IEEE Geoscience and Remote Sensing Magazine - September 2021 - 9

SYNTHETIC APERTURE RADAR
SAR has been widely used for Earth remote sensing for
more than 40 years. SAR is unique in its imaging capability:
it provides high-resolution, 2D images regardless
of daylight, cloud coverage, and weather conditions. A
wealth of excellent literature exists on the topic, as in
[1]-[6]. Spaceborne SAR systems have a side-looking
imaging geometry and are based on a pulsed radar installed
on a platform with a forward movement. The
flight direction is denoted as the azimuth and the line
of sight as the slant range direction. The time associated
to the propagation of the electromagnetic waves at the
speed of light is commonly
referred to as fast time,
whereas the time related to
the-much slower-platform
movement is the slow time.
The radar system transmits
frequency-modulated
electromagnetic pulses (commonly
referred to as chirps)
with high power and receives
the echoes of the backscattered
signal in a sequential
way. The swath width of
spaceborne systems typically
varies from 30 to 400 km.
A transmitted pulse interacts
with the Earth surface,
and only a portion
of its energy is backscattered
to the receiving antenna. The
amplitude and phase of the
backscattered signal depend
on the physical and electrical
properties of the imaged
object. Commonly used frequency
bands in SAR systems
range from the P band
(250-500 MHz) to the X
band (8-12 GHz), but even
frequencies up to the Ka band
(26.5-40 GHz) are being considered
for future missions.
More penetration of the electromagnetic
pulses in media
will occur for radar systems
using longer wavelengths.
The simplest radar system
provides a 2D reflectivity map
of the imaged area; i.e., targets
with high backscattered
signals are identified as bright
spots in the SAR images, and
flat, smooth surfaces appear
as dark areas.
SEPTEMBER 2021 IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE
DIGITAL BEAMFORMING
FOR REFLECTOR-BASED SAR
In recent years, a trend toward SAR systems with multiple
digital channels has emerged, a paradigm shift that resulted
from research activity
and following similar trends
in communication. The main
innovative characteristic of
forthcoming generations of
SAR instruments is the use
of multiple elevation and/
or azimuth receiver channels
equipped with DBF capabilities
[7]-[12]. Even multiple-input,
multiple-output
(MIMO) SAR becomes possible
[13]-[16] by utilizing
manifold transmit channels. The virtue of DBF SAR is that
it enables the conception of new systems that extend the
dimensionality of the trade space [13], [18], enabling a
substantial performance improvement [19]. DBF SAR overcomes
the fundamental limitations [17] of single-channel
SAR and allows for a high-resolution wide-swath imaging.
Large unfurlable reflector antennas are not new in
spaceborne applications, as they have been used since the
early 1960s [20]. To accommodate reflectors with diameters
larger than that of the launcher shroud, reflectors
with a rigid central part and a deployable mesh rim are
typically used. The virtue of unfurlable reflectors, compared
to planar, direct-radiating, phased-array antennas
of comparable size, is their low mass and compact storage
volume during launch [21]. For SAR, reflector antennas
have been shown to be especially advantageous
when combined with a digital feed array [22]-[24]. The
feed consists of multiple antenna elements arranged as
an NN
az # el array in the azimuth and elevation, respectively.
Figure 1 shows one feed column of Nel
elevation
channels, where each feed element is followed by a radiofrequency
(RF) front end, which may be implemented as
a simplified transmit/receive module (TRM). This is followed
by a digitizer and a digital processing unit. Such a
system facilitates the use of DBF techniques by utilizing
the data samples of the SAR echo signal received by each
antenna element of the array.
DBF for SAR can be realized with parabolic reflector antenna
systems combined with digital feed arrays and planar
antenna systems. The configuration in Figure 1 is representative
for either antenna type; however, the technique used
for the beam pointing is rather different. As will be detailed
later, the beam position of a reflector is varied by changing
(electronically) the effective position of the active feed element.
In planar antennas, the beam pointing is determined
by the slope of a linear phase excitation applied across all
antenna elements.
The RF hardware complexity of DBF systems is reduced
by moving the digital processing closer to the antenna
9
SAR IS UNIQUE IN ITS
IMAGING CAPABILITY: IT
PROVIDES HIGHRESOLUTION,
2D IMAGES
REGARDLESS OF DAYLIGHT,
CLOUD COVERAGE, AND
WEATHER CONDITIONS.
COURTESY OF DLR-MAX
IEEE Geoscience and Remote Sensing Magazine - September 2021

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