## 4 Discussion  
The EUREC\({}^{4}\)A-OA/ATOMIC campaign highlighted the effectiveness of the saildrones in measuring SSS across the tropical North Atlantic Ocean. Saildrones were directed to areas of significant interest in the NBC region and were able to provide exceptionally high spatial and temporal resolution data. The high quality of the data was reflected in the low RMSE values produced during the comparison of duplicate sensors on each saildrone (Table 1). The saildrones also augmented the near surface data collected by the four research vessels that took part in the campaign as they generally start observations about 5-7 m above and below the sea surface [41]. A few months after the campaign, the shutdowns associated with the COVID-19 pandemic restricted field campaigns including ship-based research activities. As a result, remotely piloted or autonomous data collection platforms, such as saildrone, have become increasingly valuable given their remote operation and their ability to continuously collect in situ data over periods extending from months to years. The aforementioned benefits of the saildrones made them ideal candidates for collecting data to validate the satellite-derived marine products. In this study, SSS from three saildrone vehicles were used to assess the quality of satellite and ocean model data. The differences seen in Table 2 show that the RSS70 and JPL estimates were more closely correlated to the saildrone data than equivalent HYCOM and RSS40 products and the STD values indicate that the RSS70 SSS may have been most accurate to the saildrone data [22]. The RSS40 data was the noisiest out of all four platforms and this high noise content was represented by its STD values and Spearman correlation values. This noise is most likely due to reduced spatial smoothing. However, some of this noise could be groundwater discharges and other land-based discharges from small river systems which are not visible in the lower resolution RSS70 and JPL output. Figure 7: Time series of (**a**) the salinity measured by the SBE37 sensor onboard the saildrone 1026 compared with that of HYCOM, RSS70, RSS40 and JPL satellite salinity products and (**b**) the sea surface temperature measured while saildrone 1026 crossed the fresh tongue southeast of Barbados from 16 February 2020 12 UTC to 19 February 2020 9 UTC. These results suggest that for most applications, the RSS70 data should be preferred to RSS40, which is in agreement with previous validation papers [20; 42]. There also appears to be a correlation between the spatial resolution of the SMAP satellite and the precision of its salinity estimates. It was found that the satellites with coarser resolutions (JPL at 60 km resolution and RSS70 at 70 km resolution) and less noise resulted in more accurate output. Additionally, the mean difference values from all saildrones minus satellite/model values showed that SMAP and HYCOM overall tended to record more saline conditions than the saildrones. The most prominent feature to occur over the course of the saildrone cruise was the salinity minimum experienced during 16-19 February 2020. This sharp drop in salinity was spatially visualized in Figure 6 to examine how well the SMAP products reproduce the saildrone data. The fresh tongue was adequately displayed by the JPL and RSS products, showcasing the products' ability to resolve fresh mesoscale features. The RSS40 produced the most accurate salinity data inside the fresh tongue. It was the only product that was able to depict the sharpest rise in salinity inside the fresh tongue, which is presumably because it has the highest spatial resolution of the satellite datasets. This lines up with [PERSON] et al. [22], where one of the suggested applications of RSS40 are areas of freshening events of the surface layer of the ocean. Thus, RSS40 may be suitable for investigating small scale salinity anomalies, but RSS70 should be the preferred datasets for most ocean salinity research and investigation in the western Tropical Atlantic [22].