Measuring bottom-trapped currents with excessively tilted ADCP

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Measuring bottom-trapped currents with excessively tilted ADCP

Posted by Rafael Reiss at December 11. 2017

Hey,

 

we are using an Aquadopp profiler (HR firmware, 6 m range) to measure bottom trapped density currents in a lake.

The instrument is bottom mounted (no gimbal). However, since the the slope at the location of the deployment is quite large the instrument is tilted (pitch and roll approx. -25°).

 

In the Comprehensive Manual

under section "5.1.6.3 Pitch, Roll and Heading" it says:


"Tilt  readings  between  20°  and  30°  affect  the  data accuracy in a way that is likely to make the data fail to meet the specifications.
Data acquired during tilts exceeding 30° are in general not reliable and should be discarded."

 

Is it correct that this is only true when talking about ENU coordinates (i.e. bin mapping)?

 

Since our current follows the bathymetry I am not particularly interested in true vertical and horizontal velocities but rather in isopycnal and diapycnal velocities near the bottom.

These should be readily obtained by recording in XYZ coordinates and performing a coordinate transformation to align the instrument with the downslope direction.

 

As there is no bin mapping involved the only limitation I see is the range of the tilt sensor (i.e. < 30°).

 

Is this correct or did I miss anything?

 

I am aware that there are many posts on tilted ADCPs already but I did not find any covering this specific question.

 

Thanks a lot

Rafael

Re: Measuring bottom-trapped currents with excessively tilted ADCP

Posted by Magnus Grøtterud at December 27. 2017

Hi Rafael,

Correct, the tilt and heading values are only used when ENU coordinates are selected. The tilt sensor is not linear beyond +/- 30 degrees. Bin mapping is also only applied in post-processing (f.ex Storm) when "Remove tilt effects" is enabled.

Best regards,

Magnus

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