ABE Navigation Data
The most fundamentally important data-set for any ABE operation - a requirement for all autonomous vehicles if missions are to be achieved successfully - is accurate seafloor navigation.
The navigation used for ABE is a long-baseline (LBL) system augmented with Doppler velocity logging. For LBL, ABE uses between 2 and 4 seafloor transponders, which are identical to those used by Alvin and Jason. The techniques used to set, survey, and recover the transponders are also identical. Presently, ABE uses four fixed frequencies, although plans are in place to expand this to eight and to allow the frequencies to be selected in software. ABE interrogates the transponders on a 10-second cycle. These interrogations are also heard at the vessel, which allows ABE to be tracked from the ship with only occasional interrogations from the ship to keep clocks synchronized. ABE does not depend on these interrogations from the vessel, however, so if the vessel leaves the site ABE's navigation is not impacted.
In real-time, ABE uses the round-trip travel times, the vehicle depth measurement and the local sound velocity profile to compute slant ranges. Based on vehicle and transponder depths, these slant ranges are projected into the horizontal plane and a fix computed using either a deterministic (two transponders) or least-squares (3 or more transponders) solution. ABE uses a series of filters to eliminate incorrect ranges caused by surface reflections and noise. The fixes are combined with the dead-reckoning solution from the doppler navigator and compass to produce the real-time position estimate, which is typically repeatable within a given transponder array to about 2 meters.
At the end of each run, the data is postprocessed to produce a more accurate track. On the first dive in each area, the compass is recalibrated using magnetometer and compass data from slow spins during descent. Acoustic travel times are refiltered to recover as many good fixes as possible. For the first run in a new net, the transponder positions are adjusted to minimize the least-squares error for fixes with three or more ranges. Finally, the refined fixes, the recalibrated compass, and the Doppler navigator data are recombined using a Kalman smoother. The final processed navigation data is reported in Latitude and Longitude in decimal degrees (suitable for importing into, e.g., GMT among other mapping tools) and is embedded within the time-stamped scientific data file for each dive.