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4.3 Lever arm offset measurement
The purpose of this section is to outline the process necessary to calculate the lever arm, which is the position of
the Merlin LT laser origin point in relation to the vessel’s IMU/CoG. The laser origin point is inside the Merlin LT, at
the centre point of the laser’s horizontal rotation.
The lever arm is expressed as an XYZ coordinate on the vessel reference frame. This coordinate is entered into
the navigation software and ensures that data from the Merlin LT is correctly positioned in relation to data received
from other onboard sensors and survey equipment.
Note that a calibration must also be carried out prior to the start of survey operations in order to fine tune the
heading, pitch and roll angles of the Merlin LT data with respect to the IMU. See the accompanying Merlin manuals
for QINSy, HYPACK and PDS for further details on this process.
The workflow for carrying out the lever arm offset measurement is as follows:
•
Secure the Merlin LT base plate in position.
•
Establish the physical XYZ lever arm to the Merlin LT measurement datum point on the base plate.
•
Position the Merlin LT at the desired vertical angle.
•
Determine the vertical laser offset adjustment from Table 1.
•
Apply the vertical laser offset adjustment to the measured XYZ lever arm.
•
Enter the final lever arm into your navigation software.
The final lever arm values are dependent on the vertical angle at which the Merlin LT is positioned. These values
will therefore change as the Merlin LT is reoriented vertically. Each time the unit is moved from its fixed position,
new values must be accounted for and entered into the navigation software.
If the mounting plate is used, then a horizontal rotation may also need to be taken into account: see section 4.4.
4.3.1 Measuring the physical XYZ lever arm
Navigation software packages require an XYZ measurement from the vessel’s IMU/CoG to the Merlin LT laser origin
point. Since the laser origin point cannot be accessed for direct measurement, establishing this lever arm is carried
out in two stages: a physical measurement to the visible Merlin LT measurement datum point in the centre of the
base plate, followed by an adjustment taken from Table 1 which accounts for the offset from the measurement
datum point to the laser origin point.
The measurement datum point is shown in Figure 16. This point must be physically measured as an XYZ offset
from the IMU/CoG.
Measurement datum point
Figure 16 Merlin LT measurement datum point
