An atmospheric satellite (United States usage, abbreviated atmosat) or pseudo-satellite (British usage) is an aircraft that operates in the atmosphere at high altitudes for extended periods of time, in order to provide services conventionally provided by an artificial satellite orbiting in space.
Atmospheric satellites remain aloft through atmospheric lift, either aerostatic (e.g., balloons) or aerodynamic (e.g., airplanes). By contrast, conventional satellites in Earth orbit operate in the vacuum of space and remain in flight through centrifugal force derived from their orbital speed.
To date, all atmosats have been unmanned aerial vehicles (UAVs).
An atmosat remains aloft through atmospheric lift, in contrast to an artificial satellite in Earth orbit which remains aloft through centrifugal force derived from its high orbital speed. Because of this high speed a satellite must operate in the vacuum of space, is expensive to build and launch, and its path is inflexible once launched. Atmospheric satellites fly much slower and are intended to provide a range of services more economically and with more versatility than current low Earth orbit satellites.
Operating altitudes are expected to be in the tropopause—at approximately 65,000 feet—where winds are generally less than 5 knots and clouds do not block sunlight. It is desirable in the United States to operate above 60,000 feet, above which the Federal Aviation Administration does not regulate the airspace.
There are two classes of atmosat, respectively gaining their lift through either aerostatic (e.g., balloons) or aerodynamic (e.g., airplanes) forces. In order to remain aloft for long periods, the NASA and Titan Aerospace designs use propeller-driven electric airplanes powered by solar cells, in contrast to Google's Project Loon which envisions using helium-filled high-altitude balloons.
To enable night time operation and ensure endurance through consecutive 24-hour day/night cycles, in daylight hours solar panels charge batteries or fuel cells which subsequently power the vehicle during hours of darkness. An atmospheric satellite may initially ascend at night under battery power, and reach altitude soon after dawn to allow solar panels to take advantage of a full day's sunlight.
Facebook's UAV-based Aquila system expects to use laser communication technology to provide Internet communication among UAVs, and also between UAVs and ground stations that in turn will connect to rural areas. The Aquila UAV is a carbon fiber, solar-powered flying wing design about the size of a passenger jet. Aquila's first test flight took place on June 28, 2016. It flew for ninety minutes, reaching a maximum altitude of 2150 feet, and was substantially damaged when a twenty-foot section of the righthand wing broke off during final approach to landing. The Aquila is designed and manufactured by the UK company Ascenta.
Luminati Aerospace claims its Substrata solar-powered aircraft could remain aloft indefinitely up to a latitude of 50° through formation flight like migratory geese, reducing by 79% the power required for the trailing aircraft and allowing smaller airframes.
A geostationary balloon satellite (GBS) flies in the stratosphere (60,000 to 70,000 feet (18 to 21 km) above sea level) at a fixed point over the Earth's surface. At that altitude the air has 1/10 of its density is at sea level. The average wind speed at these altitudes is less than that at the surface.
Proposed applications for atmosats include border security, maritime traffic monitoring, anti-piracy operations, disaster response, agricultural observation, atmospheric observation, weather monitoring, communications relay, oceanographic research, Earth imaging and telecommunications. Facebook is reportedly envisioning providing Internet access to the African continent with a fleet of 11,000 vehicles.
The initial goals under the NASA's Environmental Research Aircraft and Sensor Technology (ERAST) project were to demonstrate sustained flight at an altitude near 100,000 feet and flying non-stop for at least 24 hours, including at least 14 hours above 50,000 feet. The early development path of atmospheric satellites included the NASA Pathfinder (exceeding 50,000 feet in 1995), the Pathfinder Plus (80,000 feet in 1998), and the NASA Centurion which was modified into a prototype configuration for the NASA Helios (96,000 feet in 2001). An Airbus/Qinetiq Zephyr flew for 14 days in the summer of 2010, and in 2014 a Zephyr 7 stayed up for 11 days in the short days of winter whilst carrying a small payload for the British Ministry of Defence.
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