In the United States alone， as many as 34 million hikers hit the trails each year. Some of those hikers invariably get lost. To find them， a small army of （A） r may scour the woods in search of the missing by foot—and sometimes from the air. Scientists in Switzerland thought these rescue efforts might be better handled by a drone， an （B） a that flies without a pilot on board. Most are remote-controlled. But this Swiss team wanted to develop a craft that could find and search trails without anyone at the controls.

The scientists began with a neural network. That's a （1） c program designed to work much as the brain does. A neural network can learn to recognize （2） o and respond in a particular way when it finds them. In this case， the researchers wanted the network to be able to recognize and follow a trail through the forest.

For that to happen， the team attached three GoPro （3） c to a single headband. One camera pointed straight forward. The other two were directed 30 degrees to the right and left of center. A hiker donned the package and then hit the trails in a forested part of Switzerland. He was careful to keep his head pointed in the direction of the trail as he hiked. Over several weeks， the cameras recorded a （C） t of 8 hours of video covering 7 kilometers of trails. The researchers then divided the video into individual still images. More than 17 000 of these （D） i were used to train the neural network. The （E） n learned to identify what a trail looked like. And it learned to recognize when the trail was veering off to the left or right. Then the drone was ready to navigate the real world on its own.

Giusti and his team took their now "smart" device to a （F） f with trails that had not been used in the drones training. It flew along the path， constantly adjusting its direction based on the images coming in through its camera. The drone didn't perform as well in real-life as it did in the lab， Giusti admits. That's because the drone's camera took lower-quality images than the cameras that had been used to train it.

The research is still under way. There's still some way to go before Giusti and his team develop an （4） e trail finding drone.

（A， B， C， D， E， F FOR CROSS， 1， 2， 3， 4 FOR DOWN. The first letters of the absents were given.）