Ahmadreza SamadiWhy your phone puts you across the street
A fictional pickup explains reflected satellite signals, an uncertain map dot and a practical way to agree where to meet.
English film with synthetic narration and English captions. Fictional characters and map.
Read the English film transcript
Mina is waiting for a ride outside a busy station. Her phone puts her on the other side of the street. So why does your location go wrong among tall buildings? Your phone listens to satellites. GPS has at least 24, about 20,000 kilometres up.
Each one sends a radio message: where it is, and the precise time. It travels at the speed of light, so the delay tells the phone how far away that satellite is. With distances to at least four satellites, the phone can work out where
it is. But this assumes each signal flies straight to the phone. On Mina’s street, tall buildings hide much of the sky, and a signal can bounce off a glass wall to reach her. The bounce makes the trip longer, so that satellite seems farther
away than it is. A few wrong distances, and the dot slides, often to the wrong side of the street. It’s like a friend calling from around a corner: their voice bounces off a wall and takes the long way to you. That’s where the
comparison stops. You might notice an echo. A phone that can’t spot the bounce counts the extra delay as distance. At the speed of light, one ten-millionth of a second is about 30 metres. Phones also use Wi-Fi and mobile networks. But tall buildings can
get between you and the mobile towers too. So when the app is unsure, it may draw a light blue circle around the dot. It’s an estimate, not a promise. A smaller circle means a more confident estimate. Mina sees a big circle. She walks
along the pavement to the open corner, where more sky may help, sets the pin herself, and tells the driver: by the pharmacy. This is the simple version. Some phones can correct for bounces, but not every time. The atmosphere and the receiver matter too.
When the circle is big, trust a landmark, not the dot.