Complex sentence: For high accuracy depths, usually restricted to special purpose or scientific surveys, a sensor may be lowered to observe the factors (temperature, pressure and salinity) used to calculate sound speed and thus determine the actual sound speed in the local water column.
Split into simpler sentences: For high accuracy depths, usually restricted to special purpose or scientific surveys, a sensor may be lowered to measure the temperature, pressure and salinity. These factors are used to calculate the actual sound speed in the local water column.