Remote Sensing
Electro-optical Sensors
Vicki Drake
•DEFINITIONS FOR COMMON REMOTE SENSING TERMS
•Band (channel): A band is a section/grouping of wavelengths from the
electromagnetic spectrum.
–Landsat ETM+ has eight bands which collect radiation from different parts of
the electro-magnetic spectrum
•three bands are sensitive to visible light
•one band is panchromatic (b/w film sensitive to all visible light wavelengths)
•three bands are sensitive to near-infrared
•one band is sensitive to thermal infrared.
•Geostationary orbit: An orbit in which a satellite is always in the same
position with respect to the rotating Earth.
–The satellite travels around the Earth in the same direction, at an altitude of
approximately 22,000 miles (35,800 kilometers).
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•Sun-synchronous orbit: An orbit in which a satellite is always in the same
position with respect to the rotating Earth at the same time of day.
–The satellite travels around the Earth in the same direction, at an altitude of
approximately 438 miles (705 kilometers).
–Landsat-7 is sun-synchronous, always passing overhead at approximately
10:00 am local time.
Multispectral image: A remote sensing image created using data collected
from more than one band in the electro-magnetic spectrum.
•Nadir: the point on the Earth directly below an orbiting satellite.
•Orbital period: The time it takes a satellite to complete one revolution
(orbit) around the Earth.
–The orbital period of Landsat 7 is about 1.5 hours.
•Panchromatic: Sensitive to all or most of the visible spectrum, between
0.4 and 0.7 micrometers.
–Landsat 7 has a panchromatic band.
•Spatial Resolution: A measure of the amount of detail that can be seen in
an image; the size of the smallest object recognizable using the detector.
•Spectral Resolution: The dimension and number of specific wavelength
intervals in the EM spectrum to which a sensor is ‘sensitive’.
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•Temporal Resolution: How often a given sensor obtains imagery of a
particular area (a time reference)
•Radiometric Resolution: The sensitivity of a sensor to differences in
recorded radiant flux. Defines the number of just discriminable signal levels.
•Scenes: Each Landsat image collected is called a scene.
–Each Landsat scene is 115 x 106 miles long.
–The globe is divided into 57,784 scenes, and each Landsat scene has about
3 billion bytes of data.
•Three levels of resolution relating to objects discernability
–Detection
– an object is detected if there is an indication it is different from
surroundings
–Recognition – an object is recognized if it can be categorized
–Identification – Identification made if specific information about object can
be determined (i.e., deciduous trees, shopping malls, etc)
Remote Sensing
•Sensors currently in use
•Landsat Thematic Mapper
•SPOT Multispectral and Panchromatic Scanner
•India Remote Sensing Systems
•Canada Radar Satellite
•Advanced Very High Radiometric Resolution (AVHRR)
•NASA Prototype Sensors
–Lewis Hyperspectral Sensor
–Clark High Resolution Sensor
Electro-Optical Sensors
• Electro-Optical sensors use nonfilm detectors to record radiation from a ground
scene.
• The major types of electro-optical sensors are
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The
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The
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Video Camera
Vidicon Camera
Across-Track Scanner
Along Track Scanner
Electro-Optical Sensors Advantages
• Can operate in multiple bands of the EM spectrum, within and beyond extents of
photographic spectrum
• Image data can be transmitted over radio link – a telemetry feature required by
satellite systems
• In-flight display devices allow for images of ground scenes viewed in near realtime
• Systems operating in the thermal IR (TIR) region have day-night capability
• The detection process is renewable as detectors can be continuously used
• Photographic film serves as both detector and storage medium
Digital Conversion
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• Electro-Optical Sensors are digital systems
• Analog-to-digital conversion translates electronic signal to digital numbers (DN) on
magnetic tape
• DNs are mathematically processed by a computer to correct geometric and
radiometric errors (and enhance patterns in original images)
• DNs transformed into video image to create visible images – or – transformed into
video signals for TV screens – or – transformed to visible light for recording on
ordinary photographic film
Video Cameras
• Video cameras used for remote sensing collect reflected radiation in
the 0.4 – 1.1 m range of EM spectrum
• A multispectral video system uses an array of black/white video
cameras equipped with visible-to- near IR sensitive tubes and filters.
