The Blue Dot That Can't Sit Still
You're standing on a street corner, completely still, phone flat in your palm. The map finishes loading. And then the little blue dot slides four meters to the left, twitches back, drifts toward the building behind you like it's bored.
You haven't moved.
This isn't a bug. It's geometry, physics, and a few uncomfortable truths about how GPS actually works. The system was never designed to pin you to a single square meter. It was designed to get a bomber to a target. The fact that it fits in your pocket is the miracle. The drift is just physics leaking through.
Satellites Are Never Quite Where You Think
Your phone calculates its position by measuring how long a radio signal takes to travel from each satellite overhead. Four satellites, minimum. The math is called trilateration: each satellite's signal defines a sphere, your phone sits at the intersection of all four.
In practice, the atmosphere eats the signal.
The ionosphere, that charged layer about 80 kilometers up, slows radio waves in ways that vary with solar activity, time of day, and season. A signal that should take precisely 67.3 milliseconds might take 67.31. That extra tenth of a millisecond translates to about 30 meters of positional error. Modern phones use dual-frequency GPS (the iPhone 14 and most flagship Android phones support L1 and L5 bands) which cancels much of this out by comparing how two different frequencies are affected. But not all of it. Never all of it.
Then there's multipath: signals bouncing off glass towers, parked vans, the side of a bridge. Your phone receives the same satellite's signal twice, one direct and one reflected, and the processor has to guess which is real. It usually guesses right. Sometimes it guesses wrong by eight meters.
The satellites themselves carry tiny positioning errors too, corrected constantly by ground stations. Still, a satellite drifting a few centimeters in its orbit becomes a few-meter error on the ground. Stack all of this together and you get a position fix accurate to roughly 3 to 5 meters under open sky, on a good day, with a modern phone. That accuracy isn't constant. It flickers. And flickering position, when you're standing still, looks exactly like drift.
The Kalman Filter Is Doing Its Best
Here's where the software enters the fight.
Your phone doesn't display raw GPS. It runs the signal through a Kalman filter, a mathematical tool originally developed for NASA's Apollo guidance computer. Think of it as a very anxious editor, constantly second-guessing noisy data and trying to produce one clean, plausible sentence out of a dozen contradictory sources. It weighs GPS against your phone's accelerometer and gyroscope, then makes predictions: if you were here a second ago and the accelerometer says you haven't moved, you're probably still here.
When you're walking, this works beautifully. Motion gives the filter something to anchor to.
When you're standing still, it has almost nothing to work with. The accelerometer reads near-zero. The GPS signal keeps twitching within its 3 to 5 meter error bubble. Without movement to constrain it, the filter lets the position estimate wander within the bounds of what's plausible.
Here's a concrete case. Maya is waiting outside a coffee shop, phone flat in her hand, Google Maps open. Four satellites visible, decent signal, open sky. Her position estimate carries an uncertainty radius of about four meters. Every second, the filter gets a new GPS fix that lands somewhere in that circle. Without movement to anchor the prediction, the dot traces a lazy random walk across that four-meter bubble. It looks like she's pacing. She isn't.
Her friend Ravi is in the same spot, but near a glass-curtain office block. Multipath interference pushes his uncertainty radius to twelve meters. His dot looks like it's crossing the street.
What "Accuracy" Actually Means (and What People Assume)
Most people treat GPS accuracy as a fixed property of the phone. Flagship phone, better GPS. Old phone, worse GPS.
That's partly true and mostly wrong, and I'll die on that hill.
The single biggest factor in GPS accuracy is sky visibility. An unobstructed view of the sky beats any hardware upgrade. A phone with a top-tier Snapdragon 8 Gen processor in an urban canyon performs worse than a three-year-old budget phone standing in a field. The second biggest factor is time. A cold start, where the phone hasn't used GPS recently, means the receiver has to scan the whole sky for satellites. First fixes can be 20 to 30 meters off. A warm start, where the phone remembers the last satellite almanac, locks in seconds and starts far more accurate.
So: have you ever wondered why your dot snaps to a weirdly precise location even inside a building, then jumps the moment you step outside? That's the handoff between two completely different systems. Indoors, your phone is almost certainly using Wi-Fi positioning, not satellites at all. Google and Apple have built this by wardriving billions of access points. It's impressive and slightly eerie. When you exit the building, the phone switches to satellite positioning, and two systems with different error profiles don't always hand off gracefully.
What You Can Actually Do About It
Check your accuracy indicator if your app shows one. In Google Maps, the blue circle around the dot tells you everything: tight circle, the phone is confident; wide circle, it isn't. Under open sky, a radius smaller than 5 meters means you're in good shape. Larger than 20, move away from the building.
High-accuracy mode (GPS plus Wi-Fi plus mobile networks) outperforms GPS-only mode almost everywhere. Keep it on. The battery cost is real but modest.
For anything where position drift matters, tracking a run or navigating a trail, give the phone 30 to 60 seconds after opening the map before you start moving. Let the filter settle. The first fix is almost always the worst one.
And if the dot is drifting while you stand still? That's not your phone malfunctioning. That's the entire GPS system doing exactly what it was built to do, just visible at a resolution it was never meant to show you. The blue dot is a probability, not a pin. It was always moving.