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Gordon Farquharson

Principal Engineer

Affiliate Assistant Professor, Electrical Engineering

Email

gordon@apl.washington.edu

Phone

206-685-1505

Publications

2000-present and while at APL-UW

ATI SAR signatures of nearshore ocean breaking waves obtained from field measurements

Goncharenko, Y.V., and G. Farquharson, "ATI SAR signatures of nearshore ocean breaking waves obtained from field measurements," Proc., Geoscience and Remote Sensing Symposium (IGARSS), 21-26 July, Melbourne, 326-329, doi:10.1109/IGARSS.2013.6721158 (IEEE, 2013).

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21 Jul 2013

We present a technique to extract geophysical parameters of nearshore ocean breaking waves (e.g., wave height, wave velocity) from ATI SAR data. The technique exploits the azimuthal shift induced by moving scatterers in SAR imagery. We find that length of the azimuthal streaks are correlated with breaking wave height (correlation coefficient is 0.78) for a range of wave conditions. We also outline a technique for highlighting breaking waves in SAR imagery using the ATI SAR images.

Phase calibration of an along-track interferometric FMCW SAR

Deng, H., Y.V. Goncharenko, and G. Farquharson, "Phase calibration of an along-track interferometric FMCW SAR," Proc., International Geoscience and Remote Sensing Symposium (IGARSS), 21-26 July, Melbourne, 1649-1652, doi:10.1109/IGARSS.2013.6723109 (IEEE, 2013).

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21 Jul 2013

Techniques to estimate and correct range-dependent phase differences between receivers in an FMCW ATI SAR are studied. Both techniques reproduce the range-dependent phase ripple seem in the ATI interferograms.

Contrast-based phase calibration for remote sensing systems with digital beamforming antennas

Farquharson, G., P. López-Dekker, and S.J. Frasier, "Contrast-based phase calibration for remote sensing systems with digital beamforming antennas," IEEE Trans. Geosci. Remote Sens., 51, 1744-1754, doi:10.1109/TGRS.2012.2205695, 2013.

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1 Mar 2013

A contrast-based phase calibration algorithm for digital beamforming remote sensing radars using three contrast metrics is presented. The algorithm corrects time-varying antenna array phase errors that defocus digital beamforming remote sensing radar imagery. Amplitude errors are treated by equalizing the received powers in all elements. As such, the algorithm does not produce an absolute (or radiometric) calibration vector for the array. The performance of the algorithm is studied using a combination of simulated and real radar data under various conditions and is compared with a clutter-based calibration algorithm. An analytical proof showing that maximizing the expected value of the 4-norm metric is equivalent to phase-calibrating the image, except for a linear phase offset, is provided. We find that the clutter calibration algorithm performs best for statistically homogeneous scenes but that the contrast-calibration algorithms perform better with scenes with larger contrast ratios.

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Wave shadowing and modulation of microwave backscatter from the ocean

Plant, W.J., and G. Farquharson, "Wave shadowing and modulation of microwave backscatter from the ocean," J. Geophys. Res., 117, doi:10.1029/2012JC007912, 2012.

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4 Aug 2012

Shadowing and modulation of microwave backscatter by ocean waves are studied using coherent X-band radars. Two types of shadowing are investigated: geometric shadowing (complete blockage of incident rays) and partial shadowing (polarization-dependent diffraction combined with weak scatterers). We point out that the frequency of occurrence of zero signal-to-noise ratio samples cannot depend on the incident power level or the polarization if geometric shadowing occurs but can if partial shadowing exists. We then compare this behavior with observations, and show that the data do not support the hypothesis that geometric shadowing plays a significant role in low-grazing-angle microwave scattering from the ocean surface. Furthermore, our data indicate that partial shadowing only depends significantly on polarization for the steep waves found near shorelines. We also study the modulation of microwave backscatter by ocean waves using these data by looking at the phase differences between received power and scatterer velocity. These phase differences appear to be rather well explained by standard composite surface theory at VV polarization, having values that are positive looking up wave and negative looking down wave. For HH polarization, however, breaking effects come into play and overshadow composite surface effects of free waves. They cause the phase difference to be near zero for up wave looks and near 180° for down-wave looks. A simple model that involves both breaking and freely propagating waves but does not include any shadowing effects is shown to account for observed phase differences at both polarizations to within about 10°.

