Real-Time Ship Motion

Aside from the EKF, the SBG Systems inertial sensors compute at 50Hz ship motion data from accelerometers double integration. As this double integration generates drift due to orientation error or sensor bias, the best way to get a stable output is to use a high pass filter that will remove any constant component in the motion.

The proper behavior of this high pass filter is ensured by an automatic filter tuning, taking into account changes in sea state conditions. This feature is functional for swell periods up to 15 seconds for real time heave on an Ellipse, up to 20s on an High Performance products.

Due to high pass filter design, the heave, surge and sway data will always return to zero in static conditions.

Surge and Sway validity

Surge and Sway are only relevant in near static applications such as buoys.

Ship motion reference frame

The ship motion outputs are in a specific vessel coordinate frame :

  • Heave is the vertical position, positive pointing downwards ;
  • Surge is the longitudinal position, in the horizontal plane and positive pointing toward the vessel bow ;
  • Sway is the transverse position, in the horizontal plane and positive pointing toward the vessel starboard side.

Surge and Sway availability

Surge and Sway are not available with Ellipse series.

Heave step response

If a step is performed, the heave output will show the step and then will smoothly come back to zero. It may take a few minutes for the output to be stabilized after a step. The following graph shows a typical step response. Note that the time to recover a 0 heave output may depend on the previous sea state. The overall shape will remain the same however.

Swell Mode

On high performance products, the Swell mode is automatically enabled depending on sea conditions to optimize heave algorithm performance. This mode fuses GNSS information with accelerometer in a smart way to limit phase and amplitude errors that are inherent to heave algorithms. This mode will be particularly efficient in higher amplitude swell conditions.

The system determines automatically the best heave computation mode depending on sea state.

Delayed Heave

Available on higher grade units designed for hydrographic applications (Ekinox, Apogee and Navsight Marine for example), the Delayed Heave algorithm makes use of past measurements to greatly enhance heave performance. Common phase errors observed in real time Heave operation are seamlessly corrected and the filter will provide even better performance under long swell period conditions.

The Delayed heave algorithm has a fixed delay of 150s. The output messages have the same format as real time ship Motion mode and a time-stamp is used to correctly date the ship Motion data.

This algorithm is ideal for applications that don't require strict real time operation such as seabed mapping. The real time Heave operation remains available to get a first heave estimate before the delayed heave data becomes available.

Time considerations

As the Delayed heave is a delayed algorithm, the unit must remain turned ON in normal operating conditions at least 150s before, and 150s after the actual survey path is performed to enable full data acquisition.

Delayed Surge and Sway?

Only vertical ship motion (heave) is available in this Delayed Heave output. Surge and Sway are not provided in this mode of operation.

Center of Rotation and Deported Heave Operation

When analyzing the heave motion, we can find that part of the heave motion is due to the vessel rotations.

This part differs from one place to another, and is canceled at the Center of Rotations. Another location will affect the heave in a constant way.

The following picture shows the effect of the rotation induced heave at different locations on the vessel:

deported heave

In order to optimize the heave performance, the motion sensor placement recommendation may vary depending on type of vessel or installation.

For smaller survey vessels, placing the sensor directly at the point of interest will ensure best performance (e.g. direct mounting on top of the MBES sonar) : this location will ensure that no extra noise will be added by calculating remote heave, and the small Center of Rotation to sensor offset will not create significant errors. This setup is also the least sensitive to any IMU misalignment or lever arm residuals.

For larger vessels, with a significant Center of Rotation to point of interest lever arm (e.g. above 10m), we recommend to place the IMU closer to the vessel Center of Rotation in order to minimize constant errors due to long term roll or pitch offsets.
In that case it's important to setup correctly the IMU misalignments to cancel any constant offsets on the heave output.

In any case, it is possible to setup the heave output at multiple points of interest within the vessel (e.g. the sonar head).

IMU misalignment

When the boat is stationary, IMU misalignment with vessel should be precisely accounted for by mechanical design, or software configuration in order to provide consistent heave values on secondary points.

Center Of Gravity

The Center Of Gravity lever arm in SBG Systems products is not taken into account in the heave measurements. If required, we recommend to enter this value in the customer survey software used (Hypack, Qinsy, BeamworX, ...).

Heave Enhanced Altitude

In the marine survey industry, there is often the question whether using the heave output or the Kalman filtered altitude output is preferred. In case of good GNSS condition the RTK altitude can be very precise and simplifies the setup as the surveyor don’t need to bother with tide compensation. However, even when fused with inertial sensors, the RTK altitude might be disturbed in case of difficult GNSS environment such as bridge crossing.

On the other hand, the heave algorithm allows precise relative measurements, without specific errors during difficult GNSS conditions. However its more complex to use due to the tide compensation needed. The Enhanced Altitude mode algorithm takes the best of these two worlds by merging the heave output with the RTK altitude, providing accurate and absolute altitude measurement in both good and challenging GNSS conditions.

This algorithm can only be used with the marine motion profile and in combination with a precise position like RTK or PPP with fixed carrier ambiguities. This Enhanced Altitude mode can be disabled if needed.

Enhanced Altitude availability

This mode is only available in High performance products designed for hydrographic applications.

Post-processed Heave

In post-processing software such as Qinertia the Heave is re-calculated and improved compared to real time Heave or delayed Heave thanks to the Forward/Backward and Merged processing.