at the Naval Research Laboratory, wherethe film pressure vs. area isotherms are determined andcompared to similar data from the field sitesinvestigated in the Marine Microlayer Program.

(iii) Continuous monitoring of the surfacetension of water delivered from a floating pump to asensor mounted on the tower. The apparatus designclosely approximates that used for the same purpose inthe TOWARD experiment.

3. Catwalk Level

a) Surface Tension Measurements - The CLT gantrycrane extension, located at the north-west side of thetower, is used to hold the laser optics to deflect alaser beam down to the ocean surface. A wire cable issuspended from this location downward to the 5m catwalklevel. The end of the cable is fastened to a mountingplate that mates to a plate attached to a telescopeused to direct the laser beam to the ocean surface.The telescope is held in place by a bim extensionladder which is braced against the tower at the 5mlevel, and a guy wire added for additional stability,as shown in Figure 16. The mobile crane is used toraise and lower the telescope for daily servicing.

45

QE

Figure 15. View of Instruments as Seen Looking fromWest Elevation. 1. Wave Slope Sensor,2. Theristor Arrays, 3. PressureSensors, 4. Dopier Current Meter. 5. (a)Wave Follower in Storage Position,(b) Wave Follower in Operating Position6. Wind Drag and Short Wave Slopes,

7. Stillwell Photography, 8. SurfaceTension, 9 Stereophotography T-Bar,10. Surface Tension, 11. Not shown,12. C- to W-Band Multi-Frequency Radar.

46

.. ~-Crane Spar

-Ladder Support

,--TelescopeSupport Cable

-Telescope

O North Elevation

Figure 16. RPI Surface Tension Measurement System.

47

b) Stereophotography T-Bar - A T-Bar is mountedoff the south-west corner of the tower structure, asshown in Figure 17. This is a 6.Om x 10m metal barused for calibration of the stereo cameras which aremounted on the I-beam below the quarters deck level.

c) Wind Drag and Stability - A 19m boom ismounted off the south-east corner for instrumentmounting, as shown in Figure 17. A catwalk along theboom provides for ease of instrument servicing.

d) Wave Measurements - A 6.6m boom is installedat the corner off the east face at the south-east(just north of the beacon tower), as shown in Figure17. This boom is attached to the lowest catwalk andsupports a wave gage array. The wave measurements areused in conjunction with the nadir-looking bandradar mounted directly above at the quarter deck Level.

4. Main Deck Level

a) Stereophotography - Two synchronized camerasare deployed at this level, as shown in Figure 18.This is mounted in conjunction with the T-Bar locatedimmediately below at the cat walk level.

b) Stillwell Photography Three cameras withbreak away brackets are mounted on the hand rail atthis level, as shown in Figure 18. These arerepositioned during the day from the south-east to thesouth-west corner, depending on the sun angle.

c) C-to W-Band Radar - This U-Ka radar ismounted on a 15.Om boom built of triangular cross-sections attached to a pillar and positioned to swingout from the tower between the main deck and thequarters deck levels at the south-west corner, shown inFigure 18. The vector slope gage and scatterometersare mounted on the same boom to facilitate comparisonwith the same point observed by the stereo-system.

5. Quarter Deck Level

a) Surface Tension Measurements - A 2.1m boom isinstalled at the north-west side of the tower off thehand rail. This is mounted in conjunction with a 6.1mboom located directly below at the cat walk level, as

4'p 48

: !

0 Ramp a Up-

a2 N

0 0\Boom

Catwalk )

Figure 17. View of Instruments as Seen Looking Down-ward at Catwalk Level. 1. Wave SlopeSensor, 2. Thermistor Arrays, 3-4 (NotShown), 5. Wave Follower, 6. Wind Dragand Short Wave Slopes, 7. (Not Shown),8. Surface Tension, 9(b). Stereophoto-graphy T-Bar, 10, Surface Tension.11(c). Wave Gage Array.

