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A team of scientists and engineers sampling greenhouse gases in the remote South Atlantic have pushed the boundaries of what’s possible with lightweight fully autonomous UAV (unmanned aerial vehicles) by flying octocopters at altitudes of up to 9,000ft.
The researchers, from the University of Bristol, University of Birmingham and Royal Holloway, University of London used the octocopters to obtain samples above the trade wind inversion (TWI) on Ascension Island.
Exploiting the extraordinary ‘atmospheric reach’ of winds arriving at the island from Africa and South America, the team are investigating southern tropical sources of methane, an important factor in the variability of the global methane budget.
Together with a wider sampling strategy, the aim of this NERC-funded project is to improve understanding of southern tropical methane sources, their distribution and causes of variability.
The carbon fibre UAV Octocopter airframe was custom-designed by Bristol researchers Dr Tom Richardson, Dr Colin Greatwood and Professor Jim Freer. The airframe, with eight contra-rotating motors, was built and tested at the Bristol Robotics Laboratory (BRL), a partnership between the University of Bristol and the University of the West of England (UWE Bristol).
The platform was designed to carry equipment developed by Dr Rick Thomas of the University of Birmingham to capture air samples and allow the rapid measurement of temperature and humidity during the flight.
Co-principal investigator Professor Jim Freer of Bristol’s School of Geographical Sciences and Cabot Institute said: “The combination of a lightweight but powerful autonomous airframe with high stability and reliability coupled with rapidly responding atmospheric sensors is the reason why the campaign was so successful.
“The engineers at Bristol have developed an amazing platform that is capable of flying in challenging environmental conditions – it’s all very exciting.”
Sensors controlled by on-board computers linked to ground station laptops enable real time observations of temperature and humidity. This allows the altitude where the TWI starts and ends to be determined during ascent which informs decisions of where to sample during the descent, accurate to within a few metres.
Dr Rick Thomas, an expert in sensor technology and beyond-line-of-visual-sight UAV operations, added: “There is nothing stopping the further development of this airframe for any targeted sampling strategy as long as you have the right sensors on-board and the right team to safety fly UAVs to these extremes.”
The team conducted multiple flights daily over a 12 day campaign, reaching more than 3,000ft above the TWI. Flights to these altitudes last from 14 to 17 minutes as the UAV ascends and descends at 5 metres per second allowing for rapid sampling strategies to be deployed. At times the team encountered strong winds and significant levels of cloud but were still able to operate.
Dr Tom Richardson of Bristol’s Department of Aerospace Engineering, an expert in flight mechanics and control, said: “We occasionally encountered very strong winds above the TWI, estimated to be over 60 kilometres per hour which reduced our maximum operating altitude and endurance.
“Next year we plan to go back with a larger airframe and more powerful motors which will further expand our operating envelope. We now have a fantastic capability that allows us to pinpoint sample altitudes in real-time whilst taking continuous sensor measurements at up to 2.5 kilometres above the ground.”
Also working on the project were Professor Euan Nisbet, Dr David Lowry and Rebecca Brownlow from RHUL and Professor Rob MacKenzie from the University of Birmingham.
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