# ENDURUNS: An Integrated and Flexible Approach for Seabed Survey Through Autonomous Mobile Vehicles
## 2 The ENDURUNS Overall Approach  
The ENDURUNS approach consists of a system integrating an AUV and a USV equipped with photovoltaic panels (USV only), fuel-cells and rechargeable battery packs. The AUV is designed to navigate using its own thrusters or in glider mode. This enables the AUV to carry out autonomous long-lasting missions in vast geographical and deep-sea areas. The glider navigation mode allows the AUV to autonomously reach the survey zones, or move between different areas, without a large energy expenditure. With the thruster-based navigation the AUV can perform transects close to the seabed, increasing data acquisition richness and accuracy [47]. The USV autonomously follows the AUV, providing information for accurate geo-localisation of the acquired data and granting satellite communication with the Remote Monitoring and Control Centre (RMCC) at a land station. Data transfer between the AUV and the USV can be obtained through acoustics communication or through wireless connection when the AUV is at sea-surface. Figure 1 summarises the ENDURUNS approach. Different AUV technologies are currently available for autonomous surveys operations [14; 49] and several AUV-USV combinations were already prototyped and tested [50; 51]. Among the most relevant AUV implementations we recall the Autosub 6000 [14] and the Autosub Long Range [52; 53]. The Autosub 6000 is a 5.5 m long vehicle, with a dry weight of 1800 kg, a depth rating of 6000 m and an almost 48 h autonomy. The Autosub Long Range is a relatively slow-moving propeller-driven vehicle, 4.3 m long, weighting about 800 kg with a depth rating of 6000 m and experimented in a 7 days mission in Antarctica [53]. It is also equipped with precision-navigation and terrain-following features, as well as a sophisticated collision avoidance system. Other relevant implementations are air-deployable and  
Figure 1: The ENDURUNS vehicle configuration and the different data transfer solutions. Image reproduced from [48], with permission from IMEKO TC19. launchable AUVs [54]. Although most of these implementations focused on aerial deployment of traditional REMUS-type vehicles [55], a NERC-funded project investigated the use of highly portable small-scale AUVs, weighting just 2.5 kg, with a range of 350 km at speeds of 0.5 ms-1 [56]. Differently from the currently available systems, the ENDURUNS prototype is conceived for an easy deployment and for a quasi real-time interaction between the USV and the land station. Automatic AUV/USV docking capabilities allows power transfer from the USV to the AUV and data transfer in the opposite direction [57]. The AUV is equipped with several sensors and data-acquisition devices. An additional standardized independent payload, the SeaCube, can be installed inspired by the CubeSat space technology [58], for third party experiments. In particular, there is great interest in bathymetric and seabed survey data acquisition via multi-beams echo-sounders (MBES), as well as imaging data of the seabed [59]. The acquired data provide a rich understanding of both the geometrical and the environmental characteristics of the seabed, for example, the morphology, the sediments type, the presence/absence and characterization of living organisms or sunken debris (e.g., wrecks, barrels, containers), reaching submetric horizontal resolution [60, 61]. A large scale mapping survey can produce a large amount of data in short time [10]. This amount represents an exceptional technological challenge for data processing, storage, and transfer on board of AUVs. Currently, almost all data processing tasks are handled offline by human operators after data transfer. On the contrary, the ENDURUNS approach provides fully automatic on-board data processing [48]. Indeed, both the USV and the AUV are equipped with appropriate CPU boards allowing for the execution of simple algorithms that can interpret the acquired data, as well as simplifying and compressing it.