EEE TILANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. “-28, NO. 6, NOVEMBER 1980 75 1 Progress Toward a Practical Skywave Sea-State Radar T. M.GEORGES Absrracr-Recent advancesin propagation modeling, ionospheric theNational Oceanic and Atmospheric Administration’s diagnostics,and signalprocessing have helped overcome the lim- (NOAA’s) Coastal Ocean Dynamics Radar (CODAR) system. itations the ionosphere imposes on sea-state measurements with HF Apair of these compact transportable HF radarsproduces skywave radar.Wind-direction fields in tropicalstorms can be surface-current-vector maps out to 70 km from shore in near- routinely mapped under mostionospheric conditions, but waveheight real-time [6]. Othersurface-wave radars have beenused to andwave-spectrum extractionismore sensitive to ionospheric measure rms waveheight, as well as the scalar and directional distortions and requires care in signal processing and in selecting an ocean-wave spectra [ 71. Because surface-wave radars present ionosphericpath. Spot measurements with a high-resolutionradar have verified its ability to measure (in order of increasing dif€iculty) fewremaining problems and are now virtually operational, wind-direction fields,rms waveheight,and the scalar ocean-wave onlythose problems unique to skywave systems will be spectrumat rangesup to 3000 km using one ionospherichop. addressed here. Althoughsuch a radar can in principle map thesequantities over It is the increase in single-radar coverage area from thou- millions of square kilometers of an ocean area, the time requiredto do sands to millions of square kilometers that prompts the use of so under various ionospheric conditions remains to be determined. A ionosphericreflection with its attendant contaminations. minimumobjective of onemap of rms waveheight per day seems Except for the effects of the intervening ionospheric reflec- attainable. tions: the sea echoes received by skywave radars are identical tothose obtained with surface-wave illumination. Initially, I. HISTORICAL PERSPECTIVE researcherswere optimistic that the effects of imperfect ionospheric reflection could be easily predicted and controlled KYWAVEradars sense natural and man-made targets andthat vast oceanareas could be monitored with a few beyond the optical horizon by bouncing HF radio waves S judiciously located skywave radar stations [ 8 I , [ 91 . Some of off the ionosphere. The first skywave radars were research in- this optimism dissipated when they discovered that, although strumentsdeveloped in the late forties and early fifties to wind-directionfields can usually be mapped, ionospheric study the structure and motions of the ionosphere itself, using degradation of thesea-echo spectrum often prevented the echoesbackscattered from the earth. Many researchers tried extraction of other important sea-state information, such as to developground-backscatter radar into anoperational the rms waveheight. Therefore, progress in skywave sea-state diagnoseic toolfor ionospheric mapping or HF frequency radar development in the lastfive years might be characterized management,but because of problems in uniquelytrans- as a reassessment of the ionospheric problem and a marshalling formingradar signatures into ionospheric structure: such of forces to deal with it. effortshave been largely abandoned. Consequently, the re- Renewed optimism about skywave radar now seemswar- views of suchapplications by Hayden [ 11 andCroft [2] ranted because of four developments: 1) improved understand- remain reasonably up-to-date. ing of the spectral contamination caused by the ionosphere, In the fifties, the prospect of detecting and tracking mili- b) an ability to derive optimum radar propagation paths from tarytargets (missiles, aircraft, and ships) with an over-the- real-timeionospheric soundings, c). the availabilityof high- horizon (OTH) radar -spurred a distinct and largely classified resolution radar facilities built with defense funds, and d) the line of research that led to the construction in the sixties and evolution ofhigh-speed signal processing strategies to deal with seventies of large-aperture HF-OTH radar arrays by the major contaminated data. powers [31, 141, [511. The subject of this review is the third and most recent ap- 11. THEORETICAL BASIS plicationof skywave radar-sea-state monitoring-in which information about the wind and wave fields at the sea surface Crombie’sobservations [ 51 stimulated wave theorists to is derived from the sea-clutter echo that is regarded as noise constructbetter models of the interaction of HF electrc- in surveillance systems. magnetic waves with gravity waves on the sea surface. Their A sea-state radar need not employan ionospheric (skywave) radar-cross-sectionmodels had to explain not only the