How does my map work with no signal?

Dan is halfway up a hiking trail. He checks his phone: zero bars. “No service.”

And yet the blue dot on his map is moving, step by step, right along the trail.

“Zero bars,” he says, “and my blue dot is still moving. How?”

Liz, a few steps behind him, has an answer ready. “Let's follow your dot. Into space.”

The dot comes from space

About 20,000 kilometers (12,550 miles) up, at least 24 GPS satellites circle the Earth. Each one keeps calling out two things, over and over: where it is, and the time, read from a very precise clock on board.

Your phone has a small radio that does nothing but listen for them.

A signal from space takes a tiny fraction of a second to arrive. The satellite said what time it sent it, and your phone knows what time it heard it, so your phone can work out how long the trip took, and from that, how far away that satellite is.

Four satellites, one spot

One distance only tells your phone it's somewhere on a huge circle around that satellite. A second one narrows it down, and a third narrows it more. The fourth sorts out the last bit of error from your phone's own clock, which is nowhere near as precise as the satellites'. With four or more, your phone knows where you are: under open sky, GPS.gov says phones are typically accurate to within about 5 meters (16 feet).

The satellites are spread out so that from almost anywhere on Earth, at least four are above you.

It never talks back

This is the part that surprises people. Your phone never sends anything to the satellites. It only listens, the way a radio picks up a station.

So it needs:

• No bars. Bars measure your connection to the phone company's towers. GPS doesn't use them.
• No data. Nothing goes over the internet to work out your position.
• No fee. US law says the civilian GPS service is free to use; it's paid for through US taxes.

On an Android phone, Google says that with its extra “Location Accuracy” turned off, only GPS and the phone's motion sensors are used. That's what's happening on Dan's hike.

One thing GPS does need is a view of the sky. Apple notes that walls, roofs, tall buildings and mountains can block the satellites, and GPS.gov adds bridges and trees. Deep in a canyon or under thick trees, your dot may drift or pause. (With a connection, phones also use nearby Wi-Fi and cell towers to find you faster, especially indoors.)

So why did the map go grey?

A few minutes later, Dan frowns. “Then why has my map gone grey?”

His dot has walked off the edge of the map. It's still in the right place, but around it there's just an empty grid.

“The dot comes from space,” says Liz. “The map itself may need data.”

That's the catch. The position comes from the satellites, for free. The map, the trails, roads, lakes and names, normally downloads from the internet as you scroll and move. With no data, the app can't fetch the next piece. Depending on what it already has, you might still see the area you looked at recently, or nothing at all.

“Unless,” says Liz, “you download it before you go.”

Download the map before you go

Both big map apps let you save an area to your phone, ideally on Wi-Fi at home:

• Google Maps (Android and iPhone): search for the place, open its page and tap More, then Download offline map. Or tap your profile picture, then Offline maps, then Select your own map, and drag the box over your area.
• Apple Maps (iPhone, iOS 17 or later): tap your profile picture or initials, then Offline Maps, then the Add (+) button. Choose a place, adjust the area and tap Download.

What works offline isn't the same in both:

	Google Maps	Apple Maps
Map and blue dot	Yes	Yes
Driving directions	Yes, if the whole route is downloaded	Yes
Walking, cycling, transit directions	No	Yes

Google's downloaded maps also expire, so open the app on Wi-Fi now and then; it updates them for you when they're close to expiring.

Hiking trails can be patchy in both apps. For a serious hike, a dedicated hiking app or a paper map is a good backup.

Next hike

“Next hike,” says Dan, “I'm downloading the map first.”

Same zero bars, same moving dot. But this time the trail is drawn all around it.

And Dan sums it up: “My phone just listens to satellites. It's the map that may need data.”

What you can do

• Download the area before you leave, on Wi-Fi, in Google Maps or Apple Maps.
• Check what works offline: Google Maps gives driving directions only; Apple Maps also does walking, cycling and transit.
• Keep a view of the sky if the dot drifts: step out from under trees or away from cliffs.
• Bring a battery pack, and tell someone your route, for long hikes.