Design and Testing of an Inertial System for the Measurement of Sea waves Based on Onboard Observations

Nowadays, sea wave measurements are mainly carried out locally by dedicated buoys (moored or drifting) [1]. Dedicated wave buoys are typically constructed with a spherical geometry and have small size; due to their design they play an effective role in following the ocean circulation and surface topography variation along time. Meteorological buoy, instead, can measure weather parameters also through various sensors, including wave and wind sensors, thermometers, GNSS receiver and others. Concerning waves, raw signals may be processed on board to obtain the sea state and directional spectra measured in the buoy mooring location. Buoys rely on a communication system, or radio transmission, to send data to onshore monitoring stations. Such transmissions affect the buoy power consumption and of course data transmission is limited by the available bandwidth. For these reasons usually the buoy transmits data packages for a certain amount of time and then they turn in stand by for a period. This procedure even if advantageous for power management, may present some issues when sea state is rapidly changing. In such conditions a continuous data transmission would improve sea state estimation and forecast of met-ocean conditions. Moreover, often rapidly changing conditions present rather large spatial variations, so a close buoys distribution would be required for a suitable sea monitoring for example in front of critical coastal areas. In this scenario the National Recovery Program, PNRR, has founded the Robotics and AI for Socio-economic Empowerment, RAISE, innovation ecosystem. One of RAISE areas regards ports activities and includes a research project regarding nowcasting and forecasting of met-ocean conditions. Main goals are the monitoring and the prevention of risks for navigation and coastal environments by suitable protection technologies. To pursue these goals, several research activities are foreseen, including the development of a new sea wave measurement method, of special interest here, which could overcome some of the issues connected to buoy-based wave measurements. The research proposal is to consider small vessel navigating near the harbor as possible moving measurement system to monitor waves and sea state. This can be possible if the vessel is properly instrumented and it is possible to estimate the sea state parameters from the vessel response to the sea waves, including possibly when they are navigating [2]. In this sense often sensors onboard are used to measure the attitude and heave for the compensation of vessel wave motion especially in severe sea conditions [3]. On the other hand, there are only few reported experimental cases in literature in which the sea state is extracted from vessel motion [4]. The idea of this project is to implement a wave measurement system onboard, that consists of a high-level Inertial Measurement Unit, IMU, integrated with a GNSS antenna, to measure vessel motion and orientation. Then such data will be used offline, in this first stage, to estimate sea state parameters. The project is in collaboration with a geomatics laboratory that will introduce onboard photogrammetric system to estimate sea motion. In this case the inertial data will be used directly to compensate for the vessel/camera motion to obtain an absolute sea motion measurement. To validate the proposed approach, a reference buoy measurement system will be available with a modified signal processing and data transmission firmware. Buoy raw data will be available continuously and processed data describing sea state and direction will be available according to the standard protocols. So, the instrumented vessel will stay in proximity of the buoy for a comparison of reference and vessel data, then it will navigate in the same sea region to identify and discriminate the dynamical effects due to navigation.