The slides, written out
AI-drawn illustration. Simplified to explain the idea, not to scale.
Slide 1 · KNOW THE ERRORS
KNOW THE ERRORS. NAVIGATE BETTER.
GPS ERRORS
& ECDIS | COMPLETE GUIDE
Understand the mistakes. Make safer decisions.
THE KEY TO ACCURATE POSITIONING & SMART NAVIGATION
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Slide 2 · a)
a)
ERRORS IN
GPS POSITION
GPS is highly accurate, but the position shown is affected by signal conditions, satellite geometry, receiver performance and chart/reference data.
1
DELIBERATE ERROR
Selective Availability (SA)
• Intentional degradation of civilian GPS accuracy
• Discontinued globally in May 2000
• U.S. policy states there is no intent to use SA again
• Important today mainly as a historical example of deliberate GPS error
SA was estimated to degrade civilian accuracy by up to ~100 metres.
2
NATURAL ERRORS
Atmospheric Effects (Ionospheric & Tropospheric)
Propagation delays through atmosphere affect measured range. Magnitude depends on atmospheric conditions and satellite elevation. Low elevation satellites generally have greater atmospheric effects.
Multipath
Signals can reflect from structures before reaching the antenna. Reflected signals arrive with a different path delay and can degrade position accuracy.
Key takeaway
GPS position must always be assessed against other available means of position fixing.
EXAMPLES OF OTHER NATURAL FACTORS
Satellite clock and ephemeris imperfections
Receiver noise and internal clock errors
Poor satellite geometry (high GDOP) amplifies all errors
Signal blockage or obstruction
Accuracy of the receiver does not automatically mean accuracy of the plotted position.
Slide 3 · a) GPS ERROR SOURCES
a) GPS ERROR SOURCES
YOU SHOULD KNOW
3 RECEIVER ERRORS
Receiver noise and clock effects contribute to positioning error.
Performance depends on receiver design, signal processing and equipment quality.
Do not assume civilian equipment is automatically less accurate than military equipment; modern civilian dual frequency and augmentation systems can provide very high accuracy.
REMEMBER
Receiver quality, antenna placement and installation practices have a real impact on accuracy.
4 SATELLITE CLOCK & EPHEMERIS
Satellite Clock Error
Even atomic clocks have very small timing errors; small timing errors translate into range errors.
Ephemeris Error
Errors in the transmitted satellite orbit information affect the calculated position.
IMPORTANT EXAM POINT
Ephemeris data is short validity data; NAVCEN guidance identifies approximately 4 hours as its normal validity period.
Do NOT say that a 4 to 6 hour shutdown simply makes the almanac stale.
5 GDOP (GEOMETRICAL DILUTION OF PRECISION)
GDOP is not an independent error source. It describes how satellite geometry affects resulting position accuracy.
Well distributed satellites → better geometry → lower DOP
Poorly distributed or clustered satellites → poorer geometry → higher DOP
MEMORY LINE
SAME SIGNAL ERROR + POORER GEOMETRY = GREATER POSITION UNCERTAINTY
ALWAYS CROSS CHECK
Confirm GPS position using visual bearings, radar fixes or other available means.
Slide 4 · a)
a)
6. USER / DATA ERROR
WRONG DATUM
GPS position is normally referenced to WGS 84,
while chart data may use another geodetic datum.
If the chart datum and GPS reference are not
correctly accounted for:
GPS POSITION
(WGS 84)
DATUM
MISMATCH
POSITION OFFSET
ON CHART
EXAMPLE
Chart uses local datum but GPS is WGS 84.
Without applying the datum correction, the plotted position will be offset on the chart.
OOW CHECK – BEFORE RELYING ON GPS POSITION
1. Check the chart's "Positions / datum information".
2. Confirm the receiver's reference datum.
3. Apply the required chart datum correction where applicable.
4. Cross check the GPS position using visual or radar fixing, especially when approaching dangers.
COMMON DATUMS YOU MAY MEET
WGS 84
Default for GPS / GNSS
Local / Regional Datums
Used on many paper charts
Chart Note
Always check the "Positions note on the chart.
Even when the same datum is selected in the receiver, parameters may differ slightly. Always apply the chart correction if required.
IMPORTANT
A GPS receiver showing a precise numerical position does not automatically mean the position is correct relative to every chart.
EXAM MEMORY
Accuracy of the receiver ≠ Accuracy of the plotted position.
Slide 5 · b) ADVANTAGES OF ECDIS OVER CONVENTIONAL PAPER CHARTS
b) ADVANTAGES OF ECDIS OVER CONVENTIONAL PAPER CHARTS
ECDIS is more than a digital chart. It enhances navigation through real-time information, automation and integration.
1 REAL TIME POSITION
Continuous automatic display of own ship position from connected navigation sensors.
2 ROUTE PLANNING
Waypoints, routes, courses, distances and turn radii can be planned directly on the electronic chart.
3 ROUTE MONITORING
ECDIS continuously monitors the vessel against the planned route and provides relevant indications/alerts based on the system settings.
4 SAFETY INFORMATION
Safety contour, isolated dangers, prohibited areas, traffic separation schemes and other chart information support safer navigation.
5 SENSOR INTEGRATION
Depending on the approved system configuration, ECDIS can integrate and display data from systems such as GNSS, AIS, Radar/ARPA, Echo sounder, Speed log and other sensors.
6 ELECTRONIC CHART UPDATES
ENC updating can simplify chart maintenance, provided updates are correctly obtained, applied and verified. IMO guidance for good practice covers onboard ECDIS updates.
7 ROUTE CHECKING
ECDIS can check a planned route against safety contour, charted dangers, prohibited areas and other user-defined parameters.
8 DIGITAL STORAGE & DISPLAY
Large quantities of ENC data can be stored electronically. Display layers can be shown or hidden to suit the navigation task.
9 DATA RECORDING & PLAYBACK
Tracks, routes, user data and alarms can be recorded and replayed to support review and analysis.
BACKUP & FAILURE – CRITICAL EXAM POINT
SOLAS does NOT mean "paper charts must always be the backup".
ECDIS requires adequate independent backup arrangements capable of supporting safe navigation if ECDIS fails.
Appropriate backup arrangements can include suitable paper charts or other compliant arrangements.
THE FLIP SIDE
CONFIGURATION MATTERS
Incorrect safety settings (safety contour, alarm limits, safety depth) can affect alerts and decisions.
DATA MUST BE UP TO DATE
Outdated or incorrectly installed ENC data is a serious navigational risk.
EQUIPMENT FAILURE RISK
Power loss or system failure requires an adequate and independent backup arrangement.
OVER-RELIANCE IS DANGEROUS
ECDIS supports the OOW, but does not replace professional judgement and cross-checking.
FINAL MEMORY LINE
"ECDIS supports the OOW – it does not replace the OOW."
IMO's current ECDIS performance standards (MSC.530(106)) introduce the phased S-100/S-101 implementation. Voluntary use from 1 Jan 2026; mandatory from 1 Jan 2029.
Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.
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