Dolphins play with robotic seaplane

Autonomous sea take-off crucial
However, the ability to do so was crucial for the electrically powered Flying Fish, which the U-M researchers designed and built in just eight months in 2007 to meet the Phase 1 goals of the Defense Advanced Research Project Agency's (DARPA's) "persistent ocean surveillance" program.

"The difficulty of taking off on the ocean is that you either have to predict the oscillations of the ocean surface, or ignore them," said Atkins.

But had the pitch control of the UAV been designed automatically to respond to the vertical oscillations of the waves while taking off, response-time delay would have led to the controls over-correcting for each upward and downward wave movement.

This "induced oscillation" would have made it impossible for the aircraft to take off, said Atkins.

Instead, the researchers chose to solve the problem by effectively ignoring wave oscillations and letting the Flying Fish just plough through the waves on its short take-off run.

After performing take-offs controlled by a pilot using a remote-control radio transmitter, they fed recordings of the pilot's radio-signal inputs into the onboard processing unit that is used to control the UAV's flight surfaces during autonomous flight. Those recordings controlled each autonomous take-off.

Need to stay within a "watch circle"
A major initial goal of the demonstration, which was performed in front of DARPA officials, was to show that the UAV could monitor and remain within a defined "watch circle."

The idea was for the Flying Fish to drift at the center of the circle until its onboard GPS-based navigation system told the craft it was floating too far out of position. That triggered its autonomous take-off sequence, the little seaplane ploughing through the waves for just 10 meters before lifting off, flying and reaching a point where its GPS unit told the UAV it should land again.

The unmanned aircraft had to demonstrate that it could fly across the watch circle area completely autonomously: taking off, climbing, cruising and descending. The craft also had to acquire data all the while, through the onboard inertial gyro sensors it used to measure roll and pitch and the pressure sensors it used to measure airspeed.

Landing essentially involved a shallow descent.

"When it impacts the water, it goes, 'Oh, there's the water,'" said Atkins. "The boat has very well-designed pontoons. Because it doesn’t have a flat bottom, it cuts into the water like a diver, as opposed to belly-flopping."

The tests were successful. "The vehicle was stable and handled very well," said Atkins.

GPS navigation
GPS was suitably accurate as a navigation aid because, while the up-and-down lateral movement of the ocean waves represented a position-fixing problem, the aircraft's navigation system wasn't constrained by having to land on a defined, narrow runway — the Flying Fish could take off and land anywhere on the sea within a given area.

Following the successful tests, the Flying Fish is now back in the shop, the U-M researchers planning to outfit it with solar power and more sensors.

"One of the plans for the next generation is to do energy harvesting with solar cells," said Atkins.

While solar-powered high-altitude aircraft designed to take off from land and stay aloft for years at a time "have to charge energy quicker than they use it," the Flying Fish would have no such constraint. Every time it needed to recharge its batteries, it could simply land on the ocean and float for a while.

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