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Mapping without the sun
Zhang, Jixian

approach to the use of space-borne datasets in time-critical
The spacecraft TerraSAR-X delivers unique, novel quality
SAR satellite image data with a resolution of up to one metre,
which can significantly augment the capabilities of armed
forces, homeland security, and intelligence agencies, creating
both tactical and strategic advantages. Reduced image speckle
through multilooking and the presence of radar shadows of
man made objects are important features of the high
resolution TerraSAR-X images which will by far enhance the
image interpretability over that of medium resolution SAR
Besides of updated high resolution imagery of the Earth’s
surface, the availability of precise information on terrain
evaluation is strongly requested by users of geo-spatial data.
Currently most areas in the world are covered by an elevation
grid of poor quality in terms of point density and height
accuracy (ERS-1/2 Tandem, SIR-C/X-SAR, SRTM,
GTOPO30). In order to close this gap of information towards
the third dimension TanDEM-X (TerraSAR-X add-on for
Digital Elevation Measurements) is a German program for a
new generation SAR satellite operating at X-band in single
pass SAR interferometry. The single pass SAR
interferometric constellation, comparable to stereoscopic
survey missions, is realized by two independent X-band
satellites, TerraSAR-X together with TanDEM-X, flying in a
close formation, quite often less than 1 km. The mission goal
is to deliver a global digital elevation model characterized by
a height accuracy of better than 2 m. The additional spacecraft
TanDEM-X is already financed through a PPP agreement
between DLR and EADS Astrium and will be launched in
The increasing demand of sophisticated geospatial data sets
requires appropriate and precise planning documents in all the
three dimensions. However, a continuous and up-to-date
acquisition by optical sensors or airborne platforms is strongly
limited either by unfavourable weather conditions or
inefficient mission scenarios. Therefore, the easy access to the
high-resolution radar missions TerraSAR-X and TanDEM-X
gives the global remote sensing community the possibility to
participate in one of the most ambitious space programs.
TerraSAR-X will deliver high resolution SAR images with a
ground sampling distance up to lm, in combination with
unique satellite agility: The electronically steered antenna
allows acquiring physically separated areas of interest within
seconds without rolling the whole satellite. The satellite is
mainly described by: •
• TerraSAR-X is operated at X-band at 9.65 GHz.
Even pure visual interpretation will profit from the
fact that the short wavelength shows more spatial
detail than longer wavelengths.
• The chirp bandwidth, an indicator of the spatial
accuracy level, is with 300 MHz the best
performance currently in space.
• Single, dual and quadruple polarizations allow for
a more complex description of the areas and objects
on the ground. Their availability depends on the
imaging mode and always is a trade-off with the
spatial coverage and swath width.
• Radiometrically calibrated data allow for an easy
comparison between scenes.
• TerraSAR-X is both, daylight and weather
independent, i.e. the system can record data at
night and through clouds or dense smoke cover.
• The system features a quick site access time of 2.5
days to any point on Earth at 95% probability.
• TerraSAR-X features a unique agility: it is possible
to switch between its three different imaging modes
and various polarisations within only 1 to 3 seconds,
corresponding to surface acquisition offsets of only
7 to 20 kilometres.
• Time-critical data can be downloaded within
seconds after acquisition to mobile ground stations
on-scene, thus enabling near-real time data
• Anonymous and encrypted processes guarantee an
unobjectionable confidentiality.
• Furthermore, the satellite's very high resolution of
up to 1 m and its high radiometric accuracy, make
TerraSAR-X an ideal sensor to support sensitive
decision-making in time-critical situations.
• Satellite Tasking twice a day and priority settings
for the ordered data takes allow for quick
turnaround times in data delivery and late order
TanDEM-X will be an almost identically constructed system
with some slight modifications for the necessary
synchronization link between both satellites.
In the standard operating mode (so-called single receive
antenna) image data in three different imaging modes can be
acquired: SpotLight, StripMap and ScanSAR:
• SpotLight with up to 1 m resolution (10 km
(width) X 5 km (length) image size) - the most
sophisticated radar imagery available on the market:
In flight direction, the radar beam can be steered
like a veritable spotlight, illuminating a particular
ground scene for the longest time period possible,
thus achieving a 1 metre class resolution as well.
• StripMap with up to 3 m resolution (30 km X
50 km standard scene size, extendable up to
4,200 km acquisition length) - the ground swath is
illuminated with continuous sequence of pulses
while the antenna beam is fixed in elevation and
azimuth. 3
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