= 162 =
series. Each sensor element and each exposure time, which is also
referred to as integration period, thus generates a discrete voltage
signal, the amplitude of which determines the luminosity of the object
(Fig. 2).
If such an apparatus is installed in an aircraft, a missile or a
satellite and moved over the earth's surface normally to the sensor
line, the strip of ground scanned depends on the length of the picture
line. The signals generated by the sensor are digitized and recorded
on magnetic tape. Due to the high bit rate, storage must be effected
on what is known as "High Density Digital Tape" (HDDT).
This photography method is characterized by the following features:
a) Realization of a purely solid-state technology without moving
mechanical parts or electron-beam image tubes. This has led to a
very simple and compact structure which is particularly appropriate
for operations to be carried out under stringent environmental con-
ditions, and thus well suited to space flight and military missions.
b) Image geometry features a picture line characterized by central per-
spective,which produces distortionfree pictures, provided that a
vertical photo axis and horizontal ground are the case (see Fig. 1).
This photographic principle is clearly superior to that using oscil-
latory mirrors or rotating prisms, where the picture line is deformed.
Due to the carrier changing its inclination and speed, the film strip
is subject to distortions and changes in scale which can be minimized
in the case of satellite pictures, but become clearly visible when
photographs are taken from the aircraft, unless suitably compensated.
Here, the opto-electronical scanning offers the possibility of applying
electronic corrective measures.
c) Thanks to the discrete structure of the sensor cells and to the fact
that, during scanning, the sensor signal emitted by each image point
is digitized without delay, the line coordinate as well as the digi-
tized gray scale value of each image point are supplied immediately.
There is no need for subsequent measuring and digitizing as is, for
instance, required in the case of conventional aerial photographs.
