CH11 Navigation and Aids to Navigation Flashcards
By what means can the position of a vessel be fixed?
Cross bearing (at least 2, preferably 3). Range and bearing. Cross range. Beam bearings. Radar or visual. Satellite. Radio position-fixing.
What considerations are there when using a magnetic compass for position fixing?
Gradual change in variation, ie change of 5° in 400nm in the English Channel.
Magnetic variation charts should be consulted.
A magnetic epoch (time between recalculation) is 5 year.
Nearby wrecks may cause compass deflections.
Power cables may cause compass deflections.
Local magnetic anomalies may cause compass deflection.
What is a simultaneous bearing?
Two/three observations of fixed objects.
What is a simultaneous bearing and distance?
A bearing of a fixed object and a range.
Range is usually by radar but can also be gained using a Vertical Sextant Angle or an optical rangefinder.
What is a running fix?
Used when there is only one fixing point available.
Position lines taken at different times, allowing for ground track and ground speed.
What is a transit?
When two fixed points line up a position line can be obtained by taking a bearing. Also, compass error can be calculated.
What is a Horizontal Sextant Angle?
True bearing not necessarily required.
Bearings of three objects.
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What is a Vertical Sextant Angle?
Measurement of the angle between the horizon and the foci of a light ((for example) (MHWS)).
Height of the object must be known.
This will provide a range.
Added to a bearing, the position can be fixed.
What are the features of radar wrt navigation?
Fixing by ranges is more accurate than bearings.
Clearing ranges can be particularly useful off a straight and featureless coast.
Parallel indexing is a refinement of clearing ranges which uses lines drawn on the display.
Shape and size of an object have a greater effect than composition on echo reflection.
Radar beacons are known as RACONS.
What is Loran-C?
Coastal system requiring a chain of stations, consisting of a master and two slaves.
The signal can be received by either groundwave of skywave.
The parabolic nature of the signal is used to determine the signal.
Discontinued in USA and Canada.
What is eLoran?
Uses Loran-C infrastructure.
Provides a differential correction in the same way a dGNSS.
Uses high powered TX and low-frequency signal (opposite of GNSS) so is unlikely to be affect by the same interference.
