Directional sea spectrum determination using HF Doppler radar techniques

Trizna, D.B.; Moore, John C.; Headrick, J. M.; Bogle, R.

doi:10.1109/tap.1977.1141549

Cited by 19 publications

(7 citation statements)

References 10 publications

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“…oe,)•/2 occurring in the denominator of the coupling coefficient vanishes, making the coefficient itself very large. This condition of perpendicularity of the wave trains is referred to as the 'corner reflector' condition [Trizna et al, 1977], and it has been suggested that this effect can be a significant contribution to second-order scatter. For long waves the total energy contained in this spike is very small, less than the energy contained within a 1 o sector at the pattern maximum; this spike energy is even 4 times less at 20 MHz.…”

Section: Discussionmentioning

confidence: 99%

Methods for the extraction of long‐period ocean wave parameters from narrow beam HF radar sea echo

Lipa¹,

Barrick²

1980

Radio Science

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This paper describes inversion methods for HF radar sea echo Doppler spectra, giving parameters of the ocean wave spectrum in the important long-wavelength region. Radar spectra exhibiting very narrow spikes in the higher-order structure adjacent to the first-order lines are indicative of ocean wave components with a single dominant wavelength. In the simplest method of interpretation these components are assumed to be unidirectional; in this case we show how to extract wave period, direction, and rms wave height. If this simple model does not provide a good fit to the data or if the radar side bands have the form of broad peaks, we use a model for the wave spectrum with a cardioid distribution in direction and a Gaussian distribution in wave frequency. Parameters identifiable from this model include the rms wave height, dominant direction and period, and the angular spread in the direction and frequency distributions. In normal surface wave experiments the major source of error or noise is the random surface height of the sea; we describe the resulting statistics of the radar spectrum and trace the propagation of uncertainty to the derived ocean parameters. INTRODUCTIONCrombie [1955] discovered the mechanism behind high-frequency (HF) radar sea scatter nearly 25 years ago by spectrally analyzing the received time series. Using the gravity wave dispersion equation that relates the velocity to the square root of the ocean wavelength, he correctly deduced that the two sharp, symmetrically positioned Doppler peaks were produced by those ocean wave trains exactly half the radio wavelength, moving toward and away from the radar. We now call these prominent peaks the 'Bragg lines,' since the mechanism producing them is the first-order Bragg effect. When theories confirmed this mechanism [Wait, 1966;Barrick, 1972] and showed that the strength of these echo peaks is proportional to the heights of the corresponding Bragg-scattering ocean wave trains, scientists became enthusiastic about the prospect of using HF radars to measure sea state parameters. In order to make direct observations of the long ocean waves that are the essence of 'sea state' using this first-order Bragg effect, one This paper is not subject to U.S. copyright. Published in 1980 by the American Geophysical Union. would have to operate the radar at lower MF; huge antenna size requirements, heavy spectrum utilization in this region, and ionospheric problems all dictate against such a system [Barrick, 1977b; Barrick and Lipa, 1979a]. At mid and upper HF the first-order Bragg peaks are produced by the less interesting shorter waves, and hence using these alone, radio oceanographers resigned themselves to extracting only wind direction information because the short waves align themselves quickly with the wind [Long and Trizna, 1973; Stewart and Barnum, 1975].The more sophisticated radar systems and digital signal processors of the mid-1960's showed a lowerlevel spectral continuum surrounding the first-order Bragg peaks that was definitely established as sea ...

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Section: Discussionmentioning

confidence: 99%

Methods for the extraction of long‐period ocean wave parameters from narrow beam HF radar sea echo

Lipa¹,

Barrick²

1980

Radio Science

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“…Hence, a comparison of S values is not really realistic except in a very general way. Over the range of angles which we report our data, the two functions cos 4 NO/2 and CosNO are nearly equal when viewed on a log amplitude versus linear angle plot. Hence, our broadest exponents for the westerly swell which propagated into the test area compare with the narrower S values of Tyler et al, which is not surprising since they were probably generated under similar open ocean conditions.…”

Section: I'mentioning

confidence: 65%

“…(3) The area of the scattering patch in radial coordinates is given by dA = cAtR dO, (4) where dO is the 3 dB beamwidth of the radar antenna, At is the radar pulse length, R is the range from the radar to the center of the scattering cell, and c is the speed of light.…”

