Aerospace and Electronic Systems Magazine March 2018 - 36

Experimental Analysis of a HF Hybrid Sky-Surface Wave Radar

Figure 6.

Geographic view of experiment system.

CLUTTER MITIGATION METHOD
In this section, we focus on the clutter mitigation method to alleviate the impact of the ionosphere. The ionosphere does not always behave ideally. It may give rise to dynamic, spatially inhomogeneous, and anisotropic conditions. Under this nonstationary
situation, the clutter will spread in the spatial and Doppler dimension. Consequently, the energy of spread-Doppler clutter (SDC)
obscures the weak target in the first-order Bragg clutter regions.
How to suppress the SDC has attracted significant attention. In
the following, we introduce two methodologies to deal with the
problem of clutter suppression.

IONOSPHERIC DECONTAMINATION METHOD
From the perspective of spectral sharpening, ionospheric decontamination methods were proposed. For minor ionospheric contamination, the two Bragg peaks of a broadened sea clutter spectrum will not overlap. The classic decontamination methods such
as phase gradient algorithm [18], maximum entropy method [19],
and eigendecomposition method [20] can be used to correct the
ionospheric contamination. However, in the case of spectral overlap between the positive and negative Bragg peaks because of
large phase contamination, the traditional decontamination method
will fail. In this situation, time-frequency analysis is the preferred
method. The typical methods include short-time Fourier transform (STFT) and Wigner-Ville distribution (WVD). The STFT
has no cross terms; however, its time-frequency concentration is
poor. The WVD has perfect time-frequency concentration, but it
36

has serious cross-term interference. Based on these two methods,
Stankovic proposed the S method, which has the advantage of both
[21]. However, as the length of window function becomes longer,
the S method will produce an unavoidable cross term. On the contrary, as the length of window function becomes shorter, the cross
terms will be avoided, but the frequency concentration is not as
good as the former one. Based on the S method, researchers from
HIT proposed the S2 method to correct ionospheric contamination
with large amplitude, and the optimal path detection method is
used to extract ionospheric instantaneous frequency [12].

CLUTTER SUPPRESSION ALGORITHM
At present, the main methods of bistatic sea clutter suppression are
the clean and the root iteration cancellation algorithms. However,
the performance of these two methods mainly depends on the estimation accuracy of the sinusoid model parameters. Different from
the present methods, researchers proposed a monostatic sea clutter
suppression method based on a forward-backward linear prediction (FBLP) algorithm for HF surface wave radar [22]. For bistatic
HF radar, first-order sea clutter can be expressed as a sum of many
sinusoidal signals; thus, the FBLP algorithm can also be used to
suppress the bistatic sea clutter [12].
In a paper [23], we proposed a spread clutter localized processing (SCLP) method to suppress SDC using training samples from
only one range cell. According to the experimental study of the
real data, we found that targets have stronger directionality than the
SDC. This property motivated us to use an orthogonal projection
operator to project the data onto the orthogonal complement space

IEEE A&E SYSTEMS MAGAZINE

MARCH 2018



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