# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 2 The POSEIDON System
### The Observing System Components  
The fixed stations of the POSEIDON system are surface buoys deployed at various coastal and open sea sites in the Aegean and Ionian seas (Figure 2). The locations of the selected stations (Pylos, Athos, Mykonos, Saranikos and Cretan Sea) represent interconnected marine basins with different characteristics and governing processes. The SE Ionian is a crossroad where significant intermediate and deep-water masses meet. In particular, it is on the pathway of the Aegean Sea dense water that travels to the north along the western coast of Greece. Furthermore, it is a very geologically active area with lots of seismic activities and landslides, as well as a potential source of Tsunamis that might affect the Eastern Mediterranean Sea. The Cretan sea is an area of open sea conditions, characterized as extremely oligotrophic where dense waters with intermediate and deep characteristics are formed. A dipole gyre system in combination with the presence of a number of water masses creates a unique ecosystem governed mainly by physical processes. The network consists of three coastal platforms operating in marine areas shallower than 300 m (SARONIKOS, HERAKLEION, MYKONOS) and three multiparametric offshore platforms deployed at \(\sim\)1500 m (E1M3A, PYLOS, ATHOS). Both coastal and open-sea types of floats are manufactured in Norway by Fugro Oceanor. The coastal Seawatch stations can host atmospheric and surface scientific payloads. The multiparametric offshore Wavescan stations, apart from increased atmospheric and sea surface payloads, are capable of supporting real-time sensors at multiple depths (\(>\)1000 m) through inductive coupling telemetry. The data are transmitted in Near Real Time (NRT) to the POSEIDON operational center where they are automatically processed, quality controlled, stored to the system's database and then released to all major European Marine Data Aggregators (CMEMS, EMODnet etc.) The Argo Floats are free-drifting floats with automated profiling capabilities for sampling ocean physical and biogeochemical parameters. Today, the global Argo Network (www.argo.net, accessed on 5 October 2022) consists of approximately 4000 floats and has become a global, in situ ocean-observing network under GOOS (Global Ocean Observing System). In the framework of the national contribution, the Greek-Argo (www.greekargo.gr, accessed on 5 October 2022) activities are an important monitoring component of the POSEIDON network and a part of HIMIOFoTS. Since 2010, more than 28 floats have been deployed in the Eastern Mediterranean basin collecting more than 2500 temperature, salinity, and ocean-observing network. The ORIS network is a very complex network, which is a very complex  
ity and dissolved oxygen profiles in sub-basins of the Aegean, Ionian, Libyan and Levantine Seas contributing to the Euro-Argo Research Infrastructure (www.euro-argo.eu, accessed on 5 October 2022). The Argo data are valuable for the construction of the hydrographic picture of the region [6]. The Ferry Box (FB) is an automated flow through measuring system integrated on board ships of opportunity (ferry, cargo or research vessels) providing oceanographic parameters in NRT, collected along the ship's transect. The sensors installed on the POSEIDON ferry box system measure physical, chemical and biological parameters, while the data collected are stored in an on-board computer and transmitted to the POSEIDON operational center through GSM network. A 4H-JENNA ferry box system has been operating periodically, based on commercial ships availability and access offered, on the route connecting the ports of Piraeus (Athens) and Heraklion (Crete)--the first line established in the eastern Mediterranean--sampling the south Aegean Sea providing high-frequency sea surface data.