The Mixing Zephyr Pages
Mixing Zephyr Program
Navigation Issues
High precision navigation of ALVIN is
an essential component of this dive program, and will be
necessary within the context of RIDGE observatory measurements in the
coming decade for both ALVIN, ROVs, and other towed and moored
platforms. Navigation within a long baseline acoustic transponder
network is generally used for this purpose. In 1988 we initially
developed the capability to use sub-transponder-sub travel times from
pairs of transponders to
obtain ALVIN positions that were precise within ~1 meter. While this
represented a major advance in our ability to map and sample at fine
scales, there were two important limitations.
The accuracy of the fixes were limited by the quality of the survey
which could be performed from the surface ship--while the repeatibility
of positions was excellent there was no assurance that the grid was
rectilinear. Indeed that offsets would exist between the positions
calculated from different pairs of transponders demonstrates that the
transponder positions were not known with sufficient accuracy. When
re-establishing navigation nets during subsequent cruises there were
competing demands to have the new navigation in registration with
existing maps of the vent fields, versus working with the improved
knowledge of transponder locations as surveyed from the support vessel
brought about by the evolving technology used to establish the ship position (LORAN to GPS to
p-coded GPS).
A second issue relates to the analysis of the survey data. The
computational techniques used by the ALVIN group to survey transponders
has undergone a series of changes. The present technique is to work
transponder-by-transponder, minimizing the error in range as the ship
locates itself at different positions around the transponder. We found
(during the Coax response cruise in 1993), that this procedure was
unacceptable given the offsets inherent in conventional GPS positions,
leading to errors in transponder locations of order 25 meters which
would translate in offsets between positions computed from different
baselines of order 100 meters. In principle, these problems would be
solved with use of p-coded GPS navigation for the surface ship which
would be available for our program.
However one still would have uncertainties brought about by errors in
the ray-bending calculations necessary to convert travel times to
range.
In the summer of 1994, we began discussions with Bruce Howe of the UW
Applied Physics Laboratory on the applicability of software he had
developed for performing acoustic tomography experiments for the
purpose
of navigating the submersible (his application was correcting for mooring
motion during tomographic experiments). A desireable attribute
of his software was that it would allow us to use data from both the
ship and the submersible to compute transponder locations. With the
ship data, we would be able to locate the net in absolute coordinates,
while the submersible data would allow the most precise
relative positioning of the transponders because of having minimized
the errors inherent in ray-bending computations. In this section we
summarize the results of our experiences with this new approach.
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