IEEE Geoscience and Remote Sensing Magazine - September 2022 - 116

COMPACT POLARIMETRIC
SAR HAS BEEN WIDELY
USED FOR EARTH TARGET
DETECTION AS A MEANS TO
BALANCE SYSTEM
RESOURCES AND TARGET
INFORMATION.
the recognized " Rolls Royce " in polarization imaging [1],
and it can measure the polarization scattering matrix (HH,
HV, VH, and VV, where H and V represent horizontal and
vertical polarization, respectively), describing the observed
target by different transceiver
H/V combinations, and it organically
combines the advantages
of high-resolution
imaging technology and polarization
technology. On the
one hand, FP utilizes a large
transmitting signal bandwidth
and long synthetic aperture
to achieve high resolution
for distance and orientation,
forming a fine, high-resolution
SAR image; on the other
hand, the extraction and utilization of polarization information
allow the inversion of information that is difficult
to obtain by other techniques, such as target surface roughness,
symmetry, and orientation [2].
Compared with single-polarized SAR and traditional
dual- polarized (DP) SAR, FP SAR has the following outstanding
advantages: 1) it can obtain complete backscattering
information about the target and enrich the information
sources for target identification, 2) it can achieve more
robust detection and identification effects and improve the
detection of stealthy targets, and 3) it can obtain stronger
anticlutter and electronic antijamming capability and enhance
adaptability to complex electromagnetic environments.
However, FP requires alternately transmitting two
linearly polarized electromagnetic signals, horizontal (H)
and vertical (V), which puts forward higher requirements
on SAR system antenna technology, data downlink rates,
system power consumption, and other performance metrics,
and its pulse repetition frequency needs to be twice
that of the traditional DP SAR system, while the data coverage
area is theoretically only half that of DP, leading to
increased system design complexity, system maintenance,
and data acquisition costs, which limits the application of
FP SAR.
Considering the tradeoff between the mapping band
coverage, hardware requirements, and backscattering
TABLE 1. THE COMPARISON OF THE THREE CP MODES.
r/4
Transmit polarization
Transmitter polarization basis
Transmit ^ i| , h [27] [28]
Receive polarization
Receiver polarization basis
45c linear polarization
1
2
6@
4 , 0
` r j
Orthogonal linear polarizations
;
1
1
E
11 T
information of the observation scene, CP SAR offers a good
alternative for many polarimetric SAR applications [3]. In
theory, as a coherent DP system, CP SAR transmits the signal
with a special polarization (e.g., 45° linear polarization
and circular polarization) and receives echo waves with
two orthogonal polarizations [e.g., horizontal and vertical
polarizations as well as left-handed and right-handed
circular polarizations (LHCP and RHCP)]. Unlike conventional
DP schemes, it relies on transmitting H and V polarizations
simultaneously (see the " Transmitter Polarization
Basis " row in Table 1) rather than H or V polarization. With
this mode, CP SAR provides more polarization information
than a standard DP system.
Compared with traditional FP SAR, the system design
complexity, system maintenance, and data acquisition
cost of CP SAR are more advantageous, and the observation
coverage area is theoretically doubled with basically
no loss of target information, making it more advantageous
in wide-area target reconnaissance, thus ushering in rapid
development since the concept of CP was first proposed
at the beginning of this century. In practical applications,
there have been eight Earth observation satellites that offer
CP mode, especially the RADARSAT Constellation Mission
(RCM), launched in 2018, which provides a CP mode
with a mapping width of 350 km and a resolution of 50 m
and a low-noise mode with the same mapping width and a
resolution of 100 m. This spatial coverage is well suited for
target detection in large-scale ocean scenarios [4]. In studies,
when analyzed using appropriate methods, CP SAR
observations have been shown to be essentially identical
to those of FP SAR and almost always superior to those of
single-polarized SAR and DP SAR [1], [18]. A comparison of
FP, DP, and CP is presented in Table 2.
At present, CP SAR is widely used for target detection
on Earth's surface, especially in marine scenes, due to its
high score in terms of obtaining polarization scattering information
and measurement range. As a new type of radar
technology, CP SAR is different from both single-polarized
SAR and conventional DP SAR as well as FP SAR, and it
has its own unique basic principles and target detection
principles, based on which researchers have developed CP
target detection methods applicable to different application
scenarios. However, there has been no complete overview
CTLR
RHCP
DCP
1
2
6@- T
1
,
j
`8-- j
22 , 4
rr r
B
Orthogonal linear polarizations
;
1
1
E
RHCP
1
2
-
Circular polarizations
<
1 1 j
2 j 1
116
F
6@1 T
j
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE SEPTEMBER 2022
IEEE Geoscience and Remote Sensing Magazine - September 2022

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