Recent years have seen a significant growth of interest in the use of aerostats as solutions to long-loitering observation and border monitoring applications.
While aerostats undoubtedly have significant advantages in this application, there is often a lack of understanding in the broader issues surrounding the selection of the most appropriate system to use, and the longer term requirements in their operation and support. Lighter-than-air (LTA) systems have been used for many decades, and still offer unrivalled performance in their field.
The plethora of different systems offered by suppliers all have their own advantages, but this article hopes to provide some guidelines in the various types and their applications. There four key factors that will determine the optimum solution for your mission. These are 1) What you want to carry, 2) How high you want to take it, 3) Where in the world you are, and 4) How you want to retrieve the data from your payload. To begin the process of selecting a system, profiling your mission in these terms is the first step.
Understanding the threats you face, and the tools available to combat them is key to selecting an aerostat system. Where the threat is, the data you need to identify it, and the tools required to interpret that data will all provide you with the parameters for selecting the optimum solution.
Aerostats can operate with wide range of payloads including cameras, radar and an expanding selection of electronic surveillance devices. From Infra red to ultra-violet, there are camera solutions to meet all requirements, from detecting incursions, to pollution monitoring. Geo-location and object tracking have become standard features on most of these systems, but the higher the quality and the more sophisticated the solution, the heavier it tends to be. Inroads in this area have been considerable, and performance is advancing rapidly, so designing your system for easy upgrade makes sense.
Communication location and interception is also becoming lighter and more capable, so mixed payloads will provide a much richer picture of the world around you. Since weight is the enemy of any LTA system, select carefully to maximise your operational efficiency. Many of the tools you need might already be in use in other sections of your security toolbox, and it makes sense to go with commonality for ease of maintenance and support.
The higher you fly, the further you can see, but this also requires a more capable and consequentially heavier payload. From an altitude of 1000m, you will have a view to the horizon of over 100km. However, when flying at 1000m, the aerostat is not only lifting its own weight and that of the payload, but is now also lifting a longer tether, and this too must be part of the overall consideration. Finding the optimum altitude for operation is the product of a number of these factors, and each one can have a significant impact on the design of the system. A lightweight camera can let you go higher, but will not provide the resolution at full range to render it useful. Matching the payload to the mission is key, but it is important to understand the limitations of the platform.
Among the factors that will impact on your choices, the region and the climate for operations is one of the most important. The higher your launch site, and higher you want to fly, the larger the aerostat needed. Like other aircraft, hot and high operations require more power, which in the case of an aerostat is its lift gas. Air pressure and temperature both have a significant impact on the lift gas, so understanding these issues will assist your selection. Helium expands with both altitude and temperature, so the system must be designed to accommodate that expansion and still meet your operational needs. Systems must be selected to operate in the worst-case scenario, so your mission profile will be an important factor.
Once you have determined the location, and chosen your payload parameters, there are a number of options for getting power to your payload, and the data from it back to the ground. Most commonly used is the complex tether that can contain both power conductors and optical fibres for data. The alternatives are batteries and radio links. Both add weight to the payload, but the complex tether facilitates far longer loitering times at the expense of that increased weight. Increasingly advanced radio links are suitable for tactical systems, so the planned mission duration is key to the selection of power and data methodology.
There are many types of aerostats available these days, from the smallest spherical systems to the larger pressurised aerostats, and the common factors they all share are that they use helium as a lifting gas, and are tethered to a base of some sort. In relation to their lifting capacity and duration, they always offer a very low cost option for carrying cameras, radars and other electronic surveillance sensors to altitude.
Smaller spherical systems, even when fitted with a stabilising net, are the least stable type, and while their spherical shape is the most efficient for lift, it provides the lowest level of stability. Larger systems are based on variants of the traditional blimp design with fins and a rudder to provide both stability, and to ensure that they weathercock into the wind. These larger aerostats also generate aerodynamic lift which contributes further to their stability. As with other forms of aircraft, the most critical phases of operation are launch and recovery, where turbulent low-level winds are most likely to buffet the system. In overall terms, when carrying an expensive and sensitive military payload, the docility of the system in the launch and recovery phases is very important.
In terms of size, there are several factors that will impact on the system the client needs to select. The most important are the weight of the payload, and the height at which it is to be operated. As previously noted, currently, all aerostats use helium as their lift gas, though small inroads are being made in returning to hydrogen. The lift capacity of helium in very general terms, is one kilogram per cubic metre, and while this changes with temperature and pressure, as a general rule of thumb this ratio makes calculations easier. The weight you have to consider is not just the payload and its power and data management systems, but also that of the aerostat itself, and the weight of its tether it is lifting when at altitude.
The helium lifting gas will expand or contract in response to changes in both altitude and temperature, so the design must accommodate this. Most modern aerostats require a pressure management system to maintain the shape of the aerostat as the gas expands and contracts, and to accommodate wind conditions.
Until the use of hydrogen becomes widespread, the standard lift gas is still helium. Helium is not a renewable resource, but is a by-product of exploration of other materials. While there have been significant fluctuations in supply over recent years, the supply situation is gradually improving, but at the same time, the number of industries using helium in processes from semiconductor production to MRI medical scanners, there is an increasing demand on the suppliers. The rising cost of helium and limited supply means that designing the most efficient aerostat system is key to ensuring the long term viability of the system. An oversized aerostat may provide spare capacity to add payload in the future, but will pay the price of higher helium use and consumption.
Operating and maintaining LTA systems is not very different to operating other forms of aircraft since all require regular inspection and maintenance, with facilities required to respond to damage, component replacement and payload support. Aerostats can be large, so hangars and access equipment will form part of any committed system introduction. The suppliers approach to modular power and data systems, and to repair and maintenance requirements will also impact on both the long term cost of operation, and its reliability.
It is important to remember that all aerostats require monitoring 24/7. While most can be launched and recovered by a crew of three to five individuals, they still present a hazard to air traffic, especially in the even of a breakaway, so require constant monitoring to ensure that they are performing safely throughout the mission profile. Decisions regarding the appropriate window for launch and recovery, and the weather limitations of operations require comprehensive training, and a detailed understanding of the system itself. Another key to effective operations is therefore the quality of training that your operations team have, and whether it is conducted at the suppliers base, or preferably on your own operational site, selecting the right crew, and the quality of the training program will have a significant impact on the effectiveness of the mission.
In summary, the key to planning and implementing successful aerostat systems depends largely on you, the customer having a comprehensive understanding your mission, in the context of the advantages and the limitations of aerostat systems. As the user, the deeper your understanding of your needs, the better able you will be to select the most effective solution. Every supplier will claim to have a solution, and purchasing without a comprehensive understanding of your specific mission requirements can result in some very expensive mistakes. Knowing in detail what you need will prepare you for seeking out focussed quotations and tenders.
by Stuart Haycock, Airborne Operations