|
Excellent navigation is critical to mapping
For the vehicle to fly at a safe and uniform altitude over the bottom, missions are planned over the most reliable maps available of the area. To ensure that the vehicle executes the mission, and for the high-resolution maps to be accurate, the position and orientation of the AUV must be precisely known and logged during the mission, and this operational data is used during post-mission data processing.
Navigation during the dive
The navigation equipment includes an inertial navigation system (INS) that is integrated with a doppler velocity log (DVL) and laser ring gyros to measure the vehicle’s position and attitude (see Vehicle specifications). Control algorithms use these data to maintain a stable platform and to record the vehicle’s track.
The missions start on the surface where the vehicle achieves a valid GPS fix and begins a spiral descent. Since reliable bottom tracking is not possible during descent, the AUV relies on inertial navigation and position updates sent from the support ship: USBL tracking data of the vehicle is packaged on the ship and transmitted in messages over an acoustic modem link to the AUV. The vehicle responds to these messages with vehicle status messages.
After operational depth is achieved, the AUV starts the mission designed using the MB-System package "Mbgrdviz". Missions are typically composed of a sequence of straight lines that connect at waypoints. The control algorithm uses the navigated position to compute the distance of the vehicle from the line joining the previous waypoint to the next. This position “error” is the input to a control loop that computes a heading command and positions the rudder.
Successful navigation during the dive and all post-processing corrections require precision timing between sonar pings and periods of listening to prevent acoustic interference.
Navigation performance
The navigation requirement for MBARI seafloor mapping operations is that the real-time navigation error at the end of the survey be no worse than half a swath width. This allows the navigation post-processing software, Mbnavadjust, to locate overlapping and crossing swaths. It then matches bathymetric features and adjusts the navigation so that the precision is equivalent to the lateral resolution of the bathymetry data.
The bathymetry map above shows the original real-time INS navigation in red overlain with the final adjusted navigation in black. The navigation adjustments for this survey are modest: the largest relative adjustment required to match overlapping features is 30 m.
Map © MBARI 2005
- Real-time navigation: 0.05% of distance traveled, CEPR with continuous DVL bottom lock. After traveling 10 km there is a 50-50 chance that the accrued navigation error is more than 5 meters. There is a 1 in 100 chance that the error is more than 13 meters.
- Post-processed navigation: Approaches the lateral resolution of the multibeam bathymetry. In a 50 meter altitude survey, the relative navigation error is less than 3 meters.
>> More about processing the navigation data
Next: Data processing