How GPS Works Without Mobile Signal

Your phone shows no bars, yet the map dot still follows you down the road. People often find that odd. It makes sense once you see that phone coverage and GPS are separate systems.

Why the dot keeps working

GPS is a satellite system. Each GPS satellite carries an accurate record of its own position and time.¹ It broadcasts that data all the time, and your receiver listens.

The key point is that the signal only goes one way. Your phone sends nothing up to the satellites, and the system works independently of any telephone or Internet reception.¹ No tower is involved.

It's also free to use. Anyone with a GPS receiver can get the signal.¹

How the position gets worked out

A receiver needs at least four satellites in view to do the sums.¹ Four are needed because the receiver works out four unknowns: three position coordinates and the deviation of its own clock from satellite time.¹

You don't need to follow the maths. What matters is practical: more open sky means more satellites in view.

Problem: the dot is slow to appear

The dot takes a long time to show up, or never settles. Start with why. Every GPS device requires orbital data about the satellites to calculate its position.² The satellites send that data slowly, so getting it straight from them takes a while.²

Phones get a shortcut. With assisted GPS, the phone uses a data connection to fetch help from a server. A-GPS is extensively used with GPS-capable cellular phones.²

Here's the catch for backcountry trips. That shortcut needs a data connection. A phone that can use standalone GPS doesn't depend on the network. It can work beyond network range, though that mode is sometimes slower to a first fix.² Some vendor services supply almanac data that is effective for an extended time, enabling offline A-GPS, so results vary by phone.²

Poor conditions make it worse. In exceptionally poor signal conditions, a regular GPS unit may need as long as 12.5 minutes to download the almanac and ephemerides (the satellites' orbit data).² Give the phone a clear view of the sky before you assume something's broken.

Problem: the dot jumps or disappears

Signal quality matters. GPS works where signal quality permits.¹ Signals can be weakened by weather or tree canopies.²

Try these in order:

1. Move the phone to a spot with a clearer view of the sky.
2. Give it time to settle, with a clear view of the sky.
3. Check that location services are on, and that the app has permission.
4. Restart the phone if the dot still won't appear.

If none of that works, don't keep driving on a guess. Stop somewhere sensible. Pull out the paper map and compass and work out where you are, using the compass skills you practised at home.

What GPS doesn't give you

GPS gives a position. It doesn't draw a map. For that, the app needs map data saved on the phone. The steps for saving a map area are on their own page.

A dot on a blank screen tells you where you are but not which way the road goes. That's why many people also carry a printed map, for the reasons given in paper against apps.