Three transducers are needed to produce the X,Y and Z components of velocity. The Vectrino has four receiver arms as opposed to the more traditional three found on other acoustic velocimeters. Because of the arrangement of receiver arms, each pair (composed of two opposing receivers) can measure a horizontal component and the vertical velocity. Z1 and Z2 are the vertical velocity estimates associated with receivers 1&3 (X) and 2&4 (Y).
The two independent vertical estimates are redundant information and can be used in many different ways during post processing, and are typically used to assess data quality. Please check our Technical Notes section for references on various methods to utilize them.
We do expect the two estimates, since they are co-located, to be the same. In theory Z1 and Z2 should match almost exactly, but in real life it won’t be exact because they are measured using two different sets of beams. If the two estimates evidentially differ, this points to problems with data quality, instrument mounting relative to the flow (such as a small tilt in one direction), or possible damage to the probe head. If you see differences in your data, start with data quality and make sure everything looks OK there. Perform a probe check to make sure the instrument is performing as expected and check to make sure the instrument is mounted level and not tilted.
The side-looking probe does not have a secondary measurement of the vertical velocity, since there is no meaningful way to form two independent Vz estimates from the four beams. Z1 will be the correct vertical velocity and Z2 should be output as zero. We still report the SNR and COR for all four beams since these values refer to the individual beam measurements and not the transformed velocity data.
The 2D side-looking probe has four beams, two in the horizontal and two slanted 65° from the vertical. With the two horizontal beams we form the Vx and Vy velocity. Therefore, we do not need the slanted beams to measure the horizontal velocity, which means that the probe can act as a 2D system even in shallow water.