Phone GPS: about 4.9 m under open sky
According to GPS.gov, the U.S. government's official GPS site, GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft) radius under open sky. Accuracy gets worse when:
- buildings, bridges or trees block the satellite signals
- you are indoors or underground
- signals bounce off buildings or walls before reaching you (multipath)
If the position is off by a few meters, the speed calculated from it is off too. GPS speed is handy, but treat it as an estimate with some error and some lag.
How a GPS receiver works out your speed
As the satellite navigation magazine Inside GNSS explains, there are two main ways a receiver estimates velocity:
- Position differencing: take two positions, divide the distance between them by the time. It is the simplest method, but it inherits the position error, so accuracy is limited to roughly meters per second.
- Doppler: as you move toward or away from a satellite, the frequency of its signal shifts slightly. Measuring that shift gives velocity accurate to a few centimeters per second.
How these are combined depends on the device. Apple's developer documentation describes the speed an iPhone app receives as "the instantaneous speed of the device, measured in meters per second", notes that a negative value means the speed is invalid, and says that because the actual speed can change many times between location updates, the value is for informational purposes only.
iPhone uses more than one satellite system
"GPS" is strictly the U.S. system, but iPhone also listens to others. The iPhone 17 tech specs, for example, list GPS, GLONASS, Galileo, QZSS, BeiDou and NavIC (support varies by model). More satellites in view make it easier to keep a fix when buildings hide part of the sky.
Why the speed jumps: city streets and multipath
Between tall buildings you see only a slice of sky, and signals reflected off walls get mixed in. The position shifts for a moment, and that shift shows up as a sudden burst of speed. You may even see a few km/h while standing still at a crossing.
Watch out for top speed in particular. A single spike can be recorded as your maximum, giving a number that makes no sense. If the top speed looks impossible for the vehicle or the road, suspect a GPS jump. For the trip as a whole, the average speed is a much calmer number.
Why it freezes: tunnels, underground and indoors
Satellite signals do not reach into tunnels, underground stations or most buildings. A reading that freezes or drops to zero there is normal, not a fault. Once you are back under the sky, the fix returns after a short while. On subways and lines with long tunnels, expect gaps.
Why it wobbles at walking pace
At 4 km/h (about 2.5 mph) you move about 1.1 m per second. With a position error of a few meters, the error can be larger than the distance you move in a second, so at walking or slow running speeds the number tends to bounce around. For running pace, the average over a decent distance is a better guide than the instantaneous speed. See our km/h, mph, knots and m/s conversion chart for pace math.
Tips for steadier GPS speed
- Wait a moment under open sky before you set off. Right after you open an app, the fix may not have settled.
- Give the phone a view of the sky. A window seat on a train, the deck or a window on a boat. Deep in a bag or pocket, reception is weaker.
- Do not worry when it stops in a tunnel. It comes back when you exit.
- Take odd top speeds with a grain of salt. A maximum recorded downtown may include a jump.
- Do not expect it to match your car. Car speedometers are built not to read lower than your true speed, so a small gap is normal. See why your car speedometer and GPS disagree.
The driver does not look
Follow the traffic laws where you are. In Japan, for example, Article 71, item 5-5 of the Road Traffic Act prohibits gazing at images on a display brought into a car, moped or bicycle while driving, except when stopped. GPS speed is for passengers, or for after you have stopped. While driving, follow the speedometer in your vehicle.