Section: Trizna Bogle Moore and Howementioning

confidence: 99%

Observation by HF radar of the Phillips resonance mechanism for generation of wind waves

Trizna¹,

Bogle²,

Moore³

et al. 1980

J. Geophys. Res.

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Measurements are reported of directional ocean wave spectra made over 80° of viewing angle by an HF radar, operating in the surface wave mode in an area 22.5 km north of San Clemente Island, California. Ten azimuths from 280° to 360° true bearing were simultaneously measured for 10 wave frequencies ranging from 0.14 Hz (75 m waves) to 0.35 Hz (13 m waves). A Waverider buoy was used to measure omnidirectional energy in the region, and first‐order radar Bragg lines were used to determine the spreading of wave energy with angle. Data are presented in which a bimodal spectrum was present: an attenuated spectrum with wave components to 0.10 Hz from a storm at sea at 270° bearing; plus a transient local wind spectrum, stronger in amplitude at the higher frequencies, with wave cutoff near 0.14 Hz, and running from 315° bearing. Just after the onset of local winds, the westerly spectrum fitted a cosine squared spread at the lowest measured frequencies. With the development of local wind, which blew at a 12–14 kn (6–7 m s−1) speed for a period of 12 hours, the wave spectrum spread about the wind direction as cosine thirty‐second at the lowest frequencies measured, 0.14 Hz, and cosine sixty‐fourth at the highest frequencies measured, 0.35 Hz. For 0.28 Hz waves the Phillips resonance mechanism for wave generation is proposed to explain the twin peaks in amplitude observed, equally spaced either side of the wind direction. These were dominant for the earliest measurement period and still were major contributions for later measurement periods. This mechanism was found to contribute also at the higher wave frequencies, as predicted by theory. Coherence times are derived from the angular widths of the Phillips resonances based on predictions of Stewart and Manton and are found to agree quite well with theory.

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“…In one, Ward [1969] proposes that this higher-order echo is produced by spatial harmonics attached to some dominant fundamental wave; the Doppler spectra would thus consist of peaks at x/• times the Bragg peak frequency, where n is the spatial harmonic. In another model, Trizna et al [1977] sugg6st that a simple electromagnetic 'corner-reflector' effect accounts for th• entire second-order echo, and they recommend that an expe•nt be done to verify this proposed theory. The three experiments analyzed herein conclusively prove that those two suggested theories cannot adequately explain second-order scatter and hence cannot be employed to extract usefully sea-state information.…”

Section: F(f) • •/Tanh '('Kd) + Kd Sech 2 (Kd)/4tanh (Kd)mentioning

confidence: 99%

HF radar measurements of long ocean waves

Lipa¹,

Barrick²,

Maresca³

1981

J. Geophys. Res.

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Sea‐echo data from three separate narrow‐beam HF radar experiments on the Pacific Ocean are analyzed here by techniques presented in Lipa and Barrick (1980). Only those wave spectral components whose periods exceeded 10 s were included. Close agreement of radar‐deduced wave field parameters with surface observations confirms the validity of the second‐order theoretical solution for the echo Doppler spectrum, upon which this analysis is based. Depending on the particular experiment, a variety of wave parameters are extracted, including rms wave height, mean wave direction, dominant period, angular spread of the wave field, the nondirectional wave height spectrum, and higher Fourier angular coefficients versus wave frequency. The radar‐deduced wave parameters fall within the combined error bounds estimated for the radar and buoy wave observations; consequently, we contend that the primary source of error for radar data is finite sample size. Typical accuracies for specific parameters resulting from observations averaged over a 2‐hour period are ±5% for wave height, ±0.5 s for wave period, and ±7° for wave direction. Hence the utility of HF radars for long‐wave measurements has been validated.

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Directional sea spectrum determination using HF Doppler radar techniques

Cited by 19 publications

References 10 publications

Methods for the extraction of long‐period ocean wave parameters from narrow beam HF radar sea echo

Methods for the extraction of long‐period ocean wave parameters from narrow beam HF radar sea echo

Observation by HF radar of the Phillips resonance mechanism for generation of wind waves

HF radar measurements of long ocean waves

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