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Envisat
ENVISAT Sensors
— ASAR
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An Advanced Synthetic Aperture Radar (ASAR),
operating at C-band, ASAR ensures continuity
with the image mode (SAR) and the wave
mode of the ERS-1/2 AMI. It features
enhanced capability in terms of coverage,
range of incidence angles, polarisation,
and modes of operation. This enhanced
capability is provided by significant
differences in the instrument design:
a full active array antenna equipped
with distributed transmit/receive modules
which provides distinct transmit and
receive beams, a digital waveform generation
for pulse ?chirp? generation, a block
adaptive quantisation scheme, and a ScanSAR
mode of operation by beam scanning in
elevation.
Improvements in image and wave mode beam
elevation steerage allow the selection
of different swaths, providing a swath
coverage of over 400-km wide using ScanSAR
techniques. Transmit and receive polarisation
can be selected allowing scenes to be imaged
simultaneously with alternating polarisation.
ASAR Operation
Modes
The ASAR instrument may operate as a conventional
stripmap SAR or as a ScanSAR.
ASAR Stripmap Mode (Image)
When operating as a stripmap SAR, the phased
array antenna gives ASAR the flexibility
to select an imaging swath by changing
the beam incidence angle and the elevation
beamwidth.
In the Image Mode (IM), ASAR operates in
one of seven predetermined swaths with
either vertically or horizontally polarised
radiation; the same polarisation is used
for transmit and receive (i.e., HH or VV).
Swath width is between approximately 56
km (swath 7) and 100 km (swath 1) across-track.
Spatial resolution is approximately 30
m (for precision product).
ASAR Image Mode; VV or HH |
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ASAR ScanSAR Modes (Wide Swath and Alternating
Polarisation)
While operating as a stripmap SAR, ASAR
is limited to a narrow swath. This constraint
can be overcome by utilising the ScanSAR
principle, which achieves swath widening
by the use of an antenna beam which is
electronically steerable in elevation.
Radar images can then be synthesised by
scanning the incidence angle and sequentially
synthesising images for the different beam
positions. The area imaged from each particular
beam is said to form a sub-swath. The principle
of the ScanSAR is to share the radar operation
time between two or more separate sub-swaths
in such a way as to obtain full image coverage
of each.
ASAR operates according to the ScanSAR
principle, using five predetermined overlapping
antenna beams which cover the wide swath.
Wide Swath Mode (WS) gives a 400 km by
400 km wide swath image. Spatial resolution
is approximately 150 m by 150 m for nominal
product. VV or HH polarisation.
ASAR Wide Swath Mode; VV orHH
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Alternating polarization
An additional ASAR measurement mode, called
Alternating Polarisation Mode, has also
been defined which employs a modified ScanSAR
technique. Instead of scanning between
different elevation sub-swaths, the Alternating
Polarisation Mode (co-polar) scans between
two polarisations, HH and VV, within a
single swath (which is preselected, as
for the Image Mode). In addition, there
are two cross-polar modes, where the transmit
pulses are all H or all V polarisation,
with the receive chain operating alternatively
in H and V, as in the CO-polar mode.
Alternating Polarisation Mode (AP) gives
two co-registered images per acquisition,
from any of 7 selectable swaths. HH/VV
HH/HV or VV/VH polarisation pairs possible.
Spatial resolution of approximately 30
m (for precision product)
ASAR Alternating Polarisation Mode |
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ASAR
Products
For Image Mode and Alternating Polarization:
Level 0 (Raw)
Data from image mode after frame synchronisation.
Includes the instrument source packet and
input data required for processing.
Single-Look Complex (SLC)
For SAR image quality assessment, calibration
and interferometric or wind/wave applications
and the derivation of higher-level products.
Absolute calibration parameters are provided.
Precision Image (PRI)
A multi-look, ground range, digital image
suitable for most applications.
Ellipsoid Geocoded Image (GEC)
Similar to a Precision Image, but with
rectification to a map projection. Geocoded
using a map projection selectable by the
user, such as UTM, or Polar Stereographic.
Absolute calibration parameters are provided.
Medium-Resolution Image (MRI)
Available at 150 m resolution, specifically
aimed at sea ice and oceanography applications.
Absolute calibration parameters are provided.
Wide Swath products are available only
as Raw or PRI. |
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