What are methods of getting Ship Position if GPS fails at mid sea?

  1. By Astronomical object.
  2. A Bearing and Sounding
  3. Running Fix.

By Astronomical object

 Six common methods are used :
  1. Latitude by Polaris (Pole Star)
  2. Latitude by meridian altitude
  3. Latitude by ex-meridian
  4. Longitude by meridian passage of the sun
  5. Longitude by chronometer
  6. Intercept
Latitude by Polaris (Pole Star)

As the Pole Star is always around the North Pole at radius of 1°, so it is always on or near the meridian passage. The latitude of the observer can be determined. The position line runs in an east-west, or nearly east-west, direction.

Latitude by Meridian Altitude

This method is used to obtain the position line by taking the altitude of the celestial body when it is instantly on the same meridian as the observer’s. In this case, the position line runs in an east-west direction (90°-270°), and coincides with a parallel of latitude.

Latitude by Ex- Meridian Altitude

It is sometimes not possible to obtain the altitude of the celestial body when it is on same observer’s meridian due to cloud, environmental factors, etc. If the altitude of the celestial body can be obtained a few minutes before or after meridian passage, the Ex-Meridian method can be used to reduce the observed altitude to meridian altitude. The latitude of the observer can be determined. The position line runs nearly in an east-west direction.

Longitude by Meridian Passage of the Sun

By knowing that the sun orbits with one completed circle in 24 hours, or 15° for every hour, the observer can determine position at noon by using the chronometer. The advantage of this method is that the DR position is not required.

Longitude by Chronometer

This method is also used to determine the longitude of the observer. The position line runs through the position at DR latitude and observed longitude in a direction perpendicular to the azimuth of the celestial body from the observer.

About the author

Manish Mayank

Graduated from M.E.R.I. (Mumbai). A cool, calm, composed and the brain behind the development of the database. The strong will to contribute in maritime education and to present it in completely different and innovative way is his source of inspiration.


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