Ralf Reulke 
  
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planning departments for traffic or urban development, protection of nature and the environment). By using this 
channel combination the extent and damage of natural disasters or coastal erosion can be detected. The surface 
roughness as well as the sensible and latent heat flux resistances are usually derived from land use maps. 
For cloud investigations two or three channels (blue and green and in addition sometimes red) are routinely used. In 
the blue channel clouds can be well detected and it is used for spatial analyses of cloud structure. 
Monitoring of water areas can also be done with two or three channels: the ratio of the blue and the green channel, 
or the two channels separately, is used for testing lakes with regard to water quality and algae. For detecting oil 
spills one channel in the blue region and another one in the NIR region between approximately 760 and 780 nm are 
desirable. (But it should be mentioned that an active microwave system is more suitable for carrying out this task.) 
For monitoring vegetation and deriving plant parameters three channels are often used (green, red and NIR). 
However, many tasks can be performed with two channels in the red and NIR regions. Measurements in these 
channels, repeated several times during a season, give information about natural, anthropogenic or climatically 
induced effects on the biosphere. Typical tasks are the retrieval of phenological parameters and the determination of 
vegetation indices, leaf area index (LAI) and the absorbed photosynthetically active radiation. With these parameters 
the biomass, which is produced or destroyed within the time period investigated, can be estimated and hence the 
crop yield and the amount of carbon dioxide, which is fixed as carbon in the biomass produced, can be calculated by 
models. This last parameter is essential in all investigations of climate variability. The plant parameters mentioned 
above are often used for estimating evapotranspiration. In addition the proposed channels allow the identification of 
changes in the vegetation, such as forest damage as well as stress on drylands, and to the issuing of early warnings 
about possible food shortages. Using the red and NIR channels different vegetation types can be classified (Hahn, 
1996). This is interesting for planning offices, agricultural establishments, and institutions and ministries dealing 
with the protection of biotops, species and diversities. 
Selection of Channels 
The spectral surface reflectance and the atmospheric transmittance within the spectral region from 400 nm to 1000 nm 
is shown in figure 1. In the left part of the figure the transmittance of the atmosphere due to the uniform mixed gases 
and a typical (averaged) transmittance value of the variable constituents of ozone and water vapour is depicted. In order 
to avoid the reduction of the surface signal due to atmospheric absorption, the channels should be positioned within 
atmospheric windows. These are the regions where only minor absorption by the atmospheric constituents occurs. In the 
right part of the figure typical spectra of surface reflectances as well as the reflectances of two different types of cloud 
components are presented. 
  
  
ADT 
  
  
  
   
  
  
  
  
Figure 1. ADS40 spectral bands: atmospheric absorption bands (left) and typical spectral reflectances (right) 
The following channels have been selected for the ADS40: 
channel 1 - blue: 460nm +/-30 nm. The weak chlorophyll absorption band of green vegetation in water areas or at 
the surface (maximum between 430 and 450 nm) is located in this spectral region. This absorption band is important 
when using this channel for monitoring water areas. 
channel 2 - green: 560 nm +/-25 nm. This spectral domain is characterised by the reflectance maximum of the green 
vegetation and is of some importance for the detection of chlorophyll in water areas. 
channel 3 - red: 635 nm +/- 25 nm. The second chlorophyll absorption band (maximum: 650 nm) is located in this 
region. The selection of the red channel is also determined by the condition that the long-wave side of the red 
channel should break off in concordance with the panchromatic channel at 700 nm. 
channel 4 - NIR: 730 nm +/-25 nm. The oxygen-band is situated at the long-wave side of this channel. Since the 
oxygen concentration is known and is constant in space and time, it can be taken into account in the software 
algorithm. For low flight altitudes it can even be neglected. Unfortunately, the weak water vapour band centred at 
720 nm reaches into the proposed channel. This is inevitable, because the water vapour is above all concentrated in 
the lower part of the atmosphere and has a high variability in space and time. On the other hand this channel is 
  
246 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 
  
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