IEEE Geoscience and Remote Sensing Magazine - March 2022 - 166

residual RCM causes significant defocusing of the imaging
result shown in Figure 7, whose simulation parameters are
listed in Table 1.
Aiming to address the problem of the errors in the go-
LEO-SAR CAN
SIGNIFICANTLY IMPROVE
ITS FLEXIBILITY WHEN
WORKING IN THE SQUINTED
CONFIGURATION.
stop-go approach, [53] proposes a new echo model for
the wideband signal that
considers the motion of the
radar in the process of the
signal transmit/receive and
propagation. In addition, two
threshold expressions for
which the traditional go-
stop-go model is still valid
in low-resolution LEO-SAR
imaging are also presented
in [53]. In the low-resolution
case, the two threshold expressions are held, and the
traditional go-stop-go echo method is still applicable.
Therefore, the imaging algorithms based on the traditional
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50
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Azimuth Sample
FIGURE 7. Defocusing in the simulation result of the target.
TABLE 1. THE SIMULATION PARAMETERS.
Carrier frequency
Bandwidth
Squint angle
Reference range
Velocity
model, summarized in " The Broadside Configuration " and
" The Squinted Configuration " sections, are still capable of
processing LEO-SAR data.
EFFECTIVE VELOCITY
Since the orbit of the satellite and surface of Earth are actually
curved, the velocity in spaceborne SAR imaging is no
longer the real physical velocity but the selected virtual velocity,
also known as the effective velocity, which varies along
range. The calculation of its accuracy directly affects the imaging
quality of spaceborne SAR [54]-[56].
The formula of the effective velocity is derived in [17],
and it is approximately equal to the geometric average
of the satellite and satellite beam footprint velocities. In
addition, the effective velocity can be solved by using
second-order Doppler parameters [57], [58]. At present,
the method based on the orbit state vector is often used
to accurately calculate the effective velocity [59]. Due to
the high precision of the GPS of the satellite, it can provide
precise orbit parameters for the method of [59], and,
thus, the effective velocity can be accurately calculated.
THE BROADSIDE CONFIGURATION
LEO-SAR can work in a variety of imaging modes, such as
stripmap, spotlight, sliding spotlight, and TOPS, among
others. After considering the error of the go-stop-go echo
model and calculation of the effective velocity, some traditional
imaging algorithms can be used to process the LEOSAR
data under the broadside configuration.
5.4 GHz
50 MHz
30°
838 km
7,148 m/s
IMAGING ALGORITHMS FOR THE STRIPMAP MODE
The stripmap mode is the most commonly used imaging
mode in LEO-SAR. Some classical algorithms that are
based on the azimuth translational invariance, such as the
range migration algorithm (RMA) [60], range Doppler algorithm
(RDA) [61]-[63], and chirp scaling algorithm (CSA)
[24], are often used to process the stripmap data. Their principles
are introduced in detail in this section. In addition,
the differences among them are summarized and analyzed
in Table 2.
The Stolt interpolation is used in the RMA to correct the
TABLE 2. THE DIFFERENCES AMONG THE ALGORITHMS OF
THE STRIPMAP MODE.
RMA 1) Since there is no approximation in the formula-derivation
process of the RMA, the formulas in the RMA are more
precise than those in the CSA and RDA.
RDA 2) The amount of Stolt interpolation in the RMA is quite large.
Therefore, compared with the RMA, the CSA and RDA
have the advantages of small calculation amount and fast
processing speed.
CSA
166
3) The space variance of the RCM is considered in the CSA.
2D coupling, and then the scene image is reconstructed
in the wavenumber domain. Since the precise expression
without approximation is used in the formula-derivation
process, the RMA can achieve focusing without geometrical
deformation for the entire scene without adding other
approximate conditions. Even in highly squinted LEO-SAR
imaging, the algorithm is still applicable. However, the
amount of Stolt interpolation in the RMA is quite large,
and, if the interpolation is not accurate, it will have a significant
impact on the imaging quality. This is why other
imaging algorithms have to be studied after this theoretically
optimal imaging algorithm is available.
At the beginning, the RDA was mainly an efficient processing
algorithm proposed for processing Seasat data. The
RCMC performed in the range Doppler domain is the most
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