Abstract
Abstract. During underwater investigations, whatever the mission objective and the type of vehicle, obstacles detection and avoidance are essential tasks. They can either represent a target of interest that is the object of the mission or, on the contrary, represent obstacles that can hinder or affect the navigation of the vehicle. The underwater optical cameras that are usually fitted to underwater vehicles only offer a narrow field of view. The absorption of electromagnetic waves in the first few meters and the diffusion of light by the particles limit the use of these sensors to only a few meters range. The use of acoustic sensors, such as the forward looking sonar (FLS), is then necessary to enlarge the volume in which a target can be detected during the progression of the vehicle. Traditionally, sonars featured mechanical rotating parts, but lately bidirectional forward looking sonar, which directly produces a 2D image of the area, are becoming more and more common. Although these sonars can operate at frequency higher than 1MHz, their spatial resolution remains much lower if compared to current optical sensors and can be insufficient to identify and characterize a target. The combination of these two sensors in an operational scenario is essential to take advantage of each technology. In this paper we describe a low cost, multi-sensor, underwater survey solution for the identification, tracking, and 3D mapping of targets. After a description of the architecture of the opto-acoustics data acquisition and processing platform, we will focus on the calibration of the rigid transformation between the two sensors.
Highlights
In underwater surveying we continuously deal with the antagonism between the range and resolution of the sensors used
The performance of optical sensors is greatly affected by the level of turbidity and their measurement capacity are highly dependent on a combination of spatial, temporal and climatic factors that require a high degree of flexibility
The calibration procedure we propose requires the identification of common features between the optical and acoustic sensors and consists of the following steps
Summary
In underwater surveying we continuously deal with the antagonism between the range and resolution of the sensors used. Search, mapping identification and reconnaissance of underwater targets are often carried out by multiple vectors and at different times This means that the position of the target, in a continuously changing environment without stable references, can be highly uncertain and requires an extensive search when the target needs to be revisited (Mari et al, 2017). The targets, once identified by the ”long range” acoustic sensor, can be tracked to help the pilot of the remotely operated vehicle to get close to the object up to an acceptable distance for a high-resolution optical survey. This is a common procedure in underwater reconnaissance and mapping where a coarse-to-fine resolution approach is adopted car-
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