First-order Bragg Peaks Research Articles

Improving Data Quality for the Australian High Frequency Ocean Radar Network through Real-Time and Delayed-Mode Quality-Control Procedures

Quality-control procedures and their impact on data quality are described for the High-Frequency Ocean Radar (HFR) network in Australia, in particular for the commercial phased-array (WERA) HFR type. Threshold-based quality-control procedures were used to obtain radial velocity and signal-to-noise ratio (SNR), however, values were set through quantitative analyses with independent measurements available within the HFR coverage, when available, or from long-term data statistics. An artifact removal procedure was also applied to the spatial distribution of SNR for the first-order Bragg peaks, under the assumption the SNR is a valid proxy for radial velocity quality and that SNR decays with range from the receiver. The proposed iterative procedure was specially designed to remove anomalous observations associated with strong SNR peaks caused by the 50 Hz sources. The procedure iteratively fits a polynomial along the radial beam (1-D case) or a surface (2-D case) to the SNR associated with the radial velocity. Observations that exceed a detection threshold were then identified and flagged. After removing suspect data, new iterations were run with updated detection thresholds until no additional spikes were found or a maximum number of iterations was reached.

Wind and Current Dependence of the First-Order Bragg Scattering Power in High-Frequency Radar Sea Echoes

In addition to being able to extract current velocities and wind directions, the first-order Bragg peaks in high-frequency (HF) radar sea echoes also have the potential to independently map wind speeds. During a two-month experiment with the HF-radar ocean state measuring and analyzing radar, model S in the Taiwan Strait in 2013, the dependence of the first-order spectral power (FSP) on wind was found to be deterministic. An empirical model was thereafter proposed to estimate the wind speed, which gave a performance that was comparable with the popular second-order inversion method. Strong oscillations (up to ±3 dB) in the FSP caused by underlying currents were observed and cannot be readily explained by the classic wave–current interaction theory with the assumption of wave-action conservation. When we matched the local maximum and minimum values of the FSP with the local extreme and zero values of the radial-current velocities, we found that most of the extremes of the FSP occurred when the radial-current velocity was 0. To account for this phenomenon, additional smoothing with a 12.4-h moving window was applied, which led to a significant improvement in the wind-speed estimates. This knowledge of the FSP’s wind and current dependence helps us make another step toward the operational estimation of wind speeds using the first-order Bragg peaks.

Wave Height Extraction From the First-Order Bragg Peaks in High-Frequency Radars

The conventional second-order Bragg-spectrum-based wave height extraction method is often susceptible to external noise and spatial aliasing. To improve the wave height estimate, we turn to the first-order Bragg peaks and propose a new method which directly estimates the wave height from them. The key point is to confirm and use the unsaturated property of the first-order Bragg spectral power. The quantitative relation between the first-order Bragg peak power and the significant wave height can be established with the help of an in situ wave buoy, and consequently, the wave height is to be read out from the curve via the maximum Bragg peak power on one range cell. The first-order method is validated by a two-month-long data set collected by the OSMAR-S radar at 13 MHz. Compared with the second-order method, the improvement is obvious under low and moderate sea states. The new method opens the way for wider use of the first-order Bragg peaks in wave height extraction by high-frequency radars.

Time–frequency method to reduce contamination by ionospheric fast frequency modulation for HF skywave radars

The unstable and time-varying property of the ionosphere causes a frequency modulation on the high frequency sky-wave radar echo signals. When the modulation period is of 10 s or smaller, it is called the fast phase-path modulation. This sort of modulation will result in the broadening, the splitting and even the overlapping of the spectra, which makes target detection and sea-state remote sensing hard. Hence, it is necessary to decontaminate the echo signals before the coherent integration process. In this study, the pseudo Wigner-Ville distribution is proposed as a means of estimating the ionospheric frequency modulation. A frequency weighting method is used to obtain the instantaneous frequency (IF) from the time–frequency distribution and an adaptive low-pass filter is introduced to purify the IF. Then, the amplitude difference between the two first-order Bragg peaks is adopted in the weighting process to obtain the phase compensation function. To improve the compensation effect, an optimising process is proposed. The simulation results show that the method can improve the quality of the spectra for the target detection and remote sensing efficiently even in the cases of a high sea-state.

