Kongsberg Simrad SN90 Reference Manual page 634

Simrad SN90 Reference Manual
has been defined by the designer or shipyard. This origin is normally identified with a
physical marking, and also shown on the vessel drawings.
Frequently used locations are:
• Aft immediately over the rudder (frame 0)
• Vessel's centre of gravity
• The physical location of the motion sensor
Vessel coordinate system alternative origins
If necessary, other origin locations may be defined for specific products or purposes. One
example is the Navigation Reference Point that is frequently used. Whenever a vessel
is surveyed to establish accurate offset information, the surveyor may also establish an
alternative origin location. Whenever relevant, any such alternative locations must be
defined using offset values to the "official" origin established by the designer or shipyard.
A commonly used alternative origin is the physical location of the vessel's motion sensor.
Defining the physical location of each sensor
By means of the vessel coordinate system, the physical location of every sensor can
be defined using three numerical values for X, Y and Z. These values must define the
vertical and horizontal distances from a single reference point; the origin. The physical
location of the motion reference unit (MRU) is often the most important sensor to define.
For many systems, the vessel heading is also a critical measurement.
In this example, a second
reference point has been
established. It is defined with
three positive offset values
for X, Y and Z. All values
are positive because the new
reference point is in front of
and below the origin, and on
the starboard side.
The accuracy of the three numerical values for X, Y and Z defines the accuracy of the
sensor data. If you require a high accuracy, for example for underwater positioning,
underwater mapping or scientific measurements, you must have each sensor positioned
using professional land surveying. For such use, a good alignment survey is critical for
high quality results. Surveys are normally done by qualified and trained surveyors using
proven survey equipment and methods.
Related topics
Concept descriptions, page 625
Basic sonar theory, page 630
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