The data collected from an Aquadopp Profiler is used as an example in this case. The complete file format for each instrument can be found in the Help menu of the respective software.
The .prf file is the output from the AquaPro software, in binary format. This file has to be converted to ASCII format using the data conversion dialog to obtain the data files (.v1, .v2, .v3 .hdr ...).
The .hdr file contains valuable information about the file format of the other files, in addition to the setup and configuration of the instrument.
The .sen file contains the sensor data (also, the format of the .sen file is described in the .hdr file).
.v1, .v2 and .v3 refers to the velocity data (m/s) of beam 1, 2 and 3 respectively (in beam, XYZ or ENU coordinates, depending on your setup). Each column of the file represents one cell. Say you have set the instrument to measure in 13 cells, then v1, .v2 and .v3 should have 13 columns. Each row is one measurement. So the numbers you see in v1, v2 and v3 files represents the average of the measured velocity for a given period, according to the user specified cell size. You can not read the current direction from these numbers directly. If you notice how the instrument head is designed, you see that the beams point in three different directions. The velocity measured by these three beams are combined to calculate the direction of the current. The relative orientation of the transducers are thus important. More information about this can be found in the Principles of Operation.
.a1, .a2 and .a3 contain amplitude data, that is, the strength of the return signal. Also here, each column is a cell and each row is a measurement. The units are counts.
Note also that the .dat file contains calculated speed/direction. You can get this file when converting the .prf file, just check of the 'Profile' option (or the 'Profile CSV' for excel format).