Aerospace and Electronic Systems Magazine March 2018 - 42

Feature Article:

DOI. No. 10.1109/MAES.2018.170028

Extraction of Wind Direction from the Hf Hybrid Sky-
Surface Wave Radar Sea Echoes
Xiaoliang Chu, Ocean University of China, Qingdao, China
Jie Zhang, Yonggang Ji, Yiming Wang, The First Institute of Oceanography, SOA,
Qingdao, China
Longquan Yang, China Research Institute of Radiowave Propagation, Qingdao, China,
and Xidian University, Xi'an, China

INTRODUCTION
A high frequency (HF) hybrid sky-surface wave radar (HFHSSWR) system comprises a sky-wave transmitting channel and surface-wave receiving channel, which improves the detection range
compared with HF surface-wave radar (HFSWR) and reduces the
power loss and the influence of the ionosphere compared with HF
sky-wave radar. Thus, the HFHSSWR system integrates superiorities such as large coverage area, far detection range, and little loss as
a means to monitor the sea state and targets continuously. However,
as a special-configuration radar system, some properties and characteristics differ from the sky-wave radar or surface-wave radar.
Therefore, it is necessary to research and analyze the characteristics
of the sea echoes of the HFHSSWR and explore its application.
Since Riddolls [1] presented the configuration of HFHSSWR
and analyzed the limit conditions for ship detection from the aspect
of theory, the HFHSSWR has been given attention by a growing
number of people. Jiao et al. [2] considered the ionosphere reflection and surface-wave propagation mode and investigated the
feasibility of the new propagation mode. They also derived the
orientation equation for application of the mode. Jiang et al. [3]
analyzed the phenomenon that the first-order sea clutter changes
with orientation and verified the result with experiment data. Li
et al. [4]-[6] analyzed first-order sea clutter spectrum characteristics and the influence of ionosphere and bistatic angle on the sea
clutter spectrum for the HFHSSWR based on experimentation and
simulation of the first-order sea clutter spectrum. Zhang et al. [7]
presented a hybrid networking system for distributed HF over-the-

Authors' current addresses: X. Chu, School of Information
Science and Engineering, Ocean University of China, No. 238
Songling Road, Qingdao 266100, China, E-mail: (xlchu@ouc.
edu.cn). J. Zhang, Y. Ji, Y. Wang, First Institute of Oceanography, No. 6 Xianxialing Road, Room 635, Qingdao City 266061,
China. L. Yang, China Research Institute of Radiowave Propagation, No. 36 East Xianshan Road, Qingdao 266107, China.
Manuscript received January 29, 2017, revised August 15, 2017,
and ready for publication August 31, 2017.
Review handled by D. O'Hagan.
0885/8985/18/$26.00 © 2018 IEEE
42

horizon radars. However, research on the inversion of sea state parameters for this radar system is limited. It is worth carrying out
experiments for monitoring the ocean dynamical environment and
retrieving information about the sea state to verify the ability to
monitor sea state with HFHSSWR.
The application of HFSWR in retrieving sea state parameters
has been explored for years [8]-[11], and sea state parameters
such as current and significant height [10], [11], wind speed [12],
and wind direction [8], [13], [14] can be extracted from the radar
backscatter spectrum. In these parameters, wind direction can be
obtained using the first-order peaks and a directional wave model.
However, there is an ambiguity of wind direction when the single
radar system is used. So researchers have been developing several
methods to solve this problem. For example, Heron and Rose [13]
presented the multibeam (MB) method, and Wyatt et al. [14] developed a method that uses maximum likelihood (ML) to eliminate
the wind direction ambiguity. For HFHSSWR, it is necessary to
explore and analyze whether the system can be applied to the inversion of wind direction and whether these methods can eliminate
the wind direction ambiguity effectively.
In this article, we develop an experiment to obtain the sea state
parameters by the HFHSSWR and the in situ instruments. The
wind direction is extracted, and the results are compared with the
in situ data. In addition, the wind direction ambiguity was eliminated by the ML method. The results show that it is feasible to
extract the wind direction using the HFHSSWR.

BASIC THEORY OF WIND DIRECTION INVERSION
For the sky-wave radar, ionospheric conditions are transient in time
and space, which results in multipath and broadening of the Bragg
lines. Neither effect influences the wind direction measurements,
because the ratio between the two first-order peak values will be
independent of these effects for a typical ionospheric configuration[8],[15]. So the HFHSSWR will be the similar to the HFSWR
for the wind direction inversion. Li et al. [6] point out that the locus
of a scatter point can be described as an ellipse equation in the case
of hybrid sky-surface mode, which is similar to the bistatic HFSWR. So the inversion of wind direction of the HFHSSWR may be

IEEE A&E SYSTEMS MAGAZINE

MARCH 2018

http://www.edu.cn

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine March 2018