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Stability is the ability of an object to resist changes in position. Stability is an important feature of remote sensing platforms because platforms need to either maintain stability or precisely measure instability to ensure high-quality image capture and accurate registration of the image to the ground. The most stable platforms are fixed terrestrial platforms because they are structurally rigid and immobile, which also means that they have little or no agility. Satellite platforms are also relatively stable because they operate in the vacuum of space. Helicopters are less stable than fixed-wing aircraft because of the unequal lift and vibrations caused by the rotating blades. While balloons were an important platform in the early days of remote sensing, they are not widely used today because their flight is easily influenced by air currents and pressure changes resulting in minimal control of balloon flight path or position. Fixed-wing platforms are relatively stable airborne platforms. Because of this and their large range and speed, they remain the workhorse of airborne image collection.

Operating in the earth’s atmosphere subjects aircraft to air pressure and wind variations that can result in changes in pitch, roll, and yaw (figure 3.11), causing a variety of displacements in the collected imagery. Pitch is rotation of the aircraft about the axis of the wings. Yaw is rotation about the axis that is perpendicular to the wings and directed at the nose and tail of the aircraft. Roll is rotation of the aircraft about the axis of the fuselage.

Figure 3.12. The effects of pitch, yaw, and roll on aircraft stability

Traditionally, aerial photography missions required the precise measurement of many ground control points in each photograph to establish the exact spatial position and orientation of the photograph relative to the ground at the moment the image was taken. In the late 1950s, a technique called bundle block adjustment was developed to reduce the number of expensive control points required. This was based on finding tie points between photographs and then solving least squares adjustment formulas. In the 1990s, the number of control points required was again reduced by the advent of accurate GPS positioning of the aircraft that effectively added control points in the air, further reducing the control required. The advent of lower-cost precise IMUs (inertial measurement units) has further reduced the number of control points required, so that for many applications sufficient accuracy can be achieved using only highly accurate GPS and IMU systems, which is referred to as direct georeferencing. These orientation parameters are used in image orthorectification (see chapter 6) to geometrically correct the images so that coordinates in the imagery accurately represent coordinates on the ground.