# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 2 The POSEIDON System
### The Observing System Components
FB has proven to be a helpful tool in the study of water circulation (e.g., modified Black Sea Water flowing in the Aegean Sea) using data assimilation techniques, with a significant impact on the correct representation of the dynamic features of the South Aegean Sea [7]. In addition to the scientific importance, through the acquired data, the FB has proven an excellent testing platform for new sensors and techniques. During the implementation of the HIMIOFoTS national infrastructure, a second SubCtech Ferry Box system was introduced in the POSEIDON system. The new FB is currently being installed and will be deployed to collect data along a ship route in the north Aegean Sea in an attempt to capture the variability through the intrusion of the Black Sea Waters, which to a large extent modulate the ecosystem processes of the wider area. Ocean gliders are Autonomous Underwater Vehicles (AUV). They move in the sea without an energy-consuming propulsion system such as propellers, thus ensuring a long duration in their operational capabilities. The continuous sinusoidal motion in the water is executed by using mechanisms that change the vehicle's buoyancy and center of gravity, enabling the glider to descend or ascend in the water column. While moving, the glider uses the lift generated by the wings to convert that vertical motion to horizontal, propelling itself forward with very low power consumption. Three underwater SeaExplorer gliders, equipped with a wide variety of sensors to monitor temperature, salinity, currents, biochemical ocean conditions, as well as marine mammals (sound), have been integrated into the Poseidon network since 2017. Their operations aim to enhance the sustained observations of the Greek Seas, starting with the establishment of the endurance line operating in the Cretan Sea up to 1000 m depth. The transect crosses from west to east the whole island of Crete 26 nm north at the level where the double gyre system operates. An addition (2019) to the POSEIDON system is the integrated scientific cabled observatory at the \"EMSO Hellenic Arc\" node implemented at PYLOS, approximately 12-13 km east of Methoni at a depth of 1580 m and is part of the EMSO ERIC ([[http://emso.eu/](http://emso.eu/)]([http://emso.eu/](http://emso.eu/)), accessed on 5 October 2022). The site represents scientifically the wider area of the western part of the Greek arc from Crete to the southwestern Peloponnese as well as the geodynamic, hydrological, climate and biology processes that govern the eastern Mediterranean. The fixed seabed module is connected to an onshore terminal station through a 15 km length power and fiber optic cable capable of transferring large amounts of data in real time. The instrumentation hosted in the seabed module frame includes sensors for seismology, geodesy, sea level, fluid and gas vents, physical and biochemical oceanography and biodiversity imaging. Two state-of-the-art radar sea level recording and meteorological stations equipped with GNSS receivers for high precision positioning references have been added to the POSEIDON observatory in the framework of the HIMIOFoTS project. The two land stations are located in Skopelos island and in Palaia Fokea harbor, transmitting high-frequency data contributing to the national sea level monitoring network. The Coastal Surface Circulation Monitoring System \"Dardanos\" is a WERA High-Frequency Radar (HF radar) System, located at the northeastern and southeastern extremities of Lemnos Island. The installation of the first, and unique until now, HF radar in Greece is a collaboration between the University of the Aegean and HCMR. The system was an upgrade within the framework of the HIMIOFoTS project (2020) and it provides mapping of surface currents and wave parameters in the northeastern Aegean, a Mediterranean region that directly receives the important as the mentioned above Black Sea waters exiting the Dardanelles Straits.