Argo world 

Float Design

(Courtesy of Michael McClune at Scripps Institution of Oceanography)

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 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.

Argo Mission

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 1500m.

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
  • the APEX float produced by Teledyne Webb Research
  • the 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

The SBE 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 Data FAQ.

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 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.

The majority of floats in the Argo array now use the Global Positioning System (GPS) to establish float positions and use Iridium 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.