||It has been suggested that High-altitude airship and Stratospheric airship be merged into this article. (Discuss) Proposed since August 2011.|
A high-altitude platform (HAP) is a quasi-stationary aircraft that provides means of delivering a service to a large area while staying thousands of feet above in the air for long periods of time. A HAP differs from other aircraft in the sense that it is specially designed to operate at a very high altitude (17–22 km) and is able to stay there for hours, even days. The new generation of HAPs, however, will expand this period to several years.
Limitation due to power
A HAP can be a manned or unmanned aeroplane, balloon, or an airship. All require electrical power to keep themselves and their payload functional. While current HAPs are powered by batteries or engines, mission time is limited by the need for recharging/refueling. Therefore, alternative means are being considered for the future. Solar energy is one of best options currently being used for under trial HAPs (Helios, Lindstrand HALE).
Altitude selection for HAPs
Whether an airship or an aeroplane, a major challenge is the ability of the HAP to maintain stationkeeping in the face of winds. An operating altitude of between 17 and 22 km is chosen because in most regions of the world this represents a layer of relatively mild wind and turbulence. Although the wind profile may vary considerably with latitude and with season, a form similar to that shown will usually obtain. This altitude (> 17 km) is also above commercial air-traffic heights, which would otherwise prove a potentially prohibitive constraint.
Comparison to satellites
Since HAPs operate at much lower altitudes than satellites, it is possible to cover a small region much more effectively. Lower altitude also means much lower link budget (hence lower power consumption) and smaller round trip delay compared to satellites. Furthermore, deploying a satellite drains significant time and monetary resources, in terms of development and launch. HAPs, on the other hand, do not cost much and are rapidly deployable. Another major difference is that a satellite, once launched, does not allow for full maintenance, while HAPs do.
- For high-speed wireless communications
One of latest uses of HAPs has been for wireless communications. Research on HAPs is being actively carried largely in Europe, where scientists are considering them as a platform to deliver high speed connectivity to users, over areas of up to 400 km. It has gained significant interest because HAPs will be able to deliver bandwidth and capacity similar to a broadband wireless access network (such as WiMAX) while providing a coverage area similar to that of a satellite.
- For surveillance and intelligence gathering
One of the best examples of a high-altitude platform used for surveillance and security is RQ-4 Global Hawk UAV used by the US Air Force. It has a service ceiling of 20 km and can stay in the air for continuous 36 hours. It carries a highly sophisticated sensor system including radar, optical, and infrared imagers. It is powered by a turbofan engine and is able to deliver digital sensor data in realtime to a ground station.
- For real-time monitoring of a region
Another future use which is currently being investigated is monitoring of a particular area or region for activities such as flood detection, seismic monitoring, remote sensing and disaster management.
- For weather/environmental monitoring and studying
Perhaps the most common use of high-altitude platforms is for environment/weather monitoring. Numerous experiments are conducted through high-altitude balloons mounted with scientific equipment, which is used to measure environmental changes or to keep track of weather. Recently, NASA in partnership with The National Oceanic and Atmospheric Administration (NOAA), has started using Global Hawk UAV to study Earth's Atmosphere.
- As a space port
Due to the height more than 90% of atmospheric matter is below the HAP. This reduces atmospheric drag for starting rockets. As a rough estimate, a rocket that reaches an altitude of 20 km when launched from the ground will reach 100 km if launched at an altitude of 20 km from a balloon. It also allows the usage of (long) mass drivers for launching goods or humans into orbits.
- "CAPANINA Project Introduction". Capanina.org. Retrieved 2012-10-19.]
- "Lindstrand Technologies - HALE". Lindstrandtech.com. Retrieved 2012-10-19.
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- The airborne Remote Sensing technical system of the Chinese Academy of Sciences by Tong Qingxi, The Joint Center for Remote Sinsing of CAS China
- NASA Recruits Unmanned Aircraft for Earth Science, Space.com
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- Gerard K. O'Neill (1981). 2081 : a hopeful view of the human future.
- Mobile Telecommunications via Stratosphere by R. Struzak