Automatic Path Generation for Multirotor Descents Through Varying Air Masses above Ascension Island

As part of a NERC funded project investigating the southern methane anomaly a team drawn from the Universities of Bristol, Birmingham and Royal Hollaway flew three large multirotors from Ascension Island for the purposes of atmospheric sampling. The core objective of these flights was to collect air samples from below, within and above the trade wind inversion layer. % in order to provide a means to identify both methane concentrations and isotopic composition. These parameters allow the source of the methane in the different air masses to be tied in to specific source locations when combined with wind back trajectories.

The sampling missions required the aircraft to ascend vertically to collect samples from altitudes up to 2.5km above ground level. The descent on the return journey required careful consideration as rotary wing vehicles can become unstable when descending vertically through still air. The wind profiles over the altitudes flown above Ascension varied from persistent strong winds to areas of very still air.

This paper describes a method for estimating wind speed and direction from the multirotor's attitude during the ascent. An optimization process for determining ideal descent profiles is also presented, which allows the operator to approve automatically generated flight profiles during flight, just prior to the descent. The descent profiles enable the multirotor vehicle to avoid descending vertically through still air by commanding a horizontal translation. Horizontal deviations are minimised in order to avoid additional energy expenditure.