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Argo Floats
Argo is an
international
collaboration that collects high-quality temperature and salinity
profiles from the upper 2000m of the
ice-free global ocean and
currents from intermediate depths. The data come from battery-powered
autonomous floats that spend most of their life drifting at depth where they are stabilised by being neutrally
buoyant at the
"parking depth" pressure by having a density
equal to the ambient pressure and a compressibility that is less
than
that of sea water. At present there are three models
of profiling
float used extensively in Argo. All work in a similar fashion but
differ somewhat in their design
characteristics. At typically
10-day intervals, the
floats pump fluid into an external bladder and rise to the surface
over
about 6 hours while measuring temperature and salinity. Satellites determine
the position of the floats when they surface, and receive the data transmitted by the floats.
The bladder then deflates and the float returns to its
original density and sinks to drift until the cycle is repeated.
Floats are designed to make
about 150 such cycles.
Argo Missions
There are two different
standard mission operations. One is the simple mission operation and the other is the park and profile
mission operation. The main difference is in the depth to which the float descends. In the simple
mission, the float descends to a certain depth, often 2000m, and then begins its temperature and salinity
profile from that depth. In the park and profile mission, the float descends to a certain depth, 1000m
is recommended, and then descends to 2000m to start the temperature and salinity profile. In the beginning of 2006,
46% of floats profile to around 2000m and 66% profile to depths greater than 1500m.
Argo Float Models
The three float models in use are the PROVOR
built by KANNAD in France in close collaboration with IFREMER, the APEX
float produced by Webb Research Corporation, USA and the SOLO
float designed and built by Scripps Institution of Oceanography,
USA.
Two temperature/salinity sensor suites are used -
SBE,
and FSI. The
temperature data are accurate to a few
millidegrees over the float
lifetime. For discussion of salinity data accuracy please see the
section on the Argo data
system.
Argo Data Transmission
As the float ascends a series of typically about
200 pressure, temperature, salinity measurements are made and stored
on board the float. These are transmitted to satellites when the
float reaches the surface.
For most floats in the Argo array the data are
transmitted from the ocean surface via the Système
Argos
location and data transmission system (see also Service Argos Inc.).
The data
transmission rates are such that to guarantee error free data
reception and location in all weather conditions
the float must spend
between 6 and 12 hrs at the surface. Positions are accurate to ~100m
depending on the number of
satellites within range and the geometry
of their distribution.
An alternative system to Argos has been tested
using positions from the Global Positioning System (GPS) and data
communication using the Iridium
and Orbcomm
satellites.
This allows more detailed profiles to be transmitted with a shorter
period at the surface and even two-way
communication.
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