# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 4 System Efficiency
### Atmos  
ATHOS buoy configuration has changed several times and it was not steady unlike the E1-M3A and PYLOS. The buoy hosts CT sensors only for the first 100 m. In the other two buoys, the CT sensors used in the upper 100 m were different versions, over the years, of SBE 16 CTD IM, an instrument that hosts additional analog and digital sensors for biochemical measurements as fluorescence/chl-a, turbidity and dissolved oxygen. The price to pay for the auxiliary sensors is the required external connecting cabling that fails  
Figure 12: The multiparametric buoys of the Poseidon system (**a**). The mooring line diagram (**b**) corresponds to the E1M3A and PYLOS. The deployments depths of the sensors are 20, 50, 75, 100, 250, 400, 600 and 1000 m. The ATHOS buoy hosts sensor only for the first 100 m.  
often. This type of failure can compromise the whole CTD unit, creating data loss. The ATHOS buoy originally hosted only SBE 37 IM CTs, a more robust sensor with no external cabling, before upgrading eventually to a multiparameter observatory. That is why the NON-OPERATION percentage due to sensor failure is lower than for the other two buoys (Figure 15). On the other hand, the QC rejection is higher and this can be explained by the environmental conditions at the site of the buoy. The N. Aegean area is more productive than the Ionian and Cretan Seas and there are higher fouling growth rates. Direct comparison between observational networks performance is not straightforward. The heterogeneity both in terms of the scientific mission to be served and technological specifications of the research ocean buoy global fleet is huge compared to the Argo floats or gliders and other types of ocean platform. There is not a commonly accepted methodology for the evaluation of the performance of the ocean buoys. Data return, as the percentage of the data retrieved or data supposed to be retrieved according to the buoy sampling scheme, is used widely as an indicator of performance. For the POSEIDON multiparametric buoys, the averaged data return of quality-approved data for surface  
Figure 13: E1M3A percentage of the quality-approved data, the rejected data during quality control (QC), as well as the percentage of non-operation periods of the station or sensors for temperature (**a**) and salinity (**b**). (1 m) temperature and salinity measurements is 59.42% and 37.24%. The averaged data return for the underwater instrumentation is 64.28% for temperature and 62.64% for salinity. These numbers refer to useful data for science after different levels of quality control. The average of QC-rejected data is 12.73% for underwater temperature data and 12.28% for salinity. In general, peak performance is 70%. Figure 16, available on the Pacific Marine Environmental Laboratory (PMEL) website, presents the historical Tropical Atmosphere Ocean (TAO/TRITON) moorings network data return percentage, the number of buoys reporting data and the averaged days of field deployment from January 2011 to July 2022, highlighting the importance of the maintenance frequency for the performance of the network [22]. Figure 14: PYLOS percentage of the quality approved data, the rejected data during quality control (QC), as well as the percentage of non-operation periods of the station or sensors for temperature (**a**) and salinity (**b**). The overall Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) data return percentages from the deployment of the first buoy in September 1997 to the end of December 2006 for subsurface temperature measurements were 72% in real-time and 82% in delayed mode [23]. The buoys' delayed mode data return over the 1997-2017 period average mean value for all sensors at each moored location ranged between 72 and 93% [24]. In a comparison of ATLAS and T-FLEX buoy type data, the return rate of salinity RT data was 72% and 85% [25].