Real-Time Ship Motion

Aside from the EKF, the SBG Systems inertial sensors computes 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 design that will remove any constant component in the motion.

In addition, an automatic filter tuning ensures proper behavior is obtained with swell periods up to 25 seconds.

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

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.

Swell Mode

On high performance products, the Swell mode can be 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 (Ekinox and Apogee 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 can be 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 5 minutes before, and 3 minutes 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 part affect the whole vessel in a constant way.

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

deported heave

Normally, the heave computation is the most accurate when computed at the vessel Center of Rotation because the sensor measures less motion than anywhere else on the vessel.

However, SBG Systems have designed specific algorithms that take into account the Center of Rotation lever arm (primary lever arm) in order to obtain optimal heave performance even while placing the sensor away from this location. This algorithm requires you to correctly measure and enter the Primary Lever Arm in the sensor settings. Because of that, it is recommended to place the sensor as close as possible to the monitoring point (ie. close to the sonar head).

For large ships or long primary lever arms such as 10 meters or more, we recommend to keep the inertial sensor close to the Center of Rotation if the heave measurement should be provided.

When the point of interest is not located at the IMU position, it is possible to configure one or several external lever arms to deport the heave measurements to those monitoring points.

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.

Heave Enhanced Altitude

In the marine survey industry, there is often the question whether using the heave output or the Kalman filtered altitude output. 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.