The speed of sound through water is effected by the temperature, salinity (density) and depth (pressure). Since ROAB operates at a constant depth that still leaves temperature and salinity as variables. When either increases the speed of sound increases, and these are not small increases. For a given level of salinity, the speed of sound can change more than 10% from freezing water to warm water. The red lines on the chart below show the speed of sound as a function of temperature for two levels of salinity - the lower line is for fresh water and the upper line is for very salty water (a level of '40' on the 'practical salinity scale').
As the chart shows, the speed of sound can vary dramatically for different conditions of temperature and salinity. Any device which measures distance by timing how long it takes sound to propagate through water needs to be properly calibrated for the conditions in which it is operating. The ROAB device does that. But rather than measure the temperature and salinity and then computing what the speed of sound should be, ROAB takes a straight forward empirical approach. ROAB measures the time for sound to travel a fixed distance, then simply divides that distance by the time to determine the speed.
The green boxes on the above chart show the measurements made by a ROAB mark in fresh water. Each box displays the measurement error for the equipment used to gather the data: +/-2 degrees F, and +/-22 feet per second (which is ~0.5% of the speed). As the grouping of the boxes along the fresh water line demonstrates, ROAB's empirical approach properly tracks the speed of sound through water for the conditions in which it is operating.