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 2017, 84% of floats profile to depths greater than
Argo Cycle Timing Variables
Each Argo float cycle is composed of programmed events. Depending on float type, some of these events can be dated and sent
back by the float to aid in the calculation of velocities. The Argo Program has highlighted several cycle timing variables that
it would prefer that floats send back timing information. This cycle timing document
explains the variables and how they fit into the trajectory file.
Argo Float Models
The Argo array is currently
comprised of several float models:
- the PROVOR
and the new generation PROVOR, the ARVOR
built by NKE-INSTRUMENTATION in France in close collaboration with IFREMER
float produced by
Teledyne Webb Research
SOLO and the new generation SOLO, the SOLO-II float designed and built by Scripps Institution of Oceanography
- the S2A float is produced by MRV Systems in the USA who bought the rights to the SOLO-II and manufactures it under the rebranded name of the S2A float
- the NAVIS 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 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 frequently, often up to every 2db, resulting in several
hundred measurements per profile. For floats using Argos, a series of 200 measurements of pressure, temperature, and salinity are taken.
For 40% the 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.
The majority of floats in the Argo array now use the Global
Positioning System (GPS) to establish float positions and use
to transmit their data. Iridium is becoming a more attractive
option as it allows
more detailed profiles to be transmitted in a shorter time period at the
surface and it allows two-way communication with the float to change the float's mission if desired. In 2017, 80% of floats being deployed used Iridium communications and 20% used Argos communications.