Nortek Vector Comprehensive Manual

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Comprehensive Manual114© 2018 Nortek ASdependent largely on the quality of the dataset. Histograms are actually extremely useful for thistype of work. A lot of times the ambiguity velocity causes the tail of a histogram to be cut off andshifted to the wrong side of the histogram (so, a sign filter would probably work wonder here)meaning visually it is fairly easy to pick out wrapped velocities. At the least, this is a good way tocheck the success of any identification scheme developedTo unwrap the data: It is, in principle, possible to unwrap data when you can make assumptionsabout du/dt but only if the SNR and correlation is good.There are two ways to go about dealing withyour present data.1. Develop an outlier filter to remove the wrapped data. If there are only a few points (some smallpercentage of the total data) it's relatively simple to define a cutoff threshold outside of which youdiscard the velocity data. If needed, you can interpolate these removed points back in as long as thegaps created by removing them are not too large. The advantage of this is it is relatively simple andquick, and the impact on statistics is going to be fairly minimal because so few points are involved.2. If the number of wrapped points is fairly high, the above is problematic because the variance of thesignal is artificially constrained by the phase wrapping (i.e. the largest events are being eliminatedfrom sampling). In this case the data either needs to be corrected or retaken. For a field deploymentthis generally means correction. Below are the recommended steps for going about this.Step 1 in ambiguity correction is to get the data back into beam coordinates. Search for "CoordinateTransformation" in our FAQ database to find an explanation on how to do this, or confer our PracticalPrimer for Pulse Coherent Instruments available under Documents.Step 2 is to find the Ambiguity Velocity. Ambiguity Velocity (the velocity corresponding to pi,denoted V_amb) is the maximum beam velocity which can be measured by the instrument in itsdeployed configuration. This is set by the Nominal Velocity Range in the configuration window of thesoftware. There are two ways of finding this velocity:A simple estimate of Vamb is the average of the absolute values of the maximum and minimummeasured velocities.A more accurate expression is V_amb = c / ( 4 * f * timeLagInSeconds ), where c is the speed ofsound, f is the instrument frequency and timeLagInSeconds is pulled from the configurationinformation in the *.HDR file. (this formula is the standard Pulse Coherent maximum velocitycalculation. Search for "lag" in our FAQ database to read more about this).Step 3 is to remove the Vamb. The procedure is like this:loop over all the data pointsIf large jump in velocity, thenif the jump is negative, thennewvel = oldvel + 2*Vambelsenewvel = oldvel - 2*Vambendendend loopStep 4 is to transform the velocities back to XYZ or ENUA few words of caution. It is generally best to avoid phase wrapping issues as the time spentcorrecting them is often fairly significant. The most difficult task is identifying the points which arewrapped and automating this process. The correction is fairly simple once points are identifiedhowever.5.3.8 Mooring/mounting VibrationsExcessive mooring vibration can adversely affect the data. Vibration introduces spurious velocitiesand interferes with the proper operation of the tilt sensor. You may be able to detect intervals ofexcessive vibration by looking closely at the data. If you discover that mooring vibration is a problem,