Satellite Imagery

Satellite Imagery
- Remote Sensing Process
  - Energy Source
    - sun - all wavelengths, we see the visible range
    - earth - emits thermal (long) IR
    - sensor (active) - longer waves (micro and RADAR)
  - Interaction with Atmosphere
    - absorption - ozone absorbs most of the harmful small wave EMR (gamma & UV)
    - reflection (scattering) - Rayleigh scattering of blue
    - (refraction) - 2 different mediums (rainbows and lenses)
    - transmission (= atmospheric window)
  - Interaction with Target
    - reflection
    - absorption (later emmission)
  - Recording Energy
    - (photographic) - aerial photos
    - electronic/ digital - in narrow bands
      - R, G, B, but also ...
      - near (reflective) IR
      - far (thermal) IR
  - Transmission & Receiving
  - Processing & Analysis
  - "Flow of EMR"
    - EMR emitted from sun
    - portion passes thru' atmosphere
    - strikes an object and a portion of EMR is reflected
      - e.g. leaf absorbs blue & red - reflects green & NIR
      - e.g. water absorbe NIR & red - reflects blue & green
    - passes back thru' atmosphere to sensor
    - "brightness" gets recorded (= reflectance value)
- Platforms (space borne)
  - Geostationary
    - 36,000 km
    - aligned with equatorial region
    - in sync with Earth's orbit
    - image same spot
    - TV, communications & weather satellites
  - Polar Orbit
    - 900 km
    - N-S orbit
    - 90% coverage of Earth
    - 12 hr vs 1 yr (sun-synchronous) orbit
    - remote sensing satellites (Landsat, SPOT, Ikonos)
- Image Properties
  - Resolution
    - grid of pixels
    - pixel size
      - Landsat - 30m
      - SPOT - 10 m
      - Ikonos - 1& 4 m
    - fine (high) vs. coarse (low) resolution
  - Pixel Value
    - brightness - level of reflectance
    - digital - 0-255
    - value = 0, low reflectance
    - value = 255, high reflectance
    - e.g. a value of 246 in the red band means most of the red colour is reflected
  - Spectral Signature
    - unique (absorption) reflection of various wavelengths
    - satellite sensors collect in narrow bands
      - B, G, R, near IR, mid IR, thermal IR
      - relative reflectance in each band = "signature"
      - e.g. foliage high in green & IR but low in red & blue
      - more covered in the tutorial / lab
  - Display of Imagery
    - a word about colours
      - all colours at full intensity = white
      - all colours at zero intensity = black (i.e. lack of colour)
      - what about all colours at medium intensity?
      - what about red at full intensity and all others at zero?
      - what about red and blue at full intensity and all others at zero?
      - what are the 3 primary colours
    - channels or bands
      - B, G, R, NIR, Far IR (thermal)
      - landsat bands (layers)
    - colour composites
      - use the 3 channels (i.e. RGB)
      - red sensor into red display channel, etc.
    - false colour composite
      - drop the blue band
      - then "shift" the colours
      - put green reflectance into blue display
      - put red reflectance into green display
      - put IR reflectance into red display
- Summary
  - digital acquisition of reflectance
  - captured in several bands
  - use RGB to display
  - can 'shift' the colour spectrum (drop blue and show IR) - false colour