Your Phone Isn't Broken. Physics Is Just Honest.
You're standing in the middle of a large shopping centre, phone in hand, trying to find the escalator. The map spins. The blue dot drifts across three different shops, none of which you're in. You step outside, wait four seconds, and it snaps to your exact position on the pavement.
The satellites didn't go anywhere.
GPS signal loss indoors is one of those things most people chalk up to bad luck or a cheap phone. It's neither. It's the predictable result of how the system was built, and understanding it changes how you think about every location-aware app you've ever used.
A Signal That Was Never Meant to Penetrate Walls
GPS works by timing. A network of satellites (the US system runs 31 operational ones) broadcasts a continuous radio signal. Your phone listens to at least four of them simultaneously, measures the tiny differences in arrival time between each signal, and triangulates your position from those gaps. The whole calculation depends on signals that travel at the speed of light and arrive within microseconds of each other.
The problem is power. Each GPS satellite transmits at roughly 50 watts. By the time that signal crosses 20,000 kilometres of space and reaches your phone's antenna, it has weakened to something in the range of 10 to the power of negative 16 watts, so faint it's barely above the noise floor of your receiver. The system is engineered right to the edge of what's detectable in open sky.
Walls, ceilings, and floors aren't passive obstacles. Concrete attenuates radio signals, and so does steel reinforcement. A single reinforced concrete wall can reduce GPS signal strength by 20 to 30 decibels, and in signal terms, every 10 decibels is a tenfold reduction in power. A modern multi-storey building with steel framing, thick floors, and low-e glass windows (the kind coated to reflect heat) can collectively knock a GPS signal down so far that your receiver can't distinguish it from background noise.
The signal is there. Your phone just can't hear it over the static.
The Multipath Problem (Which Is Worse Than No Signal)
Sometimes, inside a building near a window or atrium, your phone actually does receive GPS signals. That can cause more confusion than receiving none at all, which is a genuinely underappreciated design irony.
When a GPS signal bounces off a wall, a ceiling, or a metal surface before reaching your antenna, your receiver gets a reflected copy of the signal. It arrives fractionally later than a direct signal would, because it's travelled a longer path. Your phone has no way to know it's a reflection. It treats it as a direct signal, calculates the wrong distance to that satellite, and places you somewhere you aren't.
This is multipath error. In open air near tall buildings, it can throw your position off by 10 to 50 metres. Indoors, where every surface is a potential mirror for radio waves, the errors compound. The blue dot doesn't just freeze. It actively lies.
Consider Maya, who works on the third floor of an office building with a glass atrium on the south side. She opens a mapping app at her desk, 15 metres from the atrium. Her phone picks up two satellites through the glass, both at low angles, both signals having bounced off the atrium's internal steel frame before reaching her. Her phone calculates a position 40 metres north and one floor down, placing her in the car park. She's not doing anything wrong. The geometry is just broken.
What Fills the Gap (And How Well It Works)
Smartphone makers aren't sitting on their hands. Modern phones layer several positioning systems on top of GPS to compensate.
Wi-Fi positioning is the biggest one. Your phone scans for nearby Wi-Fi networks, even without connecting to them, and compares their signal strengths against a database of known access point locations. Google, Apple, and others have spent years building these databases through wardriving (mapping networks from vehicles) and crowd-sourced data from users. In a dense urban environment, Wi-Fi positioning can get you to within 15 metres. In a well-mapped building, sometimes closer.
Bluetooth beacons go further. Airports, large retailers, and transit hubs increasingly install dedicated Bluetooth Low Energy beacons at known positions. Apps that know the beacon map can place you within 1 to 3 metres, and Apple's indoor maps for certain airports use exactly this. It works well, though it requires infrastructure investment and a database that someone has to build and maintain, which is why most buildings still don't have it.
Cell tower triangulation is the bluntest instrument. Your phone knows which towers it can see and how strong each signal is, with accuracy ranging from 100 metres in a dense city to several kilometres in rural areas. Good for knowing which neighbourhood you're in, not which aisle.
Then there's the inertial system: the accelerometer and gyroscope built into every modern phone. Once your last known GPS fix is established, dead reckoning can extend it, tracking your movement by counting steps and measuring direction changes. After about 30 seconds of walking, drift becomes significant. After two minutes, it's guesswork. Think of it as a bridge, not a foundation, and a rickety one at that.
What People Consistently Miscalculate
The common assumption is that "GPS" and "location" mean the same thing on a phone. They don't, and conflating them leads to genuine confusion about what your device is actually doing.
When your phone shows you a position indoors, it's almost certainly not using GPS at all. It's using Wi-Fi, Bluetooth, cell data, or some combination, with GPS contributing nothing or actively introducing noise. Turning on "high accuracy" mode in your phone's settings doesn't make indoor GPS work. It enables the phone to use all available sources simultaneously, and the improvement you see is entirely from the non-GPS sources doing their job better.
It also means indoor accuracy is almost entirely a function of how well that specific building has been mapped. A well-instrumented airport terminal can place you precisely. An unmapped office building in a mid-sized city might put you anywhere on the block.
So next time you're inside somewhere large: check your phone's location accuracy indicator. If you're within 10 metres, someone has done serious mapping work on that building. If you're at 65 metres or worse, you're running on cell towers and hope.
GPS is extraordinary engineering, the kind that can place you within a few metres anywhere on Earth's surface using satellites roughly the width of a bus, floating 20,000 kilometres up. It just wasn't designed to see through buildings. The remarkable thing isn't that it fails indoors. It's that we've built an entire parallel system, invisible and automatic, to paper over the gap, and most people using it have no idea it exists.