Rainer Sandau 
  
Several spectral bands between 400 nm (blue) and 900 nm (infrared) are specified with very steep spectral edges (see 
table 1 near the end of the paper), leading to band pass filters with complicated interference structures. The filters, 
which must be placed near to the CCD-pixels for constructional reasons, should not change the spectral response across 
the wide angular field of view. Therefore, each filter stripe and consequently each adjacent pixel line have to be 
illuminated by light of normal angular incidence, i.e. striking the line at right angles, which is accomplished by a 
telecentric lens. The loss of angular symmetry between object and image side, however, leads to a length of 420 mm 
(figure 3). Such a design characteristic is difficult to avoid, however, when demanding specifications are set for the 
spatial resolution across the field of view in both directions, along and across track. The resolution values must be kept 
constant over a wide range of temperature and flight altitude, i.e. pressure range. The same insensitivity must also hold 
for the registration error, i.e. the allowable change of the mapping geometry, which describes the change of the ground 
viewing angle of each pixel. 
Thus, all the requirements - wide angular field of view, broad spectral range, high resolution, telecentricity and 
environmental stability - finally lead to a lens layout of at least the same complexity as the film lenses of the RC30. The 
ADS40 uses eight parallel sensor lines: three panchromatic lines (forward, nadir, backward) and five spectral lines (red, 
green, blue, new infrared 1 and optional near infrared 2). The three colour lines, each equipped with 12000 pixels, are 
optically superimposed during the flight using special arrangement of dichroitic mirrors. The near infrared channels are 
slightly offset with respect to the panchromatic nadir CCD lines. 
  
Figure 3. View of the lens of the ADS40 Figure 4. The focal plane module of the ADS40 
5 CCD AND FOCAL PLANE MODULE 
The focal plane (figure 4) consists of four CCD housings: two of them contain single lines and two contain triple line 
configurations. Both types are specifically developed for the ADS40 . Each CCD line consists of two linear arrays, each 
with 12000 pixels but staggered by a 0.5 pixel shift. The panchromatic lines make use of this staggered arrangement 
thus halving the achievable ground sample distance. The colour lines (RGB, NIR1, NIR2) use 12000 pixel linear arrays. 
All CCDs have to be placed with a height tolerance of a few micrometres only. To achieve a dynamic range of 12-bit 
with a radiometric resolution of 8-bit, a temperature stabilisation system has been introduced. Furthermore, the focal 
plane is equipped with a ventilation and air drying system to avoid condensation of water vapour on the cover glass of 
the CCD packages and focal plane deformations. 
6 CCD SIGNAL PROCESSING 
The focal plane electronics contain the CCD lines with only a minimum of electronics necessary to operate the CCDs. 
The subsequent CCD signal processing is concentrated in separated sub-units. Each 24k CCD achieved by two 
staggered 12k CCDs is processed like two independent 12k CCDs. Each of the 12k pairs can independently be used for 
different colours, and, of course, the usage of only one 12k line is also possible. This modular concept of the ADS40 
allows, theoretically, product variants that use from one 12k CCD up to 3x24k (pan) + 6x12k (colour) CCDs. This 
  
260 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B1. Amsterdam 2000. 
  
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