# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 2 The POSEIDON System
### System Operations
#### 2.3.1 Deployment, Recovery and Maintenance  
The maintenance of the observatory platforms, after the recovery of the equipment, is the most crucial factor in order to maintain a valid operational status. The frequency and the necessary activities performed during the maintenance procedures are dictated by the type of platform and the area/type of environment (Table 3). Moreover, the ease of access to the site/platform and the required resources needed (R/V type) determine the maintenance planning.  
The buoys network is the most demanding, in terms of maintenance, compared to the other platforms of the Poseidon system. The limiting factor is the on-site maintenance during the R/V cruise and although the originally planned time window (Table 3) is adequate for the service, testing and redeployment of the equipment, all depends on the weather conditions. Furthermore, the complexity of the fixed stations modules, the extended scientific payload and the supporting equipment that needs to be maintained and tested before deployment requires a numerous scientific and technical team working closely and in coordination with the ship crew.  
The maintenance procedure In the Ferrybox system is performed in the destination harbor while the ship is docked. Although it has the advantage of immediate and easy access to the ship and land facilities with a time window of 12 h before the next transect, there is a disadvantage that the system cannot be tested in realistic conditions. If needed, the operators must travel along with the ship to test the equipment or collect validation samples for analysis. The cabled observatory is designed to operate for longer periods compared to the other platforms, without maintenance. The maintenance interval (3 years) has been met once so far for the POSEIDON cabled observatory. The challenges to operate the cabled  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Sensor** & **Accuracy** & **Manufacturer** \\ \hline Temperature & SBE 35 & 0.001 \({}^{\circ}\)C & SBE Electronics \\ Conductivity/ Salinity & AutoSal 8400 A & 0.003 ppt & Guildline Instruments \\ Conductivity & SBE 37 SIP & 0.0003 S/m & SBE Electronics \\ Dissolved oxygen & 3975 & 8 \(\upmu\)M & Aanderaa \\ \hline \hline \end{tabular}
\end{table}
Table 2: The instrumentation of the PCL.  
observatories are common as described by the community and documented [11; 12]. The gliders servicing is performed in the laboratory between the field missions and includes fouling removal, ballasting, compass calibration and batteries refurbishment.  
The maintenance of the scientific payload takes place after the recovery of the instruments, either on site during a cruise or on land facilities. Each sensor has dedicated procedures for maintenance, usually described by the manufacturer manuals or in relevant Best Practices reports available in the community. In the case of minor failures, the servicing includes replacement of parts (sealing, connectors, wipers, etc.) Especially for the sensors operating in the surface or shallow waters where the biofouling significantly affects the data quality, cleaning procedures are applied for keeping the sensing elements (conductivity cells, optical windows, etc.) free of biofouling.  
Figure 4: The operational scheme of the POSEIDON system.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
\begin{tabular}{c} 1 week \\ 1 week \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 3: The frequency of maintenance, the duration and the supporting means required per platform of the POSEIDON system.