horizontal beam separation

Posted by pukha lenee-bluhm at September 07. 2011

"Spatial aliasing prevents resolving the direction of waves with wavelengths shorter than two times the smallest horizontal separation between two array elements. For the AWAC, the two closest array positions are the center AST and one of the velocity measurements. The horizontal separation is depth dependent (d * sin 25 deg) and leads to an upper frequency limit for wave directions." (Nortek Technical Note TN-024)

I am unclear as to why depth times the sine of the beam angle is specified, rather than the tangent. I'm not sure how to insert a figure here, but it seems to me that if we sketch an upside down right triangle whose base coincides with the mean water surface, whose hypotenuse represents the off-angle beam and whose vertically oriented side represents the AST beam... the horizontal distance is d * tan 25 deg.

Can you kindly clarify this matter.

Re: horizontal beam separation

Posted by Jonas Røstad at September 08. 2011

Dear Pukha,

You are right, the Technical Note is wrong. The horizontal distance is given by d * tan 25 deg.
The consequence however is not large since the hypotenuse is not much greater than the adjacent cathetus for small angles.

Best regards
Jonas

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