Aerospace and Electronic Systems Magazine January 2018 - 44

Focus-Before-Detection: Part II

Figure 13.

Real measured radar marine target in the sea surface. (a) Marine target at sea. (b) ISAR reconstruction marine target.

distributions, they will be focused in different parts of parameter
space via coherent integration. Therefore, as shown in the title of
[9], RFT as well as other FBD methods can act as a generalized
Doppler filter bank processing to suppress the clutter by discarding the Doppler channels containing strong clutter. To demonstrate
the background suppressing potential of FBD methods, marine or
sea search radars are taken, for example, in this article, which have
been extensively used for maritime traffic monitoring, coastal surveillance tasks, remote sensing, and low flying aircraft striking etc.
[29-37].
In marine environments, radar echoes generated by the sea
surface can be significantly stronger than a target, which causes a
serious challenge to detect a low-observable marine target in high
sea state. There are many efforts contributed in this field, such as
analyzing the suitability of different statistical models like Rayleigh, log-normal, Weibull, and compound-K distributions etc.
on amplitude or echo features like fractal dimension, multifractal
analysis etc. However, without a significant increase of signalto-clutter ratio (SCR), even the fractal dimension feature cannot obtain the ideal detection performance based on the optimal
constant false-alarm rate (CFAR) processing in high sea state.
Fortunately, in many scenarios there are observable motion differences between a marine target and the sea background. They
are usually caused by nonuniform motion, vibrations, or rotations
of structures or target body itself. For example, the modern ISAR
can make use of roll, yaw, and pitch of a marine target to obtain a
high-resolution image as Figure 13 with 600 MHz bandwidth as
long as the SCR after clutter suppressing is high enough. Figure
13 also tells us that the motion differences actually exist between
a marine target and sea background. Therefore, it is possible to
adopt the proposed FBD method or one of the modifications with
high-order motions to improve the target detection in strong sea
background. Based on long-time coherent integration similar
to FBD, Carretero-Moya et al. have proposed coherent Radon
transform [1], Chen and Guan et al. have proposed Radon-linear
canonical ambiguity function (RLCAF) [30], Radon-fractional
Fourier transform(RFRFT) [31], Radon-linear canonical transform (RLCT) [32]. Compared with the existing methods, it is
shown that the proposed FBD methods can improve the detec44

tion probability with better ability of sea clutter suppression. The
application of FBD methods in complicated background needs
much more attention in the future.

MICROMOTIONS
Many moving targets have some structures with micromotions
on themselves, like propellers of a fixed-wing aircraft, rotors of
a helicopter, swinging arms of a walking man, space target with
coning motion, or a vibrated engine compressor and rotated blade
assemblies of a jet aircraft [37, 38]. For coherent radar, these micromotions plus the target bulk translation will induce some additional frequency modulation on the returned echoes, which generates sidebands attached to the target's Doppler frequency shift.
This Doppler modulation caused by the micromotion is also called
micro-Doppler, which can serve as additional target features for
automatic target recognition (ATR) [37]. For example, the microDoppler effect can be used to identify specific types of vehicles,
and determine their movement and the speeds of their engines [37,
38]. Furthermore, one can distinguish whether it is a gas turbine
engine of a tank or a diesel engine of a bus, from micro-Doppler
modulations in the engine vibration signal. Most existing works in
this field are focused on target recognition or classification based
on extracted micro-Doppler feature in high SNR or SCR scenarios.
Furthermore, joint time-frequency analysis (JTFA) methods, e.g.,
short-time Fourier transform (STFT), Wigner-Ville distribution,
discrete chirp-Fourier transform (DCFT), FRFT, chirplet transform,
local polynomial Fourier transform, S-method, and local polynomial Wigner distribution, are studied as tools for micro-Doppler
analysis. Nevertheless, to obtain the frequency representation in a
time window of JTFA may be regarded as the coherent integration
in the subapertures. Therefore, the SNR gain via coherent integration is also limited for JTFA accordingly, due to the limited window
to dynamically reflect the frequency change. Obviously, there is a
large improvement possibility by introducing FBD into the micromotion analysis. Fortunately, many micromotions [37] such as vibration, tumbling, procession, and nutation of a coning target like
a missile warhead, can be modeled with finite motion parameters
as Figure 14(a)-Figure 14(d), respectively. Accordingly, the special

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JANUARY 2018

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