Picture orientation

Orientation is the choice of directional referens to be represented by the 000º (12 o’clock) graduation on the fixed bearing scale around the CRT.

1-ship’s head -up orientation (unstabilised)

-The observer views the picture with the heading marker (and thus the ship’s head) at the top of the screen.

-There is no input of compass information to the display

-The objects on the starboard side of the ship will lie on the right and those on the port side will lie on the left.

-Ship’s head -up orientation is corresponds directly with the scene as viewed through the wheelhouse window.     -

-At every change in the direction of the observing vessel’s heading,the entire picture (apart from the heading marker), will rotate by an equal but opposite angle. This characteristic limits the usefulness of the orientation in three specific ways.

1-If a large alteration of course is made, any areas of land echoes are smeared across the screen, making it difficult to identify specific features. The afterglow created during the the alteration may obscure isolated fixed or floating targets for some time after the vessel is steady on her own course .

2- Due to the effect of wind and sea, the ship will “yaw” about the correct heading on a display using an unstabilized orientation, this superimposes an angular wander on the movement of all targets. Which limits the ease and speed with which bearings can be measured.

3-The disruption of the echo movement due to yaw limits the ease and the speed with which an observer can plot the movement of targets for collision avoidance purposes

1.4.2 True - north- up orientation

(stabilised) In true - north- up orientation, the observer wievs the picture with north at the “top” of the screen.

-During course alteration, there is no rotation off the picture .

-There is no angular smearing which is associated with any change in heading. -There is no masking of targets by afterglow generated during the alteration of course.

-True bearings can be read off directly and quickly.

-It is sometimes uncomfortable to view a north up orientation particularly when the vessel is on southerly courses

Course up orientation (stabilised) In course up orientation the heading marker is aligned to the 000º graduation on the fixed bearing scale at an instant ar which the vessel is right on the chosen course. A north marker is displayed at the edge of the screen. It eliminates the angular wander of the picture due to yaw,while maintaining the heading marker in a substantially (though not exactly) ship’s head up position.

ship’s head -up orientation (unstabilised)

True - north- up orientation (stabilised)

Course up orientation (stabilised)

Picture orientation compared

Picture presentation: The picture presentaion is used to indicate if the movement of displayed echoes is shown with respect to (a) the observing vessel (b) the water, or

(c) the ground.

The relative –motion display presentation

-In relative motion presentation the origin of the display is stationary and the movement of all targets is shown with respect to the observing vessel.

-Commonly the origin is is located at the center of the circular screen, also “off -centred” relative motion presentations are available.

-The esential feature is that the origin is stationary and as a consequencetargets exhibit their motion relative to the observing vessel.

The true motion presentation: -In true motion presentation the echo movement of all targets is rendered independent of the motion of the observing vessel.

-This is achieved by causing the origin of the picture to track across the screen in a direction and at a rate which correspond with the motion of the observing vessel.

-Question:Should the observing vessel’s course and the speed be measured with respect to the water or with respect to the ground?

“If the course and speed through the water are selected, the true motion presentation is said to be sea-stabilized.”

True motion sea stabilized presentation: To produce a true –motion, sea stabilized presentation the origin of the picture must be made to track across the screen in a direction and at a rate that correspond with the observing vessel’s course and speed through the water.

True motion ground –stabilized presentation. To create ground –stabilized of a true motion presentation, the origin of the picture is made to move across the screen in a direction and at a rate which correspond wi,th the observing vessel’s track over the ground. To achieve this, the in put data must have a component which represents the observing vessel’s course and speed through the water plus a further component which represents the set and the rate of the tidal stream or current.

The relative –motion display presentation

A) CHART1

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True motion sea stabilized presentation:

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a) CHART

True motion ground –stabilized presentation.

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Presentations: summary of features