Incoherent multibeam Doppler current profiler performance: Part II--Spatial response

We consider the spatial response and sampling properties of ship-mounted four-beam and three-beam incoherent Doppler current profilers. Spatial properties are an issue because such systems cannot make a point measurement of the current field, and because the actual current field is nonuniform. We obtain expressions for the effect of beamwidth, beam separation, and block averaging on system response. We observe that spatial aliasing may occur above some critical depth, and that pulsewidth reduction cannot improve vertical resolution beyond a limit imposed by system geometry. We also find that, for the three-beam system, the along-track component is irrevocably contaminated (to some degree) by the cross-track component as a result of its nonorthogonal beam geometry. The velocity and shear estimate variances achievable with these systems are calculated in a companion paper [1].