# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 2 The POSEIDON System
### System Operations
#### 2.3.4 Data Flow, Field Validation and Remote Monitoring  
The POSEIDON system platform data are collected by the operational center. The acquired data come in different forms depending on the type of the platform and the sampling scheme. The fixed-point stations such as buoys, cabled obs and land-based tide and HF radar stations transmit timeseries data at predefined intervals. The moving platforms, such as Argo floats, Ferrybox and Glider platforms, send the data set at the end of a mission. The real-time data sets are transmitted through satellite and mobile networks and containing full or averaged data. The raw sensor measurements are stored locally in the sensor or the platform logger for later retrieval. The retrieval of the raw data and delayed metadata is performed during the maintenance of the platform followed by a first level of process and homogenization. Then, the data set is stored in the POSEIDON database system. The IT team of the operational center developed a visualization tool for internal use that combines all the dataset sources of the POSEIDON system. This tool has proven to be very useful for supporting the delayed mode quality control of the data and evaluating the sensor performance during the field deployments. The field validation refers to the intercomparison of different data sets obtained by the POSEIDON platforms and sampling surveys. For the validation of the POSEIDON E1-M3A and HCB stations, HCMR performs monthly surveys that include CTD casts, equipped with chl-a, turbidity and dissolved oxygen sensors, as well as water samples from various depths corresponding to the sensor installation depth. The Ferrybox measurements are validated with water samples collected by automatic water samplers or manually during the ship transect. Validation experiments for optical and chemical sensors can be performed during the port visits of the ship as well. For the Glider missions, pre- and post-deployment CTD casts serve as a comparison point for the data. Performing validation on a regular basis is a way to ensure the data quality and the proper function of the equipment. In some cases, it can minimize the operational costs, allowing for longer deployments periods (Figure 8). Figure 6: The dissolved oxygen sensor validation procedure. Data from 6 sensors are compared with the results of the Winkler analysis during an experiment where the seawater temperature was technically raised in order to reduce the dissolved oxygen concentration. A major issue related to the POSEIDON platform status during operations is the ability to track incidents when they happen and proceed to proper response actions. Network tools were developed and are used to monitor the system's marine platforms in real time during the deployments. For the Fixed stations and the Argo floats, a web service and a visualization tool was developed by the POSEIDON's IT team, which provides combined information regarding the positioning and the communication status (Figure 9). The main disadvantage of the buoy's on-board positioning system is that it is dependent on the main power unit of the station. In case of a power failure-related issue, the only information provided by the station is the loss of connection in a specific time. To overcome this limitation, the POSEIDON buoys have also been equipped since 2015 with an autonomous satellite tacker. Figure 7: This figure demonstrates the radical improvement of the dissolved oxygen data in the POSEIDON buoys by the introduction of a new optical sensor in 2017. Panel (**a**) presents the dissolved oxygen timeseries form the E1M3A where the new sensors data corresponds to the period after 2017. Panel (**b**) focuses on the 50 m time series to demonstrate the better correlation of the optical sensor data with filed validation data (blue and red dots). Figure 8: The POSEIDON buoys data flow using as an example the E1M3A 20 m temperature timeseries. Panel (**a**) corresponds to the NRT data received for the station during the deployment period.