first- path.Indeed, the whole field of HFradar oceanography orderor Bragg-backscattering process that produces an echo evolved from surface-wave experiments by Crombie [ 51 who with two sharp spectral lines but also the spectral structure of first demonstrated HF Bragg scattering by sea waves of half an observed weaker second-order continuum of echoes from the radar wavelength. Refinements in the mathematical model doublescattering by the sea waves. Barrick [lo], [ 11 ] has of HF scattering from the sea and the evolution of high-speed produced the most comprehensive second-order model for the signal-processingsystems have rapidly led tothe present HF seaecho spectrum and its dependenceon sea state, and his stateofsurface-wave radar development represented by modelhas withstood numerous comparisons between ex- perimentallymeasured sea-echo spectra and in situsampling ofocean-wave parameters. According to his model,two Manuscript received April 1, 1980; revised July 10, 1980. mechanismscontribute to the second-order continuum: one T. M. Georges is with the Wave Propagation Laboratory, National Oceanicand Atmospheric Administration, Environmental Research is actuallysingle scattering from evanescent ocean-wave Laboratories, Boulder, CO 80303. componentsthat arise from nonlinear hydrodynamic inter- U.S. Government work not protected by U.S. copyright. 752 IEEE TRANSACTIONSON ANTENNAS AND PROPAGATION,VOL. AP-28, NO. 6, NOVEMBER 1980 action of two water waves; the second contribution is from radar waves twicescattered from the water surface before returning to theradar. Interpretingthe properties of thefirst-order or Bragg- scatter features of the Doppler spectrum permits measurement of surface wind direction and surface currents, whereas analy- sis ofthe second-order contributions to the echo spectrum hasled to methodsfor extracting rms waveheight and both the scalar and directional ocean-wave spectrum from measured -1 HZ 0 +lHZ sea-echo spectra. When waveheight measurements are coupled Doppler Shlt with suitable models for wind-wave generation, surface wind speed can also be estimated. A - Short wlnd-wave drrectlon (surface wnd d~recbon). B - Surfacecurrent. 111. MEASUREMENT HIERARCHY D/c - Rrns wave heght Some of the sea-statequantities just mentioned are rela- E - Scalar ocean-wave spectrum tivelyeasy to measurewith an HF radar,whereas others, namelythose derived from second-order echoes, require F - Frequency of domlnant Dcean waw more care in removing noise and interference from the radar Fig. 1. Many quantities that indicate conditions at the sea surface echo. Here we summarize the particular oceanographic quanti- can be extracted from the spectrum of HF radar sea echoes. Those ties that an HF sea-state radar can measure, briefly describe derived from the fiist-order spectral features (A and B) are easier howeach quantity is derived fromthe sea-echo spectrum, to measure in the presence of noiseand contamination than are and assess theparticular problems that arise in making that those derived from second-order features (D/C, E, and F). measurement with a skywave radar (seeFig. 1). B. Surface Currents A. Wind-Direction Fields Becausesurface currents simply transport gravity waves The short ocean waves (a few meters in length) that pro- (withminor complications introduced by current shear), duce first-order, Bragg-resonant echoes at HF respond quickly the Bragg lines of the Doppler spectrum are simply shifted in to the driving wind, so that the resonant waves of maximum frequency (from symmetry about zero Doppler) by an amount intensitytravel in the direction of the surface wind. If the proportional to the radial(from the radar) current com- directional pattern of these waves’ response to the wind can ponent[20]. Thus, mapping currents is relativelystraight- bemodeled, then the wind direction can be determined forward with a pairof surface-wave radars [61, [ 21 ], [ 531. (exceptfor a left-right ambiguity with respect to the radar With a skywave radar, one kind of ionospheric contamina- beam direction) by matching that pattern to relative strengths tion is a frequency shift of the seaecho spectrum caused by of the Bragg echo from waves advancing and receding along upwardor downward motion of the reflecting layer, Such theradar beam. (Brag-line echoes are virtually always ob- shifts are indistinguishable from a current-induced shift. Thus, tained even from visually calm seas.) The first demonstrations unlessthe target sea areaincludes a zero-Doppler reference of skywaveradar for ocean monitoring produced maps of (such as land or a moored platform), surface currents cannot wind-directionfields derived fromsuch measurements of be extracted from skywave sea echoes. Maresca and Carlson Bragg-line ratios [12], [131, [141, [151,