Your GPS Is Really a Stopwatch, and Stopwatches Get Fooled

Quick answer: GPS is a timing system. Your phone measures how long radio signals from at least four satellites took to arrive, then calculates where you must be. Anything that delays or bounces a signal changes the answer, and the phone has no way to know.

Nothing is being tracked

Start with the part most people get backwards. The satellites don't know you exist. They broadcast, constantly, to everybody, and they never hear a word back. Your phone is a listener.

More than thirty satellites circle about 12,550 miles up, run by the U.S. Space Force. Each one carries atomic clocks and repeats the same short message: who I am, where I am, and exactly what time it is. That's all.

Radio travels at the speed of light, roughly 186,000 miles per second. A signal from a satellite directly overhead takes about 67 milliseconds to reach you. Your phone notes when the message arrived, compares that to the time stamped on it, and multiplies the difference by the speed of light. Now it knows how far away that satellite is.

One distance puts you somewhere on a giant sphere. Two spheres intersect in a circle. Three narrow it to two points, and one of those is usually out in space. That's the trilateration, and it's old surveyor's geometry with a very fast stopwatch.

Why it takes four

Three satellites should be enough. They aren't, because of your phone's clock.

The satellites keep time with atomic clocks. Your phone keeps time with a tiny quartz crystal, the same kind of part that's in a cheap wristwatch. That's fine for alarms. For GPS it's a disaster, because light covers about one foot in a billionth of a second. A phone clock that's off by a thousandth of a second would put you 186 miles from where you are.

So the phone treats its own clock error as a fourth unknown, next to latitude, longitude and altitude. Four unknowns need four satellites. With a fourth signal, the phone finds the one clock correction that makes all the spheres meet at a single point. In effect, every fix also corrects your phone's watch against atomic time.

That's the trick I think is most elegant. Your phone borrows the accuracy of an atomic clock every time it checks.

The part with Einstein in it

The satellites are moving at about 8,700 miles per hour, and they sit higher in Earth's gravity well than you do. Both effects bend time. Their speed slows the onboard clocks slightly. The weaker gravity speeds them up more. Net result: the satellite clocks run fast by about 38 millionths of a second per day.

That sounds like nothing. Left uncorrected, it would pile up an error of about seven miles every day. So engineers set the satellite clocks slightly slow before launch to cancel the effect. Every time you open a map app, you're relying on relativity being correct.

Where the lie comes from

Out in the open, the government says a typical smartphone is accurate to within about 16 feet. That's the honest baseline. Now take away the open sky.

Bounced signals. Glass and steel buildings reflect radio. Your phone may lock onto a signal that took a detour, and a longer trip looks like a longer distance. This is the downtown drift, and I covered it in Why Your Blue Dot Jumps Downtown. The phone can't tell a direct signal from a bounced one by timing alone. It just sees a satellite that seems farther than it is.

Bad geometry. If your four satellites are bunched together in one patch of sky, the spheres cross at a shallow angle and the fix gets sloppy. Navigators call this dilution of precision. Surrounded by tall buildings or a canyon wall, you see only a slice of sky, so the satellites you can hear tend to be clustered. Altitude suffers most. Vertical error is usually worse than horizontal, so don't trust a phone to tell you your elevation to the foot.

The atmosphere. Signals slow down passing through the charged layer called the ionosphere, and a little more through the lower air. Receivers apply models to correct for it. Some newer phones listen on two frequencies, which lets them measure the delay instead of guessing. Older or cheaper ones guess.

Faint signals. By the time a GPS signal reaches the ground, it's weaker than the background radio noise. The receiver digs it out using a known code pattern. Tree canopy, a metal roof, a windshield with a metallic coating or a phone buried in a bag all make that harder.

Tunnels and garages. No signal at all. The phone then falls back on cell towers, Wi-Fi networks it recognizes and its motion sensors, which estimate how far you've gone since the last good fix. That's why your dot keeps gliding through a tunnel, and why it's often slightly wrong when you pop out.

When GPS is right and the map is wrong

Many "GPS lies" aren't GPS at all. The satellite system only says where you are. The map app decides what road that is, and then the routing software decides where to send you.

If your position is 40 feet off, the app snaps you to the nearest road, which may be the frontage road instead of the highway. And a map can't always know that a bridge is out, that a road is private, or that the "shortcut" is a two-track dirt trail that wants a high-clearance truck and a bad attitude. Stories of drivers following a route onto a boat ramp or a washed-out road are almost always a map or routing problem, with the satellite system doing its job perfectly. The dot was correct. The driver stopped looking up.

There's also deliberate interference. Jamming drowns out the faint signal with noise, and spoofing broadcasts fake ones. Both happen in conflict zones, and aviation authorities have warned pilots about it. You won't run into it on your commute, but it's a reminder of how weak the real signal is.

What to do about it

• Give it a clear start. After a long time off, a phone needs a moment to find satellites. Wait for the blue dot to settle under open sky before you drive off.
• Trust direction of travel over the dot. In a city, if the dot says you're on the wrong street but you've seen street signs, believe the signs.
• Download offline maps before any trip with spotty coverage. The satellites work without cell service. The map data often doesn't. Battery matters too, since constant location use drains it, as in Why Your Phone Dies at 30 Percent and EV Batteries Don't.
• Carry a paper map and a compass into the backcountry. Batteries die, and nobody ever had to recharge a compass. Learn to use both before you need them.
• Look at the road. If the app wants you to turn onto something that looks like a creek bed, it's wrong, and you're allowed to say so.

A very good guess

What makes GPS remarkable isn't that it sometimes misses by a block. It's that a device in your pocket can hear a whisper from 12,000 miles away, measure its arrival to within a few billionths of a second, and put you on the right street nearly every time.

Treat the blue dot like a sharp navigator who can't see out the window. Good. Not infallible. You're still the driver.