Hello, can anyone explain the difference between water track and bottom track speed in doppler speed logs?
Hello @Rosencratz,
Is Guildenstern with you?
Are you interested in how the meaning of the readings is different or in how the istrument’s working principle differs?
Hello Nikos I was actually asking about the speed log’s operating principle in these modes.
I think that the operating principle should be the same. A nice illustration of the doppler effect is shown below.
So consider that a beam of a certain frequency is transmitted from the instrument (located on vessel’s bottom), reflected on the sea bed and received back again. The frequency/direction of the received signal can be used to calculate the vessel’s speed. This is bottom track and should be aligned with GPS speed (speed over ground). When depth is high (e.g 200mtrs) this sea bed reflected signal becomes weak.
Alternatively to the sea bed reflections, reflections of the signal on water layers (or even suspended plankton) are possible to catch. The frequency shift of these reflections help calculate the vessel’s speed in relation to these water layers. This is water track speed (speed through water). These reflections may become also very weak to catch when in very cold and clean waters.
So to sum up, the operating principle is same, it’s based on the doppler effect. What changes is the reflective surface, either the sea bottom or water layers below vessel.