Beam pointing errors

Differences between the nominal and actual beam orientations can be introduced during the manufacturing process of an ADCP. Can these differences introduce significant errors in the data?

In some instruments this difference is corrected for onboard during data acquisition if the user programs the ADCP to log velocities in instrument-, ship- or Earth-coordinates upon deployment. However, if the data is output in beam coordinates, an error in the nominal inclination of the transducers will propagate to the horizontal (eastward/northward) velocities as

and to the vertical velocity (calculated from beams 1-4) as

For example, an inclination error of introduces an error of 3.4% in and and an error of 0.8% in .

The actual installed inclination of the beams is usually measured in the fabrication process, and a corrected version of the beam-to-instrument transformation matrix may be saved to the ADCP's firmware. If the values in the instrument's matrix do not match the nominal ones, the instrument's matrix should be used when converting velocities in beam to instrument coordinates instead of the matrix with zero inclination error.

As an example, the beam-to-instrument transformation matrix of some Nortek Signature1000 data is

>> ff.Config.Burst_Beam2xyz

ans =

    1.1831         0   -1.1831         0 ----> vx velocity
         0   -1.1831         0    1.1831 ----> vy velocity
    0.5518         0    0.5518         0 ----> vz velocity (from beams 1 and 3)
         0    0.5518         0    0.5518 ----> vz velocity (from beams 2 and 4)

At the numerical precision reported here, the 0.5518 entries are only 0.018% different from the nominal ones (), which would only introduce a negligible (often smaller than the velocity data resolution) error of about 0.1 mm/s.