Why GPS Puts You on the Wrong Side of the Street

00:00Why can GPS put you across the street?

GPS can put you across the street when buildings distort satellite signals. Reflected signals take longer routes, which can shift the calculated position.

You are standing here. But the map places you over there. A signal reaching your phone does not guarantee an accurate location.

00:22How does GPS estimate your location?

GPS estimates distance by timing radio signals from satellites. The signals include timing information and information about the satellites' positions.

Your phone compares the signal's timing with its own clock. That timing helps estimate how far the signal travelled.

But your phone's clock is not perfectly synchronized with satellite time. The calculation must account for that timing difference too.

A standard position calculation normally needs at least four satellites. Together, their measurements help estimate your position and your clock's timing error.

01:06How do buildings change satellite signals?

Tall buildings change which satellite signals reach your phone. Some direct paths disappear behind buildings. Other signals arrive after reflecting from a surface.

Here, only the reflected signal reaches the phone. The direct signal is blocked. Engineers call this non-line-of-sight reception.

The reflected signal travels farther than the direct route would. If that extra journey is unrecognized, the measured distance becomes too large.

01:40How can 100 nanoseconds add 30 metres?

Imagine the reflection adds one hundred nanoseconds to the journey. That is one ten-millionth of a second. It adds about thirty metres, or roughly one hundred feet.

That is an error in one satellite distance measurement. It does not mean your map position shifts by exactly thirty metres.

02:03Why can one bad distance move your location?

Your position comes from combining several satellite measurements. One misleading distance can shift the result when the calculation accepts it.

In the simplified example, one distance estimate becomes too large. The calculation shifts the estimated location to fit the measurements. The person has not moved. The blue marker has.

The size and direction of that shift depend on several things. Which signals are affected matters. So do the satellite positions and the receiver's calculations.

02:41How do reflections and blocked sky differ?

Reflections can also cause trouble when the direct signal still arrives. The direct signal and reflected copies combine at the receiver. That is called multipath interference.

Buildings can also hide useful satellites. The remaining measurements may constrain some directions poorly. That can make the position estimate less reliable.

03:07Is your position wrong, or is the map wrong?

A wrong map location is not always a satellite signal problem. The map itself might contain an incorrect address or road. That is different from calculating your position incorrectly.

03:21How does your phone combine location clues?

Your phone may also combine satellite information with other location clues. For example, Android location services can use GPS and Wi-Fi. The displayed blue marker may reflect that combined estimate.

Some positioning systems use building maps to help interpret satellite signals. Those maps can help predict blocked or reflected paths. But the benefits depend on the system and its surroundings.

Receiving more signals is useful only when their information is handled well. A stronger signal is not proof that it travelled directly.

04:04Why is the blue marker still an estimate?

So the phone can receive a signal and still misplace you. Buildings can hide direct signals and send others along longer routes. Those measurements can shift the calculated position across the street.

The blue marker is an estimate of your location. It is not a direct observation of where you stand.