NAVIGATION · Lesson 1 of 5
GNSS Fundamentals
25 min · Beginner friendly
You’ll learn toExplain in outline how GNSS fixes a position · State realistic accuracy figures and error sources · List sensible precautions against GNSS failure or jamming
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About 8 min
Every screen on board gets its position from — satellite navigation. It is usually superb and occasionally wrong, and the skill this lesson teaches is trusting it the right amount.
Take a guess
Decide before you read on. Which failure is more dangerous: your plotter's position vanishes, or it keeps showing a confident position that is wrong?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
How a GNSS Receiver Finds You
Each satellite broadcasts, in effect: here is where I am, and here is exactly when I sent this. Radio travels at a known speed, so your receiver turns each message's travel time into a distance — a . One range only puts you somewhere on a huge circle. Three ranges cross at a single point. Position from distances rather than directions is called .
- GNSS
- Global Navigation Satellite System — the umbrella term for all satellite positioning systems. GPS is one GNSS, not the only one.
- Constellation
- A family of satellites operated as one system: GPS (USA), Galileo (Europe), GLONASS (Russia), BeiDou (China). Modern receivers track several at once.
- Trilateration
- Fixing a position from measured distances to known points, rather than from bearings.
- Fix
- A position determined from measurement (here, satellite ranges) rather than estimated from course and speed.
- WGS84
- The worldwide reference model of the Earth used by GNSS. Modern charts are drawn to , so plotter positions and chart positions agree.
Knowledge check
Why does a GNSS receiver need signals from a minimum of four satellites for a full 3D fix?
How Accurate Is It Really?
In open water a standard marine receiver fixes you to within about 5 to 10 metres, tightening to roughly 3 metres with SBAS corrections (EGNOS in Europe). The is usually smaller than your boat. The skill is knowing what can quietly make it worse.
Knowledge check
Moored close under a high cliff, you watch your plotted position wander by tens of metres. The sky is clear and the satellites are healthy. What is the most likely cause?
| Error source | What happens | Typical size |
|---|---|---|
| Atmospheric delay | The signal slows slightly passing through the ionosphere and troposphere | A few metres |
| Multipath | The signal bounces off cliffs, harbour walls or your own rigging before reaching the antenna | Metres, worst close inshore |
| Poor satellite geometry | Visible satellites are bunched together, so ranges cross at shallow angles | Multiplies all other errors |
| Antenna siting | Antenna shaded by masts, sprayhoods or equipment | Weak or dropped fixes |
| Jamming and spoofing | Deliberate interference drowns out or fakes the satellite signals | Position lost or falsified |
Geometry is the multiplier in that table. Receivers report it as HDOP (horizontal dilution of precision): around 1 means well-spread satellites and a crisp fix; high means ranges crossing at shallow angles, magnifying every other error.
When GNSS Goes Wrong
The satellite signals arrive from space astonishingly faint, and that faintness is the weakness. Jamming drowns them in radio noise: the fix is lost, and the failure is obvious. Spoofing counterfeits them: the receiver shows a confident, believable, wrong position, and nothing looks broken. Both have affected real sea areas — and the mundane failures are far more common:
- Antenna and connection failures — corroded plugs and chafed cables cause more lost positions than anything satellites do.
- Power failure — a flat battery or blown fuse silences every instrument at once.
- Software problems — a frozen screen showing an old position is easy to miss.
- Human error — a entered with digits swapped beats any satellite fault for size of error.
The defence against every one of these is the same habit: never let your electronic position be the only thing that knows where you are. your position at regular intervals, keep a mental picture of where you are relative to visible land, and the plotter against independent evidence — the depth sounder, a of a headland, a buoy passing where you expected it. If the fix jumps or disagrees with the depth, believe the evidence of your eyes first.
Your turn
You are planning a 40-mile coastal passage from Westhaven to Salt Island in your 9-metre yacht. List at least five practical precautions you would take against GNSS failure, jamming or plotter breakdown before and during the passage.
Work it through on paper first — then check yourself against the solution. Reading a worked answer feels like learning; producing one is learning.
Knowledge check
Halfway through a passage your plotter position suddenly jumps two miles offshore, while your depth sounder still agrees with the depths charted along your planned track. What is the most seamanlike response?
Key points
- GNSS is the umbrella term; GPS, Galileo, GLONASS and BeiDou are individual constellations, and modern receivers use several at once.
- Position comes from trilateration: measured ranges to satellites; a minimum of four satellites gives a 3D fix plus receiver clock correction.
- Typical accuracy is 5–10 m, improving to about 3 m with SBAS corrections such as EGNOS.
- Main error sources: atmospheric delay, multipath near cliffs and structures, poor satellite geometry (high HDOP), antenna problems.
- Failure modes include jamming (position lost) and spoofing (position falsified) — plus mundane failures of power, cables and software.
- Defend in depth: log positions regularly, cross-check against depth and visual observations, and carry independent backups.
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