Imagery and GIS. Kass Green

Piloted or Unpiloted

      Until recently, most satellite platforms were unpiloted, and most airborne platforms were piloted. However, with the advent of unmanned aerial vehicles, most airborne platforms are now unpiloted, but piloted aircraft still capture much larger areas than unpiloted platforms. While less used, piloted satellite platforms have been very important in remote sensing. Starting with the Apollo space mission in the late 1960s and continuing with the International Space Station today, piloted satellites have completed many successful remote sensing missions including NASA’s Shuttle Radar Topography Mission, which generated global digital elevation models of the earth from 56 degrees south to 60 degrees north. In 2019, the International Space Station will deploy the GEDI lidar to produce a 3D map of the earth’s forests.

      Most of the areas captured by airborne platforms used for mapping today are flown over by a pilot residing in the platform. UASs are either autonomous or have a pilot operating them from the ground. Originally developed and used by the military, the use of UASs in civilian markets is exploding because of their low cost, their ability to collect imagery over inaccessible or dangerous areas, and their ability to fly low and slow, enabling the capture of high-resolution imagery over small areas that would be too expensive to capture with piloted aerial systems. Hobbyist use in the United States has skyrocketed since 2010, but commercial use was stalled because of cumbersome FAA regulations. In 2015, the FAA streamlined the process for gaining authorization to commercially operate UASs in the US, resulting in a 500 percent increase in applications in the first six months of 2015 over all of 2014 (Andelin and Andelin, 2015). Outside the United States, UAS use is also rapidly increasing with successful deployments to map archeological sites, establish property rights, monitor illegal resource extraction, and support disaster response (Pajares, 2015).

      While civilian drones do not currently have the capacity to capture imagery over large areas, the use of UASs is likely to continue to rapidly expand and evolve. As stated in the primer Drones and Aerial Observations:

      Technology will change. Faster processors will stitch together and georectify images more quickly. The acuity of photographic sensors will improve, as will the endurance and range of drones. Increasing levels of autonomy in both flight software and post-processing software will allow for the creation of cheap maps with increasingly less direct human intervention (Kakaes et al., 2015).

       Altitude

      Altitude is an object’s height above sea level. The altitude of a remote sensing platform can vary between below sea level (in bathymetric projects) to more than 20,000 miles above sea level. Remote sensing platforms are classed into three types based on their range of distance from the earth:

      1 1.Terrestrial and marine platforms, including elevated work platforms, mobile vehicles, buildings and towers, lampposts, buoys, boats, and humans.

      2 2.Airborne platforms including UASs, fixed-wing aircraft, helicopters, and balloons.

      3 3.Spaceborne platforms, which are either geostationary or orbit the earth.

      Terrestrial platforms operate from beneath the ocean 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