The Mixing Zephyr Pages

Mixing Zephyr Program
Tow Navigation Data Reduction:
Merging ACNAV with CTD Data

Information from the CTD data logger is recorded every two seconds and yields very complete but significantly large data files. Ideally, the fish navigation is computed and recorded every minute. Thus, navigation data for the towed package needs thus to be extrapolated from the sixty second interval to the two second interval. Unfortunately, several problems were encountered with the navigation data:

  1. Fliers / bad fixes: About five to ten percent (sometimes more, depending on the position of the package with respect to the transponder net used) of the navigation data contains erroneous fixes for the fish position. These bad fixes are usually caused by bad travel time information such as secondary echoes. This problem gets more acute when the fish is far away from the transponder net.
  2. Baseline problem: Another pervasive problem occured every time the fish crossed the baseline of the transponder net used. This led the navigation to go erratic unless the code used (ACNAV) was switched to compute the fish fixes out of another transponder pair. Even in this case a gap usually remains in the navigation information.
  3. Distance from net: Several tows extended over the limits of the transponder net used during Mixing Zephyrs Is deployement. This led to lose navigation at the ends of these tows.
In order to remedy these problems merging of the navigation was realized in two steps. The navigation was first filtered using RADs graphical interface. Most bad fixes were thrown out of the data. The filtered navigation was then used to extrapolate the fish position at the times the CTD data was recorded at. The positions were linearly interpolated in a table lookup fashion. The resulting grid positions (x,ys) were then used to calculate the latitude and longitude of the fish during the tows.

The following table summarizes the times in between which the navigation data is available and where gaps existed for each tow. It also lists the beginning and ending times of the unmerged and merged CTD data. All the resulting merged data is shown in the next plot.

Summary Table of Tow Navigation

6/18/95, AC

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