Suspending a dipole radar scanner from a helicopter – improving methods of evaluating glacial water resources
Glaciers cover about 10% of the Earth’s surface and water from melted glaciers is important for local people to drink and to irrigate their land, as well as for hydroelectric power. Understanding the thickness and change in glaciers is essential for understanding and managing risks related to glaciers (e.g. floods and avalanches, which can have a massive impact on local areas and people) and the amount of meltwater available from glaciers, which is necessary for reducing water stress in drought-prone areas. Change in glaciers is also a useful measure of climate change. Both climate change and the increased expectation of food and water shortage as a result of droughts are important ecological and social problems. Improved knowledge of the volume of glaciers can contribute to a better and quicker response to potential problems. As glaciers are in some of the least accessible areas of the world, scanning glaciers is not without problems and scientists and engineers are currently considering the best methods of scanning and how scanners may be transported. The study reported here focuses on the feasibility of suspending a dipole radar from a helicopter.
Scanning glaciers in mountainous areas is problematic because of the terrain, temperature and accessibility. The project reported here is part of a research study led by Dr Hamish Pritchard of the British Antarctic Survey (BAS), and involves a series of tests to establish how a dipole radar to scan glaciers, could be suspended from a helicopter, with a view to improving methods of measuring glaciers and to identifying practical difficulties. This project, undertaken by Allan McRobie, focuses specifically on structural aspects related to suspending the dipole radar scanner and is part of a larger project involving structural engineers, aerodynamic engineers, BAS and material scientists. Involvement in the project looking at structural factors formed part of my work experience placement.
The significance of the project is that it can contribute to more reliable, effective and efficient scanning of glaciers, increasing our knowledge of their structure, changes in structure, mass and composition and consequently, the effects of climate change. This will increase our knowledge and ability to respond to the advantages and risks of glacial melt.
Glaciers are formed (sometimes over thousands of years) when fallen snow does not melt away, but remains in one place and is compressed over years and becomes ice. Glaciers are found predominantly in the Arctic and Antarctic, but cover around 10% of land on the Earth, mostly in mountainous areas, with the highest concentration of these in Asia (1). They can be mountain glaciers, flowing down mountainsides or continental glaciers that cover huge areas of land. Glaciers are sensitive to temperature, allowing snow to last or snow and ice to melt. They can advance downhill when they reach a certain mass, or decrease in size when melting is greater than snowfall (2). Temperature affects how glaciers grow, reduce or move and so glaciers provide a useful measure of climate change over time. Different parts of a glacier respond differently to climate changes and so it is important to understand their structure to know more about climate change.
Glacial melt or meltwater is the water produced from glaciers as the temperature rises. It provides drinking water for people living near mountains and irrigates the land. Glacial melt can also provide hydroelectric power. Pritchard (1) reports that Asian glaciers provide water to meet the needs of 136 million people, which can be maintained through droughts. For those living near glaciers, there is the danger of glacial lake outburst floods (GLOFs) when glacial melt causes the amount of water in lakes at the top of the glacier to be too great for the debris that creates a natural dam (a moraine dam) that holds the lake in place(3). GLOFs can destroy villages. Avalanches also pose threats to people living near glaciers and these are linked to changes in glaciation. Understanding the structure of glaciers and changes over time is important for managing risks of floods and knowing when water supply may be short.
Although global warming has caused increased melting of glaciers in many areas, not all glaciers are reducing and it is important to know why this is. Raina and Srivastava cited by Reynolds (4) have reported information on 9,575 glaciers in India. The Karakoram and the Himalayan ranges are of particular interest. The Karakoram range crosses Pakistan, India and China, and, other than the polar regions, is the most glaciated area. Debris seems to be protecting the Karakoram from the Sun’s heat and Karakoram glaciers are not reducing to the same extent as Himalayan glaciers.
