General Hints and Tips

• Always check the IMU heading output each time you power up. Aim the ROV north (or wherever) and confirm the heading output is consistent with the way the ROV is pointed. If it is not, figure out why not and correct the problem.

• The dominant error source for dead reckoning is usually the IMU, so learning how to set up and calibrate the IMU is the key to good performance, especially with the baseline IMU. It is not terribly difficult if you read the manual, but if you make an assumption about how it ought to work and operate under those assumptions you will have problems.

• Ocean/lake bottom and water conditions such as salinity affect the maximum altitude at which you can operate reliably. When the maximum altitude is exceeded the DVL stops updating its dead-reckoning position estimate until it re-acquires the bottom.

• Accuracy of the DVL reduces as you approach the maximum altitude.

• The DVL should not be operated below its minimum altitude. It does not always detect going under minimum altitude and multiple reflections between the ROV and the bottom can appear to the DVL to be valid bottom detection and introduce spurious motion estimates. If you drop under the minimum, try to hop up to at least 2 meters above the bottom to allow the DVL to re-acquire. In a muck environment, crashing into the muck can entrain gas bubbles and particles in the thruster wash, which may be interpreted as sudden shifts in speed.

• Sustained operation with a cross-track tilt to the bottom (e.g., driving along a sharp drop-off) can introduce angular error into the dead-reckoning calculation. The error tends to bend the track toward the up-hill side of the path, so you can compensate somewhat for this effect. Normal random bottom variations, on the other hand, tend to cancel out.

• If you are sitting at the surface and using active control to keep the ROV partially out of the water, be aware that air bubbles may get entrained in the thruster wash and cause false bottom and false motion detection.

• When using an on-ROV GPS to set initial positions, be aware that many GPS units will send spurious positions during the transition from above the water to below the water. During the time the GPS has lost its satellite fix due to water over the antenna and before the GPS decides the current fix is no good, it can send erroneous position message with the fix flag showing the fix is valid. The errors can be as much as 30 meters. Therefore, when diving the user should beware of this effect and take action. See the discussion of the SUPPRESS-GPS command in this manual, and see the “DIVE” button in the CeruleanTracker manual.

• The DVL usually does not perform well in pool due to reverberations, which causes constructive and destructive interference with reflections which confuses the bottom tracker. If you need to operate in a pool, see here.

• When using the DVL for position hold in areas with heavy vegetation, it is possible to get into a positive feedback loop that introduces error. If the ROV is directly over vegetation, the ROV prop wash can cause the vegetation to move. This motion is picked up by the DVL, and the ROV can respond by traveling in the direction of the vegetation motion. This puts the DVL over previously undisturbed vegetation, and the process repeats.

• See the CeruleanTracker manual for how to install updated firmware.