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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 MissionsThere 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 ModelsThe three float models in use are the PROVOR built by MARTEC 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 TransmissionAs 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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