Resolution enhancement for Doppler beam
sharpening imaging
ISSN 1751-8784
Received on 28th February 2014
Accepted on 21st November 2014
doi: 10.1049/iet-rsn.2014.0384
www.ietdl.org
Hongmeng Chen
1
✉
, Ming Li
1
, Peng Zhang
1
, Gaofeng Liu
1
, Lu Jia
1
,YanWu
2
1
National Laboratory of Radar Signal Processing, Xidian University, Xi’an Shaanxi 710071, People’s Republic of China
2
School of Electronics Engineering, Xidian University, Xi’an 710071, People’s Republic of China
✉ E-mail: chenhongmeng123@163.com
Abstract: The efficient scan moving target indication (MTI) mode is implemented in the wide-area ground MTI systems for
the purpose of wide-area surveillance. However, because of the scanning movement of the radar antenna, it is difficult to
acquire enough coherent pulses in a single azimuth direction in the Doppler beam sharpening (DBS), so the sharpening
ratio is greatly limited. To mitigate this problem, a combined super-r esolution algorithm with aperture extrapolation for
DBS imaging is proposed to enhance the sharpening ratio in this study. I n this algorithm, aperture extrapolation
technique is utilised to increase the data length in azimuth direction and the amplitude and phase estimation of a
sinusoid, which can acquire accurate spectral estimation with much lower side lobes and narrower spectral peaks, is
applied to replace the fast Fourier transform to perform the Doppler analysis. In this way, the sharpening ratio in the
new algorithm could be enhanced by one time. E xperimental results on simulation data and real data verify the
effectiveness of the new super-resoluti on algorithm, and demonstrate that the proposed algorithm can provide sharp
and clear scene information with lower side lobes and narrower peaks.
1 Introduction
Nowadays, wide-area ground moving target indication (GMTI)
systems have become a very important tool for battlefield
surveillance or traffic monitoring, and several airborne radar
systems have already been developed for this ability. For airborne
radar systems, the wide-area surveillance ability is realised through
antenna scanning in a periodic manner. In order to estimate
accurately the azimuth positions of the objects, the azimuth beam
width of the radar antenna has to be narrow (see Fig. 1a). Doppler
beam sharpening (DBS) technique [1, 2], which is an unfocused
synthetic aperture technology, is usually utilised to observe a large
earth surface. DBS technique only uses the linear component of
the Doppler history of echo and has the following advantages: (i)
a wide-area surveillance ability of ground scene, (ii) a high revisit
rate (i.e. the inverse of the duration between two detections of the
same object) and (iii) a continuous tracking ability of interest
target. Therefore it has been widely applied in many cases. For
example, the joint surveillance target attack radar system [3]is
successfully used for the battlefield reconnaissance during the Gulf
War, the phased array multifunctional imaging radar [4, 5]is
performed for the wide-area traffic monitoring. Therefore it is of
great meaning to study the DBS imaging further.
DBS imaging generally includes four important procedures: pulse
compression for high range resolution, Doppler centroid estimation,
coherent accumulation in cross-range and sub-image stitching. High
range resolution of a DBS image [1] is directly related to the
bandwidth of transmitted radar signal. Data-based estimation
algorithm to obtain the Doppler centroid is discussed in [6, 7] and
a minimum-entropy Doppler estimator algorithm is proposed in [8]
to acquire high estimation accuracy in the case of high-scene
contrast. A fast Fourier transform (FFT) processing method with a
constant pulse repetition frequency (PRF) is used to keep the
sharpening ratio constant and to form the DBS image [9]. In [10],
RELAX algorithm is discussed for DBS super-resolution imaging.
Since the non-ideal motion of the radar platform, the DBS
stitching method has an important effect on the DBS image
quality. A stitching algorithm based on internal navigational
system (INS) information and a correction method based on affine
transformation are introduced, respectively, in order to improve the
stitching quality of DBS image [11, 12]. Here, we mainly consider
the third procedure, since it is the most important factor that
affects the sharpening ratio of DBS image. The sharpening ratio is
directly related to the coherent processing interval (CPI). Then, the
observing interval must be long enough so that a considerably
high sharpening ratio can be obtained by Doppler analysis
implemented by FFT [1]. Since the Fourier spectrum is the
convolution result of real signal spectrum with windowed function,
the Fourier spectral resolution will be constricted when the data
length is finite, which greatly limits the sharpening ratio. Here, we
call this method as conventional method. Before FFT processing,
the data in cross-range direction are always to be windowed to
reduce side lobe levels.
High sharpening ratio of DBS imaging is achievable when enough
pulses are measured. For an airborne radar system, the wide-area
surveillance ability is achieved via steering the antenna from one
azimuth angle to another periodically, as is shown in Fig. 1a.On
the one hand, in order to guarantee a high revisit rate, the
observation time of each beam at different azimuth angles is quite
finite, that is to say the CPI cannot be very long and the collected
pulses are very few. On the other hand, a long CPI data collection
(i.e. a large coherent pulse number) can ensure a high sharpening
ratio, since the sharpening ratio is inversely proportional to the
synthesised aperture length. However, by the traditional DBS
imaging method, for example FFT-based algorithm, high
sharpening ratio is unachievable with limited number of pulses,
which motivates approaches to enhance the sharpening ratio.
The main contribution of this paper is the presentation of a
super-resolution DBS imaging scheme with just a few measured
pulses. To overcome the limitation on sharpening ratio of
conventional method based on FFT, we present a combined
super-resolution algorithm with aperture extrapolation (AE-CSR),
which includes two key procedures. First, an autoregressive (AR)
model is established in the ‘angular’ domain, and the aperture
extrapolation (AE) [13–16] technique is used to extrapolate some
parts of the spectrum outside the bandpass of system by using the
measured limited pulses. Second, we utilise the spectral estimation
method rather than FFT to perform the Doppler analysis.
AE technique is utilised to extrapolate the scattering field to obtain
an enhanced image, which has been successfully applied to the ISAR
IET Radar, Sonar & Navigation
Research Article
IET Radar Sonar Navig., 2015, Vol. 9, Iss. 7, pp. 843–851
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