International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
3.2 StripMap Mode (SM)
In StripMap Mode the ground swath is illuminated with a
continuous sequence of pulses while the antenna beam is fixed
in elevation and azimuth. This results in an image strip with
continuous image quality in azimuth. The corresponding
parameters are listed in Table 2.
3.3 ScanSAR Mode (SC)
The ScanSAR mode provides a large area coverage. The wider
swath is achieved by scanning several adjacent ground sub-
swaths with simultaneous beams, each with a different
incidence angle. Due to the reduced azimuth bandwidth the
azimuth resolution of a ScanSAR product is lower than in
StripMap mode. The ScanSAR beams will be composed from
the calibrated StripMap beams. The corresponding parameters
are listed in Table 2.
Parameter Value SM Value SC
Number of sub-swaths | na 4
Swath width (ground | 30km (polarimetric 100 km
range) mode: 15-30 km)
Acquisition length € 1650 km < 1650 km
Incidence angle range | 20° - 45° 20° - 45°
Azimuth resolution 3m 16 m
Ground range 1.7m-3.5 m 1.7 m-3.5 m
resolution (45°-20°
incidence angle)
Table 2: Parameters of StripMap and ScanSAR Modes
3.4 Polarisation
Each pulse can be transmitted either vertically (V) or
horizontally (H) polarised. The back-scattered signal can be
received with either vertical or horizontal polarisation,
independent from the transmit polarisation. The resulting
product will consist of one polarimetric channel in one of the
combinations HH, HV, VH or VV.
In dual polarisation mode the radar toggles the transmit and/or
receive polarisation on a pulse to pulse basis. The effective PRF
in each polarimetric channel is half of the total PRF, which
means that the azimuth resolution is slightly reduced. The
product consists of two layers that can be selected out of the
possible combinations.
Single and dual polarisation will be available for all image
modes.
Quad polarisation is possible in the experimental dual receive
antenna mode as the signal can be received simultaneously in H
and V polarisation. By sending alternating H and V pulses, the
full polarimetric matrix can be obtained. The corresponding
experimental product consists of four layers. Currently quad-
polarisation is not operationally foreseen.
4. TERRASAR-X GEOCODED PRODUCTS
4.1 Geocoded Ellipsoid Corrected (GEC)
The GEC product is a multi-look detected product. It is
projected and re-sampled to either UTM or UPS in polar
regions. WGS84 is used as geodetic reference assuming one
average terrain height. As the ellipsoid correction does not
consider a DEM, the pixel location accuracy varies due to the
terrain. The terrain induced SAR specific distortions will not be
corrected and significant differences can appear in particular for
strong relief and steep incidence angles. The GEC is generated
by applying the interpolative ellipsoid correction approach (s.
chapter 2.4).
The GEC is the recommended product for marine and coastal
applications where topography doesn’t effect the location
accuracy. The orbit precision will be the main factor for the
achieved location accuracy. Depending on the time delay
between acquisition and processing either quick-look (£10 m),
rapid (£2 m) or science orbits (£20 em) will be used for the
geocoding.
4.0 Enhanced Ellipsoid Corrected (EEC)
Like the GEC, the EEC is a multi-look detected product
provided in UTM or UPS projection. WGS84 will be used as
the geodetic datum. Terrain induced distortions are corrected
considering a DEM of a moderately coarser resolution then the
TerraSAR-X products. For this purpose the 3D-interpolation
described in chapter 2.3 will be applied. The pixel location
accuracy in these products is highly accurate. The geometric
quality depends on the height accuracy and resolution of the
DEM in combination with the type of terrain and the incidence .
angle. DEMs from SRTM (C-band and X-SAR), ERS-derived
elevation models and GLOBE provide a global basis for a
terrain correction service.
It is expected that the EEC will become the standard geocoded
product of TerraSAR-X. Like the GEC the EEC is generated
automatically. No operator interactions are required.
4.3 Geocoded Layover Shadow and Incidence Angle Mask
(GIM)
The GIM product is generated as an optional add-on to the EEC
product. It provides information about the local incidence angle
for each pixel of the geocoded SAR scene and about presence
of layover and shadow areas (Meier et al, 1993), (Raggam &
Gutjahr, 2004).
The local incidence angle is the angle between the radar beam
and a line perpendicular to the slope at the point of incidence.
For its determination it is necessary to know the slant range
vector and the local surface normal vector.
Areas of SAR shadow are determined via the off-nadir angle,
which in general increases for a scan line from near to far
range. Shadow occurs as soon as the off-nadir angle reaches a
turning point and decreases when tracking a scan-line from near
to far range until the off-nadir angle reaches that value again,
which it had at the turning point.
Areas of SAR layover are determined via the slant range
distance, which in general increases for a scan line from near to
far range. Layover occurs as soon as the slant range reaches a
turning point and decreases when tracking a scan-line from near
to far range. In order to separate active and passive layover a
two step procedure scanning from near to far and back is
required.
The GIM product shows the same cartographic properties like
the geocoded output image with regard to output projection and
cartographic framing. The content is basically the local terrain
incidence angle and additional flags indicate whether a pixel is
affected by shadow and/or layover or not.
5. TERRASAR-X ERROR BUDGET ANALYSIS
(Frey et al, 2003) analysed the TerraSAR-X error budget for
geocoded products. The error sources orbit, radar and
processing parameters, cartography and geodesy, atmosphere
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