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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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