IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 9, NO. 1, JANUARY 201233
First Bistatic Spaceborne SAR Experiments
Marc Rodriguez-Cassola, Pau Prats, Member, IEEE, Daniel Schulze, Nuria Tous-Ramon, Ulrich Steinbrecher,
Luca Marotti, Matteo Nannini, Marwan Younis, Paco López-Dekker, Member, IEEE, Manfred Zink,
Andreas Reigber, Senior Member, IEEE, Gerhard Krieger, Senior Member, IEEE, and Alberto Moreira, Fellow, IEEE
Abstract—TanDEM-X (TerraSAR-X Add-on for Digital Eleva-
tion Measurements) is a high-resolution interferometric mission
with the main goal of providing a global and unprecedentedly
accurate digital elevation model of the Earth surface by means of
single-pass X-band synthetic aperture radar (SAR) interferome-
try. Despite its usual quasi-monostatic configuration, TanDEM-X
is the first genuinely bistatic SAR system in space. During its
monostatic commissioning phase, the system has been mainly
operated in pursuit monostatic mode. However, some pioneering
bistatic SAR experiments with both satellites commanded in non-
nominal modes have been conducted with the main purpose of
validating the performance of both space and ground segments in
very demanding scenarios. In particular, this letter reports about
the first bistatic acquisition and the first single-pass interferomet-
ric (mono-/bistatic) acquisition with TanDEM-X, addressing their
innovative aspects and focusing on the analysis of the experimen-
tal results. Even in the absence of essential synchronization and
calibration information, bistatic images and interferograms with
similar quality to pursuit monostatic have been obtained.
Index Terms—Bistatic radar, bistatic SAR processing, space-
borne SAR missions, synthetic aperture radar (SAR), time and
tion of a global digital elevation model (DEM) following the
High-Resolution Terrain Information-3 (HRTI-3) standard .
TanDEM-X extends the TerraSAR-X mission by adding a sec-
forming a single-pass interferometric system, and is therefore
capable of providing very accurate 3-D information. During the
first years of the TerraSAR-X mission, the German Aerospace
perform some innovative SAR experiments in nonnominal con-
HE FIRST bistatic synthetic aperture radar (SAR) system
in space is TanDEM-X,1whose main goal is the genera-
Manuscript received April 6, 2011; accepted May 12, 2011. Date of publica-
tion August 1, 2011; date of current version December 23, 2011.
The authors are with the Microwaves and Radar Institute, German Aerospace
Center (DLR), 82234 Oberpfaffenhofen, Germany (e-mail: Marc.Rodriguez@
dlr.de; firstname.lastname@example.org; Daniel.Schulze@dlr.de; email@example.com;
firstname.lastname@example.org; email@example.com; firstname.lastname@example.org).
Color versions of one or more of the figures in this paper are available online
Digital Object Identifier 10.1109/LGRS.2011.2158984
1TanDEM-X, acronym for TerraSAR-X Add-on for Digital Elevation
Measurements, is the name of the mission; in addition, it is also the name
with which the second satellite is commonly referred to at DLR. To avoid
ambiguities, we will refer to this second satellite as TDX; analogously, the
TerraSAR-X satellite is referred to as TSX.
figurations, i.e., spaceborne–airborne bistatic imaging  or the
demonstration of new imaging modes . The uniqueness and
the increased operational possibilities of the new mission allow
one to envisage a large amount of challenging and creative
experiments to take advantage of the potential of the payloads.
Aside from the obvious goal of testing the performance of
the system in nonnominal bistatic configurations, a further
objective should be outlined: develop and test new techniques,
modes, or algorithms which might become relevant in future
This letter addresses the first bistatic experiments performed
with TanDEM-X during its monostatic commissioning phase:
1) the first bistatic acquisition, complemented with a repeat-
pass interferometric processing of consecutive bistatic surveys
and 2) the first single-pass bistatic interferometric acquisition.
