beam velocities and ping rate

Hello, My question has two parts: 1) Are the beam velocities on the vectrino defined as: u_i=[c/(4*pi*f_vectrino)]*(dp/dt) with dp/dt = (1/tau)*atan[imag(Corr)/real(Corr)] ? 2) And if so, would tau be equal to 1 over the ping rate? For example, if I have a vectrino plus (200Hz) and I use a nominal velocity range setting of +/- 0.30 m/s, would tau (the time between pulse transmissions?) be, tau = 1/1124 = 8.897e-4 seconds? Thanks in advance, Kyle

Dear Kyle

The derivation of the velocity estimate is correct, given of course that "corr" comes from the complex autocovariance function and not the "cor" we report as part of the data.  The velocity setting is a "nominal" value and I am afraid you cannot use it to calculate the exact value for tau.  For example, the setting for 0.3 m/s actually uses two pulse pairs and the the exact distance between the pulses may change due to adpative elements in the firmware.

Best regards, Atle Lohrmann

Thanks Atle.

Is there a way to obtain or calculate tau using any of the  instrument settings or any of the information in the vectrino header file (the file that is generated using the data conversion function when generating ASCI files)?

Also, could you help me understand what the ping frequency corresponds to (I got the ping frequency of 1124 for a nominal vel range setting of +/- 0.3 m/s in the above example from the ping frequency table that was posted in another thread)?  My understanding of the ping frequency is that it is equal to one over the time between the two pulse pairs, and that the complex autocovariance function operates on the two pulse pairs separated by a time lag of tau.  Is this the proper way to understand what is going on?  I know the question has the potential to need a lengthy response, so if you would rather point me to an article or book that would help to explain this more, that would also be appreciated.

many thanks,

Kyle

Dear Kyle

I am a little hesitant because your questions are a little too close to the core design issues with the Vectrino technology.  I would like to provide the information you are looking for but maybe not in the form you suggest.  Is the purpose of this to find the effective sampling rate as a function of nominal velocity range?  If so, maybe we could provide a table that gives this information explicitly?

Best regards, Atle Lohrmann

Hi Atle,

I understand... My main two reason in trying to obtain the tau values are 1) for the purpose of back calculating atan[imag(Corr)/real(Corr)] from the beam velocities in hopes of being able to apply some post-processing de-alaising techniques, and 2) for getting a better understanding how the instrument works.  I would also be interested in knowing what the sampling frequency of the Vectrino is as a function of nominal velocity range setting if that information is available to the public.

Thanks again,

Kyle