49

(b)

(010

Figure 18. View of Instruments Mounted as Seen fromSouth Elevation. 1. Wave Slope Sensor,2. Thermistor Arrays, 3. Pressure Sen-sors, 4. Doppler Current Meter, 5 (a)(Not Shown), (b) Wave Follower in opera-ting Position, 6. Wind Drag and ShortWave Slopes, 7. Stillwell Photography,8. Surface Tension, 9. (a) Stereophoto-graphy T-Bar, 10. Surface Tension,11(a). KU-Band Radar, 11(b). Ku- BandRadar and Wave Gauge, 11(c). Wave GaugeArray 12. C- to W-Band Multi-FrequencyRadar, 13. C-, Ka- and G-Ban; Radar.

50

III

shown in Figure 18. Cables are routed into a serviceroom at this location to support a co-axial powercable for the photomultiplier tube, and a standard co-axial cable for signal return. Similar cables run fromthis location to the 6.1m boom below.

b) Ku Band Radar and Thorn Wave GaQe - This NRLnadir looking radar and wave gage are positioned on a6.6m boom located 6m from the south-east cornerextending off the east face, as shown in Figure 18.These measurements are used in conjunction with thewave gage measurements located at the catwalk leveldirectly below.

c) Ku Band Radar - This NRL radar is mounted onthe hand railing next to the Ku Band Radar and Thornwave gage boom described in b) above, and as shown inFigure 18. This radar is set to operate with a 45degree incidence angle.

6. Helicopter Level.

a) C-. ,- and G-Band Radar - This University ofMassachusetts radar is mounted on a 4.6m boom off thesouth-east corner of the tower, as shown in Figure 19.

b) NRL Radiometer System - This system isdeployed on the rail at this level and operated with a54 degree incidence angle.

B. Wave Follower

The present wave follower is deployed off the southside of the tower from the 5m cat walk level and the -6mhorizontal structural level, as shown in Figure 18. Thewave follower is fastened to the tower by an A-Frameparallelogram that extends to 17.7m from the tower. Thell.6m vertical trestle mounted to the A-Frame, provides fora continuous cable assembly allowing the C-Frame platform tomove vertically, hence maintaining a constant submergencedepth as it rides on the long waves. Upon completion of theexperiment, the C-Frame will be dismantled and the wavefollower stowed against the tower structure, as shown inFigure 15.

51

7-7

Figure 19. 13. C-, Ka and G-.. ad Radar InstrumentsIMounted as Seen Looking Down from the HelicopterDeck Level.3

525

The instruments mounted on the C-Frame include:

1. Laser Optical Sensor - This instrument weighs2.3 kg and is positioned so that the optical instrument ismounted on the upper face of the C-Frame, and the laser ismounted on the lower face of the C-Frame facing upward.

2. Surface Light Scattering Spectrometer - Thisinstrument weighs 2.3 kg and is mounted on the C-Frame.The sensor beam is positioned side by side in relation tothe laser-optical sensor beam location. The sensor isoperated to measure surface tension simultaneously withsurface slope measurements.

3. Acoustic Doppler Velocimeter - This instrumentweighs 2.3 kg, is 6.4 cm in diameter and is 76 cm long. Itis positioned along side the laser. The sensor tip ispositioned approximately 7 cm below the surface.

4. Electromagnetic Current Meter - This current meteris mounted on the lower face of the C-Frame and provides thetwo horizontal components of the current immediately belowthe laser beam spot on the ocean surface.

C. Circular Wave Generator

The system, also known as "Bobbing Buoy", was operatedin TOWARD to generate 30 cm long surface waves. In SAXON,an improved version is operated to generate circular wavesin the range 3-30 cm. The system operates as a verticallyoscillating buoy with a preset frequency. The buoyfrequency is recorded at the tower. The decay rate of thecircular waves will be measured using stereophotography.

