On the Imaging of Surface Gravity Waves byMarine Radar: Implications for a Moving

Platform

*Björn Lund, Clarence Collins, Hans Graber,§Eric Terrill

*Rosenstiel School of Marine and Atmospheric Science, University of Miami§Scripps Institution of Oceanography, UC San Diego

October 28, 2013

Outline

1 Introduction

2 Data Overview

3 Moving Platform Challenges and Solutions

4 Preliminary Results

5 Conclusions

Outline

1 Introduction

2 Data Overview

3 Moving Platform Challenges and Solutions

4 Preliminary Results

5 Conclusions

Introduction

Marine radars were developed for ship detection → Sea clutterfirst analyzed as noise (e.g. Croney ’66)

Today, marine radar data are being used to determinefully-directional wave spectra (Young et al. 1985) as well asphase-resolved maps of the surface elevation (Borge et al. 2004)

Surface currents and bathymetry may be obtained as aby-product of the wave analysis (Senet et al. 2001, Bell 1999)

In addition, marine radar data have been used to study internalwaves (Watson and Robinson 1990, Lund et al. 2012), winds(Dankert et al. 2003, Lund et al. 2012), oil spills (Gangeskar2004), ...

Motivation

Previous studies, focused on fixed platform data (e.g. from lighthouses or oil rigs), established marine radar as reliable wave andcurrent sensor (Borge and Soares 2000, Wyatt et al. 2003)

Recent results suggest that shipboard marine radar wave heightestimates cannot be trusted (Stredulinsky and Thornhill 2011)

Wave and current retrieval from ships remains challenging due to:

Ship-motion-induced Doppler shift leads to aliased wave energyJittering wave signal due to changing ship headingResults’ dependency on angle between analysis box and waves

Study goals:

Develop new techniques that address ship-motion-related marineradar wave and current retrieval issuesDemonstrate possibility to reliably determine waves and currentsfrom shipborne marine radar data

Outline

1 Introduction

2 Data Overview

3 Moving Platform Challenges and Solutions

4 Preliminary Results

5 Conclusions

System Specs

Radar antenna: Hardware diagram:

Scanner

Antenna

Radar

display

unit

Video

Trigger

Heading

Bearing

PC with

WaMoS

capture

board

Screen

Polar backscatterimage:

Furuno marine X-band radar operating at 9.4 GHz withHH-polarization and grazing incidence angle

Pulse length of 0.07 μs (short pulse mode) → 10.5 m rangeresolution

Antenna length of 8 feet → 0.75° antenna beam width

1.5 s antenna rotation period

Wave Monitoring System (WaMoS) sampling up to 4 km with cellsize of 0.2° in azimuth and 7.5 m in range

Hi-Res Experiment

High Resolution Air-Sea Interaction (Hi-Res), off San Francisco, 2010.

Map of Hi-Res experiment:

-124.40 -123.60 -122.80 -122.00

-124.40 -123.60 -122.80 -122.00

37

.80

38

.20

38

.60

39

.00

37

.80

38

.20

38

.60

39

.00

R/P Flip06/06, 19:00 UTC

06/10, 12:00 UTC

California

R/P Flip:

R/V Sproul:

Source: ucsd.edu

Marine Radar Image and Spectrum

Ramp-corrected marine radar image:

FLP, 06/11/2010, 00:00:01.3 UTC

-4 -2 0 2 4x [km]

-4

-2

0

2

4

y [km

]

Head N

Wind

0

2

4

6

8

10

Ram

p-d

ivid

ed r

etu

rn

Frequency-wavenumber slice throughcenter of 3D image spectrum:

-0.4 -0.2 0.0 0.2 0.4Wavenumber [rad m-1]

0.0

0.5

1.0

1.5

2.0

Angula

r fr

equency [ra

d s

-1]

First harmonic

Fundamentalmode

Group line

Outline

1 Introduction

2 Data Overview

3 Moving Platform Challenges and Solutions

4 Preliminary Results

5 Conclusions

Marine Radar Image Data Concept

Traditional approach (snapshotsimplification):

Source: Borge et al. 1999

Accurate view (spiraling data stream):

Antenna Heading Correction

Pre-correction wave analysis windowat t = 0 s:

-0.5 0.0 0.5x [km]

-0.5

0.0

0.5

y [km

]

Pre-correction wave analysis windowat t = 6 s:

-0.5 0.0 0.5x [km]

-0.5

0.0

0.5

y [km

]

Antenna Heading Correction

Pre-correction wave analysis windowat t = 1.5 s:

-0.5 0.0 0.5x [km]

-0.5

0.0

0.5

y [km

]

Pre-correction wave analysis windowat t = 7.5 s:

-0.5 0.0 0.5x [km]

-0.5

0.0

0.5

y [km

]

Antenna Heading Correction

Extension of Bell and Osler’s (2011) wave-signature-based heading correction.

