3.4 Running Fixes, Transferred Position Lines & Electronic Aids

3.4 Running Fixes, Transferred Position Lines & Electronic Aids

While electronic navigation systems—including GNSS/GPS, electronic chartplotters, and marine radar—dominate modern bridge operations, they introduce specialized vulnerabilities. The prudent Yachtmaster treats electronics as powerful aids while constantly validating them against visual transferred position lines, depth contours, and radar clearing ranges.

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The Geometry and Procedure of a Running Fix

A running fix transfers a line of position (LOP) through time. Because the vessel travels across the seabed between sightings, the first LOP must be translated by both the distance run through water and the tidal stream during the elapsed interval.

$\text{Transferred Point} = \text{Point on LOP 1} + \text{Water Track Vector} + \text{Tidal Stream Vector}$

Step-by-Step Construction Procedure

1. First Bearing (Time $T_1$): Sight charted mark at Time $T_1$ (e.g., 1000 UT). Correct for Variation to find True Bearing. Plot position line (LOP 1) from mark out to sea (single arrow at each end). Note log reading (e.g., 14.2 NM).
2. Run on Course: Steer a steady heading while mark draws abeam. Record speed, leeway, and course.
3. Second Bearing (Time $T_2$): When bearing changes by 30°–60° (e.g., at 1045 UT), take second bearing. Plot LOP 2. Note log reading (e.g., 17.6 NM; water run = 3.4 NM).
4. Water Track Vector: From any point on LOP 1 (Point X), draw line along True Course Steered (corrected for leeway). Measure log distance (3.4 NM) to Point W (>).
5. Tidal Stream Vector: From Point W, lay off tidal vector for elapsed interval (Set and scaled Drift) to Point Y (>>>). Vector XY represents net ground run.
6. Transfer LOP 1: Draw line through Point Y parallel to LOP 1 across LOP 2 (Transferred LOP 1, marked >>----->>).
7. Mark the Fix: Intersection of Transferred LOP 1 with LOP 2 is the Fix at Time $T_2$ (circle with dot, label "Fix 1045").

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Running Fix Procedure Summary Table

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Electronic Navigation Aids: GNSS, RAIM & Chart Datums

GNSS calculates position by timing radio signals transmitted from satellites to an onboard receiver.

Receiver Autonomous Integrity Monitoring (RAIM)

RAIM uses redundant satellite measurements to detect clock or signal errors:

• Basic 3D Positioning: Requires 4 satellites (coordinates X, Y, Z plus receiver clock bias).
• RAIM Fault Detection: Requires 5 satellites to flag a corrupt signal.
• RAIM Fault Exclusion: Requires 6 satellites to isolate and discard the faulty satellite.
• Action: If a "RAIM Alert" appears, increase clearing margins and verify position with visual sights or depth contours.

Horizontal Dilution of Precision (HDOP)

HDOP measures the geometric quality of visible satellites:

• HDOP < 2.0: Optimum geometry; high horizontal accuracy.
• HDOP 2.0 to 5.0: Moderate geometry; acceptable for coastal passages.
• HDOP > 5.0: Poor geometry; positions suspect.

The Critical Hazard of Chart Datums (WGS84 vs. Regional Datums)

Every nautical chart is constructed on a mathematical model of Earth curvature known as a Geodetic Datum:

• WGS84 (World Geodetic System 1984): The universal global datum used by GPS receivers.
• Regional Datums: Older charts were surveyed using regional datums, such as OSGB36 (UK), ED50 (Europe), or NAD27 (North America).

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Using Raster and Vector Charts

A raster chart is an electronic image of a paper chart. Magnifying it enlarges the same pixels; it does not add survey detail. A vector chart stores selectable objects and attributes, allowing layers and detail to change with display settings and scale. An official ENC is a vector chart; a recreational chartplotter is not automatically an approved ECDIS.

Before a passage, check chart coverage, edition/updates, datum, scale and display settings. Inspect the whole route at appropriate detail so that a small-scale overview does not conceal a danger. Cross-check the plotted position and route against independent observations and suitable backup navigation information.

Sources: UKHO chart formats; MCA electronic navigation guidance.

Fixing from a Known Waypoint

Check a waypoint’s charted coordinates and datum, then select it as the active destination. Set bearings to True and distances to nautical miles. Record the time, the current bearing to the waypoint and its direct distance; do not substitute the original route course or distance along a multi-leg route.

From the waypoint on the chart, plot the reciprocal bearing and mark the measured distance. For example, BRG090°T and DTW3.0NM place the yacht 3.0NM along 270°T from the waypoint. Label and time the fix. Both readings come from the same receiver solution, so they do not independently check GNSS accuracy; compare with visual/radar fixes or other suitable observations.

Sources: Garmin marine receiver data fields, MCA electronic navigation guidance.

Radar Fixing and Echo Sounder Navigation

Marine radar provides an all-weather positioning sensor independent of external satellite networks.

Radar Ranges vs. Radar Bearings

• Radar Ranges (Variable Range Marker, VRM): Highly Accurate. Radar pulses measure speed-of-light travel time to a target with an accuracy of tens of meters. Intersecting two or three VRM range arcs from distinctive headlands or breakwaters produces an exceptionally tight fix.
• Radar Bearings (Electronic Bearing Line, EBL): Subject to Error. Radar horizontal beam width smears physical targets tangentially, degrading bearing precision; heading errors also affect bearings.
• Seamanship Rule: Always prefer two or three radar ranges over radar bearings for fixing position.

Echo Sounder Contour Navigation

A depth sounder can provide a useful position check. Establish whether it reads below the transducer, keel or surface and apply the offset to obtain total water depth. Subtract the height of tide above chart datum before comparing with a charted contour. Where the seabed profile is distinctive, cross that depth position line with an independent transit or radar range. Allow for survey error, changing seabed and uncertainty in tidal height; the same depth may occur in several places.

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The Concept of the "Area of Uncertainty"

A plotted position has uncertainty; assess it from the method and conditions rather than assigning a fixed radius:

• Visual Three-Bearing Fix: Consider bearing errors, distance to the marks, cut angles and common errors.
• DR / EP: Start with the previous fix’s uncertainty and allow for uncertain heading, log distance, leeway and current over the elapsed time.
• GPS / GNSS: Consider receiver accuracy and integrity indications, installation, interference and chart datum. Good satellite geometry alone does not guarantee a safe position.

When passage planning near lee shores, sandbanks, or submerged wrecks, never draw courses that pass within the vessel's current Area of Uncertainty. Clearing lines, safety contours, and clearing bearings must be established outside this buffer zone.