Freely drifting buoys or floats in the ocean are of basically two types. The first type has a surface float, whose position is tracked by satellites at fairly regular intervals throughout the day, which is attached to a subsurface drogue element. The drogue element can be of various types including a parachute, holey sock at depths of a few meters to around 100-m below the surface. Near-surface drifters have a special cross-shaped drogue element that extends about 1-m below the sea surface. These are known as Davis-type drifters. The second type is completely subsurface and either track a constant pressure or density surface. Location of the float is found from two to three sub-surface sound sources, which are moored at fixed positions in the region where the floats are deployed. The float stores the arrival times of the "pings" from the sound sources and when it surfaces at the end of the deployment, which may last from several months to several years, the data is retrieved via a satellite link. Both types of floats are usually assigned an ID by the agency that retrieves the data through the satellites. Most often this is Service ARGOS for ocean-related position data. More recently GPS drifters have been developed where greater positional accuracy is required, such as in estuaries or coastal waters. At SAIC, drifter data files are identified by their ID and prefix which indicates the type of drifter. Service ARGOS often reuses ID's and so alphanumeric modifiers will be added to make the ID unique for a given drifter delployment. Commonly used prefixes are given below.
Prefix Description ARG Service ARGOS tracked Drifter with surface float element. GPS GPS tracked Drifter with surface float element. RFS RAFOS subsurface float (travel times stored on float). SFR SOFAR subsurface float (travel times stored at sound sources). In many deployments, a single track is obtained over the lifetime of the drifter. For surface floats, this is either determined by the battery capacity or by the number of days the satellite system is programmed to listen. For subsurface floats, the track is from deployment to the preprogrammed surfacing date. In some cases the track may be split into more than one segment because of grounding or unauthorized retrievals and subsequent redeployments for surface floats. Interruption of communications either from the sound sources or with satellites may also cause substantial gaps that require segmenting the tracks. In these cases the segments are indicated by modifiying the 3-character prefix by replacing the final letter with 1, 2, etc.
Many drifters also measure ancillary variables such as temperature. For instance, a drifter with a surface float and a subsurface element may measure temperature at several depths between the surface and the drogue. RAFOS floats usually measure temperature, pressure and sometimes salinity. In the file listings, the "Number of Scalars" greater than 0 indicates these types of data. The individual files should be interrogated to determine the types and depths of these ancillary measurements.
The location data for a given drifter is inspected and obvious bad fixes removed. A location
quality control flag is supplied for each fix, which is based on the numeric flags supplied by
Service ARGOS for the accuracy of each fix. For RAFOS floats, the location quality flag is
determined by the number of sound sources that were received by the float at each
interrogation. Details of the flag values and meanings are given in the netcdf
files, according
to the COARDS/CF-1.0 conventions. Scalar QA/QC is very similar to that for mooring
time series
with missing values flagged.
Drifter data (both locations and scalars) for each track or track segment are archived in a
netcdf
file for access by the NVODS server. The netcdf file name is the 3-character prefix
followed by an underscore and the drifter ID (all in lower case), which is appended by
".nc". SAIC's netcdf files follow the
COARDS/CF-1.0 conventions.
Thus,
arg_20015d.nc contains the data from the drifter with ID of 20015D. More than one type
of data can be present in an archive as discussed above. Netcdf files are self-documenting,
so the structure should be reasonably self-explanatory.