Page 49 - Electronic Navigation Cyber Book

MEASUREMENT OF SHIP'S SPEED RELATIVE TO BOTTOM OR TO
WATER
Owing to absorption by particles in the water at a depth of 200 to 400
meters, the so-called deep scattering layer (DSL), a Doppler log may only
function, down to about 200 meters, unless the set is equipped to work
in the layer of 10-30 meters below the surface.

When reflections are received from this layer the speed of the ship
relative to that layer, and not relative to the bottom, is obtained. Thus
uncertainty and confusion may occur.
Apart from the effect of the Deep Scattering Layer, the water at 10 to 30
meters below the keel also causes an echo and Doppler effect by
volume-reverberation.

This is called 'water track' (as opposed to 'bottom track'). In deep water
there is a considerable difference between the time of propagation for
bottom reflection and that for reflection from the mass of water at a depth
of 10 to 30 meters. Receivers can be made operative for only a short
period (a certain 'window' of time) either immediately after or a short time
after each pulse transmission.

Suppose that the receiver has bottom contact, with the
window occurring a short time after, transmission.

If the Doppler log then loses bottom contact, the window is automatically
shifted to occur immediately after pulse transmission.
As a result, the receiver reacts only to reflections from the 10-30-metre
water layer.

When this happens, 'bottom track' indicator is replaced by 'water track'.
When sufficiently low frequencies are used, echoes may still arrive from
a rocky bottom at a depth of 600 meters and more.

In some Doppler log, for depths less than 600 meters it is possible to
switch manually to the water track mode.

METHOD OF USE

a. To start using the Doppler, click the ‘Power’ button as shown in fig-4.
The LCD Screens start showing values of speed and distance run

© 2018 Digital Galaxy Index 49