Aerospace and Electronic Systems Magazine March 2018 - 46

Extraction of Wind Direction from the HF Hybrid Sky-Surface Wave Radar Sea Echoes

Figure 7.

Comparison between wind directions estimated by the ML method and
those measured by a shipborne anemometer at (a) site A and (b) site B.

to the elimination of the wind direction, and the parameter β is set
the fixed value 0.5 referred to previously.
The results from sites A and B calculated by the ML method
are given in the Figures 7 and 8. From Figure 7, it is seen that the
wind direction extracted by the ML method can agree the in situ
data for two sites. The correlation coefficient and RMSE are 0.63
and 37.1° for site A and 0.39 and 17.2° for site B, respectively, on
the condition that wind directions are smoothed using the 10-min
average filter in time. The correlation coefficients are low, partly
because of the small amount of data. However, the results will become better when the smooth temporal interval becomes longer. If
the results are smoothed with an interval of a half hour, the correlation coefficient and RMSE are 0.74 and 36.5° for site A and 0.67
and 11.0° for site B, respectively. In comparison, the correlation
coefficients with half-hour averaging are all improved. Nevertheless, the correlation coefficients improve slightly when the temporal interval keeps increasing. By contrast, Figure 9 displays the
results of applying the MB method. In the calculation, the least
square principle is used to increase efficiency of the MB method
[19]. The results are also smoothed using the 10-min average filter,
46

Figure 8.

Scatterplots of wind directions estimated by ML method versus anemometer measurements at (a) site A and (b) site B.

and the correlation coefficient and RMSE are −0.36 and 125.8°
for site A and 0.05 and 82.9° for site B, respectively. Comparing
the results from the methods ML and MB, it is clear that the ML
method is more adaptable to eliminate the direction ambiguity of
the HFHSSWR.

DISCUSSION AND CONCLUSIONS
In this article, the wind direction is extracted from the HFHSSWR
by the MB and ML methods. The results from the radar are compared with the in situ data, which show that the wind direction can
be obtained from the HFHSSWR, although the accuracy is not as
good as the HFSWR. The error may be affected by the HFHSSWR
system. In addition, the low wind speeds during the experiment
can produce large error. The ML method is more suitable for the
elimination of the wind direction ambiguity for the experiment
data. In the calculation, we find the results retrieved by the ML
method would be worse when the parameter β is not fixed and is
selected by the ML method. Whether ML method is effective for
the HFHSSWR needs more experimental data for examination. In

IEEE A&E SYSTEMS MAGAZINE

MARCH 2018



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