Ship-Based Observations of Ocean Waves Using Multiple X-Band Radars

1. Ship-Based Observations of Ocean Waves Using Multiple X-Band Radars Christa McKelvey, Shanka Wijesundara, Andrew O’Brien, Graeme Smith, Joel T. Johnson, David R. Lyzenga(1) Department of Electrical and Computer Engineering - ElectroScience Laboratory, Ohio State University, Columbus, Ohio (1) Department of Naval Architecture and Marine Engineering, University of Michigan, Ann Arbor, Michigan

2. Introduction • Short term forecasting of sea waves is possible by combining radar measurements with methods for predicting sea surface evolution in time • X-Band radar is particularly interesting for this application • Already widely used in marine navigation • Cost effective solution and simple system deployment • Two primary options available to date are based on: • Radar backscattered power measurements • Measurement of the velocity (requires coherent radar or application of coherent-on-receive techniques)

3. Instrument Description • Koden MDS-63R Marine Radar • 25 kW peak output power (non-coherent) • Operating Frequency 9.41 GHz ± 30 MHz • Ethernet Interface for Control • H-Pol Antenna

4. Instrument Description Modifications: • V-Pol antenna for better sea wave scattering sensitivity • Stable Local Oscillator (LO) for improved Doppler measurements • IF signal conditioning • Added data acquisition system: Dual channel 16-bit ADC @ 160 MSPS • Mean phase removal processing to achieve coherence

5. Instrument Description

6. Experiment Setup • Radars installed on RV Melville • Preliminary test measurements were performed over the period Sept. 6 to Sept. 17 2013 • Several in-situ sensors available for “ground truth” validation • Small boat with a radar reflector was used for validation of Doppler velocity calculations • Campaign includes a variety of wind and wave conditions

7. Results: Doppler Measurements • Successful measurements of modulations in both backscattered power and Doppler associated with waves can be observed • Relative radial Doppler fields are calculated up to 1 km in range, using IF power measurements • To produce a relative Doppler velocity field, mean phase removal processing is used to remove pulse-to-pulse variations in magnetron phase, and velocities associated with host ship motion

8. Results: Wind Speed Effects • Maximum range to which waves are observable is a function wind speed • Low wind conditions (< 5 m/s) can present a performance challenge for radar wave measurements • However, wave information is present to several hundred meters even in the lowest wind conditions

9. Results: Validation of Measurements • To validate Doppler velocity calculations, data from a small boat with a high RCS reflector is used • Relative “truth” velocity of the small boat is calculated using its recorded velocity and position, plus the host ship’s velocity • Measured velocity is obtained from Doppler velocity fields derived using mean phase removal processing • A close agreement can be seen between relative “truth” radial velocity, and the small boat velocity observed by the radar

10. Conclusions • Data from RV Melville campaign demonstrates the capability of observing sea waves using X-Band marine radars • Small boat radial velocity comparison demonstrates the viability of estimating relative Doppler velocity fields using mean phase removal processing

11. Future Work • Further improvements include measuring IF backscattered power up to 5 km, allowing calculation of Doppler velocities over full range (0.1 – 5 km)

12. References • [1] Dankert, H. and W. Rosenthal, “Ocean Surface Determination from X-band Radar-Image Sequences,”Journal of Geophysical Research, vol. 109, C04016, 2004. • [2] Nieto-Borge, J. C., G. R. Rodriguez, K. Hessner, and P. I. Gonzalez, “Inversion of marine radar images for surface wave analysis,”J. Atmos. Ocean. Tech., vol. 21, pp. 1291--1300, 2004. • [3] Plant, W. J., W. C. Keller, and K. Hayes, “Simultaneous measurement of ocean winds and waves with an airborne coherent real aperture radar,”J. Atmos. Ocean. Tech., vol. 22., pp. 832--846, 2005. • [4] Eshbaugh, J. V. and S. J. Frasier, “Measurement of sea surface displacement with interferometric radar,”J. Atmos. Ocean. Tech., vol. 19, pp. 1087--1095, 2002. • [5] Johnson, J. T., R. J. Burkholder, J. V. Toporkov, D. R. Lyzenga, and W. J. Plant, “A numerical study of the retrieval of sea surface height profiles from low grazing angle radar data,'' IEEE Trans. Geosc. Rem. Sens., vol. 47, pp. 1641--1650, 2009. • [6] Nwogu, O. G. and D. R. Lyzenga, “Surface-wavefield estimation from coherent marine radars,”IEEE Geosc. Rem. Sens. Letters, vol. 7, pp. 631—635, 2010. • [7] Trizna, D. B., “Coherent marine radar measurements of ocean surface currents and directional wave spectra,”Ocean Sciences 2012 Conference, proceedings, 2012. • [8] Dankert, H., Horstmann, J. and Rosenthal, W. “Ocean wind fields retrieved from radar-image sequences,”Journal of Geophysical Research, vol. 108, C113352, 2003. • [9] Smith, G. E., Majurec, N., O’Brien, A., Pozderac, J., Baker, C. J., Johnson, J. T., Schueller, G. (2013). “High power coherent-on-receive radar for marine surveillance,” In Radar (Radar), 2013 International Conference on (pp. 434-439). doi:10.1109/RADAR.2013.6652028

13. Acknowledgements This work was supported by the US Office of Naval Research ONR Contract Number : N00014-11-D-0370