• Four-camera array: blue, green ,red and near IR spectral bands
collected/isolate
– Reflectance data recorded as individual black/white images and
color IR composite images.
Vidicon Cameras
• Similar to television cameras- covering about same spectral region
as video cameras
• A latent image is temporarily stored on photoconductive faceplate
and scanned by internal electron beam.
• Used successfully for imaging Moon and Mars
• RBV (Return Beam Vidicon) cameras on first Landsat satellites.
Landsat 3 RBV a successful high-resolution mapping device.
Across-Track Scanners
• Across-track or whiskbroom scanners, developed for military,
originally
• Two primary types available today:
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Multispectral scanner
Thermal IR scanner
• Across-track scanners use a rotating or oscillating mirror to scan
contiguous series of narrow ground strips at right angles to the flight
path
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Forward motion of platform means new ground strips covered with successive
scan lines
Across-Track Scanners
• Two-dimensional record of reflectance and/or emittance information
built up along flight path
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• The Instantaneous Field of View (IFOV) determines how much
ground area the scanner “sees” at any given moment – ground area
is called “ground resolution”
– An object on the ground can only be resolved when its size is
equal to or greater than the ground resolution cell size
– An object on the ground can only be identified and classified if it
is 2 times the ground cell size.
Across-Track Scanners
• A small IFOV provides spatial detail – but restricts the amount of
radiation received by scanner
• A large IFOV results in a longer time for the scanner’s mirror to
sweep across the ground cell.
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A longer dwell time means more radiation enters the detector to create a
stronger signal
Large IFOVs are better at detecting small reflectance or emittance variations as
the expense of spatial resolution
Multispectral Scanners (Across-Track
Scanners)
• MSS operate simultaneously in the UV, visible, reflected IR and
thermal IR regions of EM spectrum.
• Spectral channels range from less than 5 to more than 10
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Scanners with >100 channels are hyperspectral
• MSS used routinely in both aircraft and spacecraft
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Satellite-mounted scanners collect multiple lines of ground data during each
sweep
Each detector is responsive to one specific wavelength
Thermal IR Scanners (Across-Track Scanners)
• Thermal IR scanners operate the same as MSS, but are confined to
Thermal IR atmospheric windows of 3 to 5 m and 8 to 14m
• Thermal IR detectors are filtered to operate in the 10.5 to 12.5 m
due to Ozone absorption in upper atmosphere
• Narrow IR bands (in the 8 to 14 m region) have important geologic
applications for detection of silicate rocks, carbonate rocks and
altered rocks
Along-Track Scanners
• Along-track, pushbroom, scanner form images without a scanning
mirror.
• Use linear arrays of very small charged coupled devices (CCD)
• A dedicated detector element for each across-track ground
resolution cell and a linear array of detectors for each spectral band.
A single array may contain as many as 10,000 individual detectors.
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• Each array is located in the focal plane of the instrument’s imaging
optics so that an entire ground strip in the across-track direction is
focused onto the detector elements at the same time w/o
mechanical scanning
Along-Track Scanner Advantages
• Improved spectral resolution due to longer dwell time on each
ground resolution cell
• Greater reliability and longer operating life due to elimination of
moving parts
• High geometric accuracy because of fixed geometry of detector
arrays
• Light weight and lower power requirements
Electro-Optical Sensor – Earth Observation
Satellites
• Landsat Program
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Landsat – “Land Satellite” – 1st international satellite program designed
specifically for collecting a view of the Earth’s surface
Started by NASA and Department of Interior – images would be acquired on a
worldwide basis with users from any country gaining access to collected data
First known as “The Earth Resources Technology Satellite Program (ERTS) until
1975
Launched July 23, 1972 – operated until 1978 (long after expected one-year
expiration date!)
Landsat Program
• Landsat 2 launched January 22, 1975
• Landsat 3, 4, and 5 launched in 1978, 1982 and 1984, respectively
• January 1983 – operation of Landsat system transferred to NOAA
(National Oceanic and Atmospheric Administration)
• 1985 – Landsat system commercialized and Earth Observation
Satellite Company (Space Imaging, now) assumed responsibility
Landsat Systems Characteristics
• Landsats 4 and 5 carry both MSS and TM sensors - however,
routine collection of MSS data terminated in 1992
• Satellites orbit at an altitude of 705 km, with a 16-day cycle.
