Excellent navigation is critical to mapping
The current navigation system used on the mapping AUV is the Kearfott SeaDevil inertial navigation system (INS). It also includes the doppler velocity log (DVL) as well as a ring laser gyro. If the DVL continuously tracks the seafloor, the real-time navigation deviation is 0.05% of the total distance traveled. The Inertial Navigation System also provides data on the vehicle attitude (pitch, heading, and roll). A Paroscientific Digiquartz pressure sensor can precisely measure vehicle depth at a standard deviation of 0.3 meters from depths of 3000 to 6000 meters. MBARI AUV technicians plan missions by using the interactive application MBgrdviz, which is part of the MB-System. 1
For the vehicle to fly at a safe and uniform altitude over the seafloor, 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 altitude (see Vehicle specifications). Control algorithms use this 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 global positioning system 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: ultra short baseline (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 multibeam procesing package MB-System "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.
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 meters.
© MBARI 2005
- Real-time navigation: 0.05% of distance traveled, CEPR with continuous DVL bottom lock. After traveling 10 kilometers there is a 50-50 chance that the accrued navigation error is more than 5 meters. There is a one 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.