IEEE Geoscience and Remote Sensing Magazine - March 2013 - 36

helicopter [173] and later elaborated
in more detail for a 3-D imaging system on a UAV [177]. In a SAR system,
the possible benefits of using multiple
ii i
transmitters range from an increase of
t
ij j
the coverage area to the suppression of
range and azimuth ambiguities to the
provision of additional baselines for
interferometric or even tomographic
Narrow
Rx Beams
ik k
applications [178], [181], [189].
To separate the echoes from the
Point
different
transmitters, some publicat
i j
k
Scatterers
tions suggest to simultaneously transmit mutually orthogonal waveforms
FIGURE 28. MIMO-SAR based on two short-term shift-orthogonal transmit signals. The Tx
[190], [194]. While this condition
waveforms are designed such that their cross-correlation vanishes for small time offsets
allows perfect separation of the scat(as it applies to scatterers i and j). For larger time offsets, the otherwise correlated signal
tered waveforms from a single point
returns are separated via narrow Rx beams (as it applies to scatterers i and k). This exploitatarget, it is not sufficient to separate the
tion of the side-looking imaging geometry together with appropriately designed transmit
signals in case of an extended scatterwaveforms enables the simultaneous use of multiple Tx signals and the unambiguous
ing scenario [178], [198]. The reason is
separation of their radar echoes also in the realistic case of a distributed scatterer scenario.
that the orthogonality is not ensured
Note that both waveforms have the same spectral support.
for arbitrary shifts between the transmit signals. As a result, the energy
from spatially separated scatterers illuminated by the other
joint Tandem-L/DESDynI study [83]. JPL suggested an
waveform(s) will not vanish after range focusing but appear
analogue switching between the feed elements [185],
either smeared or at different positions. This reasoning is
while DLR argued in favor of connecting each feed eleevident by considering range focusing as an all-pass filter in
ment with its own A/D converter [179]. The former may
the frequency domain. Depending on the number of transallow a cheaper implementation, while the latter enables
mitters, the smeared energy from the orthogonally illua performance improvement and increased flexibility by
minated scatterer ensemble may even exceed the focused
combining multiple feed signals in a digital signal procestarget impulse response [195].
sor [184], [192]. The solution with the digital feed is also
As an alternative, it was suggested to employ transmit
in favor of more advanced modes like the multiple-beam
waveforms that are mutually orthogonal for arbitrary
technique introduced above. Compared to a direct radiatshifts. This implies, however, that the waveforms cover
ing array, the computational requirements for real-time
disjunct frequency bands which prevents their use for
beamsteering are significantly reduced since only few
many of the aforementioned applications which rely on
feed signals have to be combined at each instant of time.
a coherent combination of spectrally overlapping signals.
A drawback of the multi-beam mode are the blind
To avoid this deterioration and to allow nevertheless a
ranges that are due to the fact that the radar cannot transcoherent combination of multiple transmit signals within
mit and receive at the same time. This can be overcome in a
the same frequency band, it has been suggested in [178],
bistatic SAR where the transmitter is sufficiently separated
[198], [199] to employ especially designed waveforms
from the receiver [175]. To avoid a separate transmit satel(e.g., chirp-based OFDM signals) in combination with
lite, one can employ a variation of the PRF which shifts the
digital beamforming on receive. The basic idea behind
blind ranges across the swath (Fig. 27, lower right). The PRF
this approach can be expressed by a short-term shiftvariation could either be implemented in discrete steps
orthogonality condition where two different transmit
leading to a multiple-beam ScanSAR mode or a pulse-tosignals are required to be only orthogonal for a subset of
pulse variation of the PRI. The latter provides better permutual shifts which are typically provided by a symmetformance but requires a dedicated SAR processing which is
ric interval [-x s, x s]. The short-term shift-orthogonality
currently under development [193].
enables a perfect separation of the echoes from neighborC. Multiple-Input Multiple-Output (MIMO) SAR
ing scatterers as long as the relative radar signal delay is
A receiver with n subapertures allows a simultaneous samwithin the interval [-x s, x s]. The echoes from more distant
pling of the arriving wavefronts with n phase centers. The
scatterers can then be separated by digital beamforming
on receive as illustrated in Fig. 28. For this, the antenna
effective number of phase centers can be increased by
should be of sufficient height and it may again be advanusing additional transmitters [180]. Such an extension was
tageous to use a large reflector instead of a planar array
already suggested for resolution improvement in the conantenna to provide the required aperture.
text of a forward looking imaging radar experiment on a
i Received Signals

MIMO Antenna Wide Beam
Illumination
with Orthogonal
Tx Signals

ieee Geoscience and remote sensing magazine

march 2013

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2013