I53

NN

VII. EXPERIMENT COORDINATION AND MEASUREMENT STRATEGY

For the field operations a coordination center isestablished at the U.S. Coast Guard Light House Stationlocated on Cape Henry, Fort Story Army Post. This locationserves as the staging area fc- on/off loading of all heavyequipment being transported to and from the Chesapeake LightTower. Transportation to and from the tower is provided bythe U.S. Coast Guard and a chartered vessel. The U.S. CoastGuard has two 12m and one 25m cutter (Figure 20) servicingthe area. Departure is from the Little Creek Coast GuardStation. The 12m cutters can moore along side of the tower.The 25m requires that personnel on/off load at least 100maway from the tower via a rubber utility raft, as shown inFigure 21.

The scientific and logistical considerations arecoordinated in accord with the organization plan, shown inFigure 22. The Principal Investigator in consultation withindividual investigators reviews flight availability andreadiness, surface instrument readiness, and environmentalconditions to formulate decisions for use of aircraft and/orcoordination of tower measurements. Daily weather and waveforecasts are obtained from the Navy Eastern OceanographicCenter at Norfolk (NEOC) and used to determine strategy ofmeasurement during the following day.

The Test Director is responsible for implementation ofthe experiment plan in accord with the above decisions. TheTest Director is the single point of contact for directivesregarding the day-to-day operations of the experiment.

Experiment readiness reviews are held each day duringthe test period. Direct teleconference links areestablished with each of the staging element areas.

The SAXON I Project Schedule of Events is shown inFigure 23.

L54

I

II

Figure 20. U.S. Coast Guard 25m Cutter Point Huron. One ofThree Cutters will be made Available forTransportation to the Tower on a non-interference basis. I

55

I1

Figure 21. Rubber Raft Used by 25m Cutter to On/Off LoadPersonnel to and from the Chesapeake LightTower. 12m Cutters Can On/Off Load directly atthis Tower Landing.

56

A

0 )-: c

C13~~~C. 1 90 -E.

-j 2 0Q aC Ca n )U )U

0

a))

cc. 00

0,-

L' ccc c

>0: 00c

-- 0u a a )14

0c U' -0

00< 4o

V)) C.

-4

cc

00 m

> a)-- E

Cc = 00L~ 0H-iCO-

*0- dIw -

cn nccD0Z57

6861 Iaqwn3d-

6861 atnr I

68T.d -12a

6861 ANIP

8861 aurICea~ ___________

L8861 . eW - 18861 11~d 1 _

L86 qreOi C____

U)6UIi 44 "---

r- cc -i--

co~. 00c e

.4.j 14 :3 ) -4

4~C -4 c Qm 3mr-

>4> 4C) C) -L Q

0~ 0) 2

58

VIII. DATA MANAGEKENT

It is planned that a Data Management Center, Figure 24,be established for the purpose of collecting, organizing,safeguarding, and accounting for the distribution of thevarious data sets obtained from each participatingorganization.

Each investigator shall be responsible for providingthe Data Center with the predetermined data logs and allinformation related to their activities that is required byother investigators.

The Data Center will provide each investigator with thedaily environmental measurements. All aircraft images willbe collected and maintained on file for distribution toinvestigators, as needed. Further, the Data Center is tomaintain logs of all data sets collected for distribution toinvestigators. Data reports submitted to the Center by eachinvestigator will be made available to all the other SAXON Iparticipants without cost.

The Data Center will utilize a computer system that isadequate for meeting the above requirements. It is expectedthat the Data Center will be staffed with a qualifiedindividual responsible for management of the Center inaccordance with the overall objectives of the SAXON IScience Plan.

All data acquired in SAXON I will be made available atlarge, at the expense of the requester, in accordance withthe Freedom of Information Act, one year after instrumentsare pulled out of the water. A data survey report will beprepared by 1 January 1989. Data reports are to besubmitted by June 1989.

