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Egypt Maritime COC Deck study materials
Egypt Maritime COC Deck study resources

Egypt Maritime COC Deck: Celestial and Terrestrial Position Fixing

Egypt Maritime COC Deck · Work through your exam, one topic at a time.

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Celestial and Terrestrial Position Fixing

This domain tests whether you can produce a position without GPS and can audit a GPS position you do not trust. Examiners ask for sextant correction chains, a meridian-altitude latitude, intercept plotting, and terrestrial fixes such as three-point bearings or horizontal sextant angles.

Sextant correction chain

  1. Apply index error: subtract if on the arc, add if off the arc.
  2. Apply dip = 1.76 x sqrt(height of eye in metres), always subtracted, giving apparent altitude.
  3. Apply the main correction from the almanac/tables: refraction (subtract), semi-diameter (add for lower limb, subtract for upper limb), parallax (add, negligible for stars). Result is Ho, observed altitude.

Intercept method (Marcq St Hilaire)

Take an assumed position, derive the body's GHA and declination from the almanac, compute calculated altitude (Hc) and azimuth (Zn). Intercept = Ho − Hc, plotted toward the azimuth if positive and away if negative, from the assumed position along the azimuth line. Two or more intercepts give the fix; the cocked hat should be small and its centre used.

Terrestrial fixes

Three-point bearing fixes need well-spread objects; a large cocked hat usually means a systematic error such as a compass error, not random plotting error. Horizontal sextant angles plotted with a station pointer beat compass bearings because they are independent of compass error. Cross-check every GNSS fix against a terrestrial or celestial source and monitor RAIM/jamming alerts.

Worked example - latitude by meridian altitude

Sun at meridian passage, bearing south, Ho = 82°14.0', declination 20°10.0' N. Zenith distance = 90° − 82°14.0' = 7°46.0'. Latitude = ZD + declination (same name, body bearing south) = 7°46.0' + 20°10.0' = 27°56.0' N, consistent with a position in the Gulf of Suez.

Traps that cost marks

  • Sign errors on dip and on index error "on/off the arc".
  • Plotting the intercept away from the azimuth when Ho exceeds Hc.
  • Mixing same-name and contrary-name rules for meridian altitude.
  • Treating a single GPS position as a verified fix without any cross-check.