AWAC velocity orientatio UPlooking  vs Downlooking

Hello

I am a new user of AWAC ADCP instrument, and I am proccesing velocity data of the over-the-side AWAC deployment (down-looking).

I am very confused about the geographical (N-E) direction of the currents when the ADCP is looking down because, apparently the current direction measurement (southward) doesn't fit to know direction of the area (northward). Are there any differences between Up-looking and Down-looking   current orientation  with repect to N-S or W-E components calculated by the instrument or the internal compass?

Thanks

Efrain

Dear Efrain,

The AWAC senses itself whether it faces up or down. In other words, you don't need to set any switches or parameters to tell the system how it is oriented. However, it is not clear from your posting whether you are using the VM-profiler software/firmware, or if your system is just a standard AWAC.

If you haven't installed the VM-profiler software/firmware the boat must be lying still when you measure the currents. Otherwise, the system will measure not only the currents but also the motion of the boat, and you have no means of subtracting the vessel motion from the measurements.

If you are using the above mentioned software and firmware, the most common reason for the problems you are reporting is that you have mounted the AWAC backwards, without calibrating for the installation angle. There is an X mark on top of the sensor, and this should point as closely as possible in the direction of the bow. Having done that you also have to calibrate the system, and this is done from the VM-profiler software.

I hope these comments will be of help.

Best regards,
Ketil Horn

Dear Ketil

Thank you very much by your answer.

Indeed the ADCP that I am using is a standard. In order to be able as far as possible to avoid the contamination of the measurement with the speed of the ship, the crew of the ship tries to maintain the speed of the ship in zero. (ok, that is not the best deploy)

According to your explanation if I use a standard AWAC ADCP, the geographic direction E-W or N-S does not change it does not matter if it is placed upwards or downwards. On the other hand, if the X-direction indicator orientation changes during the sampling (not always it aims towards the prow of the boat), the ENU coordinate doesn't change too?  

In the attachments, I put an explanatory graph abouth the deployment.

Sincerely yours

Efrain

Dear Efrain,

When seeing your explanatory drawing the only reason I can think of that would give you the wrong direction for the current data is if the magnetic parts of the boat (= the hull) interferes with the AWAC compass. You may send us an email (inquiry@nortek.no) with some data, such that we can take a look at these, but generally speaking it is hard to judge whether the compass is offset or not due to nearby magnetic sources. The best way to eliminate this potential problem completely is to upgrade the system to a VM-profiler. This is just firmware and software, and it would make life easier for you.

Best regards,
Ketil