In measuring glaciers, Reynolds (4) indicates that size, shape, height, depth and volume are important, as well as other details, such as snow and debris (which shield the snow and can dam lakes) and the effects of tributaries to a main glacier. Structure (thickness and change in thickness) is important as it gives information on climate change, meltwater and flow that impacts surrounding areas.
The change of glaciers is difficult to measure because of limits to technology, although these have been reduced over time, Reynolds(4). Measurements are also difficult due to the terrain, which is mountainous and hard to access. For structural information, surveys have, until quite recently, been limited to photographs from helicopters.
In earlier surveys, Kennett, Laumann and Lund (5) describe the scanning of glaciers in temperate regions from the ground, using snow-scooters to move the equipment. To overcome the difficulties of steep slopes, as with Norway’s glaciers, they considered scanning from the air but recognised the difficulties of managing antennae measuring 15m in length in mountainous regions. They reported scanning from fixed-wing aircraft since 1981(6), but commented on the problems of these in navigating narrow gaps and flying through regions with mountains. Kennett et al (5) write of the use of a helicopter with suspended antennae, consisting of dipoles, from the main hook 20m below the body of the helicopter, with the receiver linking a digital oscilloscope to a portable computer. They found the signal to be both reliable and strong, scanning up to 350m of ice in a temperate region. This seemed to be a reasonable strategy and one that has been built on in this study.
Reynolds(4) reports imaging using radiowaves through ice sheets since the 1950s, based on the principle of echo-sounding, where radiowaves from a transmitter are reflected back from structures, in this case below the ice, to a receiver, which then allows imaging. The ice itself is quite transparent to radiowaves, but water and land absorb and reflect radiowaves differently. Radio-Echo sounding (RES) can be used for distances up to 5km. Since the 1970s, Ground Penetrating Radar (GPR) has been used with frequencies in the range 50-700 MHz and there has been a lot of development of this since the 1990s. RES tended to be used from the air because of the extent of the area to be covered and their inaccessibility and GPR from the ground or water. Reynolds (4) indicates that satellites are also used to assess changes in water volume via Synthetic Aperture Radar (SAR) which detects features of water different to ice and snow and can measure surface height. He also suggests that using a helicopter allows GPR antennae to be attached under the fuselage.
Current developments in scanning glaciers focus on the use of GPR on helicopters because of the helicopter’s manoeuvrability in mountain ranges, and because the use of GPR antennae have made attaching radar to helicopters easier. McCarthy, Pritchard, Willis and King (7) have shown that GPR is reliable, effective and efficient in measuring glacial debris thickness in comparison with invasive methods such as digging pits or measuring the thickness of debris on ice cliffs. However, they note the difficulties in mountain areas.
A structure to hold a dipole radar underneath a helicopter based on the concept of a bridge was modelled and then tested at full size.
The initial design consisted of two, 20-metre, lightweight, telescopic, tubular poles at the base, which had a tapered shape, with the dipole radar going through the poles. The poles were made out of GFRP (Glass Fibre Reinforced Plastic) not CFRP (Carbon Fibre Reinforced Polymer) because GFRP is light weight (and so can be carried by the helicopter) and CFRP is conductive and would have interfered with the radar’s signal and therefore the radar would have been ineffective. The poles needed to have a great length so that the dipole radar could be longer to transmit lower frequency radio waves which would be absorbed less by the ice and therefore a better signal would be received. These two poles were connected with a coupler and at the end of the poles an arrow shape was needed in order to reduce rotational vibration. String or wire was attached at different points on the poles and met at 10 metres from the pole. A carabiner clip had the string/wire attached to it and was connected to a cable which could hang from the helicopter.
In order to test that the structure holding the dipole radar would be effective, four experiments were carried out to determine any changes that needed to be made to the initial design.
Experiment 1: A very small model of the structure was made using a quad-copter, two much smaller poles (attached with white tape) and fishing line.
