# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 1 Introduction  
The European Global Ocean Observing System (EuroGOOS) includes a broad network of organizations and initiatives operating numerous platforms, such as drifting instrumented floats reaching previously unexplored areas (EuroARGO), fixed-point surface and called ocean floor observatories providing real-time data from the bottom layer to the surface of the oceans (EMSO), coastal infrastructures to study marine coastal systems and evaluate the combined effects of natural and anthropogenic changes (JERICO) and high-tech autonomous vehicles such as gliders and AUVs (GROOM). The resolution and the data quality of the Essential Ocean Variables (EOVs) is continuously improving, and efforts are made to introduce new biological, genetic and deep-ocean EVs, together with the Marine Strategy Framework Directive (MSFD) descriptors. Part of this joint international effort is the POSEIDON system (www.poseidon.hcm.gr, accessed on 5 October 2022) developed in 2000 to fulfil the ambition of an innovative operational oceanography system in the Eastern Mediterranean Sea [1]. The evolution of POSEIDON in compliance to the policy frameworksuggested by EuroGOOS and Group on Earth Observations (GEO) resulted in an end-to-end operational system covering the full value chain, from observations to data processing and handling and to services with a suite of modelling products. At its initial phase, the POSEIDON observing system consisted of eight moored buoys deployed in the Aegean Sea, while during the first major upgrade (2005-2007 period), three multi-parametric stations were deployed in key positions (Athos basin, Cretan Sea and SE Ionian) for the continuous recording of the water column down to 1000 m. In the following years, the system, in accordance with the global practices, adopted a multiplatform multiparametric strategy with oceanographic buoys network, Argos profiling floats, the ferry-box system, gliders, cabled seabed platform, tide gauges stations and an HF radar network, which are operated and supported by several land base facilities. The POSEIDON system actively contributes to the European Ocean Observing System (EOOS) implementation [2], focuses on biogeochemical (BGC) observations and monitors deep sea ecosystems and geological processes [3; 4]. The POSEIDON fixed stations are part of the global network of OceanSTTES. At a regional level, the POSEIDON team collaborate with scientists from the multiparametric, ocean buoy network for the Mediterranean basin (W1-M3A, E2-M3A and DYAMED). Moreover, the Hellenic Centre for Marine Research (HCMR) is participating with the POSEIDON System in the national research infrastructure HIMOFoTS (The Hellenic Integrated Marine Observing, Forecasting and Technology System) that links together ocean observing and forecasting systems, coastal zone monitoring and management practices, as well as ocean engineering testing facilities [5].  
The present work consists of a short description of the observing components of the POSEIDON system, including their operation and challenges, the data return of temperature and salinity timeseries as efficiency indicator for the multiparametric buoys, the tool of the access offered to the POSEIDON nodes. The strategy of its future technological developments and upgrades is also described in terms of national and international policy requirements.