IEEE Geoscience and Remote Sensing Magazine - March 2020 - 46

mode is related to the carrier wavelength and the length of the
rotating arm. In the case of fixed radar parameters, the longer
the length of the rotating arm, the higher the resolution.
SIGNAL SYSTEM OF GB-DInSAR
The commonly used transmit signals of the GB-DInSAR
system include SFCW and FMCW. The SFCW echo signal is
the range frequency-domain sampling of the target, and its
pulse-compression process is implemented by the inverse
Fourier transform (FT) in the range frequency domain [33].
The FMCW echo signal is the range time-domain sampling
of the target, and the pulse-compression process is implemented by dechirp processing and FT.

SFCW SIGNAL
The SFCW signal synthesizes the bandwidth by combining
a series of continuous narrowband signals to achieve range
resolution. Combined with azimuth sampling, 2D resolution can be achieved and applied to SAR imaging [34]. Suppose that the SFCW has N frequency sampling points with
a step size of Tf [35]. The ith narrowband signal with step
frequency fi has a duration of T, an amplitude of A i, and a
phase of i i . The signal expression of SFCW is expressed by
S(x) =

/ S i(x) = A i $ rect: (x -T iT ) D $ exp" j $ (2rfi x + i i) ,,

N -1
i =0

(4)
where x is the range time

δarc

fi = f0 + iTf, (5)

i i = rK (x - iT )2, (6)

where f0 is the carrier frequency of the transmitted signal.
The time-frequency relation of the SFCW signal is displayed
in Figure 4 [36].
R (t )

δr

FIGURE 3. A schematic diagram of the arc scanning mode.

FMCW SYSTEM
Because the instantaneous frequency of the FMCW signal
is a linear function of time, the uniform signal bandwidth
can be obtained for the GB-DInSAR system. The expression
of the FMCW signal in the time domain is

f
fN-1
f4

∆f

f3
f2
f1

FIGURE 4. The time-frequency relation of an SFCW signal.

f
Bw

x
S(x) = rect ` T j $ exp" jrK r x 2 ,, (7)

where K r is the linear frequency modulation rate and T is
the duration of the FMCW signal. Because of the frequency's linear modulation, the phase is a quadratic function of
time. The relation between the frequency and time of the
FMCW signal is shown in Figure 5.
The hardware implementation of the SFCW system is
slightly less difficult; however, the signal acquisition time of
the SFCW system is much longer than that of the FMCW system due to the step-by-step transmission of each frequency
signal. In practice, the synthetic aperture time usually reaches
tens of minutes. The synthetic aperture time of FMCW can be
less than 1 min. FMCW signals have a longer time width, and
signal transmission and reception are in the course of platform
motion, so, theoretically, there is an intrapulse delay. However,
because the detection distance of ground-based radar is only
several kilometers compared with that of spaceborne SAR, the
impact of platform motion on echoes can be neglected. Therefore, FMCW system is the future developmental trend.
GEOMETRIC AND SIGNAL MODELS

...

FIGURE 5. The time-frequency relation of an FMCW signal.

GEOMETRIC MODEL
Figure 6 shows the geometric model of the linear scanning
mode. Its radar antenna moves uniformly along a straight
track [37]. Because the synthetic aperture of the linear scanning mode is generated by the movement of radar on the
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

MARCH 2020

IEEE Geoscience and Remote Sensing Magazine - March 2020

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