Ship motion measurements are essential for marine operations requiring precise navigation and survey stability. SBG Systems provides real-time ship motion data using advanced inertial sensors. These sensors compute ship motion at 50Hz by double integrating accelerometer signals. Double integration produces drift due to orientation errors or sensor bias. A high pass filter stabilizes the output by removing constant motion components. Automatic tuning ensures the filter adapts to varying sea state conditions. This feature supports swell periods up to 20 seconds in real-time heave estimation. As a result, ship motion measurements remain accurate and stable during operations.
The high pass filter design ensures heave, surge, and sway return to zero in static conditions. The reference is always the vessel center of rotation. Only marine-qualified units provide heave output. Surge and sway are unavailable on Ellipse series units. Surge and sway are valid only in near-static applications such as buoys. These values remain sensitive to orientation errors. The outputs are reported strictly at the IMU measurement point.
Ship motion reference frame
Ship motion outputs follow a specific reference frame definition. The frame origin is located at the output point position. Heave is the vertical displacement, positive downward. Surge is the longitudinal displacement, positive toward the vessel bow. Sway is the transverse displacement, positive toward the vessel starboard. This consistent frame ensures reliable interpretation across multiple vessel types.
The heave output exhibits a step response under sudden motion changes. When a step occurs, heave rises and then smoothly returns to zero. Recovery may require several minutes depending on sea state history. The output shape remains consistent despite environmental differences. Heave measurements do not include tide contributions. Tide compensation must be applied separately for accurate altitude determination.
The centre of rotation and the operation of deported heave
Heave is also affected by vessel rotation. At the center of rotation, rotational heave cancels completely. Away from this point, roll and pitch induce dynamic heave components. Semi-static effects from wind, ballast, or load imbalance further influence results. Different locations produce heave signals with varying shapes and amplitudes.
Sensor placement strongly affects heave performance. Mounting near the center of rotation ensures maximum accuracy. Users can configure a monitoring point for equipment such as sonar systems. Only heave measurements can be deported to this monitoring point. Surge and sway must remain referenced at the IMU location. Lever arms must be measured precisely to avoid estimation errors. Even small dimensional or angular inaccuracies propagate into heave, surge, or sway results. It is important to correct any misalignment between the IMU and the vessel frame, either mechanically or through software configuration.
Some older versions ignored lever arms in heave computation. This limited accuracy during wind or current induced vessel motions. Current firmware versions account for lever arms, improving heave estimation under dynamic conditions.
Heave Enhanced Altitude
Users often compare heave output with Kalman-filtered altitude. RTK altitude provides precise absolute measurements in favorable GNSS conditions. Tide compensation is unnecessary when using RTK altitude. However, RTK altitude may degrade under GNSS-challenged environments.
The heave algorithm provides precise relative measurements without GNSS dependence. It requires tide compensation but remains reliable under GNSS outages. Enhanced Altitude mode merges heave with RTK altitude. This approach ensures absolute accuracy even in poor GNSS environments. Enhanced Altitude requires marine motion profiles and precise RTK or PPP positioning. This feature can be disabled if not required.
Delayed Heave
Delayed Heave improves accuracy for hydrographic surveys. The algorithm uses past data to correct phase errors. It provides better performance under long-period swell conditions. Delayed Heave introduces a fixed 150-second output delay. Output messages include timestamps for consistent data dating. We recommend this mode for seabed mapping, as it does not require real-time operation. Real-time heave remains available for preliminary estimates. For full operation, the unit must remain active 150 seconds before and after surveys.
Post-processing heave estimation
Post-processing produces the most accurate heave estimation. Software such as Qinertia recalculates heave using forward and backward analysis. Merged processing enhances precision beyond real-time or delayed methods. This approach delivers the highest accuracy for hydrographic survey operations.
SBG Systems provides advanced ship motion measurement solutions combining real-time outputs, enhanced filtering, delayed algorithms, and post-processing improvements. These features ensure reliable performance in navigation, hydrography, and offshore operations.