II. TANDEM-X IN THE PURSUIT MONOSTATIC
For approximately one month, TDX followed a specific test
schedule while approaching TSX from the original 16000 km
to the final flight formation with 20-km separation planned for
the monostatic commissioning phase. Both satellites operated
independently in pursuit monostatic mode for the following
months, TDX completing a monostatic test program to vali-
date its expected performance. The monostatic commissioning
phase ended in early October 2010, after which TDX ap-
proached TSX to a distance of some hundred meters to achieve
the so-called close flight formation. In this configuration, and
from a purely geometrical point of view, TanDEM-X can be
well considered as monostatic.
Despite the tight operational schedule, some room was re-
served for innovative experiments during the first months of the
mission. As an example, the first interferometric experiment
was carried out on July 16, while TDX and TSX were still
separated by 370 km . Some two weeks later, the first
bistatic acquisition of TDX was conducted. The monostatic
commissioning phase was very appealing to perform nonnomi-
nal bistatic SAR experiments because of the longer along-track
baselines. It has been under these circumstances that the first
bistatic experiments with TanDEM-X have been conducted.
Fig. 1 shows the reference configuration formed by the TSX
and TDX satellites at the period.
III. EXPERIMENTAL RESULTS
As previously stated, two sets of innovative bistatic ex-
periments have been carried out during the monostatic
1545-598X/$26.00 © 2011 IEEE
34 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 9, NO. 1, JANUARY 2012
monostatic commissioning phase.
Formation of the TSX and TDX satellites during the TanDEM-X
commissioning phase. In both cases, the geometrical config-
uration coincided with the one shown in Fig. 1: The beams
used in pursuit monostatic operation are represented by solid
lines; the dashed ones correspond to a bistatic operation with
symmetric azimuth steering. Table I lists the main parame-
ters of the acquisitions. The column “Experiment 1” refers to
the first bistatic imaging acquisition, with which the repeat-
pass bistatic interferometric results were produced; the column
“Experiment 2” refers to the first single-pass bistatic interfer-
ometric acquisition. All data takes were acquired using the
regular strip-map modes of the satellites. The data have been
processed using the experimental TanDEM-X interferometric
processor (TAXI), which is a flexible and versatile processing
suite particularly developed for the evaluation of TanDEM-X
experimental data products , .
A. Experiment 1: Bistatic Imaging and
The acquisition, carried out for the first time on August 8,
2010, was planned over Brasilia city, Brazil . For this first
bistatic experiment, TSX operated monostatically with a squint
of −0.8◦, whereas TDX was set in receive-only mode with
a squint of 0.8◦. Due to the small bistatic angle, no relevant
modifications of the timing schemes were required. Synchro-
nization pulses were exchanged during the data take using the
TanDEM-X direct link (SyncLink), from which the differential
and a nonsquinted bistatic one were obtained, but with no
spectral overlap between them. The same acquisitions were
conducted in consecutive passes of the system over the same
area to produce bistatic repeat-pass interferograms, i.e., after
The first bistatic image acquired by TanDEM-X is shown in
Fig. 2, where the famous airplane-like shape of the Brazilian
capital appears in the center of the image. The color coding
is used to discriminate homogeneous areas (bluish color in the
coordinates (horizontal bistatic range; vertical azimuth time). Bistatic range
increases from left to right. Color coding is used to distinguish (bluish color)
homogeneous areas from (yellowish color) structured areas.
First TanDEM-X bistatic image, showing the Brasilia city area in radar
image) from structured areas (yellowish color in the image).
For better comparison with the monostatic image, Fig. 3 shows
a zoom over the city with the monostatic and the bistatic images
overlaid and appearing in magenta and green, respectively. Two
different aspects of the previous images can be outlined. In
the city center, the dominant scattering mechanism seems to
be monostatic, but there exist distinct building areas near the
lake, where the bistatic scattering dominates. The conclusion
is that, even for the small bistatic angle of the experiment,
which is about 1.6◦, significant changes in target reflectivity,
particularly in man-made structures, can be expected between
monostatic and bistatic observations. This feature might be very
helpful to enhance the performance of existing identification or
classification algorithms. The second one is the distribution of
the azimuth ambiguities. Considering the azimuth ambiguities
of the point target of opportunity in the center of Fig. 3, which
are mapped on the lake and zoomed within the ochre rectangle
at the bottom right corner of the figure, a range difference in
the positions of the monostatic and bistatic ambiguities appears.