• Satellites designed and operate to collect data over a 185 km swath.
• MSS and TM sensors detect reflected radiation from Earth’s surface
in visible and near IR wavelengths.
Acquisition of Data - Landsat
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• TM data is received directly from Landsat 5 to a network of 15
worldwide ground stations
• Data is recorded on high density magnetic tapes and sent to Space
Imaging’s Image Data Processing Center in Maryland
• Earlier, Landsat 4 and 5 TM data was sent to TDRS (Tracking and
Data Relay Satellites) then relayed to Space Imaging’s data
processing facility.
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Failure of K-band transmission on Landsat 4 and 5 stopped this transmission
Sample Landsat TM image along Missouri River
Landsat 1,2, and 3 Swathing Pattern
Landsat 4 and 5 Swathing Pattern
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Current Landsat TM Ground Receiving Stations
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Landsat Characteristics
• The wavelength range for the TM sensors is from visible through
mid-IR into the Thermal-IR section of EM spectrum
• All five Landsats have been in Sun-synchronous orbits, providing
global coverage between 81 degrees north latitude and 81 degrees
south latitude
Landsat TM image of San Francisco Bay
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Domestic Satellite Relay
Across-Track Scanner Mirror System
• Landsat 7 getting ready for Launch
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Landsat Schematics
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SPOT –(Systeme Pour l’Observation de la
Terre)
• Began in France in 1978 – a long-term commercial system
• Carries two identical HRV pushbroom scanners (High Resolution
Visible) operating in one of two modes:
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Panchromatic – a single visible band (0.51 – 0.73m) with a spatial resolution
of 10 m x 10 m
Multispectral – three images produced: one for each band – 0.50-0.59m
(green), 0.61-0.68m (red) and 0.79-0.89m (near IR) with a spatial
resolution of 20 m x 20 m
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SPOT Satellite Characteristics
• A sun-synchronous orbit at an altitude of 832 km
• Capability to view same area from two widely separated locations
enables full scene stereo images to be produced (Figure 3.19)
• SPOT 2 launched January 21, 1990 with SPOT 1 and SPOT 2 orbiting
concurrently in identical orbits 1800 apart
• SPOT 4 launched mid 1990s with increased spectral capabilities
Sample Spectral Signatures for SPOT
GIS: Land Use and Land Cover
• Land use is a description of man’s relationship with the land.
• Land cover is the natural or man-made composition that cover the
Earth’s surface at a certain location
• Major emphasis of classification system research is to incorporate
information derived from remotely sensed data
• System devised by USGS represents a national classification scheme
being used in operational mapping programs, and GIS
Features of USGS System
• The USGS system is hierarchical and incorporates features of several
existing classification systems amenable to remotely sensed data
• Criteria:
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Identification of land use and land cover should an accuracy of 85% or greater
Different interpreters should be able to repeat someone else’s identification
Categorization should permit vegetation and other land cover types to describe
present activities
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USGS Classification System
• Comparison should be possible with future land use data
• Minimum areas and Image Resolution
• The minimum area that can be classified as to land use and land
cover depends on:
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The scale and resolution of the original sensor image or data source
The scale of data compilation image interpretation
The final scale of the land use information or map
USGS Land Use Classification
• Analysis of area development means to be able to identify and map land use and
land cover change over time
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These changes are indicators of rural, urban and industrial growth in an area
GIS must handle both positional data and attribute data as databases may contain
many variables including soil, rock types, land ownership, land use/land cover,
population demographics, image data, etc.
Planning organizations need vast amounts of accurate and timely information on
physical resources and related socioeconomic factors to help guide management and
planning decisions
A State-of-the-Art GIS capability
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Accept data inputs in one or more formats
Store and maintain information with necessary spatial relationship
Manipulate data (search, retrieve, compute)
Develop models and ‘what if’ scenarios
Present data as output in various ways
Five technical elements of GIS:
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Encoding (data structure – Raster or Vector)
Data Input
Data Management
Data Manipulation/Analysis
Output
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