THE NODC STANDARD SUBSET OF SHIPBOARD ADCP DATA
The NODC provides shipboard ADCP data in two forms:
1) a high-resolution set in CODAS binary format consisting of
currents and ancillary parameters at the sampling interval
with which the data were recorded and processed and 2) a
standard subset of absolute currents at hourly and
10 m intervals which is available as ASCII or NetCDF.
If absolute currents are not available due to lack of navigation,
then relative currents are provided. If NODC receives data sets
that are not of high resolution, then the standard subset may
be of a slightly different timing sequence than hourly and the
depth spacing may be other than 10 m. For instance, if a data
set is received as gridded averages in space, then a nominal
time will be assigned. Similarly, if a data set is received with
8 m depth spacing, then this interval is used in the subset.
The standard subset includes the hourly means
and standard deviations of ship velocity components and the
transducer temperature, when available. A description of the
methodology and format of the standard subset is given below.
METHODOLOGY
The standard subset will consist of absolute currents
if the ship navigation was satisfactory. Otherwise the
currents will be given relative to the mean over a reference
layer, usually 50 to 150 m.
In the vertical, each velocity profile is linearly interpolated
to a regular grid at integral multiples of 10 m. The shallowest
level is the first multiple of 10 m that is greater than or equal
to the depth of the first good bin of the recorded data, typically
a function of the instrument configuration and the transducer depth.
The deepest level is that at which at least 50% of the ensembles
over the course of the cruise have a Percent Good greater than 30%.
For each depth bin, the Percent Good is the percentage of individual
pings in an ensemble with valid data. As noted above, for data
sets received at NODC in a reduced form (ie., not high-resolution), the
original depth interval is maintained.
In time, averages are calculated for one-hour intervals centered
on the hour. The time column in the ASCII subset gives the mean of
the times of the ensembles that fall into the interval. Hence the
mean may be slightly off the integral hour, even when ensembles
are evenly available over the entire hour. The mean time may be
even less centered in the case where data are only partially available
over the interval because instrument recording was turned off (e.g.,
at port calls). In the case where data are totally unavailable over
the entire interval so no mean time can be calculated, a "placeholder"
entry is made and assigned a time value exactly at the middle of the
interval, i.e., on the hour, followed by bad value flags for the
longitude, latitude and velocity fields. It is left to the user to
manipulate the time column if a truly even time grid is desired.
At each level, a bad value flag is given if less than 50% of the
ensembles in the hour have valid data. Bad value flags (99999)
for velocity components also occur at depth layers where less than 50%
of the ensembles in the interval contribute to the average because of
low percent good and other editing criteria, or in the case of
absolute currents, occur over the entire depth range because ship
velocity was not available as a reference.
The transducer temperature is provided as a mean and standard
deviation over the hour. The ship velocity is provided as means
and standard deviations of the components over the hour and is
derived from the final absolute ship velocities of each ensemble
as stored in the CODAS high-density database. The ship velocity
information allows one to see at a glance if the ship was on-station,
underway, or both over the hour.
FORMAT DESCRIPTION
====> HEADER RECORD
sac_id=xxxxx yr_base=yyyy start_lev=ddm num_lev=ddd type
< > denotes optional
e.g.
sac_id=00001 yr_base=1993 start_lev= 20m num_lev= 45 absolute depth_int= 8m
where sac_id is a unique identifier for the cruise (NODC bookkeeping)
yr_base is the year inwhich the cruise begins
start_lev is starting level (meters)
num_lev is the number of levels
type means either "absolute" or "relative" currents
depth_int is the depth interval or spacing between levels
and is left blank if equal to 10 m (default)
If a data set is received at NODC in a reduced form (ie., not high-
resolution), then a comment will be appended to the header to define
the timing scheme and the depth spacing if it is other than hourly
or 10 m.
A complete documentation file which describes the cruise, instrumentation,
processing, and other pertinent comments for the given cruise will be
available separately.
====> DATA RECORD
The format is designed for ease of application; therefore,
the date-time and positions are provided in decimal form as a
single field. Each data record will be fixed length although
the length may vary from cruise to cruise due to differences in
the number of depth bins.
The order of each record is as follows:
__
With a template such as:
ddd.fffff dddd.dddd ddd.dddd tt.t tt.tt suu.u su.uu svv.v sv.vv uuuuu vvvvv ..
e.g.
350.00002 157.9365 6.9120 28.9 0.01 -4.6 0.11 -3.4 0.09 419 177 ..
field template comment
ddd.fffff: decimal day; e.g. 0.50000 is
1200 UTC on January 1 of the year
given by yr_base; always present
dddd.dddd ddd.dddd: longitude and latitude decimal
degrees; lon positive east,
lat positive north;
missing flag= 1E38;
e.g. -145.1667 is 145 deg 10' W
tt.t tt.tt transducer temperature mean and
standard deviation (deg C);
missing = 1E38
suu.u su.uu svv.v sv.vv absolute ship velocity components
means and standard deviations in
m/sec; missing = 1E38
____ uuuuu vvvvv current components in mm/sec;
bad or missing = 99999;
e.g. 121 is 12.1 cm/s
__