Preface The expedition ARK XIX/3 with the German icebreaking RV ...

Navigation
High resolution mapping is not possible without precise navigation information. The
multibeam resolution (~10cm horizontally) and the one of video mapping (~1cm
horizontally) ideally imply navigation data of the same level of accuracy.
Underwater positioning, with the absence of radio-electric signal transmission in
seawater, is based on acoustic measurements with respect to reference transponders
that are mounted under the vessel or moored on the sea-floor.
On the present cruise, "Victor 6000" was positioned by the USBL system "Posidonia"
installed onboard "Polarstern" which yields a positioning accuracy of about 0.5% of the
slant range – depending on ship noise, meteorologic conditions (turbulent water around
the transducers, roll, pitch and heave dynamics), and system calibration 5-sonic velocity
profile, geometric adjustment).
Acoustic positioning, which is not sufficient for map construction, is completed by
"Victor’s” inertial navigation system. Using a 3D fibre-optic gyro and motion-sensor
(Octans/Ixsea) and a Doppler Velocity Log (DVL - Workhorse600/RDI), this relative
navigation technique integrates the displacements of the ROV in time, starting from a
given USBL-position. The integration of measurement noises allows positioning error to
drift, at a rate of less than 10m/hour.
The mapping procedures use the inertial gyro-Doppler navigation, which is reset on
USBL-positions at given intervals (1-2 hours) before or after the individual survey lines.
Reset of the gyro-Doppler navigation during a track line would lead to map distortions
difficult to correct in post-processing.
Roll and pitch compensation is carried out with data from the Octans system, which
measures these angles with an accuracy of 0.5mrad. The measurement of vehicle depth
is obtained by a Digiquartz – Paroscientific piëzo-depth cell with an accuracy of a few
centimetres. Tidal corrections can be carried out in post-processing, if corresponding tide
estimations are available.
What can we expect from ROV-borne seabed mapping?
Compared to surface-borne mapping, mapping with an ROV increases the spatial
resolution of the observations with high frequency bathymetry as well as sonar or video
images. This gives an insight to the local morphology of the seabed, allowing recognition
of structures with only a few centimetres of relief.
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