Line scanners - University of New England

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Transcript Line scanners - University of New England

Line scanners
Chapter 6
• Frame capture systems collect an image
of a scene of one instant in time
• The scanner records a narrow swath
perpendicular to the flight path to build up
an image.
– Scanning rate is adjusted to the ground speed
so that successive scans view adjacent
swaths
• Four types of scanning systems commonly
used.
– Across track (whisk broom).
– A long track (push broom)
– Spin scanner
– Conical scanner
Across track scanner
• Record brightness values along a scan
line 1 pixel the time.
• The rotating or oscillating mirror directs the
sensors field of view to repeatedly sweep
across the terrain.
• A single detector up her band continuously
outputs of voltage proportional to the
incoming energy
Across track scanner
• Collects data from the point directly below
the sensor (nadir), and also to each side.
– Therefore the distance between the sensor in
the ground increases as the sensor moves
along the scan line to each side
– Therefore there is and across track change in
scale
Across track scanner
• Ideally, the scanning rate is adjusted so
that the forward motion causes an
advance over the ground that is equal to
the size of the ground resolution
• The dwell time is time it takes to collect a
measurement for a single ground cell
– Smaller ground resolution size, means shorter
dwell time
– Short time reduces the amount of energy
captured – may lead to low signal:noise
Across track scanner
• Instantaneous Field of View (IVOF) is
the ground cell size
– The smaller the IVOF, the better the spatial
resolution
– The larger the IFOV, the more energy is
captured
Satellites are very stable platforms, but aircraft are not. This leads to a number of
distortions.
Scanner geometry is different from frame capture systems.
One-dimensional Relief Displacement
Air photo
Scanner
Along-track Scanners
• Pushbroom scanners capture narrow
strips of data to build up an image
– No mirrors, just a line of sensors
– So the entire line gets captured
simultaneously
– Multiple linear arrays are used for
multispectral data (or multiple viewing
directions)
Across track scanner
• Several advantages over along-track
systems
– Scanning mirror can introduce errors, so the
array is more accurate geometrically
– No moving parts means more durable, and
lower cost
– All the sensors are individually calibrated, so
has higher radiometric accuracy as well
– Have longer dwell time
– Smaller size
Forward
Nadir
Backward
Spin Scanner
• Geostationary satellites such as GOES
spin in place, because the earth is not
moving below them
– A single line was captured east to west as the
satellite rotated
– Stepper motor changed the angle of view to
record subsequent lines
• 1,812 lines while rotating at 100 rpm
• Technology was abandoned after 1987
Conical Scanner
• Record circular or elliptical images
– Scans a circular track around the nadir
• This is also used on meteorological
satellites
– Usually microwave or infrared at coarse
resolution
• Used on Tropical Rainfall Monitoring
Mission (TRMM), Along-Track Scanning
Radiometer (ATSR) on the ERS-2 satellite
Hyperspectral Scanner
• Airborne Visible Infrared Imaging
Spectrometer (AVIRIS)
– 224 spectral bands, 0.4-2.45 μm
• Compact Airborne Spectrographic Imager
(CASI)
– 228 bands, 0.4-1.0 μm
• HYMAP
– 126 bands, 0.45-2.5 μm
• Others produced by GER, AISA
Stereo Scanner
• Like aerial photography – allows elevation
data to be determined
• Must capture scene from two different
angles
– For nadir viewing – only overlap can be
used, and weak stereo effect
• Off-nadir viewing is better – greater relief
displacements
Thermal Scanner
• Thermal scanners have been applied to
– military
– sea surface temperature
– heat loss in buildings
– geologic mapping
– soil moisture
– soil type
– fire mapping