2162 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 14, NO. 11, NOVEMBER 2017
Ionospheric Decontamination for HF Hybrid
Sky-Surface Wave Radar on a
Shipborne Platform
Yongpeng Zhu , Yinsheng Wei, and Lei Yu
Abstract— This letter describes a method of correcting
ionospheric frequency modulation for a high-frequency hybrid
sky-surface wave radar mounted on a shipborne platform. In the
proposed method, azimuth-dependent sea clutter signals are first
decomposed into monocomponent signals based on distinguish-
able differences in their directions of incidence. Afterward, based
on the decomposed monocomponent signals, the statistical mean
of the time derivatives of the signal phases, weighted by the
signal amplitudes, is used to estimate the ionospheric frequency
modulation. Finally, the estimated result is applied to the received
data to compensate for the ionospheric contamination. Numerical
results on simulated data demonstrate the effectiveness of the
proposed algorithm.
Index Terms— High-frequency (HF) hybrid sky-surface wave
radar, ionospheric decontamination, multicomponent signal
decomposition, shipborne platform motion.
I. INTRODUCTION
A
HIGH-frequency (HF) hybrid sky-surface wave radar
system is composed of a skywave transmit channel
and a surface wave receive channel. This combination
of propagation modes enables long-range detection, low-
velocity target detection, and oceanic remote sensing [1]–[4].
Depending on where the receiving platform is mounted, such
a radar system can be classified as either land-based or ship-
mounted. Previous deployments have been largely limited
to oceanic observations of near-coastal areas due to their
fixed platforms. Therefore, as a natural extension of this
approach, a radar system mounted on a moving platform can
be employed for agility and maneuverability to improve the
capacity for oceanic radar observations of areas far from the
coastline.
Temporal phase-path variations due to the use of the
ionospheric transmit channel can produce frequency modula-
tions of ionospherically propagated HF signals, resulting in
spectral broadening [5]. To alleviate this effect on oceanic
remote sensing and target detection, a Hankel rank reduc-
tion (HRR) method was proposed in [6]. In this method,
the ionospheric contamination is estimated by tracking the
instantaneous frequency (IF) variations in sea echoes, and
Manuscript received December 26, 2016; revised May 18, 2017 and
August 4, 2017; accepted September 21, 2017. Date of publication October 12,
2017; date of current version October 25, 2017. This work was supported by
the National Natural Science Foundation of China under Grant 61471144.
(Corresponding author: Yinsheng Wei.)
The authors are with the Department of Electronic Engineering, Harbin
Institute of Technology, Harbin 150001, China (e-mail: zhuyp@hit.edu.cn;
hitweiys@sina.com; yu.lei@hit.edu.cn).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LGRS.2017.2757000
the estimated IFs are used to correct the sea clutter spectra
perturbed by nonlinear ionospheric motion. The performance
of this method dramatically decreases when the frequency
modulation is caused by fast phase-path disturbances. An effi-
cient way to address this problem is to use a time–frequency
distribution (TFD)-based method for ionospheric decontam-
ination [7]. However, the determination of an appropriate
window function length to suppress the effect of cross-term
interference on the TFD is a challenging issue. The methods
proposed in [6] and [7] were derived for the case of a fixed
platform. By contrast, a moving platform will cause the sea
clutter spectra to be modulated in both amplitude and Doppler
frequency [8]–[10]. Regardless of the influence on sea echoes,
conventional ionospheric decontamination methods will
likely fail.
This letter proposes a novel ionospheric decontamina-
tion algorithm. The method first considers the spatial dif-
ferences between individual contaminated Bragg peaks and
then decomposes azimuth-dependent sea clutter signals into
several monocomponent signals using a least-squares (LS)
method. In this way, the modulation effect on the received
sea echoes due to the platform motion can be greatly reduced.
Next, the ionospheric frequency modulation is estimated via a
statistical averaging operation performed over the decomposed
sea clutter signals. Finally, the estimated result is applied to the
received data to compensate for the ionospheric contamination.
II. S
IGNAL MODEL
In this section, we present a signal model explaining how
ionospheric contamination disturbs the sea echoes received by
a moving platform. We begin by studying the separate effects
of platform motion and ionospheric frequency modulation on
sea clutter spectra. Then, a signal model considering both
effects is derived.
Regarding the effect of the platform motion, we study the
experimental spectra of signals received by a shipborne HF
surface wave radar (HFSWR) system with the transmitting
and receiving antennas mounted on the same platform. The
experiment was conducted on November 28, 2005, in the
East China Sea, with a Douglas sea state [11] of three.
A frequency-modulated interrupted continuous wave was used
as the transmit waveform in the radar system, with a frequency
modulation period of 0.024 s and a bandwidth of 30 kHz.
The radar carrier frequency was f
0
= 13.4MHz.The
number of elements in the antenna array was M = 10, with
an element interval of d = 11 m. The platform velocity
was v
p
≈ 15 knots. The coherent integration time (CIT)
was 98.28 s.
1545-598X © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.