26 1 Introduction
a Cartesian coordinate system is constructed, the axes of which represent spectral
resolution along the flight track and geometric resolution across the track in com-
bination with the swath width, thus representing the number of detector elements
schematically.
Since it is easier to construct an array comprising a multitude of faultless detector
elements in a single dimension (line) with good yield than in two dimensions simul-
taneously (matrix), lines are bound to provide higher resolutions at a given stage in
technological development. The current state of the art is lines with 12,000 detec-
tor elements which, in staggered array (see Section 2.5), can accomplish 24,000
scanning operations along the line (see Section 2.5 and Chapter 7).
Matrices with roughly 9,000 × 9,000 detector elements are available today,
although their application in digital airborne cameras is not yet economical.
Currently, matrices with about 7 × 4 k are used (see Section 1.5). Thus, depending
on the required number of sampling points in the swath direction, the appropriate
number of matrix cameras is arranged side by side. As shown in Section 1.5, several
variants are possible.
There are also two basic concepts (matrix or line) with respect to the spectral
co-ordinates shown in the flight direction in Fig. 1.4-7. In the case of the matrix
concept, the appropriate number of cameras must be used to achieve the required
swath width with the specified GSD for each spectral channel or, if several channels
are combined in a camera using special optical processes, then for each channel
combination.
In the case of the line concept, additional lines – corresponding to the number
of required spectral channels – are arranged in the focal plane between the lines
(usually panchromatic) used in topographic mapping. It should be noted that in both
alternative solutions shown in Fig. 1.4-7, the differences in angles of convergence
and exposure time between the spectral channels, on the one hand, and between
spectral channels and the panchromatic channels, on the other, lead to pixel cover
problems during data processing or presentation. These systemic pixel cover prob-
lems arising from angles of convergence and varying recording time points also
occur with analogue airborne cameras, however, when images from several flight
missions using different films (panchromatic, colour, IR) are joined. A major advan-
tage of digital airborne cameras is that all data obtained with analogue airborne
cameras using three different films from three photo flights can be generated on
a single flight. Here, the pixel cover problems result from the overlapping of the
results obtained not on three photo flights but on only one. Also, greater differ-
ences in illumination conditions inevitably occur when images are generated on
three flights in the case of analogue airborne cameras than those generated on a
single flight using digital airborne cameras.
These systemic errors can be avoided only if the angles of convergence between
channels and different time points of imaging can be avoided. This can be done
with camera systems that convey the different spectral light components (R, G, B,
NIR) to different detector systems (matrices or lines) via polychroitic beam-splitting
devices. In large-format digital cameras (see Section 1.5) this can be achieved more
readily by using detector lines and polychroitic beam splitting devices, which cover