You're standing on a corner with a clear patch of sky directly overhead. Four bars of signal. Your phone's blue dot is drifting half a block south, pointing you into a wall. The satellite is fine. The sky is open.
So what exactly is going wrong?
Not a weak signal. Too many signals, arriving at once.
The Echo Your Phone Can't Ignore
GPS works by timing how long a signal takes to travel from a satellite to your phone. Four satellites minimum, each broadcasting a precise timestamp, and your phone compares the arrival times, does some geometry, and triangulates your position. The whole thing rests on one assumption: the signal traveled in a straight line from space to you.
Near tall buildings, that assumption collapses.
A glass-and-steel tower reflects GPS signals the same way a bathroom mirror reflects light. Your phone receives the original signal from a satellite above, plus one or two reflected copies that bounced off a building facade before reaching you. Those copies traveled a longer path, maybe an extra 50 to 200 metres of detour, so they arrive fractionally late. Your phone can't always tell the original from the echo. It averages them together.
The result is a calculated position that's physically impossible.
This is called multipath interference. Think of it like trying to find the stage at a concert when the sound is ricocheting off every wall in the venue. You hear the music fine. You just can't locate where it's coming from.
The error it produces isn't random noise, either. It's systematic and directional, which is what makes it so disorienting. You're not just imprecise. You're confidently wrong in a specific direction, which is arguably worse.
A Scenario Worth Playing Through
Take Maya and Daniel, both using the same navigation app on the same model phone, trying to find the same coffee shop in a downtown financial district. Maya approaches from the south, walking through a wide plaza with low surroundings. She gets 3-metre accuracy. Daniel approaches from the west along a narrow corridor of 40-storey towers. His blue dot swings 60 metres off course, sends him past the entrance, and briefly insists he's inside a parking structure.
Same satellites overhead. Same app.
The difference is that Daniel's phone is receiving four or five reflected signals for every clean direct one, and the geometry of that canyon is creating errors that stack rather than cancel.
Modern phones use chips that track 20 or more satellites simultaneously, pulling from GPS (American), GLONASS (Russian), Galileo (European), and BeiDou (Chinese). More satellites means more redundancy, and the algorithms can reject outliers. But in a severe urban canyon, so many signals are multipath-corrupted that the outlier rejection starts throwing out good data alongside bad. The system defeats itself.
What People Assume (and Shouldn't)
The common instinct is to blame signal strength. You open the settings, see four or five bars, and assume everything's fine. Signal strength and signal accuracy are completely different measurements. A strong reflected signal is worse than a weak direct one. The bars tell you the radio is receiving something. They say nothing about whether that something took the scenic route off a skyscraper.
There's a second assumption worth killing: that GPS is the only thing your phone uses for positioning. It isn't. Smartphones layer in Wi-Fi positioning (matching nearby network IDs against a crowdsourced database), cell tower triangulation, and the accelerometer and gyroscope in a system called sensor fusion. When GPS gets confused in a city canyon, a well-designed app leans harder on those other inputs.
That's why your position sometimes snaps back to accuracy the moment you step into a coffee shop with a dozen Wi-Fi networks visible.
The phone quietly switched its primary reference. It didn't tell you.
Ask yourself this: if your position drifts badly in tight streets but holds steady on open ground, do you actually want to spend ten minutes poking through location settings? Multipath is almost certainly the culprit, not a hardware fault, and no toggle fixes physics.
The real fix is geometry. Step into an intersection, or find a gap between buildings where you have a wider cone of open sky, and watch the blue dot settle. You're not improving the signal. You're removing the mirrors.
The satellites are doing their job perfectly. The buildings just turned your phone into someone sitting in a room full of echoes, trying to work out where the music is actually coming from.