In the past two decades, the Argo Program has collected, processed, and distributed over two million vertical profiles of temperature and salinity from the upper two kilometers of the global ocean. A similar number of subsurface velocity observations near 1,000 dbar have also been collected. This paper recounts the history of the global Argo Program, from its aspiration arising out of the World Ocean Circulation Experiment, to the development and implementation of its instrumentation and telecommunication systems, and the various technical problems encountered. We describe the Argo data system and its quality control procedures, and the gradual changes in the vertical resolution and spatial coverage of Argo data from 1999 to 2019. The accuracies of the float data have been assessed by comparison with high-quality shipboard measurements, and are concluded to be 0.002 • C for temperature, 2.4 dbar for pressure, and 0.01 PSS-78 for salinity, after delayed-mode adjustments. Finally, the challenges faced by the vision of an expanding Argo Program beyond 2020 are discussed.

15The international Argo program, consisting of a global array of more than 3000 free-drifting 16 profiling floats, has now been monitoring the upper 2000 meters of the ocean for several 17 years. One of its main proposed evolutions is to be able to reach the deeper ocean in order to 18 better observe and understand the key role of the deep ocean in the climate system. For this 19 purpose, Ifremer has designed the new "Deep-Arvor" profiling float: it extends the current 20 operational depth down to 4000 meters, and measures temperature and salinity for up to 150 21 cycles with CTD pumping continuously and 200 cycles in spot sampling mode. High 22 resolution profiles (up to 2000 points) can be transmitted and data are delivered in near real 23 time according to Argo requirements. Deep-Arvor can be deployed everywhere at sea without 24 any pre-ballasting operation and its light weight (~ 26kg) makes its launching easy. Its design 25 Manuscript (non-LaTeX) Click here to download Manuscript (non-LaTeX) Manuscript_Serge_Le_Reste_3.doc 2 was done to target a cost effective solution. Predefined spots have been allocated to add an 26 optional oxygen sensor and a connector for an extra sensor. Extensive laboratory tests were 27 successful. The results of the first at sea experiments showed that the expected performances 28 of the operational prototypes had been reached (i.e. to perform up to 150 cycles). Meanwhile, 29 the industrialization phase was completed in order to manufacture the Deep-Arvor float for 30 the pilot experiment in 2015. In this paper, we detail all the steps of the development work 31 and present the results from the at sea experiments. 32 33

The scientific community observes the ocean for applications in the fields of oceanography and climate research. To recover in situ data, more than 3000 profiling floats are operated in the framework of the Argo program. Each float performs cycles between the sea surface and a depth of 2000 m. Scientific data are gathered while the float is traveling upward from the depths of the oceans and are then transmitted via a satellite communication system at the end of each cycle. During its time at the surface, mainly dedicated to transmissions, the float is vulnerable and subject to drift, which limits its use in many studies. Moreover, transmission times are becoming longer due to a trend toward high-resolution or multisensor profiles. Consequently, the transmission system embedded in the profiling floats had to evolve. Argos-3 is the latest generation of the Argos satellite communication system. It has been designed to allow instruments to transmit more data in a small time budget and as an alternative to Iridium, already implemented on profiling floats in restrictive applications. This study aims to evaluate the implementation of Argos-3 on Arvor profiling floats. Tests were carried out first in the laboratory, before being implemented on the Arvor float and deployed at sea. This study proves that the high-data-rate mode suffered from European electromagnetic noise, which is incompatible with this application. The interactive low-data-rate mode was successfully qualified; it is capable of transmitting an entire dataset in a few minutes, compared to 8–10 h for the previous Argos-2 system.

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