T=0h. Autonomous (hand-) launch.

One year after having demonstrated the 81-hour continuous solar-powered flight that is still the current world record in flight endurance for all aircrafts < 50kg total mass, the AtlantikSolar UAV has completed its next milestone by demonstrating the first-ever fully autonomous (from launch to landing) solar-powered perpetual flight with significant payload (Color + Thermal Camera) in a 26-hour Search-and-Rescue (SaR) mission.

While the 81-hour and 28-hour endurance record flights were important milestones that demonstrated the perpetual endurance capability of AtlantikSolar, they required manual pilot control for launch and landing and did not carry any aerial imaging payload. However, the missions that our team really cares about – Search-and-Rescue missions relying on long-endurance aerial sensing to support authorities e.g. in the European refugee crisis currently unfolding over the Mediterranean Sea – require both of these elements, i.e. ease-of-use through full launch-to-land aircraft autonomy as well as significant payloads to help the rescue teams with the detection of victims on land and sea.

The 26-hour solar-powered Search-and-Rescue flight performed by AtlantikSolar AS-3 from July 19^th – 20^th 2016 demonstrated exactly that, and is, therefore the first-ever flight worldwide to combine:

• Perpetual flight: 26-hours of solar-powered day/night- and thus energetically-perpetual flight.
• Full aircraft autonomy: No pilot stick moved within 26-hours of flight.
• SaR payload: The aircraft carried a 10 Watt 300g payload (1 Color camera, 1 Thermal Camera, 1 ODROID onboard computer with WLAN) and performed victim detection from the air during day and night.
• Environment-aware: The aircraft performed automatic thermal updraft tracking for increased energetic efficiency and to speed up the battery recharge.

Flight summary

The 26-hour flight started on July 19^th at 18:02 local time in Hinwil, CH with full batteries. The launch was performed fully autonomously: After all system checks were complete, the aircraft was tossed in the air via a hand-launch and then continued automatically and without pilot interaction towards its first loitering waypoint.

T=2h. Circling above while live-streaming camera data.

The respective flight control technology is based on an ETHZ/3DR Pixhawk autopilot with a custom flight controller designed at the Autonomous Systems Lab. Equipped with a color camera, thermal camera, an onboard computer and wireless LAN, the aircraft began live-streaming the respective images. The full payload was operated until around 21 o’clock, after which the color camera was switched off because of the insufficient lighting conditions. The batteries were at 95% state of charge at that time, having started their discharge shortly before 20 o’clock. Note that operating a fully-fledged SaR payload consuming 10W is a significant challenge for perpetual flight on a small-scale solar-powered UAV such as AtlantikSolar, and the overall power consumption of ~60W was thus closely tracked during the first hours of the evening flight.

T=5h. Flying at night with position indicator LEDs on.

T=5h. The airplane is seen loitering in the top left, performing the SolAIR Search-and-Rescue mission.

The actual testing of the Search-and-Rescue capabilities started just before 23:00. With the aircraft now flying in total darkness, the infrared camera served as the only source of aerial imaging information. As visible in the video, the payload system clearly manages to find the victim lying in low grass (and surrounding houses, tents, cars, the streets and especially the warm electric generator) at 23:05. The victim detection was performed manually by the ground station operator based on the live-streamed images this time. However, the next AtlantikSolar test flight will have automatic on-board victim detection (as already tested in the ICARUS search and rescue project) implemented. These demonstrated capabilities are integral elements for perpetual flight and aerial victim detection missions. We are convinced they can be of great help in solving pressing issues, such as the European refugee crisis of 2015 and 2016 above the Mediterranean Sea.

T=5h. Live-streamed thermal camera images with annotation. Note the victim that can easily be detected.

T=13h. Ground Station on the next morning. We’ve received first sun here.

Despite heavy winds of up to 8m/s, the Search-and-Rescue support activities with the infrared camera payload were continued throughout the rest of the night. The sun hit at 06:20 local time on July 20^th and the minimum battery state of charge was reached with 26% battery energy remaining at 08:04. Considering the additional payload power consumption (and mass) of 10W , this energetic margin – achieved on July 20th, and thus about one month after the solar solstice on June 21st – is a success and even exceeds the margin of 23% predicted by our simulations. To accelerate the battery recharging process, the aircraft also implemented autonomous thermal updraft tracking that was enabled during the late morning once thermal updrafts were encountered (see video). Multiple “free” altitude gains of >100m were achieved this way – again without any pilot interaction. The batteries were fully re-charged at 15:30.

T=26h: Final approach during the autonomous landing.

