The Prudent Mariner’s Insurance Policy: Reintegrating STCW Competencies for Navigation in a GNSS-Compromised World
In the landscape of modern maritime navigation, the Global Navigation Satellite System (GNSS) has transformed bridge operations, providing continuous, high-precision position data directly to the Electronic Chart Display and Information System (ECDIS). This seamless efficiency, however, has inadvertently created a profound vulnerability. As reliance on satellite-derived position grows, the art of traditional position fixing—enshrined in the International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW)—has dangerously atrophied on many bridges.
The prudent mariner of the 21st century must recognize that technology, while essential, is a primary tool, not an absolute guarantee. In a world of increasing electronic warfare, jamming, and spoofing, the ability to rapidly and accurately revert to independent, non-electronic methods is not merely a regulatory compliance issue—it is an immediate operational necessity. This discussion outlines the specific STCW competencies required of the Officer in Charge of a Navigational Watch (OICNW) and details the indispensable, prudent manner in which these foundational skills, especially celestial navigation, must be integrated into daily watchkeeping to ensure true safety at sea.
Part I: The Mandate of Competence and the STCW Standard
The STCW Code, specifically Table A-II/1 for the OICNW, provides the definitive framework for navigational competence. It is a common misconception that ECDIS and GNSS satisfy the entire navigational function; in reality, they are merely tools used to demonstrate a broader required proficiency. The core function of “Plan and conduct a passage and determine position” is supported by several non-electronic skills that serve as the foundation for navigational resilience:
- Celestial Navigation: This competency is unequivocally mandatory: the “Ability to use celestial bodies to determine the ship’s position” is explicitly listed. This skill demands the use of a sextant, precise timing, and the Nautical Almanac to determine a Line of Position (LOP). Crucially, this method relies solely on human observation and calculation, rendering it immune to electronic interference that affects all radio-based aids.
- Terrestrial and Coastal Navigation: The OICNW must be able to determine position using independent observations of landmarks and aids to navigation (lighthouses, beacons, buoys). This translates to the practical use of radar and visual bearings and ranges to plot LOPs.
- Dead Reckoning (DR): The code requires the ability to maintain a DR plot, “taking into account winds, tides, currents and estimated speed.” DR is the immediate and indispensable backup position used the instant a GNSS failure or doubt is encountered.
- Compass Errors: The OICNW must possess the “Ability to determine errors of the magnetic and gyro-compasses, using celestial and terrestrial means.” This ensures the mariner maintains a truthful heading reference when electronic systems fail or provide misleading information.
The overarching theme of the STCW standard is verification. The criteria for evaluating competence mandate that “The reliability of the information obtained from the primary method of position fixing is checked at appropriate intervals,” necessitating a “confirmation of vessel position by alternate means.” This is the core principle that must drive the daily routine on the bridge.
Part II: The Modern Threat—The Failure of Trust
The imperative to use alternative means is elevated by the increasingly hostile electronic environment. Intertanko’s Jamming and Spoofing of Global Navigation Satellite Systems (GNSS) guide highlights the two primary man-made threats that mandate the immediate reversion to STCW skills.
Jamming (Signal Denial)
Jamming involves overwhelming the weak GNSS signal with stronger radio frequency noise. A jammed receiver typically registers a “No Fix” status, a high Horizontal Dilution of Precision (HDOP), or a complete loss of the position display, often accompanied by system alarms like “RAIM Failure.” This threat is disruptive but straightforward to detect.
Spoofing (Signal Deception)
Spoofing is far more dangerous. It involves broadcasting false GNSS signals designed to mimic authentic ones, which tricks the receiver into calculating an erroneous position that appears perfectly valid. The ECDIS will display a “good fix,” alarms will be silent, and the unsuspecting mariner may proceed on a dangerously false track, potentially heading toward land or hazards while the chart display shows the vessel safely offshore.
Furthermore, GNSS Time Spoofing can corrupt the essential time synchronization required by the entire bridge suite—AIS, radar, VDR, and ECDIS logs—leading to critical failures in target correlation and data integrity.
Against these threats, the prudent mariner understands that the solution cannot be another electronic system. The ultimate backup must be independent of the electromagnetic spectrum used by GNSS. This is where the paper chart and the sextant become the final line of defense.
Part III: Prudent Integration—Daily Verification Routine
Prudent watchstanding is characterized by a disciplined routine of position verification that actively forces the OICNW to compare the GNSS position against an independent source.
| Interval | Method | STCW Competency Applied | Purpose |
| 15 Minutes | Visual/Radar Cross-Reference (Piloting) | Terrestrial Navigation | Confirm immediate track-keeping integrity and check radar overlay accuracy against the ECDIS chart display. |
| 30 Minutes | Manual LOPs (Coastal) | Terrestrial Navigation | Plot single/multiple radar or visual bearings/ranges (LOPs) on the ECDIS to form a fix, independently verifying the GNSS input. |
| 60 Minutes | DR Plot & Manual Fix (Ocean) | Dead Reckoning/Celestial Navigation | Comparing the actual GNSS position with the Dead Reckoning (DR) position; or plotting a Celestial Fix to establish an independent position on the high seas. |
The most important integration is the mandatory use of Celestial Navigation to provide this independent position check when GNSS is lost, or even when it is fully operational.
Part IV: Celestial Navigation—The Manual Fix and the Universal Plotting Sheet
In a true GNSS-denied environment, the ECDIS cannot be trusted to plot the complex LOPs generated by sight reduction. The prudent mariner reverts to the most reliable manual tool: the Universal Plotting Sheet (UPS). This ensures that the position is determined from first principles, providing maximum certainty before any data is transferred to the electronic system.
