Nortek Vector Comprehensive Manual

Also see for Vector: Quick start guide

Contents

Nortek Vector Comprehensive Manual Manual pdf 60 page image

Comprehensive Manual60© 2018 Nortek ASnot to an orthogonal coordinate direction such as X, and any screening should take this intoaccount. Therefore, we recommend looking at each beam individually when looking at correlationand SNR rather than an average across two or more beams. Problems can occur in only onebeam and averaging the data across beams may obscure potential problems.Check each beam individually. It does occur that just one beam is bad, and this can be indicatedby a signal level near the noise floor, for example. The data may still usable, but if using theVector one has to process the data with the two other beams only; a solution that is known as atwo-beam calculation. This will result in 2D currents only (since there is only data from two beamsavailable. Note that the processing makes one important assumption; the vertical currents arezero. This is a reasonable assumption in the vast majority of current flow. For the 4-beamsystems, one can discard data from one of the beams and still get 3D velocities out.Removing low correlation measurements is a good idea because correlation is a strong indicator ofdata quality in the sense of a valid Doppler phase shift determination.Lower correlation means more noise in the data.The SNR is calculated by subtracting the noise level in counts from the amplitude levels measuredin counts during velocity measurement, and it tells you the rate of signal over noise. The noiselevel is measured at the beginning of each burst (and once at the beginning of continuousmeasurements). A rule of thumb is that SNR should be >15 dB. The noise level should be around50 counts for the Vector.Correlations above ~70% is considered to be generating good quality data. The value of correlationthreshold of 70% is actually fairly stringent and are may be throwing away good data. A closeexamination of the dataset is the best way to set a correlation threshold for discarding bad datapoints.Screening data: There is no point using the SNR values to screen data, use the correlation valueinstead. When you discard data with a low correlation, you are screening the data with respect toSNR as well.Typical processing tasks:1. Assess data quality (QA/QC)2. Data screening3. Statistics to describe the flow (mean, variance)4. Spectral Analysis (turbulence, waves)2.8 Common Data Analysis ScenariosCorrelation looked good during data collection, but there are several range cells with poorcorrelationLow correlation in only a few range cells is symptomatic of weak spots. Unfortunately, there is not away to correct for this in post processing. Please see the Configuration Guide for how to eliminateweak spots before recording data.SNR looked good during data collection, but there are several range cells with muchdifferent SNR (higher or lower)This is also symptomatic of weak spots or an obstruction (e.g. an object) within the beam. Weakspots will generally see higher SNR because the strong boundary echo is causing problems, butmay be associated with lower SNR depending on the nature of interference.A beam obstruction will typically produce a strong echo and high SNR. It may create an acousticshadow resulting in low amplitude and SNR in bins behind the object as well.Weak spots can not be corrected in post processing. Removing biased velocities associated with anobject in the beam is possible by determining which range cells are affected and discarding themfrom analysis.Mean velocity profiles do not look good even though correlation and SNR are highIt is important to assess data quality and perform some basic quality control on a dataset beforeattempting to interpret the results. At minimum, discarding very low correlation measurements(<40%, adjusted as needed based on data quality) and very low SNR values (<15 dB, again adjusted