IEEE Geoscience and Remote Sensing Magazine - March 2013 - 35

recently also in a spaceborne experiment using the dual
receive antenna mode of TerraSAR-X [196].

shorter but higher SAR antenna. Such a more compact shape
may have constructive advantages, avoiding, e.g., a complicated folding for the satellite launch. Note that this mode
makes also effective use of the hardware already available
for digital beamforming in elevation and avoids its duplication to implement multiple azimuth channels. The required
ultra-wide swath illumination can either be accomplished
by a separate small Tx antenna, or by using a combined Tx/
Rx antenna together with (a)
phase tapering, (b) spectral
Tx diversity or (c) an illumiUnfoldable reflector
nation with a sequence of
antennas will provide
sub-pulses [178].
An interesting alternalarge apertures and
tive to a planar antenna is
high sensitivity for
a reflector that is fed by a
spaceborne SAR systems.
multichannel array as illustrated on the lower left of
Fig. 27. A parabolic reflector focuses an arriving plane wave on one or a small subset of feed elements. As the swath echoes arrive as plane
waves from increasing look angles, one needs only to
read out one feed element after the other to steer a high
gain beam in concert with the arriving echoes. This technique was originally suggested in [170] and then reinvented independently by DLR and NASA/JPL during a

B. Advanced Concepts for
Ultra-Wide Swath Imaging
The HRWS system from the previous section requires
a very long antenna to map an ultra-wide swath with
high azimuth resolution. As a rule of thumb, a length of
10 m is required for every 100 km swath extension. To
avoid an undue increase of the antenna length, several
new instrument architectures and modes have been suggested in [179]. One example is the combination of the
displaced phase center technique described in the previous section with a ScanSAR or TOPS mode (cf. Fig. 27, top
left). As in classical ScanSAR, azimuth bursts are used to
map several swaths. The associated resolution loss from
sharing the synthetic aperture among different swaths is
compensated by illuminating a wider Doppler spectrum
and collecting the radar echoes with multiple displaced
azimuth apertures. Such a system is currently considered
by ESA as a possible successor to Sentinel-1 [188]. The
goal is to map a 400 km wide swath with 5 m resolution.
The peculiarities of the multichannel ScanSAR processing and their impact on system performance have been
analyzed in [186]. A possible drawback of this multichannel ScanSAR approach is the rather high Doppler centroid
with which some targets will be mapped
if a high resolution is desired. The situaScanSAR with Multiple
Single-Channel SAR with
tion becomes even worse in a multichanAzimuth Channels
Multiple Elevation Beams
nel TOPS mode. High squint angles may
moreover challenge co-registration for SAR
Multiple
interferometry [197].
Bursts
Rx-Beams
(4
Azimuth
Besides multichannel ScanSAR, several
in Elevation
Channels)
alternative concepts have been suggested in
[179]. A common feature of these alternatives
is that they record not only one but several
radar echoes arriving simultaneously from
different directions. For this, multiple narrow
elevation beams are formed where each beam
follows the echo of a different pulse transmitted by a wide beam illuminator. This enables
Blind Ranges for Given PRF
an increase of the coverage area without the
Digital Beamforming with
Multiple Beam SAR
necessity to either lengthen the antenna
Reflector Antenna
with Variable PRI
or to employ burst modes. The top right of
Fig. 27 provides an illustration, where three
Blind Ranges
Instantaneous
Move Across
narrow Rx beams follow the echoes from
Rx Beam
Swath
(2 Feed Elements)
three simultaneously mapped image swaths
PRF 2
that are illuminated by a broad Tx beam. A
sufficiently high antenna is needed to separate the echoes from the different swaths by
PRF 1
digital beamforming on receive. An alternaA
A
tive is range variant null steering as already
D
D
suggested for the quad-element array SAR in
Digital
[171]. Since the azimuth resolution is, as in a
Beamforming
classical stripmap SAR, given by half of the
antenna length, this will typically lead to a FIGURE 27. Advanced instrument architectures for ultra-wide swath SAR imaging.
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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2013