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 Northrop Grumman 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.
An unmanned lighter-than-air vehicle, the HAA, is intended to operate above the jet stream in a quasi-geostationary position to deliver persistent orbital station keeping as a surveillance aircraft platform, telecommunications relay, or a weather observer. They proposed to launch their HAA in 2008.
The airship would be in the air for up to one month at a time and was intended to survey a 600-mile (970 km) diameter of land. It will use solar cells to provide its power and would be unmanned during its flight.
It is designed to be 500 feet (150 m) long and 150 feet (46 m) in diameter. To minimize weight it is to be composed of high strength fabrics and use lightweight propulsion technologies.
The HAA is intended to operate at a height of above 60,000 feet (18,000 m) and to have a payload for military use.
Lockheed Martin and the U.S. Army launched the HALE-D sub-scale demonstrator on July 27, 2011, demonstrating key technologies critical to the development of unmanned airships. The airship was supposed to reach an altitude of 60,000 feet, but a problem occurred at 32,000 feet. It descended and landed without damage.
High altitude airships can improve the military’s ability to communicate in remote areas such as those in Afghanistan, where mountainous terrain frequently interferes with communications signals.
A stratospheric airship is a powered airship designed to fly at very high altitudes (30,000–70,000 ft or 10 – 20 km). Most designs are remote operated aircraft/unmanned aerial vehicles (ROA/UAV). To date none of these designs have received approval from the FAA to fly in U.S. airspace.
Stratospheric airship efforts are being developed in at least 5 countries.
The first stratospheric powered airship flight took place in 1969 reaching 70,000 feet (21.3 km) for two hours with a 5 lb (2.2 kg) payload. On December 4, 2005 a team led by Southwest Research Institute (SwRI), sponsored by the Army Space and Missile Defense Command (ASMDC) successfully demonstrated powered flight of the HiSentinel stratospheric airship at an altitude of 74,000 feet (22.6 km).
Lockheed Martin has been contracted by the United States Department of Defense Missile Defense Agency to construct a high-altitude airship (HAA) to enhance its Ballistic Missile Defense System (BMDS).
- Myasishchev M-55
- 21st Century Airships
- Integrated Sensor is Structure
- Orbital airship
- Long Endurance Multi-intelligence Vehicle
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