59

03

LL 0 )0

(I))

0c C) C0iIU

C:)

c .44

0)0

0 Ca

--- 4

CLL

600

-t: . 5 -

IX. DATA ANALYSIS

Two primary areas of analysis will be emphasized in thedata analysis phase. These are:

1. Hydrodynamics of very short waves.

2. Radar backscatter modulations due to internal andlong surface waves.

The above will require supporting analyses related to:

1. Subsurface hydrodynamics.

2. SAR imaging.

Tables 7a and 7b show the participating investigatorsand related areas of analysis.

61

Table 7a. Investigators and Corresponding Areas of Analysis.

Supporting Supporting

Analysis Analysis

Organization Subsurface Surface Radar SAR/RARHydro- Hydro- Backscatter Imaging

dynamics dynamics

APL/JHU B. Gotwols

R. SternerR. Chapman

DTI P. Jang

GSFC/NASA T. Liu

ISA 0. Lee

JPL R. Martin T. ThompsonW. Brown

R. GoldsteinH. ZebkerF. Li

LJI F. Henyey J. Wright M.S.Longuet-

Higgins

MIT J. Henry

MPL K. WatscnP. Hanson P. Hanson

NADC A. Ochadlick

NORDA P. Smith

6I!

62!

Table 7b. Investigators and Corresponding Areas of Analysis(Continued).

Supporting SupportingAnalysis Analysis

Organization Subsurface Surface Radar SAR/RARHydro- Hydro- Backscatter Imagingdynamics dynamics

NOSC J. Rohr

NPGS K. DavidsonE. ThorntonT. Stanton

NRL W. Barger W. Keller W. Plant

G. Geernaert C. Sandlin/D. Schuler j. Hollinger

ORE P. Hwang D. Kaslingam D. HaytM. Tran

ORINCON W. Garrett

RpI J. KorenowskiW. Asher

SIC S. Beck R. Guza

U TEXAS A. FungARLINGTON

U DELAWARE J. Wu D. Lyzenga

U KANSAS R. Moore

U MASS C. Swift

USGS S. Wu

63

REFERENCES

Dynamic Technology, Inc. (1987) Perspectives on FutureResearch Directions in Ocean Remote Sensing, Report No. DTN-BP701-01, 23 pps.

Geernaert, G.L., K.L. Davidson, S.E. Larsen and T. Mikkelsen(1987) Wind Stress Measurements During the Tower Ocean Waveand Radar Dependence Experiment, Accepted for Publication inJGR.

Holiday, D.G. St-Cyr and N.E. Woods (1986) A Radar OceanImaging Model for Small to Moderate Incidence Angles, Int.J. Remote Sensing, 7, pp. 1809-1834.

Hsiao, S.V. and O.H. Shemdin (1983) Measurement of WindVelocity and Pressure with a Wave Follower, J. Geophys.Res., 88, 9841-9849.

Hughes, B.A. (1978) The Effect of Internal Waves on SurfaceWind Waves, 2. Theoretical Analysis, J. Geophys. Res., 83,455-465.

Hwang, P.A. and O.H. Shemdin (1988) The Dependence of SeaSurface Slope on Atmospheric Stability and Swell Conditions,Accepted for Publication in JGR.

Jang, P.S. (1986) Comparison of Composite Two-Scale RadarScattering Model with Full Wave Approach, DynamicTechnology. Inc. Report DT-8607-I0.

Kasilingam, D.P. and O.H. Shemdin (1988) A ComprehensiveTheory for Synthetic Aperture Radar Imaging of the OceanSurface: With Application to TOWARD on Focus, Resolutionand Wave Height Spectra, Accepted for Publication in JGR.

Lubard, S. C., R. W. Ziemer and J. B. Swan (1974) OWEX/FLIPRadar Experiment Summary Report (U), RDA Report No. RDA-SR-010-ARPA, 71 pps.

