Remote sensing is the acquisition of information about an object without physical contact. This term is mostly in contrast to on-site observation.

Categories

Source of radiation:

  1. Passive remote sensing: record emitted or reflected radiation, typically sunlight.
  2. Active remote sensing: emit signal from aircraft or satellites and measure reflected radiation

Type of radiation:

  • visible light: CCD (Charge-coupled device, 感光耦合组件)
  • Infrared
  • Sonar: (passive) vessel, animal; (active) underwater objects
  • Ultrasound sensor: (active) sea level, tide

radiometer radar altimeter: seafloor, ocean wave height and wavelength

Photogrammetry

Photogrammetry (摄影测量法) is a traditional and less sophisticated method of remote sensing, compared to satellite imagery.

Private vendors: Pictometry (Southwest US, ~95 aircrafts)

Drones have been used for photogrammetry in recent years.

Types and uses:

  1. Orthophoto: for measurement and map making;
  2. Oblique image: for (human) validation.
  3. Stereophotogrammetry: topographic maps (地形图), rapid 3D mapping (Saab Group)

Earth Observation Satellite

Earth observation (EO) is the acquisition of satellite imagery of Earth.

Instrumentation technology and applications:

  • Radar (radio detection and ranging, 雷达):
    • aerial traffic control (speed limit enforcement),
    • large scale meteorological data (precipitation, wind velocity)
  • LiDAR (light detection and ranging, 光学雷达):
    • weapon ranging (测距), laser-homing projectiles (激光自动寻的导弹),
    • vegetation, chemical concentration in the atmosphere
  • InSAR (interferometric synthetic aperture radar, 干涉合成孔径雷达):
    • precise, large-scale digital elevation model, land cover and land use

Data processing

Data resolutions:

  1. Single band
    • spatial: meter per pixel, 1 ~ 1000;
    • radiometric: bit, 8 ~ 14; [256 ~ 16384 levels]
  2. Spectral: μm per band, 0.10 ~ 2.1; [visible spectrum 0.39 ~ 0.70 μm]
  3. Temporal: for time-series studies, cloud-averaged image for deforestation and mapping.

Corrections:

  • Georeference: matching points on image to established benchmark
  • Radiometric correction: convert monochromatic scale to radiance values
  • Topographic correction: recover reflectivity in horizontal conditions from terrain-affected radiance values
  • Atmospheric correction: transform gray-scale value to eliminate atmospheric haze

Data processing levels†:

  1. Level 0: Raw data
  2. Level 1: Reconstructed data
    • (1a): Unprocessed data at full instrument resolution, time-referenced and annotated with ancillary information, including radiometric and geometric calibration coefficients and georeferencing parameters
    • (1b): Level 1a data processed to sensor units
  3. Level 2: Derived geophysical variables, at full instrument resolution.
  4. Level 3: Variables mapped on uniform space-time grid scales.

Level 1 data is the most fundamental record with significant scientific utility. Level 2 data is the first directly usable data for most scientific applications, variables including ocean wave height, soil moisture, ice concentration, etc. Level 3 data is smaller and have regular spatial and temporal organization.

† As defined by NASA and EUMETSAT

Earth Observation Data Products

See the main article about Earth Observation Data Products.

Elevation Models:

  • Digital elevation model (DEM)
  • Digital surface model (DSM)
  • Orthorectified radar intensity images (ORI)

🏷 Category=Geographic Information System