Unmanned aerial vehicles are a valuable tool in the monitoring of weather systems. Greg Holland looks to the skies and examines their benefits.

Unmanned aerial vehicles (UAVs) vary and come from a broad range of families. Of particular interest are those UAVs which have a capacity to monitor severe weather systems and provide data both on the system and on the impact of the system on vulnerable communities. To be able to effectively collect relevant data, such UAVs must have long endurance and a capacity to either withstand severe weather conditions, or to operate at very high levels, well beyond the dangerous conditions below. UAVs which have this capacity vary from the diminutive Aerosonde, with its wingspan of less than 3m; to the Global Hawk (picture above) with a wingspan similar to a Boeing 737 and the giant Helios/Pathfinder series, which are nearly 50m across.

Candidate UAVs

The first unmanned flights, by fixed wing aircraft, were conducted more than 70 years ago, using manned aircraft which had been specially modified. Nowadays, UAVs are specialised systems developed to exploit niche areas. By their very nature these aircraft excel at dull, dirty, dangerous and debilitating operations. Advances in small, powerful electronics have enhanced them, via the use of precision navigation systems, such as the Global Positioning System (GPS) and by advances in global communications, such as the Low Earth Orbiting Satellites (LEOS) and the internet.1

Designed specifically for environmental surveillance, the Aerosonde has been utilised extensively in meteorological operations since 1995. The aircraft

(figure 1) has a 2.9m wingspan, a maximum weight of 15kg, is equipped with meteorological sensors as standard, and has a wide range of other sensors or payloads incorporated for specialist requirements. Its major feature is its long endurance, which can be extended beyond 30 hours, and which enables operations in remote and extreme conditions.

The Aerosonde was designed specifically to fly into the weather of interest and take observations directly. By comparison, high-flying aircraft such as the Global Hawk can operate above the weather using remote sensing systems or specially deployed packages (called dropsondes) to observe the conditions below. To date, Global Hawks have not been used directly in such programs, but there are several programmes seriously considering the potential for such deployment.

Monitoring giantsSitting right at the top of the pecking order, in terms of size, flight altitude and endurance, is the Helios/Pathfinder aircraft. These are huge flying wings, powered by a combination of solar power and hydrogen fuel cell technology, which have flown to nearly 100,000ft and will be capable of missions extending for months at time. These UAVs are still at the developmental level, and have not yet been deployed in severe weather operations. They will have the capacity to utilise sophisticated remote sensing instruments and remain above a particular system, such as a tropical cyclone, for its entire lifetime.

These aircraft provide excellent examples of the different evolutions of UAVs. The Global Hawk occupies a niche similar to that of the aged U2 aircraft or its civil version, the ER2. The big advantage of removing the pilot is that much longer endurance is possible and it takes away the need to protect humans against the severe conditions found at high altitudes. The Aerosonde and Helios UAVs, by comparison, represent aircraft which can only exist without pilots. For the Aerosonde this meant a much smaller and cheaper aircraft, but one with long endurance, which can operate in dangerous conditions. Indeed, the Aerosonde was designed specifically to be dispensable so that it could be used for those missions where the risk of loss is high. The Helios fills a niche between manned aircraft and satellites. It is capable of carrying substantial payloads of remote sensing instruments, operating at the edge of space, but it can also be retrieved and upgraded with relative ease.

Operating systemsWhile the aircraft tend to be the focus of attention, equally important has been the major advances in ground systems which provide both UAV command and interactions with the customers and users of the system. We illustrate the sophistication of these systems by way of a description of the Aerosonde ground system. The ground system uses the following components: designated launch and recovery sites; command centre; communications through a combination of radio, landline, satellite and internet; and provision of data and mission information to users through the Aerosonde Virtual Field Environment.

The Aerosonde can be operated from a variety of locations, requiring little more than a large field for launch and recovery. During a mission, a command centre has responsibility for mission coordination, a monitoring, regulatory and safety watch, the tasking of multiple mission requirements, and monitoring the flow of data. Thanks to modern communications, this command centre can be located in a variety of locations. The user of the UAV accesses relevant data and monitors the progress of missions in real time from their own personal computers or office workstations. Users can also develop modified missions using a mouse `point and click' and upload these to the command centre for implementation. The major advantage of this approach is that users can have complete interaction with the mission without leaving their home office, thus obviating the need for unnecessary travel to remote locations.

Weather/disaster monitoringAs noted earlier, major uses for UAVs in severe weather and natural disaster monitoring involve observing the weather system and documenting the impact. While there have been several limited examples of UAV use in these circumstances, the full capability of UAV technology has not yet been adequately explored.

In the observing mode, UAVs excel at what is known as targeted observations. Such observations are aimed at providing data in regions that are not well served by routine observing systems and where there is the greatest benefit to be gained. For example, a computer forecast-model can define where the maximum growth in forecast errors will occur and the aircraft can be sent to this region to take saturation observations. Alternatively, an insurance company, or a building regulatory authority may need improved information on the detailed structure of the lower wind fields in tropical cyclones to be used in analysis of building codes or insurance risk.

The substantial benefits to be gained from such observations have been demonstrated by the manned aircraft observations of hurricanes taken by the US over the past 40 years. Unfortunately, the cost of such an operation has meant that similar observations have not been undertaken in other regions. UAVs will be providing this role routinely within the next several years and this has been a major basis of the development of the Aerosonde. Analysis presented by us in the Bulletin of the American Meteorological Society has shown that forecast improvements of up to 30% could be achieved. Routine UAV observations will also provide the first objective definition of wind structure and intensity of cyclones in many affected regions.

During and immediately after impact by a natural disaster, normal community systems are in disarray and it is very difficult for authorities to effectively manage both preparations and evacuations, and the post-event community support. Here UAVs have a major role. They can be deployed to monitor evacuation routes and provide detailed information on problem areas. Long-endurance flights can be maintained over affected areas to provide visual and infrared imagery of damage patterns, of problem areas (such as looting), and of deployed emergency response systems. During weather-related events communications are often badly disrupted. UAVs have the capacity to act as repeaters for voice communications and as an internet node connecting emergency management systems with the outside world.

References
1 www.uavforum.com
2 www.aerosonde.com

Greg Holland works for Aerosonde Pty Ltd, Melbourne, Australia. www.aerosonde.com