As a final step, and after 26-hours of solar powered flight, the aircraft performed a fully autonomous landing at the Hinwil, CH airfield. Using its lightweight LIDAR (Light Detection And Ranging) sensor to measure its distance to the ground, AtlantikSolar – a hard to fly aircraft that can usually only be steered and landed by extremely experienced pilots – could safely perform the automatic landing. We consider the demonstrated full flight autonomy a vital step to allow search-and-rescue support teams, which usually do not possess extensive UAV flight training, to benefit from the significant advantages of solar-powered and in general high-performance UAVs.

T=26h. Landed autonomously.

AtlantikSolar AS-3 after its 26-hour fully autonomous solar-powered SaR mission.

The small operating post in Hinwil, CH.

Live-streamed RGB camera image. Our tents are clearly visible.

Further information

Detailed design and technical information on the UAV platform can be found in “Oettershagen P, Melzer A, Mantel T, Rudin K, Lotz R, Siebenmann D, Leutenegger S, Alexis K, Siegwart R (2015), A Solar-Powered Hand-Launchable UAV for Low-Altitude Multi-Day Continuous Flight. In: IEEE International Conference on Robotics and Automation (ICRA)” [Download] .

An additional up-to-date publication, including the research for this flight, is planned.

Some more impressions from the flight were broadcasted live through our twitter account.

Acknowledgements

This research was funded through the research project SolAIR – Solar-powered Automated Aerial Imaging and Reconnaissance Using Infrared Cameras. SolAIR is a project funded under Armasuisse contract #043-12. The AtlantikSolar project is in addition funded with ETH Zurich’s internal resources, private supporters, and the European Union FP7 Search-And-Rescue research projects ICARUS and SHERPA .

Multiple project partners and collaborators have contributed towards making this important milestone possible, and we’d like to thank all of them for their various and ongoing support. Finally, we are grateful towards the Hinwil and Rafz model aeroplane clubs for providing the airfield!

Pilots: Rainer Lotz, Adrian Eggenberger. Development and Operations Team (Autonomous Systems Lab): Philipp Oettershagen, Rainer Lotz, Amir Melzer, Thomas Mantel, Bartosz Wawrzacz, Konrad Rudin, Thomas Stastny, Timo Hinzmann, Dieter Siebenmann, Dr. Gregory Hitz, Prof. Dr. Roland Siegwart.

The AtlantikSolar project was developed at the Autonomous Systems Lab at ETH Zurich. 

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T=0 hours. AtlantikSolar team hand launches the AS-2 at 09:32 on July 14th, hoping to set a new world record.

Two weeks after demonstrating AtlantikSolar’s first 24-hour flight, the fixed-wing team of ETH Zurich’s Autonomous Systems Lab has reached another milestone: 81.5 hours and 2316 km of continuous flight for its 6.8kg AtlantikSolar 2 Unmanned Aerial Vehicle (AS-2). This breaks the flight endurance world record in its class.

The AS-2’s fifth test flight:

• sets a new world record for the longest ever demonstrated continuous flight of all aircrafts below 50kg total mass, and is also the longest-ever continuous flight of a low-altitude long-endurance (LALE) aircraft (the previous record being a 48-hour flight by the 13kg SoLong UAV ).
• is the second-longest flight ever demonstrated by an Unmanned Aerial Vehicle (behind Airbus Space’s 53kg Zephyr 7)
• is the third-longest flight ever demonstrated by a solar airplane (behind Airbus Space’s 53kg Zephyr 7 and the 2300kg Solar Impulse 2)
• is the fifth-longest flight ever demonstrated by any aircraft (both manned and unmanned).

In addition, the flight is a first important milestone to verify the UAV’s ability to stay airborne for multiple days while providing telecommunication services in large-scale disaster-scenarios, or live-imagery during industrial sensing and inspection missions.

T=2 hours, shortly after launch. Weather conditions: occasional clouds, and strong winds up to 40km/h.

Flight Summary

The flight was performed at the Rafz, Switzerland, RC-model club airfield from July 14^th-17^th, of which the first three days provided sunny conditions. Take-off was performed via hand-launch at 09:32 on July 14^th, and after 2316km and 81.5 hours – 4 days and 3 nights – of flight, the aircraft landed safely and with fully charged batteries at 18:56 on July 17^th. The fully charged batteries would in theory have enabled to continue the flight through the night again. With the exception of take off, the aircraft was in fully-autonomous operation 98% of the time, and less than 2% in autopilot-assisted mode via its Pixhawk autopilot.

T=8 hours. Monitoring the aircraft and its energy generation and storage system. The batteries are fully charged.

The long-endurance flight provided helpful insights on the flight performance: The average level-flight power consumption in calm conditions (e.g. during night) was shown to be between 35-46W. Maximum power input throuh the 88 SunPower E60 cells during the day was around 260W. With this performance data, the aircraft managed to fully charged its batteries (100% SoC) at around 13:05 local time, before the time of maximum solar radiation (solar noon, occurring around 13:30). After flying through each of the three nights, the aircraft on average reached a minimum state of charge of 35% at around 07:45 local time, showing sufficient energetic safety margins for worse environmental conditions (such as longer nights, cloud cover or winds).

