We all know that pressure sensors on the sea floor cannot be used as an accurate estimate of sea surface elevation due to atmospheric variations (can be as much as 0.3-0.5 meters). It therefore it has been asked on more than one occasion if the Acoustic Surface Tracking for the AWAC or Signature may be used to measure surface elevation.  We have generally cautioned against this as variations of the speed of sound in the water column can lead to similar errors as the pressure sensor.

Thanks to some work conducted by MetOcean Engineers and Tremarfon we can now whistle a different tune... well, maybe a portion of the melody has changed.

If we can assume two characteristics then we get a very good estimate of absolute distance to the free surface.  The first characteristic is if the water is well mixed then the speed of sound is uniform and thus constant.  The second criteria is if we have a good estimate for the salinity (user defined) and temperature (measured) then we can fairly accurately estimate the speed of sound in the water.

However if there is substantial layering and the temperature is quite different from the top to the bottom (where temp is measured) then errors will be seen in this estimate. Generally speaking, places with lots of current have good mixing and are not as sensitive. Also more shallower places tend to have less gradient with temperature as well.

The good news is that it has little effect on the estimates of wave parameters, which is what the AST is really used for. Wave measurements are less sensitive because we are more interested in the change of distance to the surface and not the absolute distance. So 1% error in 50 meters of water with 1 meter waves means that the error for waves is 1 cm whereas the error for distance is 50 cm.