ul 2004
nerated
pach (s.
coastal
location
for the
> delay
+10 m),
for the
product
used as
orrected
then the
polation
location
ometric
1 of the
cidence -
-derived
s for a
:ocoded
'nerated
e Mask
he BEC
ce angle
resence
gam &
ar beam
cidence.
Y range
r angle,
r to far
'aches a
om near
e again,
t range
| near to
'aches a
om near
yover a
back is
ties like
tion and
| terrain
pixel is
IS
dget for
lar and
iosphere
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
and relief were investigated and published in (Frey et al, 2004).
Assuming that the Sampling Window Start Time bias (SWST)
will be determined very precise during the commissioning
phase and the consideration of zero Doppler instead of the
Doppler Centroid as the reference function the main error
sources are the availability and accuracy of the DEM, the orbit
and - to some extent - the atmospheric path delay.
The estimated error contribution of the orbit is slightly less then
the specified accuracy of the different orbit products quick-look
(+10 m), rapid (+2 m) and science (£20 cm), e.g. up to 1.9 m in
range in case of rapid orbit and an error in cross track direction.
The height accuracy and resolution of the DEM in combination
with the type of terrain and the incidence angle determine the
error contribution of the elevation model. For steep incidence
angles (<23°) a height errors converts with a factor of 2.4 into a
location error. For 45° the factors goes down to 1.
The ionospheric path delay will be modelled and considered
during the SAR processing (Jehle et al, 2004).
6. PROCESSING ENVIRONMENT
DLR's German Remote Sensing Data Center operates a satellite
ground station in Neustrelitz which will serve as the main
ground station for TerraSAR-X. Beside data reception the
payload ground segment will comprise screening and
processing as well as archiving capability. Screening,
processing and geocoding will be performed by the TerraSAR-
X Multimode SAR Processor (TMSP). Annotated level 0 data
will be generated from every acquired TerraSAR-X data set and
will be stored within DLR's long-term archive.
6.1 TerraSAR-X Multimode SAR Processor (TMSP)
The TMSP will be integrated and operated in DLR's ground
receiving station in Neustrelitz to routinely screen and archive
all received TerraSAR-X data. On request the data will be
further processed. The system design foresees the CEOS Level
Ib processing of 70 scenes from TerraSAR-X per day mainly
acquired in SpotLight and StripMap modes. This corresponds to
a data volume of approximately 13.5 GBytes of raw data. Level
Ib comprises Single Look Slant Range Complex (SSC), Multi
look Ground range Detected (MGD), GEC and EEC products.
This amount of data will be handled by a multi-node processor.
The target hardware are multi-processor Unix computers.
Beside screening the TMSP will comprise the focussing of the
radar data, the geocoding and product formatting. The GEC and
EEC generation as well as the elevation data base will be
integrated as separate components into TMSP.
6.2 GEC & EEC Generation
The ellipsoid correction and the terrain correction using a
coarse elevation model will be implemented in one component.
Both procedures geocode the input SAR image block by block.
In case of the enhanced ellipsoid correction a DEM is provided
in geographic co-ordinates from the DEM data base in the best
available spacing. During the block-processing the DEM is
transformed into the output map projection and the required
pixel spacing by a bilinear interpolation. Then the SAR image
is transformed by applying the 3d interpolation approach.
Optionally the incidence angle is calculated. Utilising the multi-
processor environment the individual blocks are processed in
parallel. Finally layover and shadow conditions are determined
and flagged in the GIM if required. The GEC processing is
similar but simplified as no DEM needs to be considered.
843
The SAR processing of TMSP will provide the SAR data, the
orbit, geometry and order parameters to the geocoding modules.
After geocoding the “GEC/EEC generation” component
delivers the geocoded image and annotation data to the product
formatting component of TMSP.
6.3 Digital Elevation Model Data Base
The task of the Digital Elevation Model Data Base (DEM-DB)
is the storage and provision of elevation information for the
ortho-rectification process and geometric calculations. The
DEM-DB supports multiple resolutions and keeps the elevation
data in tiles. The data files are stored on disk via the
conventional Unix file system. The data base organisation as
well as the elevation data representation is in geographic co-
ordinates. DEMs of different sources can be organised in
separate projects. Within each project a tree structure from
North to South is created in 1° steps. Beneath this directory the
next level is sorted from East to West, again in 1 degree
increments. At this level DEMs are stored provided in 1¢
resolution or lower (e.g. 3”). One tile completely covers 1°x1°.
Areas where no elevation information is available are masked.
Each tile contains an overlap of 30” to the south and the west to
the adjacent tiles. Higher resolution DEMs lead to 0,01° cells
below the 1°- level. The finest resolution supported is 0,01”.
Different elevation models like the SRTM C-band and the
SRTM X-band derived elevation products are stored in different
projects. This enables the separation of different qualities, DEM
sources and the minimisation of disk space.
The DEM data base itself consists of the file configuration and
management system and two levels of software modules for file
access and data manipulation.
The utility level provides all functions for data access and data
base maintenance based on a predefined geographic area. An
“import” function for example creates the directory structure,
splits the DEM file into the corresponding tiles and inserts those
tiles. The “get” function supplies the DEM tiles of a geographic
area on local disk.
The application level provides further processing capability like
transformations into different map projections and geodetic
datum, merging and mosaicking of different resolutions and
qualities (Roth et al, 2002) as well as DEM colour shading and
visualisation (Knópfle & Dech, 1999).
The operational EEC generation will utilise DEMs in
= DTED level 2 (SRTM X-band globally, C-band for the
US, regionally ERS-derived),
=» DTED-1 (SRTM/C-band globally, regionally other DTED-
products),
= DTED-0 (GLOBE) where no other DEM information is
available.
6.4 Experimental GTC Processor
The standard geocoding does not consider high resolution
elevation models. The TerraSAR-X ground segment service
focuses on an automated and therewith cost efficient
processing.
Furthermore very high resolution DEMs are rarely available
and expensive. Nevertheless a geocoding processor will be
implemented that will generate SAR products with higher
geometric accuracy on an experimental basis. It considers high
resolution elevation data, tie-pointing and image adjustment as
well as correction capabilities of the atmospheric in particular
the tropospheric path delay (Jehle et al, 2004). The
corresponding output will be a Geocoded Terrain Corrected
(GTC) product.
