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 several models of profiling float
used 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 or GPS determine the position of the floats when they
surface, and the floats transmit their data to the satellites. 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 2014, 80% of floats profile to depths greater than
1500m. Another 12% profile to between 1000 and 1500m.
Argo Float Models
The Argo array is currently
comprised of several float models: the PROVOR
and the ARVOR
built by NKE-INSTRUMENTATION in France in close collaboration with IFREMER, the
float produced by
Teledyne Webb Research, the
float designed and built by Scripps Institution of Oceanography, USA and the
built by MRV Systems in the USA.
The ARVOR is a new generation PROVOR float and the S2A is a new generation SOLO float.
is a new float being built by Sea-Bird in the USA.
temperature/salinity sensor suites is now used almost exclusively. In the beginning, the
FSI sensor was also used.
The temperature data are accurate to a few millidegrees over the float
lifetime. For discussion of salinity data accuracy please see the
Argo Data Transmission
As the float
ascends, a series of 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 floats using high speed communications with more bandwidth capabilities,
measurements are taken more frequently, often up to every 2db, resulting in several
hundred measurements per profile.
For 70% of floats in the Argo array the data are transmitted from
the ocean surface via the
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 using positions from the Global
Positioning System (GPS) and data communication using the
satellites now comprises 30% of the Argo array. 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. In 2013, 60% of floats were
been deployed with Iridium antennas and 40% with Argos antennas.