The Steps for Deriving a Celestial Fix:
- Sight Reduction (The Calculation):
- The OICNW measures the angle (Observed Sextant Altitude, $\text{Hs}$) of a celestial body (Sun, Moon, Star, or Planet) using a sextant at a precisely recorded time (UTC).
- Corrections are applied to $\text{Hs}$ to obtain the Observed Altitude ($\text{Ho}$).
- An Assumed Position (AP), typically the ship’s latest DR position, is selected.
- Using the Nautical Almanac and sight reduction tables (or a non-GNSS electronic calculator), the Computed Altitude (Hc) and the True Azimuth (Zn) of the body from the AP are determined.
- The difference between Ho and Hc yields the Intercept (Ho – Hc), measured in nautical miles (NM). This value is Towards the body if Ho > Hc and Away if Ho < Hc.
- Plotting the Line of Position (The Plotting Sheet):
- The OICNW uses a Universal Plotting Sheet (UPS), a specialized form of Mercator projection designed for plotting in any latitude.
- Plot the AP: The Assumed Position (Lat/Long) is marked on the center of the UPS.
- Draw the Azimuth Line: The True Azimuth (Zn) is laid off from the AP.
- Measure the Intercept: The Intercept (a) is measured along the Zn line, either towards or away from the celestial body, depending on the Ho vs. Hc comparison. This new point is the LOP terminal point.
- Draw the LOP: A line is drawn through the terminal point, perpendicular to the Azimuth line. This line is the Celestial Line of Position (LOP).
- Determining the Fix:
- The process is rapidly repeated for a second, and ideally a third, celestial body.
- The intersection of the two or three LOPs on the UPS marks the ship’s definitive Celestial Fix (or Cocked Hat).
- The latitude and longitude of this intersection point are then read directly from the graduated scale of the Universal Plotting Sheet.
Step 5: Transferring the Fix to the ECDIS
The final, critical step is to transfer this independently derived, manually plotted position onto the electronic chart. The mariner must understand that the paper chart, in this instance, is the source of truth, and the ECDIS is merely the display medium.
- Retrieve Coordinates: Read the precise latitude and longitude of the Fix from the UPS.
- Access Manual Input on ECDIS: Navigate to the ECDIS function that allows manual position input—typically found under “Position Sensor Selection,” “Manual Position Input,” or “User Position.”
- Enter the Fix: Manually enter the derived latitude and longitude coordinates.
- Confirm and Shift: This action either enters the new position into the ECDIS log (as an independent manual fix) or, in the event of an emergency, is used to manually shift the ship’s symbol on the screen to this new verified position.
By forcing the execution of the manual plot, the mariner maintains cognitive command of the navigational process and bypasses the possibility of electronic spoofing. The resultant fix becomes the gold standard against which all other electronic systems must be evaluated.
Part V: Emergency Action—STCW in Crisis
The Intertanko guide on GNSS interference mandates an immediate, comprehensive response that relies heavily on manual competence. In a situation of suspected spoofing or loss of position, the OICNW must move immediately:
- Switch to DR: Immediately transition the ECDIS to Dead Reckoning mode, relying on the last known credible position and the ship’s course and speed inputs.
- Safety First: As noted in the Intertanko guide, the OICNW must STOP the ship, if necessary, to gain more time for safe situation reassessment, especially in restricted waters. The main engine must be ready for immediate manoeuvring.
- Independent Directional Checks: Immediately verify the ship’s heading. Determine the Gyro Error using either celestial means (a sun azimuth) or terrestrial means (bearing to a known object) and cross-check the output with the fully independent Magnetic Compass.
- Determine New Position: Immediately initiate manual position fixing using terrestrial LOPs (radar/visual) or, if offshore, a full Celestial Fix using the Universal Plotting Sheet.
- Manual Data Feed: Recognize that GNSS feeds more than just the ECDIS. Manually feed the Estimated Position (EP) from the Celestial/Terrestrial Fix into critical dependent equipment such as the GMDSS, Autopilot, and any other systems relying on position data (e.g., ODME). Also, Verify GNSS time data against ship’s chronometer to check for Time Spoofing, which is crucial for AIS and VDR integrity.
- Record and Report: Record evidence of GNSS interference (signal loss or misleading information) and Report the event to the coastal state, UKMTO, Flag Administration, and the NATO Shipping Centre (NSC).
- Contingency Planning: Make a passage plan to a safe anchorage in case a lengthy denial of navigation is expected, and keep it on stand-by.
Conclusion
The reliance on a single, primary source of navigation data—GNSS—has inadvertently created a navigational single point of failure. The STCW Convention’s requirements for competence in celestial, terrestrial, and dead reckoning are not outdated concepts; they are the necessary, highly resilient skills required for commanding a vessel in the modern world.
The prudent mariner adopts the following mindset: The ECDIS is a tool for monitoring, but the determination of position must always be subject to independent verification. By diligently integrating the plotting sheet and the sextant into the daily routine—by taking, reducing, and manually plotting a celestial fix to obtain a Lat/Long that is then transferred to the ECDIS—the OICNW ensures that the vessel’s safety is ultimately secured by human competence and not by an electronic signal that can be easily compromised. This resilience is the hallmark of true seamanship.
Additional Reading and Links
OCIMF – Recommendations on Usage of ECDIS and Preventing Incidents (2020)