# ENDURUNS: An Integrated and Flexible Approach for Seabed Survey Through Autonomous Mobile Vehicles
## 1 Introduction
Indeed, they are equipped with downward looking altimeter for seafloor collision avoidance purposes, but do not employ monitoring acoustic devices that need to be operated along the isobaths, keeping a constant distance from the seafloor as much as possible. These instruments are widely used on AUVs, which are able to run parallel to the ocean floor ensuring the quality of the measurements. Commonly installed sensors on board of AUVs or stand-alone fixed platforms for autonomous underwater operations of ocean mapping and monitoring are summarised in Table 1. Examples of operative contexts where AUVs are employed are summarised in Table 2, and discussed in [14, 16, 17]. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**Sensor Type** & **Use \& Description** \\ \hline \multirow{6}{*}{**Active sonar**: This class of instruments are used to measure reflected acoustic signals documenting the objects on the seabed or in the water column. & **Multibeam echo-sounders** (MBES): acoustic beams with known directions are used to estimates the backscattering of the particulate suspended in the water column and/or the back scattering of the seabed. These sensors produce a 2.5D model bathymetry and the terrain characterization. & **Reference** \\  & **Side scan sonars** (SSS): this sensor measures the acoustic surface reflectance of the seabed producing a map of the seabed’s acoustical reflectivity. & **Side scan sonars** (SSS): this sensor measures the acoustic surface reflectance of the seabed producing a map of the seabed’s acoustical reflectivity. & **Side scan sonars** (SSS): this sensor measures the acoustic surface reflectance of the seabed producing a map of the seabed’s acoustical reflectivity. & **Side scan sonars** (SSS): these systems use several pings’ simultaneously for each seabed point establishing a virtual transducer array providing better range and increased seabed resolution compared to conventional SSS. & **Single scan sonars** (SSS): those systems use several pings’ simultaneously for each seabed point establishing a virtual transducer array providing better range and increased seabed resolution compared to conventional SSS. At present, AUV-based operations generally require the support of surface vessels and are extremely complex, high-risk, and expensive [9, 17]. The use of sophisticated AUV vehicles, which can be either propelled purely via electric thrusters or based on gliding, is also expensive because they need support vessels for their deployment and recovery increasing mission costs [43]. Therefore, new developments in AUV technology are needed to improve their endurance and autonomy, especially with respect to deployment and recovery methods such to meaningfully reduce the mission costs. At the same time AUV technology should achieve noteworthy improvements in the capability to explore the ocean and inspect offshore infrastructure, in comparison to currently available state-of-the-art commercial systems. The objective of this article is to present and describe the achievements of the H2020 ENDURUNS project, grant agreement n.824348, 2018-2022, [44], which is a novel scientific and technological approach for autonomous seabed surveying in deep ocean or in coastal areas. The proposed novel approach overcome the limits of the current state of the art combining a hybrid AUV capable of moving using either thrusters or in glider mode [45], with an Unmanned Surface Vehicle (USV) equipped with satellite communication facilities to interact with land stations, assuring quasi real-time data transfer and mission control. A valuable characteristics of the proposed approach is the use of clean and renewable energy provided by photovoltaic solar panels for the USV, and hydrogen fuel cell