Pressure and AST Differences

There have been a few questions recently as to why there can be a difference between the mean pressure and the mean AST.  This is sometimes noted to be as much as a meter.  When the time series is plotted, it appears that there is a vertical offset between the measurements.

There are probably three sources of this bias or difference between the two.

1) The pressure sensor can have an offset at the time of the deployment if it is not set to zero.  Typically the AWACs are sent from the factory with a 0.3 meter offset.

2) If there are large atmospheric pressure variations, then this would change the mean pressure for a given burst, and this would not be seen in the mean AST.  The difference can be as much as 0.5 meters, but typically is no more 0.25 meters.

3) The AST could show a bias in its estimate of distance to the surface due to variations in the speed of sound.  Remember that the AST is really measuring the time of travel from when it sends and receives a pulse.  This assumes a constant speed of sound for its travel path.   If there are any variations (gradients) in the water column then there could be bias in the distance measurement.  Errors would also be seen if the initial estimate for the speed of sound is incorrect.  This would occur if there was something wrong with the temperature sensor (this would have to be a lot), or if the estimated for sality was way off.


A "bad" combination of all three of these could lead to a meter or more difference between the pressure and AST.

The good news is that it has little effect on the estimates of wave parameters, which is what the AST is really used for. " />'>

-Torstein