## 3 Prototype Testing  
### July 2018 Deployment at the Acqua Alta Oceanographic Tower  
After extensive testing of individual components and the full system in the laboratory and outdoors on land, a first prototype system test was performed on the Acqua Alta Oceanographic Tower (AAOT) in July 2018 during the international Field Inter-Comparison Exercise (FICE) organized by the FRM4 SOC (Fiducial Reference Measurements for Satellite Ocean Colour) project. AAOT is an oceanographic platform in the Northern Adriatic Sea 15 km offshore from Venice in water of 16 m depth. It was used extensively over the last 20 years for oceanographic data collection including optical oceanography [29] and was used for a number of multi-partner optical radiometry inter-comparison exercises [30; 31]. It is the location of the first AERONET-OC site and provides such data since 2002. The above-water platform structure was entirely reconstructed in 2018, but the new structure closely follows the design and layout of the original platform. The PANTHYR system was deployed on the east corner of the top deck of the platform (Figure 4), with the irradiance sensor collector 2 m above top deck floor and, hence, about 14 m above sea level. The system ran for six days continuously from 12 to 17 July 2018, under the supervision of the developer for the first three days and with non-specialized supervision for the remaining days. Some test conditions were not the most challenging that can be expected in the future; grid power was available, and data were not transmitted over a 4G link but stored onboard. However, the basic functions of autonomous pointing and data acquisition were fully and successfully tested. This deployment for prototype testing was also installed only at moderate height and not above the height of the top deck masts for practical reasons; for future operational deployments, the PANTHYR system should ideally be positioned above the height of all such masts and structures or with a careful analysis of any shading of downwelling irradiance. For subsequent data comparison, only PANTHYR measurements at relative azimuth angles of 225\({}^{\circ}\) or 270\({}^{\circ}\) were used. Using the angular field-of-view data (7\({}^{\circ}\) full-width half-maximum) for the radiance instrument supplied by the manufacturer suggests that the water-viewing radiometer sees an elliptic patch of water with about 3 m diameter. Typical integration time for the water-viewing measurement is \(\leq\)512 ms during this experiment. Spatio-temporal variability caused by the surface wave field is, thus, mainly averaged and not resolved. The FICE campaign further involved the deployment of multiple supervised radiometer systems, including seven above-water radiometer systems, one shipborne free-fall underwater radiometer system, and one platform-deployed vertical profiling system. Inter-comparison of all these systems will be the focus of a separate paper. In the current paper, results of the PANTHYR system are compared only to one supervised above-water radiometer system, the manually supervised RBINS (Royal Belgian Institute of Natural Sciences) hyperspectral above-water three-radiometer TRIOS/RAMSES system, hereafter called the \"M3 TRIOS\" system, and the autonomous multispectral AERONET-OC system operated by the Joint Research Centre (JRC) of the European Commission at Ispra. The five TRIOS/RAMSES sensors used here (two for PANTHYR, three for M3 TRIOS) were radiometrically calibrated at the Tartu Observatory laboratory just before the FICE campaign, thus minimizing calibration-related differences. Only measurements from the five days with best sky conditions (13-17 July) are used here. For those days and during the period of day used for comparison in the present study, sun zenith angles ranged from 23.4\({}^{\circ}\) to 46.7\({}^{\circ}\) and wind speed ranged from 0.1 to 4.3 m/s. Figure 4: Photographs of PANTHYR prototype deployment at Acqua Alta Oceanographic Tower (AAOT).