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Question 1

Describe the working principle of a sextant. Advantages and drawbacks ?

Answer 1

Working Principle of a Sextant:

• Measures the angle between a celestial object (like the Sun or a star) and the horizon.
• Adjust the index arm to align the object with the horizon using mirrors.
• The angle is calculated using Snell-Descartes law (index of refraction).
• Time (usually GMT) is used with the angle to calculate latitude and longitude.
• Advantages : Low cost, Portable, Easy to use, Robust (no electronics)
• Disadvantages : Accuracy (max. 0,1’ for sextant), Labour-intensive (no automation)

Question 2

Describe how a horizontal and/or vertical angular measuring device (e.g. a sextant or electronic
angular measuring device) can aid the planimetric positioning.

Answer 2

Horizontal Angular Measurement:

A sextant or electronic device measures the angle between the observer and a landmark or reference point (e.g., a beacon).
This angle helps determine the direction to the landmark.
When combined with known distance or position, it helps find the observer’s location on a map.

Vertical Angular Measurement:

Measures the vertical angle (altitude) to a landmark or reference point (e.g., a tower or hill).
By knowing the height of the landmark, you can calculate the distance using trigonometry.

Question 3

Describe how a horizontal angular measuring device (e.g. a sextant or electronic angular
measuring device) can be combined with a distance measurement for the planimetric
positioning.

Answer 3

Reflector-less Systems:

Manual orientation to point on the vessel.
Electronic distance measurement (EDM) used.

Reflector on Vessel:

Automation is possible.
Electronic distance measurement using phase, pulse, or carrier wave (e.g., IR).

Determining Planimetric Position:

Use an angular measuring device (e.g., sextant) to measure the angle to a known reference point.
Use a distance measuring device to find the distance to the same point.
Apply trigonometry or triangulation to calculate your position and determine the planimetric coordinates.