ISPRS Commission III, Vol.34, Part 3A ,,Photogrammetric Computer Vision“, Graz, 2002
2. BASIC PRINCIPLES
2.1 Push broom Principle
For triangulation a push broom scanner must have three sensor
lines in the focal plane. They are mounted in such a way that
one is looking forward, one is looking backward and one is
looking in the nadir direction. The distance between the forward
and the nadir line and the distance between the backward and
the nadir line need not be equal. For the ADS40 the forward and
backward views use different angles. The scan of each line is
called a scene. The scene of a sensor line is similar to a digital
image of a frame sensor.
Figure 2: Continuous scan of three scenes compared to
discrete photos of a frame sensor.
2.2 Measuring Tie Points in Scenes
To measure homologous points in the scenes the corresponding
mm line and sample must be found. The matching process is very
similar to current matching processes. Owing to the complexity
of raw aircraft motion in the Level 0 scenes, it is convenient to
perform matching on the Level 1 rectified scenes.
Ra
Forward scene | |
Oo
Pram,
d
Nadir scene | |
zm n
Backward scen | | | 5
RE,
Different area from same
{ sensor location
Same area from different sensor locations
(measuring homologous points)
Figure 1: Backward, nadir and forward scenes. ; E
Figure 3: Measuring homologous points in three scenes.
A. scene differs from a frame photograph in the way that it is
scanned continuously rather than taken at a nearly discrete
position. All scenes are scanned synchronously.
2.3 Orientation Fixes
The GPS and IMU data, which is measured at high rates during
image acquisition, yields a continuous position and attitude of
the ADS40. During the triangulation process, we want to
update this continuous stream of data based on the principles of
least squares bundle adjustment. In this case we are using
“orientation fixes” at regular intervals along the flight path of
the push broom scanner.
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