The SAR instrument may be operated in one of two modes:
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Single Beam
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ScanSAR
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Single Beam Mode
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In Single Beam Mode operation, the beam elevation and profile are maintained constant throughout
the data collection period. A beam is characterized by its:
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nominal incidence angle
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nominal swath width
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nominal spatial resolution
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Single Beam Operational Modes
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Fine Beam
The Fine beam mode is intended for applications which require the best spatial resolution
available from the RADARSAT system. The azimuth resolution is 8.4 m, with range resolution
9.1 m to 7.8 m from F1 to F5. Since the radar operates with a higher sampling rate in this
mode than in any of the other beam modes, the ground swath coverage has to be reduced to a
nominal 50 km in order to keep the downlink signal within its allocated bandwidth. Originally,
five Fine beam positions, F1 to F5, were available to cover the far range of the swath, the
incidence angle range from 37 to 47 degrees. By modifying timing parameters, 10 new
positions have been added with offset ground coverage. Each original Fine beam position can
either be shifted closer to or further away from Nadir. The resulting positions are denoted by
either an N (Near) or F (Far). For example, F1 is now complemented by F1N and F1F.
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Standard Beam
Standard beam mode operates with any one of seven beam positions, referred to as S1 to S7.
The nominal incidence angle range covered by the full set of Standard beam is from 20 degrees
(at the inner edge of S1) to 49 degrees (at the outer edge of S7). The spatial resolution is 27 m
in azimuth and 26 m (near) to 20 m (far) in range direction. Each individual beam covers a
minimum ground swath of 100 km within the total 500 km accessibility swath of the full set of
Standard beams. The nominal spatial resolution in the range direction is 26 m for S1 at near
range to 20 m for S7 at far range. The azimuth resolution is the same, 27 m, for all beam
positions.
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Standard beam modes allow imaging over a wide range of incidence angles with optimum
system image quality.
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Wide Beam
Wide beams are similar to the Standard beams except that the swath width achieved by this
beam is 150 km rather than 100 km. As a result, only three Wide beams, W1, W2 and W3 are
necessary to provide coverage of almost all of the 500 km swath range. They provide the
comparable resolution as the standard beam mode although the increased ground swath
coverage is obtained at the expense of a slight reduction in overall image quality. W1 and W2
are available for single beam products. W3 is used as one of the beams to form SWA product
but not recommended for individual image because it contains a nadir ambiguity (narrow white
vertical strip in the image).
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Extended High Beam
Six Extended High beam positions, EH1 to EH6, are available for collection of data in the 49 to
60 degree incidence angle range. Since this beam mode operates outside the optimum scan
angle range of the SAR antenna, some minor degradation of image quality can be expected
when compared with the Standard beam mode. Swath widths are restricted to a nominal 80 km
for the inner three positions, and 70 km for the outer three positions. The operational beam
positions on satellite are EH3, EH4 and EH6, which can cover the complete Extended
High beam swath.
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Extended Low Beam
A single Extended Low beam position, EL1, is provided for imaging in the incidence angle
range 10 to 23 degrees with a nominal ground swath coverage of 170 km. As with the
Extended High beam mode, some minor degradation of image quality can be expected due to
operation of the antenna beyond its optimum elevation angle range.
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ScanSAR Mode
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In ScanSAR operation either two, three or four single beams are used during data collection.
Each beam is selected sequentially so that data is collected from a wider swath than is
possible with a single beam. The beam switching rates are chosen to ensure at least one
"look" at the earth's surface for each beam within the along track illumination time or dwell time
of the antenna beam. In practice, the radar beam switching is adjusted to provide two looks per
beam.
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The beam multiplexing inherent in ScanSAR operation reduces the effective sampling rate
within each of the component beams, hence the increased swath coverage is obtained at the
expense of spatial resolution.
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