How a phone's GPS works at sea
A modern phone does not rely on the American GPS alone. It picks up four satellite constellations at once: GPS (USA), GLONASS (Russia), Galileo (Europe) and BeiDou (China). That is about thirty satellites visible on average, against a dozen with GPS alone. More satellites = better accuracy, better reliability, faster fix.
In open water it is the ideal environment for GPS reception: no buildings, no relief, clear sky. Accuracy typically reaches 3 to 5 metres in normal conditions. The phone locks on in a few seconds if a recent fix exists ("hot start"), and in 30 seconds to 2 minutes if the phone has not seen the sky for a while ("cold start").
The GPS receiver is passive: it receives signals emitted by the satellites, it emits nothing. That means two useful things:
- GPS works even in airplane mode, with no subscription, no cellular network.
- Tracking does not reveal your position to a third party as long as no network connection actively transmits the data.
Real accuracy: what you get, what you don't
Three nuances to know so you don't get it wrong.
Typical horizontal accuracy: 3 to 5 metres
In open water, clear sky, multi-constellation on, you get 3 to 5 metres of horizontal accuracy. That is more than enough for sailing, where metre-precision has no practical use (you do not moor to an exact stake 10 miles out).
Accuracy degraded near obstacles
At anchor under a high cliff, alongside between two metal boats, under a powerful radar antenna, accuracy can drop to 10-15 metres or more. The phone then receives fewer satellites, or reflected signals that skew the calculation. Offshore it is invisible. To within 10 metres on a swinging radius, it is simply a matter of common sense.
Lower vertical accuracy
GPS altitude is less accurate than horizontal position: typically 5 to 10 metres of error. For sailing it is a non-issue (altitude is zero). For aviation or mountain hiking, yes.
Instantaneous speed vs average speed
The instantaneous speed shown by the phone is computed by the GPS receiver itself, with typical accuracy of 0.1 to 0.3 knot. The average speed (over a passage) is computed afterwards from the track, and it is very accurate. The phone's SOG (Speed Over Ground) is usable directly for navigation, with no correction.
Battery use: the real numbers
This is the question that comes up most. Here are the orders of magnitude observed on a recent phone (iPhone 14+, Samsung Galaxy S22+) with an optimised app:
| Mode | Hourly drain | Run time from 100% |
|---|---|---|
| Continuous GPS tracking, screen on | 5 to 10% / hour | 10 to 20 hours |
| Continuous GPS tracking, screen off | 2 to 5% / hour | 20 to 50 hours |
| Tracking every 30 s, screen off, app in background | 1 to 3% / hour | 30 to 100 hours |
Practical conclusion: a phone alone, without a power supply, comfortably lasts a one-day passage, lasts two days with a little care (screen off, recording rate at 30 s), and does not last three days without a recharge.
The simplest solution: a wired 12 V supply at the chart table, USB-C or USB-A depending on the phone. For 30 euros of kit and 30 minutes of installation, the phone stays charged permanently and tracking can run indefinitely. It is the most cost-effective fit on board for anyone who wants to track their passages.
App-side optimisations
A well-designed app minimises drain three ways:
- Background tracking: the app keeps recording the track while the screen is off and you use the phone for something else (or it is locked).
- Adaptive rate: slow the recording when the boat is not moving (at anchor), speed it up when it moves.
- Batch writing: group position writes to spare storage cycles.
Offline mode: what works, what can break
GPS works everywhere, independent of the network. What may not work at sea out of coverage is whatever depends on the network:
What always works offline
- GPS position acquisition (passive, independent of the network)
- Recording the track in the app (local storage)
- Displaying maps if they were preloaded before departure
- All log entry functions (watch, observed weather, sails, crew)
What can break offline
- On-demand map display (if the app loads tiles from an online server)
- Real-time weather (marine warnings, GRIB) if not preloaded
- Sync between crew accounts
- On-demand address and point-of-interest search
The absolute rule: offline-first
Choosing an app whose architecture is offline-first means everything works locally by default, and the network only enriches. No blank screen at sea out of coverage, no blocked feature. That is the difference between an app that works ashore in a café and an app that works offshore.
Which recording rate to choose
The recording rate (how many positions per minute) affects track quality, storage and battery drain.
- One position every 10 seconds: very smooth track, heavy storage (one 24-hour passage = 8,640 points), good for racing or fine analysis.
- One position every 30 seconds (recommended for cruising): smooth track, reasonable storage (2,880 points / 24 h), accuracy enough for any later analysis.
- One position every 60 seconds: acceptable track but cuts tight tacks, reserve for long passages without fine manoeuvres.
- Adaptive rate: the best approach, the app speeds up or slows down with speed and course change.
Phone, tablet or dedicated GPS
Phone
The most accessible, already in your pocket. Small screen, but excellent multi-constellation GPS. Ideal for coastal sailing, the solo skipper, the co-skipper filling in the log from the deck. Limit: screen too small for hands-free charting.
Tablet
The optimal compromise for cruising. Large screen readable in the sun (models at 1000+ nits), GPS often slightly less accurate than the phone (except high-end models), good run time if powered. Allows charting + log at the same time. The most common solution on modern cruising boats.
Dedicated GPS
For professional boats, top-level racing, or the hardware redundancy some ocean races require. NMEA integration with the other instruments. More expensive, more specialised, more durable. Not essential in leisure cruising but still a standard for demanding uses.
How Ekynavy handles tracking
- Multi-constellation: uses all 4 satellite systems (GPS + GLONASS + Galileo + BeiDou) for the best accuracy and reliability.
- Background tracking: the passage keeps recording even when the app is in the background or the screen is off, on iOS and Android alike.
- Adaptive rate: positions every 10 to 30 seconds depending on speed and manoeuvres, optimised for sailing.
- Offline-first: maps preloaded before departure, GRIB weather stored locally, log 100% functional offline. Automatic sync when coverage returns.
- Controlled drain: 2 to 4% of battery per hour in standard mode, compatible with a one-day passage without a power supply.
- NMEA WiFi compatible: for boats with a NMEA WiFi network, the app completes the phone's GPS data with that of the multifunction display (compass heading, log speed, depth). Self-service setup in 3 steps.