Page 184 - Digital Analysis of Remotely Sensed Imagery

Image Geometric Rectification 149

image (Fig. 5.5b). This kind of distortion is especially severe toward
the end of the scan line, where q is maximal.
Scanning Mirror Inconsistency
Two problems related to the scanning mirror emerge during scan-
ning, velocity inconsistency and scan-skew. The mirror rotates non-
linearly across a scan like a pendulum. Its velocity is the maximum at
the nadir and gradually decreases to zero at the extreme. The velocity
always alternates between these two extremes. Consequently, the
ground is not swept linearly, causing distortion along the scan direc-
tion. This systematic distortion can be eliminated completely through
application of some kind of mathematical formula.
Scan-skew refers to the motion of the spacecraft away from the
planned direction along the ground track during the time required to
complete a scan. This causes the ground swath scanned, not normal
to the ground track but slightly skewed (cross-scan geometric distor-
tion). It causes random errors that are impossible to deal with.

5.1.3 Errors Associated with the Platform
Two types of parameters determine the status of the sensor in
space, position and orientation, each being associated with three
components.
Position
Among the three parameters (X-easting, Y-northing, and Z-altitude)
defining the position of the platform in space, altitude (Z) is the most
critical as it affects the scale of the obtained imagery (Table 5.1). A
higher altitude than the nominal causes a larger ground area to be
covered, resulting in the image having a smaller scale. Conversely, a
lower altitude leads a smaller ground area to be sensed. Hence, the
image has a larger scale. The departure of a spacecraft or aircraft from
its nominal altitude is translated into a scale variation. The other two
coordinates (easting and northing) govern the geographic area to be
covered. A change in the position causes a slightly different area from
the planned one to be sensed. There is no change in image scale.

Orientation
Sensor orientation is defined by three parameters, roll (w), pitch (f ),
and yaw (k ). Roll refers to the rotation around the flight direction
(X-axis), whose increment points to the right. As illustrated in Table
5.1, this rotation occurs clockwise. It results in a change in scale in
the direction perpendicular to the flight direction. The scale is either
larger or smaller than the nominal, depending upon the location in
relation to the X-axis. The scale along all lines parallel to the flight
direction is constant. Pitch is the rotation around the Y-axis, or a
direction perpendicular to the flight direction. Its effect on scale dis-
tortion is identical to roll except that the distortion occurs in the