Why GPS is weak indoors: satellites and open sky explained

Your GPS signal is weak indoors because the signals from navigation satellites are very faint by the time they reach the ground, and roofs, walls and tall buildings block or bounce them. Your phone needs a clear view of several satellites at once. Stepping outside or near a window, away from tall buildings, usually gives a faster and more accurate fix.

How phone location actually works

People say "GPS", but modern phones listen to several satellite systems, together called GNSS (Global Navigation Satellite Systems):

• GPS, run by the United States.
• Galileo, run by the European Union.
• GLONASS, run by Russia.
• BeiDou, run by China.

Each satellite sends a precise time signal. Your phone compares the arrival times from several satellites to work out where it is. It needs at least four satellites for a position with height, and more satellites spread across the sky give better accuracy.

GNSS is receive-only. Your phone does not send anything back to the satellites, and it does not need mobile data or Wi-Fi to calculate a position. That is a separate system from your mobile signal, so you can have full bars and poor GPS, or no bars and a good fix.

Why the GPS signal is weak indoors

The signal is faint. Navigation satellites orbit very high above the Earth. Their signals are extremely weak at ground level, far weaker than your Wi-Fi or mobile signal.

Roofs and walls block it. Concrete, metal, brick and energy-efficient windows with a metallic coating all absorb or reflect satellite signals. Deep inside a building, there may be nothing usable at all.

Buildings bounce it. In city streets with tall buildings, signals reflect off walls before reaching your phone. This is called multipath. The phone thinks the signal travelled further than it did, so your dot jumps across the street.

The sky view is narrow. In a valley, a forest or between tall buildings, you can only see a small patch of sky. The satellites you can see are clustered together, which makes the maths less accurate.

How your phone helps itself

Phones use a few tricks to find you faster and indoors:

• Assisted GPS (A-GPS): the phone downloads satellite position data over the internet, so it does not have to wait to receive it from the sky. This makes the first fix much quicker.
• Wi-Fi and mobile network location: the phone can estimate where you are from nearby Wi-Fi networks and cell towers. On most phones, a setting for Google location accuracy uses this. It works indoors but is less precise.
• Motion sensors: the phone can guess small movements between fixes using its accelerometer and compass.

So indoors, the blue dot you see is often a Wi-Fi or network estimate, not a satellite fix. That is why the circle around it is large.

How to get a better GPS fix

1. If your GPS signal is weak, go outside, or stand right next to a window, ideally one facing open sky.
2. Move away from tall buildings and out from under trees or bridges.
3. Keep the phone still for a moment and let it settle.
4. Make sure location is on, and on most phones turn on improved accuracy, which uses Wi-Fi and network data alongside satellites.
5. Make sure the app has precise location permission, not only approximate.
6. Remove thick metal cases or car mounts with metal plates, which can block signals.
7. If the compass seems off, recalibrate it. Many map apps prompt you to move the phone in a figure-of-eight.

What does "open sky" mean in practice?

Open sky means you can see most of the sky from horizon to horizon. A field or a beach is open sky. A narrow street between tall offices is not. More visible sky means more satellites spread out above you, which means a faster, more accurate position. Satellite view tools show this as a sky plot, a circle with each satellite marked by where it sits overhead.

Is weak indoor GPS the same as weak indoor signal?

They have similar causes, but they are different systems. Satellite signals are much weaker to begin with, so they suffer more indoors. Mobile signal is also blocked by walls and metal; our guide on why signal is weak inside buildings and basements covers that side. If you want to see your mobile signal rather than GPS, see how to check your real signal strength on Android.

How CellRadar helps

CellRadar has a Satellite & Sky View that shows the satellites your phone receives and how open your sky is, so you can see why a fix is slow. Its signal map plots your own mobile signal readings, which is a separate measurement. If you want to map weak coverage too, see how to make your own signal map.

Frequently asked questions

Does GPS use mobile data?

Calculating a position from satellites does not need data. Phones use data for assisted GPS to get a faster first fix, and map apps need data to load maps unless they are saved offline.

Why is my GPS accurate outside but jumps around in the city?

Tall buildings block parts of the sky and reflect signals, so the phone receives bounced signals that make it think you are somewhere else.

Can I improve GPS signal with an app?

No app can make satellite signals stronger. An app can show which satellites you receive and how open your sky is, so you can move to a better spot.

How many satellites does my phone need?

At least four for a position with height. More satellites, spread across the sky, give better accuracy.