ARGO Technology

Author: Adrian Drago

How does an ARGO profiler work?

Advances in technology have made a big impact on the methods of how we observe the sea. The Argo programme is an example of the capacity to make direct measurements of ocean currents, temperature and salinity, by making use of devices which float, profile, move and span all areas of the world’s oceans. The programme brings together efforts from a number of countries, including a number of European states, that have committed to maintain a minimum of 3000 profiling floats to collect high-quality temperature and salinity profiles from the upper 2000m of the ice-free global ocean and currents from intermediate depths. This report is intended to explain the principles on which ARGO floats work and on how they manage to measure and transmit data about the oceans.

Argo profiler structure

Figure 1: Structure and main components of an ARGO profiler

The ARGO profilers are battery-powered autonomous floats, just about 25 Kg in weight, shaped like a vertical torpedo as shown in Figure 1. There are three types of float models in use: PROVOR built by MARTEC and IFREMER in France, the APEX float produced by Webb Research Corporation, USA and the SOLO float designed and built by Scripps Institution of Oceanography, USA. All profilers are essentially made up of three main components:

  1. the hydraulics system which controls the buoyancy (the result of weight against upthrust) that allows the profiler to go up or down the water column;
  2. the sensors that make the measurements, and the electronics that controls the functioning and scheduling of the profiler; and
  3. the data transmission system which communicates via satellite to deliver data to a land station.

ARGO measurement cycle

 Figure 2Measurement cycle of a typical ARGO float

Figure 2 shows the measurement cycle of an ARGO float which typically repeats the cycle every ten days. The cycle starts by the float at the surface. After transmitting the data collected in the previous cycle and its current position (latitude and longitude), the float starts its new cycle by descending at a speed of about 10 cm/s taking high precision measurements of temperature and conductivity (from which salinity can be calculated) as it descends. The descent stops at a pre-defined depth, known as the parking depth, where it continues to drift under the effect of currents at that depth. The profiler spends most of its life at this depth where it is neutrally buoyant with the surrounding water density. At the end of the cycle the profiler starts to move up again and takes measurements as it rises towards the surface. Once arrived at the surface it takes again a bearing and transmits its new position. It subsequently transmits the temperature and salinity profiles. The difference between the initial and final positions of the cycle, combined with the time of the cycle, allows the deep currents to be measured.

The vertical movement of the profiler is controlled by a hydraulic system making use of the simple law of flotation. The floats pump fluid into an external bladder which inflates and thus raises the buoyancy (upthrust) on the profiler; the profiler experiences a net upward force which allows it to rise to the surface over about 6 hours. When the bladder deflates the float returns to its original density and sinks back to its parking depth. Floats are designed to make about 150 such cycles until the battery power is used up.

MedArgo active floats

Figure 3: Picture showing positions of ARGO floats that are currently active in the Mediterranean (from MedArgo)

Geostationary satellites are used to determine the position of the floats when they surface, as well as to allow data transmission to a land station. Data are processed and elaborated to produce maps such as those shown in Figure 3. These maps plot the successive positions of the ARGO floats producing a ‘spaghetti’ picture. These plots show the movement in time of the profilers and thus show the trajectories followed and hence the currents experienced.

References

Official website of the ARGO programme

MEDARGO website

CORIOLIS website

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