# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 2 The POSEIDON System
### Land Base Facilities  
The land base facilities in HCMR Athens and Crete serve and support the POSEIDON system operations. The real time data flow, monitoring, engineering and technical services are performed in specialized laboratories and facilities. The operational center of POSEIDON is located at HCMR's facilities in Anavyssos, Attica. It consists of a cluster of servers and storage media which provide cloud services of high availability and load distribution with the use of a series of virtual machines together with High Performance Computers capable to support the timely provision of the POSEIDON forecasting products. The servers, the storage media and the network devices are connected through multiple and high-speed links in order to achieve the highest availability and fastest response time for the hosted services. All the POSEIDON servers are running on community-driven free operating systems, while all the services and the dissemination tools are built on open-source software. The POSEIDON operational center receives, processes and analyzes all the data collected from the observing network operations. These data are archived and utilized for the production of forecasts, scientific research and outsource.  
The calibration laboratory tests and calibrates sensors for a range of oceanographic parameters focusing on the unique environmental conditions of the Eastern Mediterranean Sea. The calibration facilities at the HCMR Thalassocosmos complex in Crete include fully equipped laboratories with a custom-made large calibration tank, a number of reference sensors and equipment, and all the necessary apparatuses for temperature, salinity, chlorophyll-a, turbidity, dissolved oxygen, CO\({}_{2}\) and pH sensors calibration and validation experiments. The activities are supported by engineers and scientists, who prepare the instrumentation, perform field experiments, service and maintain the instruments and assist external users during experiments in the calibration facility. The Poseidon Calibration Lab (PCL), through transnational access activities and participation in research projects, has proven to be a powerful tool for the calibration of sensors deployed in the wider Mediterranean Sea. The instrumentation of the PCL is presented in Table 2.  
Figure 3: The global ocean in situ NRT data files per parameter as presented to the Copernicus Marine Service (CMEMS) In Situ TAC monitoring webpage (www.marineinsitu.eu/monitoring/, accessed on 5 October 2022). The temperature and salinity data files are significant more than the other parameters.  
The buoys laboratory (located in Anavyssos) performs maintenance work in a specially designed enclosure of 210 sq.m., which has an 8 ton crane bridge and portable lifting machines. The electronics lab is used for fault detection and repair of various electronic parts. The station dataloggers, telemetry modules and power management units are checked at a specially designed simulation benchmark; for most sensors, the manufacturer's specified control procedures are performed. The buoy laboratory deals with the new modules and sensor integration, prepares the mooring line and anchoring lines of the fixed stations and handles the equipment storage and the consumables provision for the field operations. The glider lab is where preparation procedures such as ballasting, compass calibration and a stability test are carried out before every mission.