Argo FloatsArgo 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.
Thestandard 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 KANNAD in France in close collaboration with IFREMER, the APEX float produced by Teledyne Webb Research and the SOLO 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 KANNAD. 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.
An alternative 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.