GNSS: how satellite positioning works
1The lesson in plain words
GNSS receivers time weak signals from several satellite systems to find your position, usually to a few metres. Precise is not the same as safe.
Four systems
GNSS (global navigation satellite system) is the general name for satellite positioning. There are four global systems: GPS (USA), Galileo (European Union), GLONASS (Russia) and BeiDou (China). Most modern receivers use several at once, which gives more satellites in view and a more reliable fix, meaning a calculated position. People still say "GPS" for all of them.
How it works
Each satellite broadcasts its position and a very precise time. Your receiver measures how long each signal took to arrive and turns that into a distance. With signals from at least four satellites it works out latitude, longitude, height and the error of its own clock. By the time the signals reach Earth they are extremely weak, which is why they are easy to block or jam.
How accurate
In open sky a modern receiver is usually within a few metres. A clear view of the sky matters: cliffs, high quays, a metal or solar-panel canopy or an antenna mounted below deck can block satellites and make the position worse. SBAS (satellite-based augmentation), such as EGNOS in Europe and WAAS in North America, sends corrections that can bring it to about 1–3 m.
What it feeds
GNSS supplies position, time, COG (course over the ground) and SOG (speed over the ground). AIS (automatic identification system) shares vessel information. DSC (digital selective calling, your radio’s digital alert system) sends alerts, including distress alerts. AIS and DSC use built-in or connected GNSS receivers. Equipment using the same faulty source may share its wrong position.
Precise is not safe
A position can be accurate to metres and still put you in danger. The chart may be old, use a different datum (the reference system for chart positions) or be shown at the wrong zoom. Signals can also be jammed or faked. Treat GNSS as one source and check it against the depth, what you see and the chart. More on these limits next.
2Key data
- Global systems
- GPS, Galileo, GLONASS, BeiDou
- Satellites for a fix
- At least 4
- Typical accuracy, open sky
- A few metres; about 1–3 m with SBAS
- SBAS
- EGNOS (Europe), WAAS (North America)
3Key points
- 1
Four global systems: GPS, Galileo, GLONASS, BeiDou; receivers use several together.
- 2
A fix needs at least four satellites: position plus the receiver's clock error.
- 3
Typical accuracy: a few metres in open sky; about 1–3 m with SBAS (EGNOS, WAAS).
- 4
GNSS can feed AIS and DSC: equipment using a faulty source may share its wrong position.
4Frequent mistakes
Wrong: The receiver is broken if the position wanders near a cliff.
Right: Cliffs and high quays block satellites. Move into open water and check the receiver's accuracy display.
Wrong: A position shown to 0.001′ is exact.
Right: The decimals show the display's resolution, not its accuracy. Expect a few metres, and more when signals are blocked.
Wrong: GNSS and GPS are two different things you must choose between.
Right: GPS is one GNSS. Most receivers combine GPS with Galileo, GLONASS or BeiDou.
5Practice case & exam keys
Moored under a 60 m cliff, your plotter shows the boat jumping 20–30 m around, and the accuracy display reads poor. A friend says the receiver is broken. Is it?
Show the answer
Probably not. The cliff hides much of the sky, so the receiver sees fewer satellites, all on one side, and the fix gets worse. Out in open water it should settle to a few metres again. Near hazards, rely on the depth, your eyes and bearings rather than the boat symbol.
Exam keys
- GNSS systems: GPS (USA), Galileo (EU), GLONASS (Russia), BeiDou (China).
- Ranging: travel time from at least four satellites gives position and time.
- Accuracy a few metres; SBAS (EGNOS, WAAS) improves it to about 1–3 m.
- AIS and DSC use built-in or connected GNSS receivers; errors can spread from a shared source.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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