Does GPS work without a signal, or in airplane mode?
Yes — and the reason why explains most of what people find confusing about phone navigation. Your phone works out where it is by listening, not by asking. What a data connection actually buys you is speed and a map.
Short answer: yes. Your phone can work out where it is on a mountain with no bars, in a country where your SIM does nothing, and with airplane mode switched on — as long as airplane mode has not also switched off Location Services, which on iOS it does not.
The confusion is worth unpicking, because almost everyone has had the experience of opening a map app in a dead spot and seeing a blue dot on a grey rectangle, and concluding that the positioning was broken. It was not. The map was missing. Those are two entirely different systems that happen to be drawn on the same screen.
Your phone listens; it does not ask
A satellite positioning receiver is exactly that — a receiver. Roughly thirty GNSS satellites are continuously broadcasting a signal that says, in effect, "I am satellite number eleven, here is my orbit, and my clock reads this". The signal travels one way, from space to you. Your phone never transmits anything back, and no satellite has any idea you exist.
From the tiny differences in arrival time between four or more of those broadcasts, the receiver solves for four unknowns: your latitude, your longitude, your altitude, and the offset of its own cheap clock against atomic time. That is the whole trick. It requires a clear-ish view of the sky and nothing else. No SIM, no data plan, no wifi, no account, no subscription.
This is also why GNSS is free everywhere on earth and why your position cannot be "used up". The satellites broadcast whether anyone is listening or not.
So what is mobile data actually doing?
Two things, and neither of them is the fix itself.
A faster first fix
To find satellites quickly, a receiver wants to know roughly where they are right now. That data — the almanac and the ephemeris — is itself broadcast by the satellites, but slowly: acquiring it from cold over the air can take thirty seconds to several minutes. Assisted GNSS simply downloads the same orbital data over the network in a fraction of a second, so the receiver knows where to look.
Without assistance you get a cold start. The fix still arrives; it just takes longer. In practice this is the thing people actually notice in a dead spot, and it is why standing still for a minute at the trailhead before you set off is worth doing.
The map underneath
Map tiles — the streets, the contours, the satellite imagery — are pictures fetched from a server. They have nothing to do with positioning. With no data you get your coordinates and no background to draw them on, which is exactly the grey-rectangle experience, and is a missing picture rather than a missing position.
What still goes wrong without a signal
- A cold start takes longer, as above. Give it a minute in the open before you start.
- Deep canyons, dense forest, tunnels and the inside of a vehicle all block or reflect the signal. This has nothing to do with cellular coverage — it happens in the middle of a city too, which is why a route through a built-up area sometimes wanders through buildings.
- Wifi positioning, which phones use indoors to place you from nearby network names, needs a database lookup. Offline, indoors, you fall back to satellites alone, which is to say you fall back to almost nothing.
- The map is blank unless the app has stored tiles in advance.
Recording a route with no signal at all
For a recorder, the question that actually matters is not whether the fix arrives — it does — but where the app puts it. An app that batches your ride in memory and uploads it at the end has built a dependency on the network that the positioning system itself does not have. An app that reads your list of past rides from a server has built a second one, and that is the version that hurts: you go into a tunnel, open the app, and your rides appear to be gone.
Roam takes the other approach, structurally. Every fix is committed to a database on the phone the moment it arrives, in the same transaction as the ride’s running totals, so there is no window in which a recorded kilometre exists only in memory. The screen that lists your rides reads that same local database and has nowhere else to read from — the repository contains a build gate that scans the whole feature for networking symbols and fails the build if one ever appears, precisely because an earlier version of this app did read its list from a server and produced exactly the empty-list-in-a-tunnel experience described above.
The honest exception is the map’s background imagery, which is fetched from a tile endpoint like any other map. With no data your route line is still drawn, measured and saved; the streets behind it may not be. That is a picture of the world going missing, not your ride.
Frequently asked questions
Does GPS work in airplane mode?
Yes. Airplane mode turns off the transmitters — cellular, and by default wifi and Bluetooth — but GNSS reception is passive and stays on. On iOS, Location Services is a separate switch and is not disabled by airplane mode. You will get a slower first fix, because assisted GNSS cannot download orbital data, and no map tiles.
Does GPS use mobile data?
The position fix itself uses none. Assisted GNSS downloads a few kilobytes of orbital data to speed up the first fix, and map tiles can use a great deal of data — but that is the map, not the positioning.
Why does my phone say "no location" when I have no signal?
Usually it is a cold start that has not finished yet, or you are somewhere with no view of the sky. Go outside, stand still, and give it a minute. If an app shows an empty screen rather than a slow one, the app is probably waiting on a server rather than on a satellite.
Can I record a hike abroad without roaming charges?
Yes. Put the phone in airplane mode and record normally — positioning costs nothing and involves no network. Only a map that fetches tiles will be affected, so download any offline maps you want before you leave, or accept a route line on a blank background.
A recorder that does not lose the ride
Roam commits every GPS fix to the phone the moment it arrives, marks signal gaps instead of drawing a straight line through them, and reads climb from the barometer rather than from GPS altitude. No account, no sign-in, no sync. Recording a ride, viewing it and replaying it are free.