The flight also subjected the aircraft to a wide range of environmental conditions. Among them were thermal updrafts during the first evening/night (causing a remaining state of charge of 40% ), and downdrafts during the second night (remaining state of charge 32%). The last hours of the flight were marked by upcoming thunderstorm clouds and the strongest winds – up to 60 km/h – the aircraft was ever subjected to. Although the ground station was partially damaged by the winds, the airplane could be landed safely in autopilot assisted mode once the winds had calmed down.

T=41 hours. Drawing circles in the night sky using the onboard position indicator lights.

T=81 hours. Thunderstorm clouds and winds up to 60 km/h make landing the AS-2 a challenge.

T= 81.5 hours. Landing.

T=81.5 hours. A happy AtlantikSolar team after a record flight.

Future work

Having demonstrated the multi-day endurance capability of the bare UAV platform, the AtlantikSolar UAV project will now focus on extended endurance flights with payloads including optical and infrared cameras, and atmospheric sensors. These payloads will also be carried during a long-endurance and long-distance mission of more than 12 hours and 400km that is planned for later this year in the Brazilian rain forest.

Further information

Detailed design and technical information on the UAV platform can be found in “Oettershagen P, Melzer A, Mantel T, Rudin K, Lotz R, Siebenmann D, Leutenegger S, Alexis K, Siegwart R (2015), A Solar-Powered Hand-Launchable UAV for Low-Altitude Multi-Day Continuous Flight. In: IEEE International Conference on Robotics and Automation (ICRA)”

Acknowledgements

This research was funded through ETH Zurich’s internal resources, private supporters, and the European Union FP7 Search-And-Rescue research projects ICARUS and SHERPA . In addition, multiple project partners and collaborators have contributed towards making this important milestone possible, and we’d like to thank all of them for their various and ongoing support. Finally, we are grateful towards the Rafz model aeroplane club for providing the airfield and the Aero Club Asas da Planície (Portugal) for providing the backup airfield!

Pilots: Rainer Lotz, Adrian Eggenberger, Philipp Oettershagen, Bartosz Wawrzacz. Development and Operations Team (Autonomous Systems Lab): Philipp Oettershagen, Rainer Lotz, Amir Melzer, Thomas Mantel, Bartosz Wawrzacz, Konrad Rudin, Thomas Stastny, Raphael Schranz, Jan Steger, Lukas Wirth, Dieter Siebenmann, Dr. Stefan Leutenegger, Dr. Kostas Alexis, Prof. Dr. Roland Siegwart.

The flight was performed at the Rafz RC-model club airfield in excellent sun conditions. After take-off at 11:14, with batteries charged to 57%, the aircraft was quickly setup to follow an efficient and fully-autonomous loitering path, which allowed a completed battery-charge by 14:08 o’clock.

The midday and afternoon were characterized by strong thermal up- and downdrafts, but enough power was generated by the solar panels to keep the batteries full. Their discharge started only when the sun slowly went down at around 19:30.

The night flight provided calm conditions, with the autopilot keeping the aircraft stable despite horizontal winds of up to 5m/s, and the safety pilots keeping a good eye on the aircraft using its position indicator lights. Flying at an average airspeed of 8.4m/s (the point of minimum sink rate) and an average power consumption of 43W during the night, the aircraft received first sun at around 5:50 o’clock, and maintained a minimum charge of around 35% until the solar modules regenerated enough power to stay airborne.

After 24 hours of continuous flight, the aircraft had recharged its batteries to 84%, a significantly higher state-of-charge than the day before. The batteries were in fact fully charged by 12:43 o’clock – one hour and twenty-five minutes earlier than the day before.

AtlantikSolar landed safely at 15:35, thereby setting a new Swiss endurance record for unmanned solar-powered flight, and improving upon the previous internal record (ASL’s Sky Sailor) by a over an hour.

The project’s next goal is to extend the flight duration to more than 80 hours (3 days), in order to beat the old endurance record for solar-powered UAVs below 20kg (48h flight by the 13kg SoLong UAV in 2005) by more than a day. If it is able to achieve an 80-hour flight endurance, the 7kg AtlantikSolar would be the third-longest flying aircraft in the world, only behind Airbus Space’s 53kg Zephyr and the 2300kg Solar Impulse 2.

We’d like to thank everybody who made this flight possible — including all project partners and collaborators, the Rafz Model airfield club, and our safety pilots — for working so hard and making this big step in the project possible.

The AtlantikSolar project was developed at the Autonomous Systems Lab at ETH Zurich.