Paskausky, D., J.D Elms, R.G. Baldwin, P.L Franks, C.N.Williams, Jr., and K.G Zimmerman (1984) Addendum to Wind andWave Summaries for Selected U.S. Coast Guard OperatingAreas. National Climatic Data Center Asheville, NC 28801,Report No. CC-D-05-84, 523 pps.

Pierson, W.J., Jr. and R.A. Stacy (1973) The Elevation,Slope and Curvature Spectra of Wind Roughened Sea Surface.NASA Tech Report No. CR - 2247, 126 pps.

SARSEX Interim Report by the SARSEX Experiment Team, JHU/APLSTD-R-1200, May 1985.

64

Shemdin, O.H. (1966) The Dynamics of Wind in the Vicinityof Progressive Water Waves, Ph.D. Dissertation, StanfordUniv., Stanford, California.

Shemdin, O.H. (1980) Measurement of Wind and Surface WaveSlopes with a Wave Follower, JPL Report No. 715-123,Pasadena, California.

Shemdin, O.H. and P.A. Hwang (1988) Comparison of Measuredand Predicted Sea Surface Spectrum of Short Waves, Acceptedfor Publication in JGR.

Shemdin, O.H., S. Wu and M. Tran (1988) Stereo PhotographicObservations of the Sea Surface in TOWARD, Accepted forPublication in JGR.

Synthetic Aperture Radar Working Group Interim Report,Volume I (S), 1986.

TOWARD Experiment Team (1986) TOWARD Field ExperimentInterim Report: Volumes I and II, JPL Report, Pasadena.

Valenzuela, G.R. (1978) Theories for the Interaction ofElectromagnetic and Ocean Waves: A Review, Boundary LayerMeteoroloQy, 13, 63-85.

Watson, K. (1986) Some Surface Wave Modulation MechanismsRelating to the JOWIP and SARSEX Observation, Mitre Corr.JSR-85-203.

II

II

LIST OF TABLES

TABLE PAGE

A-1 Other Aircraft and Respective Frequencies .............. 69

A-2 System Characteristics of CCRS C- and X-Band SAR SystemOn Board CV-580 Aircraft ................................ 70

A-3 CCRS Data Processing Characteristics ................... 71

A-4 System Characteristics of MIT Ka-Band SAR/RAR System OnBoard Gulfstream Aircraft ............................... 73

A-5 Gulfstream SAR Data Processing Characteristics ......... 74

A-6 System Characteristics of JPL P-, L- and C-Band SARSystem On Board DC-8 Aircraft ........................... 75

A-7 JPL-SAR Data Processing Characteristics ................ 76

A-8 System Characteristics of DFVLR X- and Ka-Band RARSystem On Board Do228 Aircraft .......................... 77

A-9 System Characteristics of Intera X-Band SAR System OnBoard Conquest Aircraft ................................. 79

A-10 Intera STAR I System Data Processing Characteristics ... 80

67

APPENDIX A

DATA ON OTHER AIRCRAFT

I. Introduction

The inclusion of the airborne radar systems describedbelow would have greatly enhanced the overall objectives ofSAXON I. The consequence would have been a morecomprehensive intercomparison of SAR and RAR imaging at abroader range of radar frequencies.

II. Desired Airborne Measurement Systems

The desired radar systems are shown in Table A-i. Theradar characteristics for these are discussed below:

1. CCRS: C- and X-Band SAR on Board CV-580

A new generation Synthetic Aperture Radar (SAR) hasbeen installed into the CCRS aircraft. It is a digitallycontrolled, two-channel radar, operating at C-Band (5.3GHz), transmitting either H or V polarizations, andreceiving both simultaneously. A breakdown of thespecifications are shown in Table A-2. The system featuresan on board 7-look, real-time processor and display for onereceive channel with data acquisition geometry in threemodes; nadir, narrow swath, and wide swath. The system dataprocessing characteristics are shown in Table A-3. Duringflight, data is sent to a dry silver image recorder and toHDDT. Measurements for this output show that the aspectratio, averaged from 25 km section imagery, is correct towithin 0.5%.

