High Altitude Long Endurance (HALE) Unmanned Aerial Vehicle

The high altitude long endurance (HALE) unmanned aerial vehicle (UAV) is designed to fly at 60,000 feet for months at a time, providing a carrier for a highly accurate and low cost alternative to expensive satellite (Launching cost is around $25000/kg of payload). HALE systems use advanced aircraft or airship technologies to provide mobile, usually uninhabited, platforms operating at altitudes in excess of 20 Km (stratospheric platforms).

The HALE platform would have many of the advantages of both terrestrial and satellite systems, while at the same time avoiding many of their pitfalls. Additional applications of HALE platforms include the ability to provide superior remote sensing performance compared to satellite systems for a wide range of civil applications involving time-varying or emerging phenomena (e.g. environmental science, atmospheric science, communications, ocean monitoring, law enforcement, customs, immigration, urban planning and monitoring, road traffic monitoring, pollution control and others) and for a wide range of military applications including imagery intelligence, signal intelligence, electronic warfare and military search and rescue.

HALE unmanned missions appear to be feasible using a lightweight, high efficiency, span-loaded, Solar Powered Aircraft (SPA) which includes a Regenerative Fuel Cell (RFC) system and novel tankage for energy storage. However, this design has complexity and weight penalties associated with thermal management, electrical wiring, plumbing, and structural weight. Another way to enable practically unlimited endurance is to supply the energy required for propulsion and payloads remotely. The dominating attention in this area has been given to the use of microwave transmission from ground, through a focused beam, to a receiving antenna on-board the aircraft.

None of the aircraft can carry large payloads (2,000 kg or more) at high altitudes and remain aloft for months at a time. An airship can do this. Because the airship uses buoyancy for its lift, it does not require as much power as a vehicle that derives its lift by propelling itself through the atmosphere. Consequently, airships do not need to stay in motion to remain aloft. Therefore, they can loiter over a specific location as well as move to a new location. In addition, airships can carry large-volume, heavy payloads. These characteristics make airships superb candidates for long-endurance surveillance missions.

However, a renewable energy airship, issues a challenge to design the power system, the propulsion system, and the craft’s aerodynamics as an organic whole. This yields the minimum mass system that can balance solar power generation against propulsive energy consumption given seasonal variations in winds and daylight. The current thinking for an airship’s renewable energy system is to employ a photovoltaic array coupled to an electrochemical energy storage system such as a fuel cell or battery. The other most frequently studied alternative energy production scheme considers beaming power from the earth’s surface to the airship. This would eliminate the mass penalties for energy storage, but requires significant investments to develop a safe and effective power beaming system.

The best approach for all weather, coastal surveillance is to use strategically stationed radars. Radar positioned at high altitudes permits viewing a large area with few stations. The coverage area of the airship is determined by calculating the distance to the horizon from the airship. This radial distance (S) is calculated based on the height of the airship (h) and the Earth’s radius (r).

S = Cos-1(r / r + h) r

Where earth radius r =6378.1 x 103 m.

A stratospherically stationed airship’s radar can observe approximately 500 km in any direction. With this viewing ability, a fleet of seven airships could provide continuous coverage of the entire Indian coastline. In contrast, it would take approximately 60 ships or land-based towers to observe the same territory. Besides the littoral coverage, the Stratospherically stationed airships observe 500 km out to sea. This translates to additional reaction time for intercepting unknown vehicles.

Our atmosphere is a very dynamic environment with great fluctuations in temperature, density, pressure, wind speed, and solar intensity. The environment’s influence is greater on a long-endurance, renewable energy airship than it is on conventional aircraft. This is due to two factors: the airship’s large size making it very sensitive to atmospheric winds and available sunlight limiting the power produced by the airship’s solar panels. In general, the airship can operate at any location that has sufficient solar intensity to generate the power needed to overcome wind drag and an atmosphere dense enough to maintain buoyancy.

Another operational approach is to employ multiple airships, which cycle through the high wind area. This would allow them to drift with the wind and still maintain continuous coverage. Once they drifted out of observational range, they could move to a low wind area and fly back inland for another cycle.The last operating option is to change altitude to avoid the high wind conditions since the high winds are transient and do not occur at all altitudes simultaneously.

As increasing numbers of countries see UAVs as an entry-level reconnaissance technology, small procurements worldwide will become more and more common in the next 10-20 years, if not in the next five.