Heading errors:

0 20 40 60 80Time [s]

-2

-1

0

1

2

Radar

puls

e h

eadin

g e

rror

[°]

Heading pre- and post-correction:

0 20 40 60 80Time [s]

110

111

112

113

114

115

116

Ship

headin

g [

°]

Before heading correction After heading correction

Antenna Heading Correction

Post-correction wave analysis windowat t = 0 s:

-0.5 0.0 0.5x [km]

-0.5

0.0

0.5

y [km

]

Post-correction wave analysis windowat t = 6 s:

-0.5 0.0 0.5x [km]

-0.5

0.0

0.5

y [km

]

Antenna Heading Correction

Post-correction wave analysis windowat t = 1.5 s:

-0.5 0.0 0.5x [km]

-0.5

0.0

0.5

y [km

]

Post-correction wave analysis windowat t = 7.5 s:

-0.5 0.0 0.5x [km]

-0.5

0.0

0.5

y [km

]

Dependency of Results on Analysis Box Position

Traditional analysis window setup:FLP, 06/11/2010, 00:00:01.3 UTC

-2 -1 0 1 2x [km]

-2

-1

0

1

2

y [km

]

Head N

Wind

0

1

2

3

4

Radar

retu

rn [

×10

3]

Test setup:FLP, 06/11/2010, 00:00:01.3 UTC

-4 -2 0 2 4x [km]

-4

-2

0

2

4

y [km

]

Head N

Wind

0

1

2

3

4

Radar

retu

rn [

×10

3]

Dependency of Results on Analysis Box Position

7

8

9

10

Pe

ak w

ave

pe

rio

d [

s]

-100 0 100Box − peak wave direction [°]

305

310

315

320

325

330

335

Pe

ak w

ave

dire

ctio

n [

°]

0

2

4

6

8

Sig

na

l-to

-no

ise

ra

tio

Downwave Crosswave Upwave Crosswave Downwave

Color legend:Near range,mid range,far range.

Signal-to-Noise Ratio Dependency on Box Orientation

-100 0 100Box − peak wave direction [°]

0

2

4

6

8S

ignal-to

-nois

e r

atio

Downwave Crosswave Upwave Crosswave Downwave

t = 0.5 h t = 1.6 h t = 2.7 h t = 3.8 h t = 4.9 h t = 6.0 h

Outline

1 Introduction

2 Data Overview

3 Moving Platform Challenges and Solutions

4 Preliminary Results

5 Conclusions

Example of Marine Radar Wave Spectrum

Ramp-corrected radar image:FLP, 06/11/2010, 00:00:01.3 UTC

-4 -2 0 2 4x [km]

-4

-2

0

2

4

y [km

]

Head N

Wind

0

2

4

6

8

10

Ram

p-d

ivid

ed r

etu

rn

Marine radar frequency-directionspectrum:

Comparison with Datawell Buoy and ADCP Data

Time series of waves:Time series of currents:

Datawell buoy data courtesy ofThomas Herbers.

Comparison with Datawell Wave Spectra

Frequency spectrum at t = 0 h:

0.05 0.10 0.15 0.20 0.25 0.30Frequency [Hz]

0.0

0.2

0.4

0.6

0.8

1.0

Norm

aliz

ed w

ave e

nerg

y

Marine radar Datawell buoy

Frequency spectrum at t = 5 h:

0.05 0.10 0.15 0.20 0.25 0.30Frequency [Hz]

0.0

0.2

0.4

0.6

0.8

1.0

Norm

aliz

ed w

ave e

nerg

y

Marine radar Datawell buoy

Outline

1 Introduction

2 Data Overview

3 Moving Platform Challenges and Solutions

4 Preliminary Results

5 Conclusions

Summary / Outlook

Identified ship-motion-related marine radar wave and currentretrieval issues and proposed solutions:

Aliasing due to Doppler-shift from vessel motion→ Geo-referencingJittering wave signal due to Gyro compass errors→ Antenna heading correctionWave retrieval dependency on range and antenna look direction→ Correction function

Shipborne marine radar waves and currents are in goodagreement with measured and modeled reference data

Current research focuses on modeling of wave signal’sdependency on range and antenna look direction

Acknowledgment

This work was supported by theCenter for Southeastern TropicalAdvanced Remote Sensing (CSTARS)of the University of Miami and theU.S. Office of Naval Research (ONR)under grants N000140510758,N000140710650, N000140810793,and N000140910392.