Principles: Electromagnetic Energy

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Sensors
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Sensor Basics

• We see colour because our eyes detect the entire
  visible range of wavelengths and our brains
  process the information into separate colours.
• Most remote sensors work by seeing only very
  narrow ranges of wavelengths or colours.
• The information from a narrow wavelength range is
  gathered and stored in a channel, also sometimes
  referred to as a band.
• We can combine and display channels of
  information digitally using the three primary
  colours (blue, green, and red).

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TM Ch 1
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TM Ch 3,2,1
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TM Ch 4,3,2
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Multi-spectral Scanners

• There are two types of multi-spectral scanning
  systems
• Across-Track
• Along Track
• Each system can operate from either an Aeroplane
  or a Satellite-based platform

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MSS Systems

• There are three parts to across-track
  multi-spectral scanning systems (MSS) system
• Mirror
• Detector array
• Amplifier

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Mirror

• Using an oscillating mirror, this system can scan
  terrain along a series of scan lines that are at
  right angles to the flight line of aeroplane or
  satellite.
• Data are normally collected with an arc of 60
  degrees either side of nadir.
• As the aircraft or satellite moves forward
  successive scan lines are collected, yielding a
  contiguous 2-dimensional array of lines.

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Detector Array

• Once collected, the incoming energy is separated
  using a diffraction grating or prism into several
  spectral components (wavelengths).
• By placing an array of electro-optical detectors
  (sensors) geometrically after the prism or
  grating, the incoming energy is essentially
  split into multiple narrow bands or channels.
• Each detector is designed to have its peak
  spectral sensitivity in a specific band or
  wavelength range

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Instantaneous Field of View (IFOV)

• The ground area that is covered by the IFOV can
  be expressed as follows
• D Hb
• D Diameter of circular area viewed on the
  ground.
• H Flying height
• b IFOV of the MSS
• D is commonly referred to as the Spatial
  Resolution of the system, and the ground segment
  sensed at any instant is termed the Resolution
  Element.

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Along Track multi-spectral scanning systems

• Push-broom and MSS systems are quite different.
• In a push-broom system there is no mirror, and
  instead a linear array of detectors is used to
  scan perpendicular to the flight line.
• The liner array of detectors usually consists of
  numerous charge coupled detectors (CCD)
  positioned end-to-end.

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Pushbroom IFOV - Resolution

• In the case of a push-broom system, the IFOV or
  spatial resolution is determined by the size of
  the detectors.
• Each spectral band has its own array of sensors,
  each of which is placed in the focal plane of the
  scanner.

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Active or Passive Sensing ?
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Passive Remote Sensing

• The sun provides a very convenient source of
  energy for remote sensing.
• The sun's energy is either reflected, as it is
  for visible wavelengths, or absorbed and then
  re-emitted, as it is for thermal infrared
  wavelengths
• Remote sensing systems which measure energy that
  is naturally available are called Passive Systems
• Passive sensors can only be used to detect energy
  when the naturally occurring energy is available.
  For all reflected energy, this can only take
  place during the time when the sun is
  illuminating the Earth

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Passive Sensor
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Active Remote Sensing

• Active sensors provide their own energy source
  for illumination. The sensor emits radiation
  which is directed toward the target to be
  investigated. The radiation reflected from that
  target is detected and measured by the sensor.
• Advantages for active sensors (a) the ability to
  obtain measurements anytime, regardless of the
  time of day or season, (b) examine wavelengths
  that are not sufficiently provided by the sun
  (microwaves), (c) to better control the way that
  a target is illuminated.
• However, active systems require the generation of
  a fairly large amount of energy to adequately
  illuminate targets.

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Active Sensor
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Platforms
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Airborne Platforms

• Aerial platforms are primarily stable wing
  aircraft, although helicopters are occasionally
  used.
• Aircraft are often used to collect very detailed
  images and facilitate the collection of data over
  virtually any portion of the Earth's surface at
  any time.

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Satellite Platforms

• In space, remote sensing is sometimes conducted
  from the space shuttle or, more commonly, from
  satellites.
• Man-made satellites include those platforms
  launched for remote sensing, communication, and
  telemetry (location and navigation) purposes.
• Because of their orbits, satellites permit
  repetitive coverage of the Earth's surface on a
  continuing basis. Cost is often a significant
  factor in choosing among the various platform
  options.

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Orbits

• The path followed by a satellite is referred to
  as its orbit.
• Orbit selection can vary in terms of altitude and
  their orientation and rotation relative to the
  Earth.
• Satellites at very high altitudes, which view the
  same portion of the Earth's surface at all times
  have geostationary orbits.
• Many remote sensing platforms are designed to
  follow a near polar orbit (basically north-south)
  which, in conjunction with the Earth's rotation
  (west-east), allows them to cover most of the
  Earth's surface

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Images

• Electromagnetic energy may be detected either
  photographically or electronically.
• The photographic process uses chemical reactions
  on the surface of light-sensitive film to detect
  and record energy variations.
• It is important to distinguish between the terms
  images and photographs in remote sensing

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Displaying Images

• An image refers to any pictorial representation,
  regardless of what wavelengths or remote sensing
  device has been used to detect and record the
  electromagnetic energy.
• An image may be represented and displayed in a
  digital format by subdividing the image into
  small equal-sized and shaped areas, called
  picture elements or pixels, and representing the
  brightness of each area with a numeric value or
  digital number.

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The Image Display

• The computer displays each digital value as
  different brightness levels.
• Sensors that record electromagnetic energy,
  electronically record the energy as an array of
  numbers in digital format right from the start.
• The two different ways of representing and
  displaying remote sensing data (either
  pictorially or digitally), are interchangeable as
  they convey the same information (although some
  detail may be lost when converting back and
  forth).

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Data Formats

• RASTER DATA
• Digital data uses raster data structure
• Image an array of values
• Each pixel a separate unit
• Origin often upper left hand corner of image

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BIPBAND INTERLEAVED BY PIXEL

• Line 1, pixel 1, band 1
• Line 1, pixel 1, band 2
• Line 1, pixel 1, band 3
• Line 1, pixel 1, band 4
• Line 1, pixel 2, band 1
• Etc.
• Values for all 4 bands written before values for
  next pixel

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BILBAND INTERLEAVED BY LINE

• Line 1 for band 1
• Line 1 for band 2
• Line 1 for band 3
• Line 1 for band 3
• Line 2 for band 1
• Line 2 for band 2
• Etc..

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BSQBAND SEQUENTIAL

• Each band treated separately
• Band 1
• Band 2
• Band 3
• Band 4
• For many applications, most practical structure