Experiment 2: Improvements were made to the first experiment in order to reduce the pendulum motion of the structure, which involved attaching a pole to the quad-copter and attaching the structure to the pole with extra string.
Experiment 3: To try to reduce the amount of resources needed to make the structure, only one fishing line was used. This was a fifth scale model using 4m poles.
Experiment 4: The next experiment to try was the full-scale model. In place of the lightweight, telescopic, tubular poles, masts designed for use as vertical antennae supports, were used. Before carrying out this experiment, a risk assessment was done and measurements were taken in order to do calculations to see whether the model would buckle. Unfortunately, the outcome of the calculations showed that the model would most likely buckle. To prevent the model from buckling the height was increased from 10 metres to 14 metres in order to reduce the compressions that would contribute to the buckling of the poles. Instead of a helicopter, a cherry picker was used to lift the structure and an aluminium coupler was used to connect the two poles.
Experiment 1: This experiment concluded that the structure was effective, however the structure moved in a pendulum motion, which needed to be reduced
Experiment 2: Attaching a pole to the helicopter would not be feasible as there would only be one attachment point on the helicopter, so the other pole could not be attached to it. A bundle of three poles might be used to add extra weight, making the pendulum motion less easy to excite, and this would also add stiffness to help inhibit any potential for buckling and reduce the amplitude of any vibrational modes.
Experiment 3: The two, 4 metre, poles self-buckled, which would not be appropriate for the structure for the dipole radar.
Experiment 4: The poles did not buckle and the structure was successful.
Overall the bridge-like structure to suspend a dipole radar in a pole casing was successful, suggesting that this may be a good way to attach a radar to a helicopter to scan glaciers. However, the pendulum motions may need to be reduced by, for example, adding weight before the dipole radar can be used suspended from a helicopter in this way, and consideration will need to be given to the properties of the materials used under extreme cold temperatures.
The tests highlight possible ways forward in improving scanning of the structure of glaciers, with a positive impact on our understanding of global warming, climate change, meltwater and other aspects of the effects of glaciers on ourselves and our environment.
- Pritchard, H.D. Asia’s glaciers are a regionally important buffer against drought, Nature, 545, 2017, 169-174
- Hays, J, Glaciers: Their Mechanics, Structure and Vocabulary, Facts and Details ,last updated January 2012, accessed 27 July 2017 http://factsanddetails.com/world/cat51/sub323/item1314.html
- IRIN, The Inside Story on Emergencies, Himalayan glaciers melting more rapidly, last updated 20 July 2012, accessed 27 July 2017 http://www.irinnews.org/report/95917/climate-change-himalayan-glaciers-melting-more-rapidly
- Reynolds, J.M. Ground Penetrating radar surveys for detailed glaciological investigations in the Polar and Himalayan regions. In: Ramesh, R., Sudhaker, M and Chattopadhyay, S. (eds). Scientific and geopolitical interests in Arctic and Antarctic. Proceedings of International Conference on Science and Geopolitics of Arctic and Antarctic, (iSaGAA), March 2013, LIGHTS Research Foundation, 296:273-288.
- Kennett, M., Laumann, T. and Lund, C. Helicopter-borne radio-echo sounding of Svartisen, Norway Annals of Glaciology, 1993 17, 23-26
- Watts, R.D. and Wright , D.L. Systems for measuring thickness of temperate and polar ice from the ground or form the air. Journal of Glaciology, 27 (97), 1991, 459-469 in Kennett, M., Laumann, T. and Lund, C. op cit
- McCarthy, M. Pritchard, H. Willis, I and King, E. Ground-penetrating radar measurements of debris thickness on Lirung Glacier, Nepal, in Journal of Glaciology, 2017, 63, (239), 543-555
About the Author
Amy Mackie is a 15 year old student in Year 11 at Bolton School. Amy hopes to study Engineering at University and is very grateful for the amazing opportunity to undertake this work experience placement with Allan McRobie, at the University of Cambridge, Department of Engineering.