A dual-channel, X-Band system has recently beencompleted for installation into the aircraft.

68

Table A-I. Other Aircraft and Respective Frequencies.

SAR Mode RAR Mode

Aircraft P L C X Ka C X Ka

1. CCRS CV-580

2. MIT Gulfstream

3. NASA DC-8

4. DFVLR Do228

5. INTERA CONQUEST

IAvailable in 1989

6II

69

U)4-

N M L N ID I .

-S4

-44

-0 75 Ln

: .-) Q

N

a).- .. E-'4

m 4-0

a) C)-lw a) (n m 3

-0- a) a ~

im

4 --4 S a I 0 O - ,

C: C: 44 M-

a) a a) 4 0) --4 -J 0 -4 M q1)9( C: 4- ) 4 ) E/ -- E m U)-00-, C

01 w m - C 0 a): ) 0 -4

ux 2:a m C" K4 <J zz4 U'0 4 4

E- ~ t ~-

a)0 - a -- 4 400 t 4 0~' a) --70

Table A-3. CCRS Data Processing Characteristics.

Polarizations HH - VV -V- VH

Angle of Incidence at Center of Image 350

Digital Slant Range Coverage 22 km

Digital Along Track Coverage (Raw) 16 km

Digital Along Track Coverage (Image) 16 kin

Digit Image Size 4096 pixels rangex 4096 pixels azimuth

PRF 2.57 x (Ground Speed in m/s) 337@ 131 m/s

Digital Image Pixel Spacings 4.0m Slant Rangex 3.89 azimuth

7III

T .m , - -- --- m 71

2. MIT K -Band System on Board Gulfstream Aircrafta

This system is in its final stages of development andis expected to be in operation by Summer 1988. The systemcharacteristics are shown in Table A-4; it can be operatedin both SAR and RAR modes, simultaneously. The dataprocessing characteristics are shown in Table A-5.

3. JPL P-, L- and C-Band Radar on Board NASA:DC-8 Aircraft

The JPL L-Band system on board the NASA: CV-990 wasflown in TOWARD with a high degree of success because of thesimultaneous digital and optical recordings and imageprocessing capabilities available with the system. Thesystem is expanded to include C- and P-Bands with multi-polarizations and is deployed on the NASA: DC-8. Table A-6shows the system characteristics. The data can be obtainedwith various imaging geometries such as aircraft altitudeand azimuthal angles relative to surface waves and/or winddirection. The data processing system characteristics areshown in Table A-7. The versatile polarimetric capabilitiesof this radar system will allow determination of thecoherence time of surface scatterers. This importantinformation will be compared with other determinations ofthe same quantity using tower-based radars.

Both the L- and C-Band radars have front and backantennas. The system is operated as an interferometric SARwhere phase difference measurements are obtained on a pixel-by-pixel basis. These are converted to line-of-sightvelocities for use in verifying velocity bunching and othervelocity-related aspects in SAR imaging.

4. DFVLR X- and Ka-Band RAR on Board Do228

It would havtz been desirable to have the participationof the German Aerospace Research and DevelopmentInstitution. Their X- and Ka-Band RAR system would bedeployed on a Do228 aircraft capable of making a flightacross the Atlantic Ocean. The RAR has been operated fordetection of sea slicks and has produced good qualityimages. The specifications of this system are detailed inTable A-8. A C-Band SAR will be available in 1989. Thecorresponding resolution will be better than 10.0m.

