Missions for small unmanned aircraft (Group 1 and Group 2) include over-the-hill surveillance and providing airborne communications relay points. Greater endurance is almost universally desired by operators to increase the time spent on-station performing the mission and reduce the number and frequency of takeoffs and landings.

Autonomous soaring techniques were first proposed by Wharington and refined by Allen as a method to find convective thermal updrafts and gain altitude energy. While several autonomous soaring algorithms have been implemented on unmanned aircraft and demonstrated significant endurance gains, the technology of autonomous soaring has not yet bridged the gap from research to practical application for a mission.

Rather than providing a fixed geometric orbit to continuously track a target on the ground, an aircraft in autonomous soaring mode maneuvers into and moves with thermal updrafts. This maneuvering is often believed to be counterproductive to a surveillance mission, especially if the ground target is moving against the wind, since thermals tend to drift downwind. However, this research shows a surveillance mission is still achievable while performing autonomous soaring.

Cooperative autonomous soaring is a technique in which multiple aircraft flying in close proximity share information about the local conditions in order to improve each individual aircraft’s performance. Theoretical and implemented demonstrations have shown promising results of two vehicles sharing soaring information. Depenbusch demonstrated multiple aircraft flying simultaneously and sharing soaring data, while also using memory of prior soaring conditions. Storing and remembering areas of lift is a way for a single agent to cooperate with itself and should be explored in future research.

No research has previously tried to use autonomous soaring techniques to carry out a specific mission. Autonomous soaring with mission constraints has been demonstrated, but did not include any attempt to quantify the performance of the mission payload itself. This research attempts to quantify the effect of autonomous soaring on an imagery mission using imagery resolution and time-on-station as metrics. For additional realism, the demonstration also includes a communications relay payload to further add real-world transmission effects.

A single-vehicle communications relay concept of operation (CONOP) is shown in Figure 1. This setup uses a single airborne asset providing service to two remote sites. However, maneuvering within the given airspace constraint area may not provide sufficient coverage, and requires the aircraft to operate over the obstruction. Instead, this report proposes two aircraft: one over the base and one over the remote user.

Figure 2. Notional imagery and communications relay mission with autonomous soaring.

A notional over-the-hill surveillance and communications relay mission is designed to show the potential mission performance enhancement offered by using autonomous soaring technologies. Figure 2 shows a command site that is out of radio frequency (RF) line of sight (LOS) to a remote unit. The RF blockage could be a mountain range, significant distance, urban obstructions, or any other number of complicating factors. In this scenario, two unmanned aircraft provide a communication link between the command base and remote user. Also, the remote user has direct access to the video product, since s/he is on the same network.

A typical Group 1 UAV has only two to four hours of endurance. If the remote site is one to two flight hours away from the launch site, this leaves little or no time on-station actually performing the mission. This research shows how a Group 1 UAV using autonomous soaring and a solar power system can extend the aircraft’s four hours battery-only endurance to more than 12 hours. This will provide uninterrupted imagery over the remote target site.

This work was done by Daniel J. Edwards, Aaron D. Kahn, Sam V. Carter, Phillip Jenkins and David Scheiman for the Naval Research Laboratory. NRL-0078

This Brief includes a Technical Support Package (TSP).
Quantifying Autonomous Soaring on a Surveillance and Communications Relay Mission

(reference NRL-0078) is currently available for download from the TSP library.

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This article first appeared in the June, 2020 issue of Aerospace & Defense Technology Magazine.

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