Photonics-based real-time and high-resolution
ISAR imaging of non-cooperative target
Fangzheng Zhang (张方正), Qingshui Guo (郭清水), Ying Zhang (张 营), Yao Yao (姚 瑶),
Pei Zhou (周 沛), Daiyin Zhu (朱岱寅), and Shilong Pan (潘时龙)*
Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing University of
Aeronautics and Astronautics, Nanjing 210016, China
*Corresponding author: pans@ieee.org
Received July 6, 2017; accepted September 22, 2017; posted online October 19, 2017
Real-time and high-resolution imaging is demonstrated based on field trial detection of a non-cooperative target
using a photonics-based inverse synthetic aperture radar (ISAR). By photonic generation and de-chirping of
broadband linear frequency modulation signals, the radar can achieve a high range resolution thanks to the
large instantaneous bandwidth (8 GHz at the K band), as well as real-time ISAR imaging using low-speed
analog-to-digital conversion (25 MSa/s). A small-size unmanned aerial vehicle is employed as the non-
cooperative target, and ISAR imaging is realized with a resolution far better than those achieved by the previously
reported photonics-based ISARs. The capability for real-time ISAR imaging is also verified with an imaging frame
rate of 25 fps. These results validate that the photonics-based radar is feasible in practical real-time and high-
resolution ISAR imaging applications.
OCIS codes: 280.6730, 060.5625.
doi: 10.3788/COL201715.112801.
Inverse synthetic aperture radar (ISAR) imaging, which is
implemented based on an advanced signal processing algo-
rithm rather than large aperture antennas, is a promising
technique to identify moving targets
[1]
. In applications such
as pilotless automobiles, unmanned aerial vehicles (UAVs),
and quick security checks, real-time ISAR imaging with a
high-resolution is highly desired
[2–4]
. To achieve this goal,
the ISAR should first emit a microwave signal with a large
instantaneous bandwidth and then process the received
echoes rapidly in the receiver
[5]
. However, direct generation
of a linear frequency modulation (LFM) signal, a waveform
that is commonly used in high-resolution ISARs, by means
of a direct digital synthesizer (DDS) is limited to a few
gigahertz (GHz)
[6]
, so high imaging resolution is difficult
to be achieved using conventional electronic technolo-
gies. To break the instantaneous bandwidth limitation ,
one method is to apply a high carrier frequency in the
ISAR system. For example, a terahertz ISAR with a
carrier frequency of 321 GHz and an instantaneous band-
width of 10 GHz was reported, where a range resolution
of ∼1.5 cm was achieved
[7]
. However, the required multi-
ple stages of frequency conversion, filtering, and ampli-
fication greatly increased the system complexity and
severely deteriorated the signal quality and imaging per-
formance. In addition, the detection range is limited for
radars operated at the terahertz band because of small
emission power and large atmospheric absorption, espe-
cially in extreme weather conditions. On the other hand,
to achieve fast or even real-time ISAR imaging, de-chirping
was applied in the receiver
[8]
, but this technique still suffers
from bandwidth and performance limitations of electronic
signal processing, especially when a very large instantane-
ous bandwidth is adopted.
Microwave photonic technologies have been proposed for
generating and processing high-frequency and broadband
radio frequency (RF) signals
[9–12]
, which provide solutions
to overcome the bandwidth limitations in modern radars.
For example, photonic generation of LFM signals with a
bandwidth over 10 GHz can be easily implemented
[13,14]
,
and microwave photonic frequency converters can be oper-
ated with a frequency range of tens of GHz
[15,16]
. The great
potential of microwave photonic technologies in future ra-
dar applications has been proved in a photonics-based fully
digital coherent radar
[17]
. However, the signal bandwidth is
only tens of megahertz (MHz), and the signal processing in
the sampling receiver still restricts the operation band-
width. To take full advantage of microwave photonic tech-
nologies in broadband radars, we have proposed a
photonics-based ISAR, which has the ability for real-time
and ultra-high-resolution imaging
[18]
. In this system, the
broadband LFM signal is generated by photonic frequency
quadrupling, and the received radar echo is de-chirped to a
low-frequency signal based on photonic frequency mixing.
Thanks to the large operation bandwidth of the photonic
signal generation and de-chirp processing, a high range res-
olution can be achieved. Besides, sampling and processing
of the low-frequency signal after de-chirping makes it
feasible for the radar receivers to implement fast or even
real-time ISAR imaging, which is verified through a turn-
table imaging experiment. However, in realistic scenarios,
the targets to be detected are usually non-cooperative,
i.e. the distance, speed, and moving direction are
unknown. Thus, ISAR imaging of non-cooperative targets
is necessary
[19,20]
. Evaluation of the performance of non-
cooperative-target ISAR imaging is of great importance
for practical applications of the photonics-based ISAR.
COL 15(11), 112801(2017) CHINESE OPTICS LETTERS November 10, 2017
1671-7694/2017/112801(4) 112801-1 © 2017 Chinese Optics Letters