In short: Bin mapping is the process of remapping velocity cells to correct for an instrument's tilt during data collection.

A little longer: During profiling current measurements, the water column is divided into several segments. These segments go by many names, such as "cells", "bins", "range cells", "velocity cells" and "depth cells". This division allows velocities to be measured at various depths.  

              Figure. Position of cells for a leveled and tilted instrument. 

For the best data quality, instruments should be kept still looking straight upwards or downwards without any tilt. This will often be difficult and sometimes impossible. What happens then is shown to the right in the figure above. Some cells will be shifted higher and others lower in the water column which can cause a smearing of shear, which means that the shear layer will look thicker than it really is, since the measurements are retrieved at different depths. Velocity outputs from the same numbered cells will thus represent currents from different depths. Eventually, two differently numbered cells might be aligned to the same depth. An example of this is shown for the tilted instrument above, where cell number 6 from the left beam and cell number 7 from the right beam are at the same depth. With severe tilt and/or at great range, the difference in vertical position will become even bigger. 

It is possible to remap the cells for each beam, this is called bin mapping (or "remove tilt effects", "cell mapping", "depth cell mapping", "tilt compensation", "map to vertical"). In this process, cells at equal depth will be matched and velocities will be computed at that level. Velocity corrections and depth corrections are made by using information about the instrument's tilt (pitch and roll) at the time of data collection. This is provided by a built-in tilt sensor in the instrument. For Midlife instruments, this procedure can be carried out as processing options in the Storm and Surge software packages. This means that for Midlife instruments the correction can only be applied to the averaged product from each average interval. For Signature instruments, which store raw data, the corrections can be done at the same rate as the tilt sensor update rate, which is the same as the sampling rate  (max 1 Hz for the standard tilt sensor, for more information consult the instrument technical data sheet).

For Siganture instruments with an AHRS (Attitude and Heading Reference System) installed, replacing the traditional tilt sensor (and magnetometer), it is possible to enable real-time bin mapping on a ping-by-ping basis and have the output data in telemetry files given directly with depth corrections. When enabling serial output on an instrument with AHRS, the configuration is by default set to bin map. This can nevertheless be turned off with the command "SETTMAVG, MAPBINS=0", and reset with the command "SETMAVG, MAPBINS=1". However, bin mapping will never be executed in the provided raw data files. Without an AHRS, the output is never automatically corrected for, neither in telemetry or raw data.

Since remapped cells no longer will be in relation to the instrument head, the velocity data cannot be given in BEAM or XYZ coordinates after bin mapping. For that reason, the bin mapping procedure will also convert to ENU coordinates, if the data is not already in ENU coordinates. To clarify any misconceptions, transformation to ENU coordinates and bin mapping are two different things. To bin map, it is also necessary to convert to ENU coordinates, but data can be given in ENU coordinates without bin mapping. To carry out the cell position maneuvering, tilt is the only sensor directly required, but since the data after bin mapping must be given in ENU coordinates, the instrument also needs to have a compass that measures heading.