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Mixing Zephyrs
Use of Satellite and Acoustic Navigation

With the exception of one off-axis tow (mz1t22), all vertical and tow-yo CTD-T casts were continuously tracked using an acoustic navigation system (insert FIGURE (overview of satellites, ship, net, and fish)). During each tow-yo, the "real" (latitude and longitude) position of the Atlantis II was determined using P-code (highest available accuracy) data from the Global Positioning System (GPS) satellites. Simultaneously, the ship's position was established relative to a network of ocean bottom transponders by measuring the elapsed time taken by each of three transponders to return a characteristic sound pulse in response to a common acoustic "trigger" from the ship. Between each determination of the ship's position relative to the transponder network, the position of the fish was monitored: hydrophones in the ship's hull listened for both the acoustic signal (ping) emitted by the fish (CTD-T/Niskin bottle package) and the corresponding responses from the three transponders.

With these travel time observations, the position of the fish relative to first the transponders, and then the ship were calculated by a software package called AcNav (a DOS/Windows program customized for the Atlantis II navigational equipment). Depth readings from the fish were input by an AcNav operator to constrain the navigational computations, and signal degradation was mitigated by switching between different in-hull hydrophones and/or adjusting the gain on individual transponder frequencies. On long linear tow-yo's, the triad of transponders used for navigation was altered as fast as possible to optimize transponder geometry and minimize data loss. During the off-axis tow (and on occasions when the transponder navigation was inadequate) the position of the fish was estimated using the Pythagorean theorem; the depth of the fish and the wire out (measured at the winch spool) were used as a leg and hypotenuse of a rough triangle (approximation of the catenary as a straight line seemed appropriate after comparison with reliable transponder-determined fish positions). GPS latitudes and longitudes were noted at the beginning and end of each tow, at the top and bottom of each vertical pass through the water column, as well as whenever an anomaly was detected or sampled. Additionally, each tow was plotted in real time on a Seabeam bathymetry map of the ridge, with the dual-intention of forecasting seafloor "close encounters," and monitoring and planning track coverage accumulated during the cruise. Using the maps and GPS annotations, the instantaneous, global (lat/lon) position of the fish --and therefore the vent plumes-- can be determined retroactively, but without real-time logging of the GPS information the potential accuracy of such locations is not fully realized.

One additional source of information was the Precision Depth Recorder (PDR), which provided a rough idea of instantaneous bottom topography during a tow, as well as a measurement of the minimum distance above the seafloor attained by the fish on each yo. Bottom depths computed by summing the maximum depth recorded by the fish and the distance above the seafloor (from the PDR) were used to roughly characterize the bottom topography in the contoured anomaly plots. Future incorporation of Seabeam bathymetry data may offer improved topographic resolution in such plots.

SRV, 8/15

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