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 determinethe 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.
The standard Argo mission is a park and profile mission
where the float descends to a target depth of 1000m to drift and then
descends again to 2000m to start the temperature and salinity profile.
In the beginning of 2010,70% of floats profile to depths greater than
1500m. Another 20% profile to between 1000 and 1500m.
Argo Float Models
Most of the Argo array is currently comprised of three float models: the PROVOR
built by NKE-INSTRUMENTATION in France in close collaboration with IFREMER, the
APEX float produced by
Teledyne Webb Research and the
float designed and built by Scripps Institution of Oceanography, USA.
Two current floats now have new models available. The ARVOR
is a new generation PROVOR float and it is also being built by NKE-INSTRUMENTATION.
The SOLO-II is a new generation SOLO and is being built by MRV systems.
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. 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.
system to Argos has been tested using positions from the Global
Positioning System (GPS) and data communication using the Iridium
satellites. Iridium is becoming a more attractive option as it allows
more detailed profiles to be transmitted with a shorter period at the
surface and even two-way communication. As of 2010, 250 floats have
been deployed with Iridium antennas.