Mapping wind by the first-order Bragg scattering of broad-beam high-frequency radar
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Abstract: Mapping wind with high-frequency (HF) radar is still a challenge. The existing second-order spectrum based wind speed extraction method has the problems of short detection distances and low angular resolution for broad-beam HF radar. To solve these problems, we turn to the first-order Bragg spectrum power and propose a space recursion method to map surface wind. One month of radar and buoy data are processed to build a wind spreading function model and a first-order spectrum power model describing the relationship between the maximum of first-order spectrum power and wind speed in different sea states. Based on the theoretical propagation attenuation model, the propagation attenuation is calculated approximately by the wind speed in the previous range cell to compensate for the first-order spectrum in the current range-azimuth cell. By using the compensated first-order spectrum, the final wind speed is extracted in each cell. The first-order spectrum and wind spreading function models are tested using one month of buoy data, which illustrates the applicability of the two models. The final wind vector map demonstrates the potential of the method.
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Key words:
- high-frequency radar /
- first-order Bragg peak /
- broad-beam /
- wind field /
- wind speed
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Figure 2. Attenuation of first-order spectrum power (FSP) result from the wind spreading function (WSF) with different
$s$ .
Figure 3. Normalized surface impedance for 13 MHz radar wave under different wind speed,
$ {0}^{\circ } $ represents radial wind and$ {90}^{\circ } $ represents cross wind. The solid and broken lines represent the real and imaginary parts of the impedance, respectively.
Figure 4. Added loss for a rough sea under different winds. a. Under radial wind; b. under cross wind; c. under 20 kn wind speed; and d. under 30 kn wind speed.
Figure 5. Fitting of the parameters under different radial winds (a), and discrepancy loss between radial wind and cross wind under different wind speeds (b).
Figure 6. Modulation process of a gate signal to a receiving signal (a), effect of the gate signal on the radar echo power (b), and basic transmission loss for a monostatic 13 MHz OSMAR radar (c). Each range cell is
$ 2.5 $ km. In a, AT is the transmit pulse, GR is the receive pulse, t0 is the delay corresponding to distance R, AR is the effective receive pulse for distance R, and Tp is the sweep period.
Figure 7. Total transmission loss for a rough sea for the monostatic 13 MHz OSMSAR radar. a. Under radial wind; and b. under cross wind.
Figure 9. Map of the HF radar deployed on the Fujian coast of the Taiwan Strait. Radar station is marked by the star named XIAN, and the three buoys are marked by dots.
Figure 11. Number of DOA solutions in each measurement bin, where the total number of measurements is 6 202. a. For positive FSPs; and b. for negative FSPs.
Figure 12. The maximum FSPs versus wind speeds for different range cells (a); and the maximum FSP loss (relative to the maximum FSP in range cell
$ 2 $ ) versus range cells under different wind speeds (b). Each range cell is$ 2.5 $ km.
Figure 13. Comparison of the theoretical and the radar extracted propagation attenuation relative to the range cell 2 with different radial wind.
Figure 15. Wind speed and the second-order integration versus time (a); scatter plot and linear fitting result (b).
Figure 16. Wind speed and the maximum first-order peak versus time (a); and scatter plot and fitting result (b).
Figure 17. Buoy wind speed versus the second-order integration (a); and buoy wind speed versus the radar wind speed from the second-order model (b).
Figure 18. Comparison of the wind speed and the radar wind speed from the first-order model.
Figure 19. Sample comparisons of the in-situ measured and estimated wind direction at Buoy A in March. Note the sample numbers do not represent a time series but are individual, independent estimates of half an hour time period with wind speed above
$ 6.5 $ m/s.Table 1. Radar parameters of the OSMAR-S
Parameters Value and setting Center frequency/MHz 13 Bandwidth/kHz 60 Transmit antenna monopole Receive antenna cross-loop/monopole Sweep period/s 0.38 Average power/W 100 Range resolution/km 2.5 Coherent integration time/min 6.5 Normal direction/(°) 100 Transmitted waveform FMICW pulses Technique of azimuthal resolution direction finding 
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