7

(DD

a) C-.- 00C D a

co D CD CD r-aLC) I' CD C)C 'o

00) cDi z D N 'r Lfl --i U) CD +1 +1 (\j +-

$-4

u'c

0 000E. Q ' a 0

(0 4- 410 0L44 U) -- a, a) a))

0 0 U) Z --0 V) 423 00

M -I a) 8 - 0) 0 V)~ E

4~ - 3 C~E C:- --4 00 U 2

0) M0 0.0 Cl) ) a )a) () 0.. M . ( U ) 4 -4

a)4 0) C: Z V) Mar ) Cl)4 T :3 4. -,- --i w a

G) 0 : ma r 0 0 m Z .) u1a-' 4-- C a) M4 0 4* (1 00 - H V) x -

1C : (a 0 < 0 (a C- c I- a) V) 42m) 4 20' C) V ! -1 a) ( -- I --I Lm -- 42-

8(a~ a)- 0 0 0 ' :3 a 2

a)0 .4 2 ) ( (a 44 (aZ- 4 U 4 0 >

a) ) 0 000000 )U 0~a)-73-

Table A-5. Gulfstream SAR Data Processing Characteristics.

Polarization HH, VV, HV, VH

Angle of Incidence at Center of Image 60(can be adjusted)

Digital Slant Range Coverage 0.7 kin

Digital Along Track Coverage (Raw) 30 km7

Digital Along Track Coverage (Image) 30 km

Digital Image Size

Optical Image Size

PRF (pps) 1500

Digital Image Pixel Spacings

74

c (V

CD -fl '. Lfl >

LIN) "I Ic~

E

4-1

co W

c]LI)

CD Ln C C--r4

CN L N

N - D L fl >co *D CD d) u ( C r-

-44- -

C,4

r 4

EC) 41 a r

a) -0 E-, C:o :

CO0 a) 0

:3 0 NV

a4 M ~ 0) -.

-~~~ (o a)a 0 C)

-~~~ ~u a)Ca-;.0 QV .

0) --4 00 3: W 00cw Z~.-. Z a)V Z QU)--

a) ~U ~V 0 0 ~ 2 75

Table A-7. JPL-SAR Data Processing Characteristics.

Four OPTICAL Channels HH - VV - HV - VH

4 DIGITAL SUBFRAMES HH - VV - HV - VH

Angle of Incidence at Center of Image 350

Optical/Digital Slant Range Coverage 7 km (Quad Pol Mode)

Optical/Digital Slant Range Coverage 12 km (Quad Pol Mode)

Optical Along Track Coverage 60 km

Digital Along Track Coverage (Raw) 30 km

Digital Along Track Coverage (Image) 11 km

Digital Image Size 927 pixels rangex 1024 pixels azimuth

Optical Image Size 50 mm cross track,1:250,000 along track

PRF 3 x (CV-990 Ground Speed in m/s)600 @ 200 m/s, 750 @ 250 m/s

Digital Image Pixel Spacings 7.5m Slant Rangex 11.0 Azimuth

76

I

(D IU)l

CD-- I- n D c

0

a)o E

-n '0- 0 0 CL-

U) 0 - C 0UN :3 a4 -7 0 m

-4, .-j -1 -'Z

(v 3 : i : C - C - - C '0 IN0 -14 a ) -4 4 T 0 M M

C: > C M E 0j 4-) Cje E 1 v

0- mJ D :r- 0 ~~ 0 0 . a) ) 0E-00 X N1 Z~ Z 44 Z C Za

L. ~J 0.0 COO .77

A new instrument, Rotating Synthetic Aperture Radar(ROSAR), which allows SAR imaging from a stationaryplatform, has been developed. This system provides a SARimage at 360 degrees around its position. It operates at L-Band, 1.15 GHz and can be operated from a tower togetherwith a rotating RAR.

5. Intera X-Band On Board Conquest

The Intera STAR I, X-Band SAR system on board Conquestis a new generation SAR providing wide or narrow swathcoverage with digital processing. The systemcharacteristics are shown in Table A-9. Data recorded onthe aircraft may be down-linked as imagery to a designatedfacility in real-time mode or produced as hard copy. SARimagery is recorded on HDDTs and displayed on a continuousstrip image recorder. Imagery produced on HDDTs can betranscribed to industry standard 6250 or 1600 bpi tapes.This production system is shown in Table A-10.

