Offline charts on a phone: what works without signal, and what does not
GPS never needed a phone signal. Nearly everything else on the screen does, unless you fetched it before you left the marina.
In this piece
The GPS in your phone works at sea with no signal, no SIM and no data. It is a receiver: it listens to satellites and works out where you are, and it never needs to talk to a phone mast. What stops working when the bars disappear is everything else on the screen that comes from the internet: map tiles you have not stored, new forecasts, anything that sends a question to a server, and any alert that has to reach another phone.
So most searches for an offline nautical charts app are really two questions. Will the position work offshore? Almost always. Will there be a map under it? Only if you downloaded that area before you left. Below is what we download, what we check at the dock, and what we never ask a phone to do on its own.
Does phone GPS work without signal or data?
Yes. GPS, and the other satellite systems most current phones use alongside it (Galileo, GLONASS, BeiDou), are one-way broadcasts. Each satellite transmits its time and position; the receiver in your phone times the signals from four or more of them and computes a fix. No mobile network is involved at any point. In the open with a clear view of the sky, a phone is typically good to within a few metres.
Where it struggles is below decks, especially in a steel or aluminium hull, and under a hard top covered in solar panels. Glass fibre lets the signal through, but not as well as open sky. A phone in a pocket in the cockpit is usually fine; one in a drawer under the chart table may give you a fix that wanders by tens of metres.
What A-GPS does, and what happens without it
Phones use assisted GPS (A-GPS) to get the first fix quickly. Over the data connection they fetch the satellites’ orbit data and a rough position, so the receiver knows where to look. Without it the phone has to collect that information from the satellites themselves, which is slow by comparison: a first fix can take anything from under a minute to several minutes. Many phones also store predicted orbits that stay usable for some days, so a phone that was online in the marina usually finds itself quickly offshore. The simple habit is to open your navigation app and get a fix before you lose signal, and to leave location switched on.
What airplane mode turns off
Airplane mode turns off the radios that transmit: the mobile network, and on most phones Wi-Fi and Bluetooth too, though you can switch those two back on by hand. On current iPhones and most Android phones it leaves the GPS receiver working, because the receiver only listens. Location is a separate setting; leave it on. On a long passage out of range, airplane mode also saves battery, since a phone searching for a phone mast that is not there works hard at it.
What to download before you leave
Map tiles are the big one. A map app draws its map from small squares fetched as you pan and zoom. Online, you never notice. Offline, anything you have not stored is blank, and it tends to be blank at exactly the zoom you need to see a harbour entrance. Download the whole area you might reach, including the bail-out harbours from your passage plan and the coast beyond the destination, at the level of detail you would use coming in to anchor.
Then collect the rest of what normally lives online: the latest forecast for the whole time you will be out of range, the pilot book or cruising guide, tide tables and tidal stream data where there are tides, the manuals for the engine and the electronics, and the phone numbers you might need, the coastguard’s included. Note the forecast’s issue time, so you know how old it is when you read it again on day three. A screenshot of the forecast table is crude, but it stays on the phone after any app’s cache has gone.
For somewhere with patchy coverage, the San Blas islands for instance, do the downloads on a fast connection a day or two ahead. Marina Wi-Fi on a busy evening is not the place to fetch a whole coastline.
Phone maps versus official nautical charts
An official nautical chart, on paper or as an electronic navigational chart (ENC), comes from a hydrographic office and exists for one purpose: keeping ships off the bottom. It carries soundings and depth contours, lights with their characteristics, buoyage, wrecks, cables, anchoring restrictions and the date of the survey behind it, and it is kept up to date through Notices to Mariners. The vector charts in a chartplotter are mostly built from the same survey data.
A general map on a phone is a different thing. Many are drawn from map data made for the land, with the coastline done well and the sea left blue and empty. It shows you where the bay is, where the village and the road are, and where the boat is relative to them. It will not tell you whether there is 2 metres or 20 under the keel, and it will not show the rock 100 metres off the point unless someone happened to map it.
The maps in SafeBoat are maps too, not official nautical charts: there are no soundings and no ENC data in them. We use them to see where we are, where the anchor is and how the bays lie, and we navigate from a chart.
A good second source, and a poor only one
A phone has a good GPS, a bright screen and a battery that lasts hours, not days, with that screen on. It overheats, it dislikes salt water, it goes over the side, and when it dies it takes the map, the position and often your only way of calling anyone with it. That makes it a first-rate backup to a plotter and paper, and a poor sole means of navigation. On passage we still write position, course, speed and log reading in the logbook every hour, so that a dead phone and a dead plotter leave us with a dead-reckoning position less than an hour old.
Battery, heat and water
The GPS receiver is a small load. The screen is the big one. A phone showing a map at full brightness in the cockpit can flatten itself in a few hours, and the sun that makes you turn the brightness up is heating it at the same time. What works for us:
- On passage, run it from a 12 V USB socket or a power bank, and charge it somewhere shaded.
- Let the screen time out between looks rather than keeping it lit all day.
- Keep it out of direct sun. A hot phone protects itself by dimming the screen, slowing down and refusing to charge, and in the end it shuts down with a temperature warning until it cools. In a 35 °C cockpit that can take a while.
- Treat water resistance ratings with suspicion. They are tested in fresh water in a laboratory, not in salt spray, and seals age. A waterproof pouch on a lanyard costs less than a phone, and the lanyard matters as much as the pouch.
- Carry a second device with the same maps downloaded: an old phone or a tablet, charged, in a drawer.
What works offline, and what does not
| Thing | Without signal | Notes |
|---|---|---|
| GPS position | Works | The first fix may be slower without A-GPS |
| Map areas you downloaded | Works | Only the area and the detail you saved |
| Map areas you did not | Blank or blurred | Usually right where you zoom in |
| Forecasts | Last saved copy | Older by the hour, with no updates until you are back in range |
| An anchor alarm running on the phone | Works | It needs GPS, not the network |
| Alerts to a phone ashore | No | The phone aboard needs data to send them |
| AI assistants and route advice | No | They run on servers |
| Calls, messages, email | No | Unless you carry a satellite device |
In SafeBoat, specifically: you download map regions in the phone app, and once downloaded they load instantly and work with no connection, as does the anchor alarm. A banner warns you when you leave the downloaded area, which is your cue to look at the chart. Forecasts are cached for up to 3 hours for when the signal goes. The route planner’s AI, BoatGPT and alerts to paired phones need a connection, so if you want someone ashore keeping an eye on the boat, read our notes on leaving the boat at anchor first.
One test before any trip out of coverage: at the dock, put the phone in airplane mode, open the map and scroll along the whole route at the zoom you would use to enter each harbour and bay. Anything blank is something you still have to download, and the dock is a far better place to find that out than the harbour entrance.