# Edge Computing Architecture for the Management of Underwater Cultural Heritage
## 4 Buoy System for Underwater Location: A Case Study
Of the three buoys, one acted as the  
Figure 4: The electronic architecture of the buoys, integrating acoustic and LoRa communication, enables precise real-time communication and localisation of underwater elements. Figure 5: This assembly of buoy components illustrates the robust design and adaptability of the proposed solution for real-time underwater localisation, essential for effective UCH site management. The three-dimensional design of the buoy components (**a**), assembled buoy components (**b**), and functional prototype of the buoy (**c**). central unit, responsible for orchestrating the range measurements and processing the data collected. This central buoy communicated with the two other buoys, which functioned solely as measurement nodes. When prompted by the central unit, these measurement nodes conducted range calculations relative to the UAE and promptly transmitted their results back to the central buoy for processing. The UAE, positioned on an object equipped with an acoustic transmitter, was guided through the water by a surface boat. This configuration allowed the system to assess the UAE's precise underwater location as it moved along its designated trajectory. The structured setup and communication between the central buoy, measurement nodes, and UAE enabled the collection of essential data regarding the system's accuracy, responsiveness, and effectiveness in tracking and monitoring underwater acoustic elements. Below, one of the successful messages sent by the buoy system is analysed:  
24/08/2023 12:23:19 B1, RNG, 4.22, 45.779338836669918, 15.936838150024414, 81  
24/08/2023 12:23:19 B2, RNG, 10.1, 45.779228210449218, 15.936708450317382, 81  
24/08/2023 12:23:19 B3, RNG, 1.88, 45.779338836669918, 15.936790466308594, 81  
The message was composed as follows:  
<time> B#, RNG, <range>, <lat>, <lon>, <e_GPS>  
where  
* Time: date and time formatted as dd/mm/yyyy hh:mm:ss. * B#: buoy to which the message belongs (B1: central unit, B2: bouy node 1, B3: bouy node 2). * RNG: range correctly calculated. Otherwise, it shows LOG. * Range: distance between the buoy and the underwater acoustic element, in meters. * Lat: latitude, in decimal degrees. * Lon: longitude, in decimal degrees. * e_GPS: error, in centimeters. Each point, corresponding to the location of the buoys, was overlaid with a sphere that symbolised the distance to the UAE. The radius \(r_{i}\) of these spheres equaled the sum of the distance \(d_{i}\) between the buoy and the UAE plus the GPS error \(e_{GPS}\) margin and the error in the acoustic range measurement \(e_{Acoustic}\) (0.1 m according to the manufacturer), thus offering a detailed three-dimensional representation of the position and the uncertainty associated with each measurement (Figure 6). Figure 6: The trilateration algorithm used for underwater localisation provides accurate spatial mapping of buoys and UAEs, enhancing the platform’s capacity to precisely track assets in submerged environments. Location of the buoys (**a**). Data processed using the trilateration algorithm (**b**). \[r_{i}=d_{i}+e_{\text{GPS}}+e_{\text{Acoustic}} \tag{6}\]  
To obtain a graphical representation of the results, the data were processed using a spherical trilateration algorithm (Equation (6)), in which each of the obtained points was accurately represented in space based on the known distances \(p_{i}\) between the buoys, taking one of them as the origin. \[(x-p_{i,0})^{2}+(y-p_{i,1})^{2}-r_{i}^{2}=0 \tag{7}\]  
Figure 6 provides a 3D spatial visualisation of the buoy network and how the relative buoy distances are used to pinpoint the object's position accurately. The approach effectively accounts for and visualises the uncertainty inherent in the measurements, offering a comprehensive view of the spatial configuration and accuracy of the localisation system. In the future, buoys equipped with acoustic sensors can be utilised not only for localisation but also for the detection of diver bubbles.