78

CD)-H

CD) <D() n L'741

C:C

r - -

41

-H-S

2 - E W

~~0

-~-~75 0 - x)

CQ m( 0 roa) C: 0) c U _- j=fC a) 7j c. 04 C E

N: :3 0 N

-C~~C ~- ~~ -40-3 C-) 0 -0) U) OCO E

M) lzu -= c c - cu0)0~r <, -14 C-C 0 fl ( C 0

C~~: 8- N 3>U1 0 O ~ -00)0~ ~ -4 -4 a~C-4 I ~ 9 'i -

C : (1 (Z ' cz -4 :, ~C N

- -. C C C: 0 : M ~ C C0) -- 1 0 Cu 0 i 1- J 1

>~~- C:E vuc0)) a~ 0 Cu 0 C C O 0 3: X CO 0

U~~~ 3z z a, ~~ l *4 Qj

E0

791

Table A-10. Intera STAR I System Data ProcessingCharacteristics.

OPTICAL Channels 0

1 DIGITAL SUBFRAMES HH

Angle of Incidence at Center of Image 550

Digital Slant Range Coverage 23 km

Digital Along Track Coverage (Raw) 46 km

Digital Along Track Coverage (Image) 25 km

Digital Image Size 4096 pixels rangex 96 pixels azimuth

Optical Image Size

PRF

Digital Image Pixel Spacings 12.Om Slant Rangex 12.0 Azimuth

80

APPENDIX B

SAXON I

DIRECTORY

81.

Mr. Bill Asher Mr. Mike CliftonRensselaer Polytechnic Institute University of CaliforniaDepartment of Chemistry San DiegoTroy, New York 12180-3590 Scripps Institution of

(518) 276-8982 OceanographyLa Joila, California 92093

Dr. William Barger (619) 534-0593Naval Research LaboratoryChemical Division Mr. Frank Dareff6175 Naval Air Development CenterWashington D. C. 20375 Code 3024

202-767-3573 Warminster, PA 18974

Mr. Steve Beck Dr. Roger F. DashenScripps Institute of Oceanography University of California,Marine Physical Laboratory San DiegoSan Diego CA 92152 Physics Department(619) 534-1797 La Jolla, California 92093OMNET - MPL.SIO (619) 534-6659/2230

Dr. Erik Bock Professor Kenneth DavidsonRensselaer Polytechnic Institute Naval Postgraduate SchoolDepartment of Chemistry Code 63-DS110 8th Street Monterey, CA 93943Troy, New York 12180-3590 (408) 646-2309(518) 276-8457 OMNET - K.DAVIDSON

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Dr. Gary Geernaert Dr. Eric 0. HartwigNaval Research Laboratory Director, Environmental SciencesCode 7784 office of Chief of NavalWashington, D.C. 20375 Research(202) 767-1602 or 2095 Code 112OMNET - G.GEERNAERT Arlington, VA 22217-5000

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Mr. Don Hoff Professor Haralambos ff. FritikosJet Propulsion Laboratory Moore School of Electrical Eng.Mail Stop 125-177 University of Pennsylvania4800 Oak Grove Drive Philadelphia, PA 19104Pasadena CA 91109 (215) 898 8112 I(818) 354-3138

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Mr. Les Mc Cormick Mr. Bill O'ReillyOcean Research and Engineering Scripps Institute of255 South Marengo Avenue OceanographyPasadena, California 91101 La Jolla CA 92093(818) 568-1800 (619) 534-0226OMNET - L.MCCORMICK

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Dr. Jim Rohr Mr. Bob SteinbacherNaval Ocean Systems Center Jet Propulsion LaboratoryCode 634 Mail Stop 125-177San Diego, California 92152-5000 4800 Oakgrove Drive(619)-553-1604 Pasadena, California 91109

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Dr. Howard ZebkerJet Propulsion Laboratory4800 Oak GrovePasadena California 91109(818) 354-8780

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