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

     3                     doc: Fri Oct 19 10:08:19 2012

     4                     dlm: Fri Oct 19 12:13:59 2012

     5                     (c) 2012 A.M. Thurnherr

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

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     8 

     9 =Introduction=

    10 

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

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

    13 correlations between the two corresponding time series of vertical

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

    15 time lagging is accomplished WITHOUT regard of the clock time reported

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

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

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

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

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

    21 fields in the processing output are always consistent with the CTD

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

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

    24 include:

    25 

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

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

    28 records.

    29 

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

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

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

    33 

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

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

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

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

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

    39 

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

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

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

    43 constructed manually.

    44 

    45 

    46 =Solving Time-Lagging Problems=

    47 

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

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

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

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

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

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

    54 

    55 

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

    57 

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

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

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

    61 show immediately whether there are problems with the CTD pressure data

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

    63 pressure values to nan.

    64 

    65 

    66 -Step 2: Manually Determine an Approximate Time Lag-

    67 

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

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

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

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

    72 <estimated lag>".

    73 

    74 

    75 -Step 3: Manually Determine an Accurate Time Lag-

    76 

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

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

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

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

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

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

    83 lag>" option.

    84 

    85 

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

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

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

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

    90 correctly. 

    91 

    92 

    93 =Patching Together CTD Time-Series Files=

    94 

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

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

    97 data acquisition is restarted during a cast, multiple files are

    98 produced. The resulting files cannot simply be pasted together because

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

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

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

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

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

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

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

   106 Fractional seconds can be ignored.