sensed images are taken from high altitudes, their lenses are commonly designed for an infinite object distance; i.e., the lenses are fixed.
Shutter speeds are usually extremely fast because most platforms are moving at high speeds.
Remote sensor camera bodies must be able to withstand the extreme temperatures and vibrations encountered by the vehicle, boat, aircraft, or satellite platform. Additionally, for mapping purposes, the precise internal geometry of the sensor components within the body must be known as well as the location of the imaging surface when an image is collected so that the imagery can be accurately terrain corrected and georeferenced to the earth.
Platforms
This section reviews remote sensing platforms by examining platform features. Seven major features distinguish platforms from one another: whether they are manned or unmanned, and their altitude, speed, stability, agility, and power.
Different Types of Platforms
Geosynchronous — 22,236 miles
Satellites that match Earth’s rotation appear stationary in the sky to ground observers. While most commonly used for communications, geosynchronous orbiting satellites like the hyperspectral GIFTS imager are also useful for monitoring changing phenomena such as weather conditions. NASA’s Syncom, launched in the early 1960s, was the first successful “high flyer.”
Sun synchronous — 375-500 miles
Satellites in this orbit keep the angle of sunlight on the surface of the earth as consistent as possible, which means that scientist can compare images from the same season over several years, as with Landsat imagery. This is the bread-and-butter zone for earth observing sensors.
Atmospheric satellite — 100,000 feet
Also known as pseudo-satellites, these unmanned vehicles skim the highest edges of detectable atmosphere. NASA’s experimental Helios craft measured solar flares before crashing in the Pacific Ocean near Kauai.
Jet aircraft — 90,000-30,000 feet
Jet aircraft flying at 30,000 feet and higher can be flown over disaster areas in a very short time, making them a good platform for certain types of optical and multispectral image applications.
General aviation aircraft — 100-10,000 feet
Small aircraft able to fly at low speed and low altitude have long been the sweet spot for high-quality aerial and orthophotography. From Cessnas to ultralights to helicopters, these are the workhorse of optical imagery.
Drones — 100-500 feet
Drones are the new kid on the block. Their ability to fly low, hover, and be remotely controlled offer attractive advantages for aerial photography, with resolution down to sub-1 inch. Military UAVs can be either smaller drones or actual airplanes.
Ground based/handheld — ground level
Increasingly, imagery taken at ground level is finding its way into GIS workflows. Things like Google Street View, HERE street-level imagery, and Mapillary; handheld multispectral imagers; and other terrestrial sensors are finding applications in areas like pipelines, security, tourism, real estate, natural resources, and entertainment.
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