RODRIGUEZ-CASSOLA et al.: FIRST BISTATIC SPACEBORNE SAR EXPERIMENTS WITH TANDEM-X 35
Radar illumination from the left. Note the significant differences in the scattering of some buildings between the monostatic and bistatic images. Note also the
different range positions of the ambiguities of the images as a consequence of the different squint angles (see rectangle at the bottom right).
Zoom over Brasilia. (Green) TanDEM-X bistatic over (magenta) TerraSAR-X monostatic. Radar coordinates (horizontal range; vertical azimuth time).
Thishappens because themonostatic image issquinted whereas
the bistatic is not, and therefore, the 2-D monostatic impulse
response, unlike the monostatic, is skewed. This feature might
be exploited to develop ambiguity identification/suppression
Fig. 4 shows the repeat-pass interferogram generated with
two bistatic images acquired with a time lag of 11 days. Note
that the images are rotated 90◦with respect to Figs. 2 and 3.
Before interferometric combination, the bistatic data have been
calibrated in phase and time with the use of the SyncLink (cf.
Section III-B) information . The SRTM (Shuttle Radar To-
pography Mission) DEM has been used to remove topography
. The residual fringes correspond mainly to a priori DEM
errors and (possibly) marginally to unaccounted atmospheric
effects. The mean value of the coherence is 0.35; in urban
areas, this value increases to about 0.5. There are no significant
differences between the values obtained from the monostatic
repeat-pass and the bistatic interferograms. Concerning the
interferometric performance, the baselines are practically the
same, as is the SNR of both acquisitions. Note that the monos-
tatic image has a squint, but since no significant changes in tar-
get reflectivity other than those for certain man-made structures
have been observed, the results are definitely consistent. Aside
from its novelty, the relevant conclusion of this experiment was
that we could obtain with the new system bistatic images and
interferograms of similar quality to the monostatic (more ma-
ture) TerraSAR-X counterparts, a quite relevant information at
over Brasilia with a time lag of 11 days. Radar coordinates (vertical range;
horizontal azimuth time). Radar illumination from the top.
Bistatic repeat-pass interferogram generated with two bistatic images
B. Experiment 2: Bistatic Single-Pass Interferometry
Following the success of the bistatic imaging acquisitions
(cf. Section III-A), a natural step was to perform a single-pass
bistatic interferometric experiment before the end of the pursuit
monostatic commissioning phase, i.e., profiting of the 20-km
along-track baseline. However, a way to overcome the spectral
decorrelation of the previous bistatic configuration was needed.
Because of the small bistatic angle, simultaneous monostatic
and bistatic images with similar equivalent squint angles have
Doppler spectral overlap, which further suggests coherence
36 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 9, NO. 1, JANUARY 2012
acquisition. The figure shows the color-coded height with the bistatic reflectivity overlaid.
Geocoded (north points rightward) DEM of the area surrounding the Turrialba volcano, the first single-pass bistatic interferometric TanDEM-X
between the two images. This equivalence is shown in Fig. 1.
To achieve this, an imaginative command of the satellites
was designed, with a switch of the azimuth antenna patterns
of TSX and TDX on a pulse-to-pulse basis. Both satellites
transmitted one pulse using the nonsquinted beams (solid lines)
in Fig. 1 (any undesired energy from the other satellite was
highly attenuated due to the lack of overlap of the nonsquinted
footprints, separated by about 17 km); in the next pulse, TSX
transmitted with a squint of −0.9◦, and TDX only received with
a squint of +0.9◦(depicted with the dashed lines in Fig. 1). All
things considered, one pursuit monostatic interferogram with
full baseline, plus two symmetric bistatic interferograms with
half baseline, could be computed.2However, the acquisition
i.e., the swath was halved, and second, due to the specifics of
Difference between pursuit monostatic and single-pass bistatic DEMs. A multilook with an effective factor of 20 has been applied to the data.
