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                    R E A D M E . Y O Y O 
                    doc: Fri Aug 10 07:07:59 2012
                    dlm: Fri Oct 19 11:05:29 2012
                    (c) 2012 A.M. Thurnherr
                    uE-Info: 56 0 NIL 0 0 72 3 2 8 NIL ofnI
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=Overview=

This README contains notes on how to process data from non-standard
casts, such as yo-yos (consecutive down-up casts collected at a given
location without restarting the instruments) and tow-yos (yo-yos
carried out while vessel is in slow motion).

Processing of yo-yo and tow-yo data requires the following two steps:
	1. Split the data files into individual down-upcast pairs
	2. Process the resulting files as described in [README.ProcessData]

It is important to note that yo-yo and tow-yo casts can be full depth
(i.e. between the sea surface and the sea bed) or partial-depth. If
absolute velocities (rather than just vertical shear) are required for
partial-depth casts, the user must make sure that there are either BT
data or SADCP data available for velocity referencing of each
down-upcast pair. Essentially this means that each down-upcast pair
must extend down to near the seabed or up into the depth range where
SADCP data are available. 


=Step 1: Splitting the Data Files=

Both data files must be split between individual down-upcast pairs, i.e.
whenever the CTD winch switches from up to down.

CTD DATA: First, find the CTD splitting times by plotting depth vs.
elapsed time. Then, split the CTD data into separate files using any
text editor. If SeaBird CNV files are use, the same header can be used
for all output files.

ADCP DATA: There is an ADCP file splitting utility for M$ Windows
provided by RDI. Alternatively, the length of each ensemble can be read
from the binary ADCP files and the UN*X utilities "split -b" and "cat"
can be used to split the files into the required chunks.


=Step 2: Processing the Split Data Files=

The split data files, each containing data from exactly one consecutive
down- and up-cast, can be processed exactly as described in
[README.ProcessData]. If the CTD time-series data used during
processing has an "elapsed-time" field all output elapsed times are
consistent with this input, i.e. the elapsed times of the output casts
are relative to the beginning of the entire yo-yo or tow-yo cast.
Otherwise, every down-/up-cast pair gets its own elapsed-time field.

Usually, in standard LADCP processing the velocity data from the down-
and upcast are combined. While this smears out any information on the
temporal variability of the velocity field during the cast, down- and
upcast only profiles are necessarily derived from much fewer samples
and, therefore, associated with considerably larger uncertainties and
errors. It has been found, in particular, that the top-to-bottom shear
in down-/upcast-only profiles is often quite bad. In the context of
partial-depth yo-yo and tow-yo profiles the severity of this problem
can be evaluated by comparing two consecutive velocity profiles at
their "unconstrained" end. E.g., in case of a partial-depth yo-yo near
the seabed, i.e. constrained with BT data, the uppermost portion of the
first upcast can be compared to the uppermost portion of the 2nd
downcast, etc. If the errors are found to be unacceptably high, the
velocity profiles from the combined down-/upcast data should be used
instead. Alternatively, multiple simultaneous velocity referencing
constraints can be applied, e.g. using the shear-inversion method
described by Thurnherr (JAOT 2010).