Origins of features in wavenumber-frequency spectra of space-time images of the ocean

Plant, W.J. and G. Farquharson, "Origins of features in wavenumber-frequency spectra of space-time images of the ocean," J. Geophys. Res., 117, doi:10.1029/2012JC007986, 2012.

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20 Jun 2012

Wave number-frequency spectra measured with remote sensing systems consist of energy along the ocean wave dispersion relation and additional features that lie above and below this relation. At low frequencies a feature passing through the origin as a straight line is observed while one or more high-frequency features exhibit substantial curvature. Here we utilize images obtained on the open ocean from microwave Doppler shifts, which are directly related to scatterer velocity and allow us to calculate expected wave-wave interaction effects. We show that the strongest features lying off the first-order dispersion relation are not primarily due to second-order interactions, breaking caused by wind turbulence, advection by turbulence, or shadowing. The low-frequency feature can be seen traveling in the opposite direction to swell when looking nearly crosswind. We show that the most probable cause of these features is the interference of long ocean waves, which causes breaking near local maxima of surface slope. Doppler spectra observed by the radars indicate that the maximum speed reached by water particles on the open ocean is less than 6 m/s and usually close to the speed of the low-frequency feature in the wave number-frequency spectrum. Since this is much less than the phase speeds of dominant wind waves and swell, neither of these waves can be the breaking wave. Rather, we hypothesize that the superposition of these waves steepens short gravity waves on the surface, which then break to produce water parcels traveling near their phase speed, the speed observed by the radar.

Observation of a boat and its wake with a dual-beam along-track interferometric SAR

Toporkov, J.V., P.A. Hwang, M.A. Sletten, S.J. Frasier, G. Farquharson, D. Perkovic, "Observation of a boat and its wake with a dual-beam along-track interferometric SAR," In Proceedings, Int. Geosci. Remote Sens. Symp. (IGARSS), Honolulu, 25-30 July, 1940-1943, doi:10.1109/IGARSS.2010.5649709 (IEEE, 2010).

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25 Jul 2010

The Dual-Beam Interferometer is an airborne instrument that combines two along-track interferometric synthetic aperture radars observing the surface below at different squints. This configuration allows retrieving vector velocities of surface flows in a single aircraft pass. The system was designed by the University of Massachusetts and saw several deployments in the early 2000s.

An imagery of a boat with a rather pronounced wake system captured during one of these flights is the subject of this paper. The velocity of the vessel is estimated based on the "train off the tracks" displacement in one of the looks. This estimate, which will be affected by uncertainty in target position due to smearing, is then used to remove unknown phase biases in the interferometric channels. Retrieved velocity variations are examined along cuts traversing the wake, with the focus on its narrow "turbulent" part. We find that the reconstructed velocities 150 m behind the boat are intuitively satisfying, but we are unable to fully account for the cross-wake component of velocity at 400 m behind the vessel.

SURF zone surface displacement measurements using interferometric microwave radar

Farquharson, G., S.J. Frasier, B. Raubenheimer, and S. Elgar, "SURF zone surface displacement measurements using interferometric microwave radar," In Proceedings, Int. Geosci. Remote Sens. Symp. (IGARSS), Honolulu, 25-30 July, 2428-2431, doi:10.1109/IGARSS.2010.5650521 (IEEE, 2010).

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25 Jul 2010

Vertically polarized backscattered power, Doppler velocity, and interferometric height were measured in the nearshore ocean region using a high-resolution imaging microwave radar. The interferometric surface displacement measurements are compared with normalized radar cross sections (NRCS) and Doppler velocities, and with in situ pressure sensor estimates of wave height. The rms uncertainty in the interferometric surface displacement is computed from the data. The interferometric surface displacement measurements are intuitively satisfying in that positive displacements are generally associated with pixels that have larger NRCS values. Interferometric surface displacements compare well with heights derived from in situ pressure sensors, and we provide some possible explanations for the differences.

Inventions

Conformal Series-Fed Electronically Squinted Aperture-Couple Patch Array Antenna

Record of Invention Number: 46904

Gordon Farquharson

Disclosure

7 Apr 2014

Shipborne Ocean Wave Measurement System

Record of Invention Number: 46763

Gordon Farquharson, Bill Plant

Disclosure

11 Dec 2013

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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