# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 2 The POSEIDON System
### The Observing System Components
The POSEIDON team design and deploys, on demand, subsurface mooring lines hosting sediments traps, ADCPs and experimental samplers in the area of E1-M3A and PYLOS sites. The moorings contribute to the multiparametric operational observational nodes with new data products for ecosystem study [8]. The parameters measured and transmitted in NRT for quality control and the oceanographic payload of the system nodes platforms are presented in the Table 1.  
Although the technological advancements during the last decade have made possible the development of many different automatic sensors, their maturity in terms of efficiency, accuracy and overall performance varies significantly. It is characteristic that the accumulated experience from the use of CTDs as basic equipment of marine observatories for decades and the establishment of international traceable standards for these parameters result in significantly higher efficiency (Figure 3). However, even for this mature technology there are only few CTD manufacturers that offer instruments able to operate in RT mode submerged in several depths, and even fewer provide detailed documentation such as application and operation guidelines, case studies report or specialized technical manuals. Sensors measuring biological, chemical and carbonate parameters although they \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Platform/Sampling** & **Sensor** & **Sensor Technology** & \begin{tabular}{c} **Manufacturer** \\ **Declared Accu-** \\ **racy/Sensitivity/Range** \\ \end{tabular} \\ \hline \multirow{4}{*}{\begin{tabular}{c} Temperature/ \\ Conductivity/Pressure \\ \end{tabular} } & Buoys/180 min & SBE 16 plus-IMP C-T-P & \multirow{4}{*}{\begin{tabular}{c} Thermistor/Electrode \\ SBE 45 \\ \end{tabular} } & 0.005 \({}^{\circ}\)C/ \\  & \begin{tabular}{c} Cabled obs/60 min \\ Ferrybox/1 min \\ Glider/30 s \\ Argo float/NA \\ \end{tabular} & \begin{tabular}{c} SBE GPCTD \\ \end{tabular} & & \begin{tabular}{c} Thermistor/Electrode \\ Cell/Strain-gauge \\ \end{tabular} & 0.0003 S m\({}^{-1}\)/0.1\% of full scale range \\ \hline \multirow{4}{*}{Currents} & Buoys/180 min & \begin{tabular}{c} Nortek single point \\ current meters/ \\ Nortek profilers \\ \end{tabular} & Acoustic/ADCP & 0.5 cm s\({}^{-1}\) \\  & HF Radar/30 min & WERA & \begin{tabular}{c} Transmission \\ Frequency 16.15 MHz \\ \end{tabular} & Maximum range 96 Km \\ \hline \multirow{4}{*}{Waves} & Buoys/180 min & \begin{tabular}{c} Fugro Directional Wave \\ Data Sensor \\ \end{tabular} & Inertial sensor & \
\begin{tabular}{c} Heave, Surge, \\ Sway \textless{}10 cm \\ /Direction 0.3°/ \\ Wave Period 2\% \\ of value \\ Maximum range 96 Km \\ \end{tabular} \\ \hline \multirow{2}{*}{Sea level} & Tide gauges station/ 5 min & OTT RLS & Pulse radar & 0.8–2 m: \(\pm\) 10 mm \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Fluorescence/ \\ Turbidity \\ \end{tabular} } & Buoys/180 min & WetLabs FLNTU & Optical & 0.025 \(\mu\)g L\({}^{-1}\)/ \\  & Ferrybox/1 min & Scufa Turner FL & & 0.013 NTU \\ \hline \multirow{2}{*}{Dissolved oxygen} & Buoys/180 min & SBE 63 & \multirow{2}{*}{Optical} & \multirow{2}{*}{2\%} \\  & Cabled obs/60 min & & & \\ \multirow{2}{*}{Ferrybox/1 min} & Aanderaa Oxygen & & & 5\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The parameters measured and transmitted in NRT for quality control and the oceanographic payload of the system platforms. are radically improving over the years, they require frequent calibrations and validation procedures. To ensure the data quality, the sensors' performance is evaluated by the scientific community with extensive field studies and intercomparison exercises such as the Fixo3 project reports for the pCO\({}_{2}\) sensors [9; 10].