RD Instruments Cobra-Tac, User Manual, Page: 12 / 62

Cobra-Tac User's Guide
page 6 RD Instruments
NOTE. The initial position is critical to the accuracy of the Cobra-Tac.
Make sure you are entering the position of the Cobra-Tac where it will
enter the water. For best results, enter/edit the initial position once you
have entered the water using a hand held Differential GPS.
•
Local magnetic variation correction can be obtained from charts
of the local area. Enter the magnetic variation for the area as ac-
curately as possible.
If you are losing the bottom, indicated by “ BAD ” displayed instead of the
depth and altitude, then one or more of the following is possible:
•
If there are abrupt depth changes, the Cobra-Tac may have trou-
ble locking on to the rapidly changing depths as you swim. If
you know where the abrupt changes are located, try to swim
slowly over these regions.
•
If there is air being pulled below the transducer it may block one
or more of the beams. Try swimming deeper in the water.
•
Check if debris has become caught on the transducer faces and is
interfering with the beams.
•
The bottom has grass, weeds, brush, or other submerged materi-
als that are disrupting the beams near the bottom: try to swim
slowly over these regions.
•
There is a high sediment concentration near the bottom, and there
is not enough contrast between the suspended sediment layer and
the actual bottom to determine the true bottom range. At some
point the sediment concentration can be so high the Cobra-Tac
will not work.
NOTE. If the Cobra-Tac looses the bottom, it will use a water-mass layer
to attempt to continue navigating. Accuracy will depend on environmental
conditions.
NOTE. Good Survey Suggestions:
Swim along a grid (parallel paths).
Swim at a high altitude (far from bottom).
Keep the Cobra-Tac level.
Slow down while marking objects.