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                    R E A D M E . T I M E L A G G I N G 

                    doc: Fri Oct 19 10:08:19 2012

                    dlm: Fri Oct 19 12:13:59 2012

                    (c) 2012 A.M. Thurnherr

                    uE-Info: 106 0 NIL 0 0 72 3 2 8 NIL ofnI

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=Introduction=

In order to derive velocity profiles the data from the CTD and LADCP

instruments need to be merged. This is accomplished by calculating lag

correlations between the two corresponding time series of vertical

velocities calculated from the two instruments. In this software, the

time lagging is accomplished WITHOUT regard of the clock time reported

by the instruments, i.e. the instrument clocks do not have to be

synchronized. Instead of clock time, elapsed time in seconds is used. In

case of the CTD data, an elapsed time field can be supplied by the user

(see [README.ProcessData]); in case of the LADCP data, the

"elapsed-time" field is calculated by the software. The "elapsed-time"

fields in the processing output are always consistent with the CTD

elapsed times. While the time-lagging algorithm implemented in the

software is fairly robust, it has been known to fail. Possible reasons

include:

1) CTD PRESSURE SPIKES. Significant pressure spikes must be removed

prior to processing, *without* adding or removing CTD time-series

records.

2) LACK OF SURFACE VESSEL MOTION. If there is no surface-wave motion

affecting the vessel, time lagging is much more difficult. In rare

cases, time lagging must be carried out manually (see below).

3) MISSING CTD SCANS. For SeaBird 911 systems, if the connection

between the CTD and the deck box is not clean CTD scans will be

dropped. For the software, this looks like the CTD clock running faster

than the ADCP clock. There are cases where the CTD clock appears to

have gained more than 5 seconds during a 2000m-deep cast.

4) MULTIPLE CTD FILES. When CTD acquisition is restarted during a cast,

multiple files are created. In order to process the LADCP data from such

a cast, a CTD time-series file without any missing records must be

constructed manually.

=Solving Time-Lagging Problems=

While there are several run-time options that can be used to help the

time-lagging algorithm, detailed knowledge of the algorithm is required

to understand when and how to use these options, i.e. the user is

referred to the code and comments in [LADCPproc.bestLag]. However, the

following method can always be used to solve time-lagging problems, as

long as the CTD time series does not have any gaps.

-Step 1: Produce and Plot a Combined CTD/LADCP Time-Series File-

This is accomplished by processing the data with the "-l 0" option and

using "-t <time-series file>" to produce the file. Plot the resulting

time series of CTD_w and LADCP_w in the same panel. The plot should

show immediately whether there are problems with the CTD pressure data

(spikes). Often, standard processing works after setting any bad

pressure values to nan.

-Step 2: Manually Determine an Approximate Time Lag-

Use the output file generated in step 1 to determine how many seconds

have to be added to the elapsed field when plotting LADCP_w to bring

the two time series into approximate (a few seconds accuracy)

agreement. Often, the data can now be processed normally by using "-i

<estimated lag>".

-Step 3: Manually Determine an Accurate Time Lag-

If preprocessing with the -i option still does not succeed, time lagging

must be carried out manually. If this happens, there is most likely a

serious problem with either the CTD or LADCP data that should be solved

before proceeding. This is done exactly as in step 2 but to higher

accuracy (as high as you can). Once the best lag has been determined

manually, the data can be reprocessed with the "-l <manually determined

lag>" option.

After solving any time-lagging problems the results should be checked by

creating a time-series file (with -t) during final processing and

overplotting the LADCP_w and CTD_w time series. If there is still a

visible lag between the time series time lagging was not carried out

correctly. 

=Patching Together CTD Time-Series Files=

The LADCP processing software requires the CTD data to be supplied as a

single time series file with a constant sampling interval. When CTD

data acquisition is restarted during a cast, multiple files are

produced. The resulting files cannot simply be pasted together because

the resulting time series would have gaps. The only way to solve this

problem is to determine separate time lags for each of the CTD files

manually (using the method described above). The difference between the

resulting time lags is equal to the length of the gap between the two

files. The user can now create a dummy (all fields set to nan) CTD file

with required number of records that must be added between the CTD

files to create a single continuous regularly-space time series.

Fractional seconds can be ignored.