by Terence Morley
They were sat huddled together when they heard the high-pitched whining noise. Looking up to the sky, they caught a glimpse of an aeroplane. No, it wasn’t an aeroplane - it appeared small but it was lower in the sky. It was a drone – an unmanned aerial vehicle. It seemed to be circling roughly above them.
“I think they’ve found us,” said John to his two daughters.
The drone pilot was about 10km away: a woman in a black jacket flying the drone by remote control. She could see the view from the drone’s camera on the mobile phone attached to the controller. Occasionally, she would use her forefinger and thumb to zoom into an area on the screen and then zoom out again after seeing nothing.
“Whereabouts are you looking now, Sue?” asked the computer scientist.
“If we show the GPS coordinates on the map, it says the drone is somewhere near that mountain, there. But, I am sure that’s not right. When I lost sight of the drone it was heading in this direction,” said Sue, drawing a line on the map with a black-painted fingernail.
“If that’s the case, and we find them with the drone, we could be sending the team in the wrong direction.” He turned around and walked back to his van.
John and his daughters were at the bottom of a crevasse. They had been camping and hiking through the mountains. It was Spring, but in some areas that were shaded from the Sun, thick snow still lay on the ground. The two girls had been holding their Dad’s hands and were singing a song that echoed around the mountains. They had been stamping out a crunchy beat in the snow. They’d had no chance to save themselves as the snow gave way beneath their feet and they slid into a deep crack in the ground.
Luckily, none of them were hurt. That was thanks to the mass of snow that had landed before them. It was impossible to climb out because the walls of ice were too slippery and too steep.
That was six days ago. They had survived so far by eating the remaining food from their rucksacks and drinking the water from the melting ice.
The computer scientist’s daughter, Sam, was sitting next to him in the van. She didn’t want to say anything because her dad looked like he was concentrating.
Her dad said, “why is the GPS wrong?” to himself, but out loud.
“What’s the GPS, Dad?” Sam was ten years old but she knew a lot about computers and mobile phones, so she was sure she would understand.
“Well, GPS is a system for finding out where you are. The biggest countries in the world have all made their own systems but GPS is currently the most popular one. You know about GPS and SatNavs” said her dad pointing to the navigation device on the van’s dashboard.
“Oh, I didn’t know you were talking about the same thing,” Sam confessed. “But we’re using a drone, not a car.”
“Yes, but it’s the same system. It can be used in the air as well as on roads. I’ll tell you how they work,” began her dad, thinking how to explain it simply.
“There are a number of satellites orbiting the Earth and they keep sending out a signal in all directions. The signal contains a message saying which satellite is sending it together with a very accurate time of when it is sending it.”
“Like a TV signal?” asked Sam.
“Sort of. Imagine that our television could receive the satellite signal and the screen says it is from Satellite Number One and the time is 11.52 and 44.7649231 seconds. It doesn’t just say it is 11.52 and 44 seconds because it needs to be much more accurate than that. But imagine if we also had the same sort of very accurate clock in our house. Our clock would say that the time was a bit later than that. Do you know why?”
Sam thought for a bit. She thought about a person clapping at the end of the 100m running track and how she hears the sound a short time after she sees the clap. “Is it because the signal takes a bit of time to get to us?”
“Exactly,” said her dad, looking impressed, “and we can subtract the satellite’s time from our time to find out how long the signal took to arrive. From that we can calculate how far away the satellite is.”
“But if we know how far we are away from a satellite, how does that tell us where we are?” asked Sam starting to look a bit confused.
“We can receive a signal from more than one satellite and find out how far away we are from each of them. Then, because we know where the satellites are, we can use mathematics to calculate exactly where we are. Or, in this case, where the drone is,” replied the computer scientist, hoping that he had explained it well enough.
When John and his daughters saw the drone for a second time, John thought about setting fire to a sleeping bag to give off a smoke signal. This was very risky because if the drone didn’t see the smoke, they would be one sleeping bag short at night. It was also very dangerous because of the poisonous fumes that would be given off by the fire. But if they did nothing it was certain that they would not be found, so John decided to take the chance.
The computer scientist sat back and said, “normally the GPS system is excellent but I don’t know why it is not giving us the correct location now.”
Sam thought again about the clapping on the running track and how you can sometimes hear two claps when only only was made. “Can you get echoes from satellite signals, Dad?”
“What do you mean?” he replied.
“Well,” said Sam, “can a satellite signal get bounced off the side of a mountain? You know, so the signal has a longer distance to travel and the drone thinks it is further from the satellite than it really is. Would that make it wrong?”
“So it’s reflections from the mountains that are confusing the drone,” he mused.
“You little genius,” he smiled and he dived out of the van, banging his head on the door frame in the process.
“Sue, I know why the GPS is wrong,” yelled the computer scientist as he ran towards her.
“I’ve got smoke!” she shouted back.
They both looked at the image on the screen. There was smoke coming out of a hole in the ground.
“Sue, can you fly the drone directly above the smoke, but fly a bit higher so you can see more of the surroundings?”
She did that and he asked, “is it possible to put a marker on the image where the smoke is coming from?”
She moved her fingers across the screen a few times and a small red flag appeared exactly on the source of the smoke.
“Capture that image and email it to me,” he shouted while already running back to his van.
In the van, he opened a website containing satellite images of the Earth that also give you the coordinates for the images just like on maps. He found the location of their camp on the website and then zoomed out a bit. Opening the drone’s image from the email, he made it partly see-through, then placed it over the satellite image.
He began rotating it, zooming it and moving it around until he found a place where it almost exactly matched up with a region of the satellite image. “Got you,” he said.
Now that the image was aligned with the satellite image, he read off the coordinates for the drone’s red flag and sprinted to the mountain rescue team. He gave them the coordinates for the smoke and watched them get their kit together and start walking into the mountains.
It was a long wait but eventually a voice came over the radio: “We’ve found them. They’re safe.” A roar of applause erupted from all over the rescuers’ camp.
A helicopter was sent to meet the rescuers and bring them all back to the camp.
When it arrived, the rescuers led the family to Sue and told them that she was the person that found them.
She said, modestly, “I can’t take all of the credit” and she looked around for the computer scientist but couldn’t see him. “We would have found you a bit sooner if it wasn’t for the problem of GPS not working properly in the mountains,” she continued, explaining the problems that they had encountered.
The computer scientist and his daughter were already sat in front of the computer in the van. Their new project was to discover methods to allow a drone to match its images with satellite images by itself.