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                    R E A D M E . P R O C E S S I N G T I P S 

                    doc: Thu Oct 20 14:11:49 2011

                    dlm: Fri Oct 21 17:33:21 2011

                    (c) 2011 A.M. Thurnherr

                    uE-Info: 18 0 NIL 0 0 72 3 2 4 NIL ofnI

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=Time Lagging Problems=

If the first pass (1Hz) fails, there are no diagnostic plots that can

be inspected. Often, however, this problem is simply due to CTD

pressure spikes, which must be removed before processing. If the CTD

pressures are good, it becomes very useful to write the

LADCP_timeseries ($out_LADCPtis =

"$data_subdir/$out_basename.LADCPtis";) and compare it to the CTD time

series (averaged to 1Hz for size), e.g. to spot obvious problems or to

determine an initial offset guess.

One possible reason for the first time-lagging pass to fail is that the

beginning of the cast is not correctly recognized. In case of

2011_IWISE (003UL, but probably also others) this is because there are

valid velocities in bin#2 even before the LADCP enters the water. The

problem was solved  by increasing $refLr_firstBin.

If the 2nd pass (24Hz resolution) of time lagging fails, the diagnostic

plots (*_TL.eps, *_TLhist.eps) should be inspected. Often, the problem

can simply be solved by decreasing $TL_required_top_three_fraction to

something like 0.4.

When profiles with very little ship motion (e.g. in/near the ice) are

processed $TL_required_top_three_fraction may have to be decreased to

very small values (0.05 in case of some stations in NBP0901). With such

a low value it is important that the diagnostic plots are inspected

carefully to make sure that there are no timelagging-related issues. It

is also advisable to plot/inspect the time-series output to make sure

that the final lag is reasonable.