## 3 Validation of the Instrument v2021
### The \"Floatenstein\" Drifting Buoy in the Caribbean: November 2021  
A small-scale, home-produced drifting buoy was built following the v2021 design in the context of the OneOcean expedition, a circumnavigation by the Norwegian tall ship Statsraad Lehmkuh. The electronics are identical to everything that was discussed above, except for the battery solution used. Indeed, since the instrument had to be transported by plane in the checked luggage of an expedition member, two traditional alkaline D-cell batteries were used instead of the Li-batteries of the other instruments. A standard rectangular rigid plastic enclosure of size 12 cm \(\times\) 12 cm \(\times\) 9 cm was used as the main body of the buoy. Since alkaline batteries are heavier than lithium batteries, this resulted in too low buoyancy. As a consequence, we added small chunks of styrofoam (wrapped in duct tape to protect from abrasion from the elements) on the sides of the rigid plastic enclosure in order to increase buoyancy. These additional buoyancy elements were set in place using cable ties and glued in position using bathroom silicone sealant. In addition, the rigid plastic box was fully sealed with epoxy glue, to make it completely watertight. Finally, a keel composed of diverse scrap metallic parts (spare screws and bolts), packed in duct tape, was fixed at the bottom of the plastic enclosure to improve its static stability. This combination of disparate building materials gave a very peculiar appearance to the drifting buoy, which got nicknamed humorously as \"Floatenstein\" (Floating-Frankenstein). A picture of the instrument, fully built before being shipped, is presented in Figure 8.  
Despite its unusual appearance, Floatenstein proved to be a robust and well-functioning drifter. Floatenstein was released in the open ocean on 11 November 2021, and, at the time of writing this manuscript (January 2022), Floatenstein has been active for little over two months and is working nominally. Data transmitted include (i) GPS position sampled each 30 min and (ii) vertical wave spectrum sampled each 2 h, similar to what is described in Section 2. In order to validate the good functioning of Floatenstein and the electronics inside, we compare the wave data transmitted by Floatenstein with direct satellite altimeter measurements. The Wavy software package (see [[https://github.com/bohlinger/wavy](https://github.com/bohlinger/wavy)]([https://github.com/bohlinger/wavy](https://github.com/bohlinger/wavy)), accessed on 15 January 2022, [PERSON] et al. [65]) is used to perform (i) gathering of relevant satellite data, (ii) collocation with the position of Floatenstein, and (iii) extraction of relevant fields. For the collocation process, the temporal and spatial constraints are 30 min and 50 km, respectively. Four satellite missions had coinciding satellite measurements: Sentinel-3A/B, Jason-3, and the altimeter mounted on CFOSAT. The satellite observations are of level 3 processing quality and are publicly available in the Copernicus CMEMS  
Figure 7: Comparison of Zeni-v2021 and SPOT-1386 spectra at 08:21 15 September 2021 (**left**) and 08:21 16 September 2021 (**right**): the distances between the buoys were roughly 1 km and 3 km, respectively. Low-frequency noise of Zeni-v2021 spectra was removed by applying an ideal filter at the lowest frequency local minimum of the smoothed spectrum, similar to what was reported in [PERSON] et al. [63], [PERSON] et al. [64], which is a common postprocessing step when working with accelerometer-derived wave data. This indicates agreement within typically 5% between the Sofar Spotter and the instrument v2021 measurements in the open water, before the instruments drift away from each other. Results of the comparison between Floatenstein and satellite data are presented in Figure 9. The satellite data are naturally spaced in time, due to the orbital pattern of the satellites collecting the data.