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Determining Sensor tilt for ATCOR2 and ATCOR3

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The calculations for atmospheric correction will depend on the sensor view angle. For sensors without tilt capability this parameter will not appear. There are two categories for sensors that do have tilt capability:

  1. Sensors with East/West tilt capability
  2. Sensors with East/West and North/South tilt capabilities (also known as omni tilt sensors). The 2 groups below show which sensors have East/West tilt capability and which satellites are omni tilt sensors.

East/West Tilting Sensors

  • Cartosat PAN
  • IRS-1C/D PAN
  • MSU-E
  • SPOT

Omni Tilt Sensors

  • Ikonos
  • OrbView
  • QuickBird

 

For East/West tilting sensors, the user must choose one of 7 tilt options:

  • Nadir
  • 10 deg East
  • 10 deg West
  • 20 deg East
  • 20 deg West
  • 30 deg East
  • 30 deg West

For omni tilt sensors, the user must choose one of 13 tilt options:

  • Nadir
  • 10 deg East
  • 10 deg West
  • 10 deg North
  • 10 deg South
  • 20 deg East
  • 20 deg West
  • 20 deg North
  • 20 deg South
  • 30 deg East
  • 30 deg West
  • 30 deg North
  • 30 deg South

 
The satellite elevation, or the elevation above the horizon, is used to determine the magnitude of the tilt. The tilt magnitude will either be Nadir (no tilt), 10, 20 or 30 degrees. The sun azimuth and satellite azimuth are used to determine the tilt direction. The tilt direction is defined by ATCOR’s discrete azimuth grid.

Discrete Azimuth Grid

  • 30 deg  = East
  • 150 deg = West
  • 120 deg = North
  • 60 deg  = South


You will need to look at the metadata ASCII file that accompanies your imagery to determine the correct tilt angle to specify for ATCOR2 or ATCOR3. The file extension containing the tilt metadata will be different for each sensor.

Below is the information needed to calculate the tilt angle for a QuickBird image. This information was taken from the IMD file accompanying the imagery.

sunAz = 163.694;
sunEl = 33.6918;
satAz = 55.439;
satEl = 72.5352;


Tilt Magnitude
A good approximation for tilt is:

  • Tilt = 90 deg – satEL
  • Tilt = 90 deg – 72.5352
  • Tilt = 17.5 deg

For ATCOR, the nearest available tilt angle is 20 deg.


Tilt Direction
The relative azimuth angle or the angle between the sensor line-of sight and the solar azimuth defines the tilt direction. The relative azimuth angle depends on the scene latitude and the season in which the  image was acquired.

  • Relative Azimuth = sunAz – SatAz
  • Relative Azimuth = 163.694 deg – 55.439 deg
  • Relative Azimuth = 108.3 deg

So the nearest angle from ATCOR’s discrete azimuth grid is 120 deg, or North.

For this particular scene, you would specify 20 deg North as the tilt angle for ATCOR.

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