Live overhead spacecraft from your location
Once a day the server downloads a list of satellites from a service called Celestrak. The list contains around 15000 objects and includes information about where each satellite is and how fast it's moving. While you're using the app, the server calculates how each satellite should have moved since then. This is done using a model called SGP4. As long as the satellite hasn't fired its thrusters, the position should be accurate to within a few hundred metres — though the error grows over time as the data ages.