Your phone opens a map in an underground concourse. There is no clear line of sight to navigation satellites, yet the familiar blue dot appears. Has the receiver become extraordinarily sensitive? More likely, the phone is combining several kinds of evidence, some of which have nothing to do with a live GPS fix.
The geography of nearby networks
Wi-Fi access points and cellular base stations are fixed, or at least often associated with known locations. Operating systems can compare nearby radio observations with location databases and other signals to create an estimate. The phone does not necessarily need to connect to every Wi-Fi network it observes. The specific approach varies by software provider and available settings.
Apple's Location Services documentation describes the use of cellular, Wi-Fi, GPS and Bluetooth information. It also notes that satellite visibility can be poor around roofs and tall buildings, making other sources more important. In unfamiliar environments or where infrastructure changes, the alternative location may be approximate or outdated.
Did you know? A device with Wi-Fi but without a cellular subscription can still produce location estimates in some settings, because the surrounding wireless environment itself provides clues.
Why two nearby phones disagree
One handset may have cached a previous satellite position. Another may have observed a different set of Wi-Fi networks. Permissions, sensor availability and privacy settings may further change what each application receives. That makes location a continuously revised inference rather than a single measurement stamped with universal certainty.
This also explains why a map can work surprisingly well in a shopping centre while still identifying the wrong floor or side of the building. Location services estimate a position; they do not inherently know the layout of every corridor or escalator.
Useful technology, deserving questions
Wireless-location databases enable navigation and convenience, but location inference can be sensitive. Ask what access an application needs and whether approximate location will do. A weather app may need the town, while a turn-by-turn navigation feature needs much better detail.
The next pieces of the puzzle are why GPS fails indoors and how tunnel navigation keeps working.
Follow the evidence, not the fiction.
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