Drag down is a typical phenomenon that can negatively affect data from ADCP measurements. As the name suggests, it is about instruments being dragged down in the water column. This means that the instrument itself is in motion and this will contaminate the data since the instrument calculates current velocities based on the premise that it is stationary. 

Because the instrument is being dragged down, spikes in pressure and tilt data can indicate the drag down effect (see Figure 1). 

ADCPs measure relative motion between an instrument and scattering particles (indirectly the water). When an instrument stays in a fixed position, the output data will represent the movement of water alone. In situations where an instrument is also moving, the output data depend both on how the instrument and the water move - and there is no way to differentiate between the two. This implies that any current data affected by drag down is invalid in the entire profile. 

Drag down typically occurs when strong currents push on a mooring with a force greater than the rig can withstand. For some, the main purpose of measurements with ADCP is to determine the maximum speed in an area. This can unfortunately be difficult to achieve if the deployment setup is susceptible to the drag down effect. 

An effective way to reduce the risk of drag down is to minimize the amount of volume that the current can take hold of. This can be to use a thinner or shorter rope. Another way to avoid drag down is to measure in areas where the currents are less brutal, but this can be at the expense of what one wants to measure. 

Figure 1. An example of how drag down can appear in the data. There is compliance between pressure and tilt spike and noticeable changes in speed.