Aquadopp Deep
Up to Current profilers and current meters
We have recovered our Aquadopp deep from its first deployment andupon reviewing the data the vertical velocities appear to be wrong. Our horizontal velocities were mostly tidally dominated with maximum velocities of <15cm/s. The vertical velocities varied by more than this and were very noisy. The signal strength for the upward beam, which is actually only 20° up from horizontal, was much higher than for the other two beams. My questions are:
1. Assuming that the increased signal strength is due to returns from a nearby instrument in the mooring (only 1.5 m away) that is in a sidelobe of the beam, why is there an associated velocity with that signal return? Both instruments are fixed to the mooring line so there should be no doppler shift of the returning signal.
2. Has the beam pattern for the transducers been acurately determined and is 20° the optimal angle given that the instrument will be used in moorings with other instruments or flotation potentially above (or below) it? Another beam angle might move instruments directly above the Aquadopp out of the sidelobe.
3. Given that we now have this data set with vertical velocites that cannot be used, are the horizontal velocites affected? Does the transformation matrix use the upward beam signal in determining the horizontal velocities or are only the horizontal beams used?
David Spear
1. Assuming that the increased signal strength is due to returns from a nearby instrument in the mooring (only 1.5 m away) that is in a sidelobe of the beam, why is there an associated velocity with that signal return? Both instruments are fixed to the mooring line so there should be no doppler shift of the returning signal.
2. Has the beam pattern for the transducers been acurately determined and is 20° the optimal angle given that the instrument will be used in moorings with other instruments or flotation potentially above (or below) it? Another beam angle might move instruments directly above the Aquadopp out of the sidelobe.
3. Given that we now have this data set with vertical velocites that cannot be used, are the horizontal velocites affected? Does the transformation matrix use the upward beam signal in determining the horizontal velocities or are only the horizontal beams used?
David Spear
I will try to at least answer a couple of your questions.
1. If you could look at the returned time series in the velocity cell, you would see that when the transmit pulse hits the other instrument there is a large phase shift in the data. This phase shift will have a large frequency content and will in the processing lead to an erroneous velocity. After that phase shift you would see a nice coherent signal that would give a zero velocity if only that part was processed.
3. If the pitch and roll values were zero the upward beam signal would not affect the horizontal signals. And given the cosine response in the horizontal direction you only get a very small contribution up to tilt angles of 10 degrees. If you used diagnostics data during the measurement you can look at those to get an idea of how the mooring moved during the deployment, in addition to the sensor data in the normal measurements of course.
Sven Nylund
1. If you could look at the returned time series in the velocity cell, you would see that when the transmit pulse hits the other instrument there is a large phase shift in the data. This phase shift will have a large frequency content and will in the processing lead to an erroneous velocity. After that phase shift you would see a nice coherent signal that would give a zero velocity if only that part was processed.
3. If the pitch and roll values were zero the upward beam signal would not affect the horizontal signals. And given the cosine response in the horizontal direction you only get a very small contribution up to tilt angles of 10 degrees. If you used diagnostics data during the measurement you can look at those to get an idea of how the mooring moved during the deployment, in addition to the sensor data in the normal measurements of course.
Sven Nylund
Powered by
Ploneboard
Forgot your password?
New user?
