Scanner configuration


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Scanner configuration

How the set-up of the scanner is naturally dependent on the type of scanner. Whisk broom scanners will often have a set of predefined bands, of which a number can be selected, push-broom scanners may have the option of programming channels by integrating spectral bins.
In the following section the configuration is discussed based on the assumption of programming a casi scanner as used in this project. More general recommendations are given in the lower part of the page.

The configuration element can be split into the interrelated components:

  • Band width and placement
  • Spatial resolution

The spatial resolution may limit the number of bands which can be acquired in a given configuration.
The actual  configuration
is very much a balance between spatial resolution, spectral resolution and the available photons to reach the sensor units.

  • A high spatial resolution will give a shorter integration time for each pixel and thus less photons to the sensor.

  • Narrow spectral bands leads to both low signal to noise level and thus also to low discriminations levels.

  • The available radiance from the bottom is determined by both irradiance, the water depth, the attenuation in the water column and the bottom reflectance. In the figure below the absorption of clear water is shown as the percentage of down-welling radiance just below the surface that is available at different depths. It is clearly seen that he region with the highest transmittance of radiance is below 600 nm, and that practically no radiance from the bottom can be expected for bands above 700 nm. Remark that the figure shows the maximum available percentage of down-welling radiance, since other constituents in the water will further reduce the down-welling radiance at a given depth. 

Based on absorption coefficients from Kirk, 1994

 

General recommendations
  • A high spatial resolution makes interpretation and geo-correction easier
  • Make the bands fairly wide 10-15 nm, in order to allow high signal to noise
  • Define several bands in the region 500-600 nm in order to allow for detection and minimization of coherent noise. Further details in Noise analysis
  • Allow at least one band in the near infrared region, e.g. around 780 nm, to be used in potential sun-glint removal.
  • Be aware of potential weak spectral regions of the scanner, such weak regions may be due to the type of detector used have a low response in e.g. the lower part of the spectrum available.   

 

 

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This page was updated: 19. September 1999
These pages are maintained by Michael Stjernholm, NERI

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