R2Sonic 2022, Operation Manual

Page: 80 / 133

Page 80 of 133
Version 3.0 Rev r000
Date 25-08-2010
Part No. 96000001
8.3 The Motion Sensor
The motion sensor is used to determine the attitude of the vessel in terms of pitch, roll and heave.
Pitch is the movement of the bow going up and down. Roll is the movement of the port and
starboard side going up and down. Heave is the vessel going up and down.
The sonar head is physically attached to the vessel; as the vessel moves, so does the sonar head.
The motion sensor reports the movements of the vessel to the data collection software; the data
collection software, using the offsets to the motion sensor and to the sonar head, computes the
movement at the sonar head to correct the multibeam data for pitch, roll and heave.
One important aspect of the motion sensor is the sign convention used by the motion sensor as
compared to the sign convention used in the collecting software. The surveyor must be aware of
the convention that is used and what adjustments are necessary, if any, to ensure that the
convention is consistent with the data collection computer.
There exist two major areas of thought as to where the motion sensor should be situated. One
group believes that the motion sensor should go as close to the multibeam as possible, even if the
multibeam is mounted on an over-the-side pole. The second group believes the motion sensor
should be placed as close to the centre of rotation for the vessel as possible.
Placing the motion sensor on the hydrophone pole would seem to solve for all movement of the
pole itself, but in fact the motion sensor, mounted in this fashion, can provide false attitude
measurements. This is particularly true when there is significant roll; the motion sensor on the pole
can interpret a portion of this roll as heave, which is not true. By placing the motion sensor as close
to the centre of rotation (also called the centre of gravity) as possible, only the real heave of the
vessel will be measured. All software will solve for the motion of the sonar head, based on the
offsets that have been entered into the setup files for the vessel configuration; this is called a lever
arm adjustment. The other consideration is that the motion data is usually applied to the GPS
antenna. The GPS antenna is usually mounted high on the vessel, so any pitch or roll will induce a
large amount of movement in the GPS antenna thus providing a false position due to the antenna
movement. If the motion sensor is mounted on the hydrophone pole, it is reporting an exaggerated
motion because it is far from the centre of motion of the vessel; this exaggerated motion then
would be applied to the GPS antenna position and the vessel position computation would be in
error.
The other consideration is that the alignment of the motion sensor must be on or parallel to the
centre line of the vessel; it is essential to prevent ‘bleed-over’ of pitch and roll. If the motion sensor
is not aligned with the centre line, when the vessel rolls some of the roll will be seen as pitch as the
motion sensor’s accelerometers and gyros are not aligned with the axes of the vessel it is mounted
on. It is more difficult to obtain this precise alignment if the motion sensor is placed on the pole.
Mount the motion sensor as close to the centre of rotation (or centre of gravity as possible) and
perfectly aligned to the centre line of the vessel.