to the highest buildings on earth and may be fixed (e.g., ATM video cameras) or mobile (e.g., cars and boats). Airborne platforms fly within the earth’s atmosphere up to an altitude of typically 9.5 miles (15.3 kilometers) and include fixed-wing aircraft, UASs, helicopters, and balloons. Fixed-wing aircraft are the most common type of remote sensing platform and are used by many private companies and governments for imaging purposes. High-altitude piloted aircraft platforms have pressurized cabins, enabling them to fly as high as 50,000 feet above sea level. Low-altitude piloted aircraft platforms operate at altitudes up to 30,000 feet (5.7 miles), but are generally used to collect data at lower elevations to gain higher spatial resolution. The hovering ability of helicopters (below 500 feet and up to 12,500 feet) allows them to collect imagery at lower speeds than fixed-wing aircraft. Balloons have a wide range of achievable altitudes, from as low as needed for a tethered balloon to around 20 km or more for a blimp. UASs can be fixed- or rotor-winged with altitudes ranging from very close to the ground to very high in the air.
At the highest altitudes, earth observation satellites carry remote sensors around the earth in orbit at altitudes ranging from 100 to over 22,000 miles above sea level. Maintaining orbital altitude is a constant requirement for satellites because of the earth’s steady gravitational pull and atmospheric drag. Lower satellites must travel at higher velocities because they experience greater gravitational pull than satellites at higher altitudes. Thus, maintaining orbit requires a constant balance between gravity and the satellite’s velocity. Satellites with fuel onboard maintain their orbital altitude by using the fuel to maintain their velocities. However, at some point all satellites fall back to earth and burn up in the atmosphere, usually in controlled descents.
Speed
Speed is the rate of motion of an object expressed as the distance covered per unit of time. It determines the level of detail and amount of area (extent) a remote sensing system can collect. The altitude and speed flown while collecting remotely sensed data are also determined by the desired resolution and coverage, as well as the sensor being used (e.g., digital or film camera, lidar). Remote sensing platform speeds can range from stationary (zero velocity) to over 17,000 miles per hour. Most terrestrial platforms are stationary. Mobile terrestrial platforms such as cars and boats tend to travel at low speeds to enable the collection of very-high-spatial-resolution imagery. Fixed-wing UASs and aircraft typically fly at 55 to 650 miles per hour. Helicopters and rotor UASs, with their ability to hover, typically fly at 0 to 150 miles per hour. The speed at which a satellite travels in orbit is determined by its altitude. The lower the altitude, the faster the satellite must travel to remain in orbit and not fall to earth. Satellites in near-circular orbits have near-constant speeds, while satellites in highly elliptical orbits will speed up and slow down depending on the distance from the earth and direction of motion.
Stability
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.
Agility
Agility refers to the ability of the platform to change position and can be characterized by 1) reach or the ability of a platform to position itself over a target, which is sometimes referred to as field of regard; 2) dwell time, which is how long the platform can remain in the target area working; and 3) the ability to slew across the target area.
Fixed platforms such as a traffic-light pole above a street intersection have no agility. Satellites are tied to their orbits, which restricts their agility. However, some satellites are pointable (e.g., able to slew off nadir), which makes them much more agile than nonpointable satellites. This, coupled with their ability to quickly orbit the earth, provides them with a long-range reach around the globe, which is not available to aircraft.
Within their range, aircraft and fixed-wing UASs are more agile than satellites, and helicopters are more agile than fixed-wing aircraft. The hovering abilities of helicopters and rotor-winged UASs allow them to obtain more target specific data than fixed-wing aircraft can collect, and they can more easily reach targets in a congested airspace. Blimps and remote-controlled balloons have greater mobility than hot-air balloons because they have engines and are more maneuverable.
Power
Power refers to the power source that runs the platform. The more powerful the engine or engines, the faster and higher the platform can travel and the greater payload it can carry. Satellites are propelled into space by launch vehicles to escape the earth’s gravity. Afterward, they use electric power derived from solar panels for operation, and stored fuel for orbital maneuvering. Of critical importance is the amount of power remaining after launch for the sensor to operate. Size, weight, and power, coupled with communication bandwidth (the ability to offload the image from the focal plane) are the biggest drivers in satellite sensor design.
Fixed-wing aircraft are powered by piston engines, turbocharged piston engines, turboprops, or jet engines in single- or twin-engine configurations. High-altitude piloted aircraft platforms are usually powered by twin jet engines or turboprops. The high power of these