2Note that the acquisition differed from a ping-pong one in which the two
monostatic images were not consecutive (one pulse delay) but simultaneous;
moreover, only a single bistatic image (instead of two) was acquired.
the command, no calibration nor synchronization pulses were
available. The acquisition was carried out over the Parque Na-
cional del Volcán Turrialba in Costa Rica, which is a gracefully
mountainous area. Note that this experiment was conducted
in early October 2010, which is about a week before the first
official bistatic TanDEM-X interferograms in close formation
were obtained, and is therefore the first bistatic single-pass
spaceborne SAR interferometric acquisitions.
TanDEM-X incorporates a direct X-band link (SyncLink) to
calibrate the time and phase references of the bistatic data, so
that no residual synchronization errors propagate in the final
DEM product . As a matter of fact, without proper clock
synchronization, no bistatic SAR interferometry is possible
. In the absence of SyncLink information, TAXI has an
automatic synchronization module which is capable of recover-
ing the synchronization error using the bistatic data , ,
which definitely substantiates the scientific character of the
experiment. The estimated clock carrier frequency difference of
about 124 Hz is also consistent with the available contemporary
RODRIGUEZ-CASSOLA et al.: FIRST BISTATIC SPACEBORNE SAR EXPERIMENTS WITH TANDEM-X 37 Download full-text
(Left) Pursuit monostatic and (right) single-pass bistatic interferograms. Note
the higher sensitivity to topography of the pursuit monostatic interferogram due
to the higher baseline.
Crop of the interferograms in the areas surrounding the volcano.
Fig. 5 shows the DEM generated using one of the bistatic
interferograms (with bistatic reflectivity overlaid). Although
the test of the performance of this automatic synchronization
procedure is out of the scope of this letter, we can show the
validity of the approach by cross-checking the results obtained
resulting from the conventional pursuit monostatic one. Fig. 6
shows the height difference between the two DEMs. A mask
has been used to avoid including values with low coherence.
No trends in range or azimuth can be identified, which quali-
tatively validates the automatic synchronization approach. The
standard deviation of the height error of the DEMs computed
using single-look interferograms is 23.3 m, which results in an
effective averaging factor of about 20 for the DEM error of the
Fig. 7 shows a crop of the (left) pursuit monostatic interfero-
area near the volcano. As expected, the pursuit monostatic in-
terferogram has twice as much height sensitivity as the bistatic.
The height of ambiguity of the pursuit monostatic acquisition is
about 85 m.Interms of coherence, we expect the mono-/bistatic
pairs to be less sensitive to volume decorrelation because of
the halved baseline. Nevertheless, the bistatic image is also
expected to have an SNR 1.2 dB worse for equivalent reflec-
tivity. This loss in SNR is attributed mainly to the electronic
antenna steering and consequently causes a proportional loss
of the overall coherence in the mono-/bistatic pairs. The mean
value of the coherence of the pursuit monostatic pair is 0.65;
for the single-pass bistatic pairs, this value drops to about
0.63. To illustrate the impact of volume decorrelation in the
pursuit monostatic and one single-pass bistatic acquisitions,
Fig. 8 shows the interferometric coherences (middle—pursuit
monostatic; right—single-pass bistatic) in a forest area north
from the volcano; the left crop corresponds to the bistatic
intensity image. Although not blatantly evident, the forest area
has a lower coherence in the pursuit monostatic pair due to
This letter has presented two innovative spaceborne SAR
experiments performed with TanDEM-X in the monostatic
intensity image of the area. The middle and right plots show the pursuit
monostatic and single-pass bistatic coherences, respectively. Note the decrease
in the coherence of the middle crop w.r.t. the right crop, caused by volumetric
Forest area north from the volcano. The left plot shows the bistatic
commissioning phase of the mission (in particular, the first
bistatic imaging acquisitions and the first single-pass bistatic
interferometric acquisition). Moreover, repeat-pass bistatic in-
terferometry with TanDEM-X has also been demonstrated.
Even in these experimental configurations, with partial lack
of synchronization and calibration information, bistatic inter-
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