HF Bistatic Ocean Doppler Spectra: Simulation Versus Experimentation

We simulate the Doppler spectra that can be obtained under experimental conditions from bistatic high-frequency oceanic radar. For this, we combine the bistatic second-order theory with the characteristics of the full radar system, such as antenna patterns, range attenuation, and postprocessing of the received signal. Because of the sharp variation of the bistatic geometry at short range, we show that it is crucial to take these effects into account for the correct interpretation of the first-order Bragg peaks. The second-order spectrum is more robust to the system characteristics but can under some conditions also suffer from its artifacts, especially in the vicinity of the secondary peaks. A comparison is made with experimental spectra acquired recently with a Wellen radar system on the Mediterranean coast. The Wave Watch III model is used to simulate directional wave height spectra after a preliminary validation with in situ buoy measurements. Experimental and simulated Doppler spectra agree generally well, except in the case where the directional wave spectrum has little energy in the line of sight of the radar.

An Inversion Method for Extraction of Wind Speed From High-Frequency Ground-Wave Radar Oceanic Backscatter

A new approach to extracting sea surface wind speed from high-frequency radar Doppler spectra is presented. Based on certain appropriate approximations associated with the Doppler region close to the first-order (Bragg) peaks, the second-order radar cross section equation is differentiated. Once Doppler shifts due to ocean currents are removed from the data, an expression that relates the wind speed to the frequency position of the second-order peak is derived. The method is applied to simulated noisy data as well as to field data obtained from a Seasonde (a product of CODAR Oceans Sensors) in Breezy Point, NY. In the latter case, the retrieved wind speeds are then compared to ground truth data measured by an anemometer from a National Oceanic and Atmospheric Administration weather station located in the vicinity of the illuminated patch of ocean. Subject to certain constraints as detailed in the manuscript, the algorithm shows significant promise.

Limits to the Inversion of HF Radar Backscatter for Ocean Wave Measurement

Abstract The power spectrum of backscatter from the ocean surface at HF radio frequencies is characterized by two large peaks called the first-order Bragg peaks. These are surrounded by a continuum due to second-order effects. The power spectrum can be described in low to moderate sea states by a nonlinear integral equation relating it to the ocean wave directional spectrum. Inverting this equation provides an estimate of the directional spectrum. A number of inversion methods have been published. In this paper, the Wyatt method is discussed. This method uses the part of the backscatter power spectrum that surrounds the larger Bragg peak. An extension of the method to include the spectrum around the weaker peak is discussed, and some improvements in the solution are demonstrated. Until recently, all published work in this area concerned solutions to a linearized version of the equation. The second extension presented here tackles the nonlinear problem but points out the limitations of the approach and sho...

Low-energy positron-diffraction study of NaF and LiF.

Using a magnetically transported beam of slow positrons, we have measured the elastic backscattering probability versus energy for positrons normally incident on air-cleaved and vacuum-cleaved (100) surfaces of NaF and LiF. We observe enormous first-order Bragg peaks and several higher-order peaks, the positions of which imply an inner potential of -0.5\ifmmode\pm\else\textpm\fi{}0.2 and -0.1\ifmmode\pm\else\textpm\fi{}0.2 eV for NaF and LiF, respectively. The atomic plane spacings are equal to the bulk values within 1/2%. There are only slight differences between the data obtained using the freshly air-cleaved and the vacuum-cleaved surfaces. Surface disorder may be responsible for the presence of some weak Bragg reflections corresponding to directions that would otherwise be inaccessible to the incident positron beam. The temperature dependence indicates that the disorder would not have a thermal origin.