Dimensional surveying
Several sensors are connected to the SU90 system to make its operation as accurate as possible. These sensors are physically located on different places on the vessel. Typical sensors are those providing navigational information (heading, speed or geographical position) or vessel movements in the sea (motion sensors).
In order to "move" these sensors to a common location on the vessel a vessel coordinate system is defined. The relative location of each sensor is then measured and referenced to this coordinate system. This is commonly referred to as dimensional surveying.
Why is this important?
When a sensor makes a measurement, the information is always related to the physical position of the sensor. This introduces an error since this is not the physical location of neither you nor the SU90 system. On small vessels, these measurement errors are often negligible. On larger vessels, however, the distances between the sensors are greater, and the errors can affect the accuracy of the SU90 system. The errors become extra noticeable when the vessel moves in heavy seas.
Example
The antenna of a position sensor is typically mounted high above the superstructure, while a motion sensor is located close to the vessel’s centre of gravity. Both of these are physically positioned far away from the transducer, which may be located closer to the bow.
If you have an echo sounder transducer located close to the bow, the measured depth will change considerably when the bow is subjected to heave in heavy seas.
In order reduce the measurement errors you can do this:
1 Install and use a motion reference unit (MRU) to detect the movements of the vessel.
2 Define a vessel coordinate system, and place all the sensors in it.
Dimensional surveying
The dimensional surveying tasks recommended for the SU90 system include:
Define the vessel coordinate system.
The vessel coordinate system is established to define the relative physical locations of system units and sensors. When you have several different sensors and transducers on your vessel, and you wish each of them to provide accurate data, you need to know their relative physical positions.
Define the location of the origin in the coordinate system.
Place it in the coordinate system.
The origin is the common reference point where all three axis in the vessel coordinate system meet. All physical locations of the vessel’s sensors (radar and positioning system antennas, echo sounder and sonar transducers, motion reference units, etc.) are referenced to the origin.
Define the vessel’s centre line.
Place it in the coordinate system.
Measure the physical location and orientation of each sensor (or sensor antenna).
Place the sensors in the coordinate system.
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 accuracy of the three numerical values for X, Y and Z defines the accuracy of the sensor data.
Measure the physical location of the transducer.
Place it in the coordinate system.