IEEE Geoscience and Remote Sensing Magazine - December 2022 - 11

high-gain receive beam (antenna radiation pattern) that
follows the reflected radar echo traversing the ground. In
its basic implementation, SCORE improves the signal-tonoise
ratio (SNR) and suppresses the range ambiguities, but
it does not allow for a substantial increase of the imaged
swath width beyond a few tens of kilometers. The latter deficiency
is significant, as one of the crucial parameters of
spaceborne SAR systems monitoring dynamic processes is
the width of the imaged swath. A wide swath enables reducing
the revisit time, shortening the time between successive
observations of any ground region, and, by this, facilitates
the monitoring of dynamic processes. Current in-orbit operational
SAR system image swaths in the order of 20 to
80 km in stripmap imaging mode. Although the imaged
swath width of conventional single-channel SAR may be
increased by utilizing burst operation modes, such as ScanSAR
and TOPS [2], [3], this increase is paid for by a degraded
azimuth resolution.
In multibeam SCORE DBF is used to image multiple
swathes simultaneously allowing for high-resolution and
wide-swath SAR [4], [5], [6]. The DBF technique detailed
in this article can overcome the preceding dilemma and
may be understood as an intrinsic stripmap-based imaging
mode allowing for an ultrawide swath, with no difference
for the SAR image processing. It is an extension to the basic
SCORE technique suggested in [7], [8], and [9]. Multibeam
SCORE is the reference operation mode of the NASA/ISRO
SAR (NISAR) mission [10], [11], [12] and is also implemented
by JAXA for the Advanced Land Observing Satellite-4
mission [13] as well as by the ESA for the Radar Observing
System for Europ (ROSE-L) mission [14]. Further, multibeam
SCORE operation is inherent to the staggered
SAR mode (see [15] and [16] and part 3 of the tutorial
article), which has been considered for other missions
(and mission proposals), such as NISAR and Tandem-L
[11], [17], [18], [19].
To explain the innovative SAR technique required for
ultrawide-swath imaging, a reference SAR system capable
of imaging a swath of 355 km (ground range) is considered.
The parameter values of this system are developed in
a step-by-step procedure throughout the article, explaining
the beamforming approach and relating it to the
instrument operation.
The overall structure of the article is as follows. In the
following section, the impact of imaging a wide swath
on the radar echo timing is analyzed. Next, an operation
mode, known as multibeam SCORE, capable of imaging
an ultrawide swath is detailed. The reflector-based beamforming
underlying the imaging mode is elaborated in
the " Beamforming Weights, Reflector Illumination, and
Secondary Beams " section, giving examples for the excitation,
resulting reflector illumination (the primary beam),
and consequent secondary radiation pattern. Multibeam
SCORE introduces gaps in the imaged swath, caused by
the transmit pulse instances of the instrument; this issue
is analyzed in detail in the " Blind Ranges " section, where
the expressions governing the extent of the gaps are derived.
Last but not least, the effort for data stream handling
and memory management is discussed and related
to basic SAR performance parameters, such as resolution
and oversampling.
AN ULTRAWIDE-SWATH IMAGING SYSTEM
In the following, the consequences of extending the swath
width on the timing are discussed; it is shown that the
single-beam SCORE technique is not suitable for ultrawide-swath
imaging and therefore needs to be enhanced
utilizing DBF. The starting point is the timing analysis
shown in Figure 1, with three subswaths (orange bars)
imaged at a pulse-repetition frequency (PRF) of 1,770 Hz.
The timing (or diamond) diagram shows the transmit
pulse instances (blue strips) and nadir returns (green
strips) versus a range of PRF values (abscissa). These are
related to the geometrical swath parameters, here, the incidence
and off-nadir look angle (ordinate). It is used to
visualize possible positions for the echo window and allows
the system engineer to decide on the number, placement,
and extent of the imaged subswaths. The PRF has
been chosen higher than the minimum value determined
by [1, eq. (1)].
DECEMBER 2022 IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE
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IEEE Geoscience and Remote Sensing Magazine - December 2022

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