Computer and Robot Vision Vol. 1, by R.M. Haralick and Linda G. Shapiro, Addison-Wesley, 1992, ISBN 0-201-10887-1.

Computer and Robot Vision

Underwater surveillance has traditionally been carried out by means of surface and undersea manned vessels equipped with advanced sensor systems. This approach is often costly and manpower intensive. Marine robotics is an emerging technological area that enables the development of advanced networks for underwater surveillance applications. In contrast with the use of standard assets, these advanced networks are typically composed of small, low-power, and possibly mobile robots, which have limited endurance, processing and wireless communication capabilities. When deployed in a region of interest, these robots can cooperatively form an intelligent network achieving high performance with significant features of scalability, adaptability, robustness, persistence and reliability. Such networks of robots can be the enabling technology for a wide range of applications in the maritime domain. However, they also introduce new challenges for underwater distributed sensing, data processing and analysis, autonomy and communications. The main thrust of this study is to review the underwater surveillance scenario within a framework of four research areas: (i) underwater robotics, (ii) acoustic signal processing, (iii) tracking and distributed information fusion, and (iv) underwater communications networks. Progress in each of these areas as well as future challenges is presented.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-rsn.2017.0074

Cooperative robotic networks for underwater surveillance: an overview

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Surveillance in antisubmarine warfare has traditionally been carried out by means of submarines or frigates with towed arrays. These techniques are manpower intensive. Alternative approaches have recently been suggested using distributed stationary and mobile sensors, such as autonomous underwater vehicles (AUVs). In contrast with the use of standard assets, these small, low-power, and mobile devices have limited processing and wireless communication capabilities. However, when deployed in a spatially separated network, these sensors can form an intelligent network achieving high performance with significant features of scalability, robustness, and reliability. The distributed information FUSION (DIFFUSION) strategy, in which the local information is shared among sensors, is one of the key aspects of this intelligent network. In this paper, we propose two DIFFUSION schemes, in which the information shared among sensors consists of: 1) contacts, generated by the local detection stage and 2) tracks, generated by the local tracking stage. In the first DIFFUSION scheme, contacts are combined at each nodes using the optimal Bayesian tracking based on the random finite set formulation. In the second DIFFUSION scheme, tracks are combined using the track-to-track association/fusion procedure, then a sequential decision based on the association events is exploited. A full validation of the DIFFUSION schemes is conducted by the NATO Science and Technology Organization—Center for maritime research and experimentation during the sea trials Exercise Proud Manta 2012–2013 using real data. Performance metrics of DIFFUSION and of local tracking/detection strategies are also evaluated in terms of time-on-target (ToT) and false alarm rate (FAR). We demonstrate the benefit of using DIFFUSION against the local noncooperative strategies. In particular DIFFUSION improves the level of TOT (FAR) with respect to the local tracking/detection strategies. In particular, the TOT is increased over 90%–95% while the FAR is reduced of two order of magnitude. The problem of communication failures, data not available from the collaborative AUV during certain periods of time, is also investigated. The robustness of DIFFUSION with respect to these communication failures is demonstrated, and the related performance results are reported here. In particular, with 75% of communication failures the ToT is over 90%–95% with a relatively small increase of the FAR with respect to the case of perfect communication.

Distributed Information Fusion in Multistatic Sensor Networks for Underwater Surveillance

Port-starboard ambiguity is an important issue in underwater tracking systems with anti-submarine warfare applications, especially for wireless sensor networks based upon autonomous underwater vehicles. In monostatic systems this ambiguity leads to a ghost track of the target symmetrically displaced with respect to the sensor. Removal of such artifacts is usually made by rough and heuristic approaches. In the context of Bayesian filtering approximated by means of particle filtering techniques, we show that optimal disambiguation can be pursued by deriving the full Bayesian posterior distribution of the target state. The analysis is corroborated by simulations that show the effectiveness of the particle-filtering tracking. A full validation of the approach relies upon real-world experiments conducted by the NATO Science and Technology Organization - Centre for Maritime Research and Experimentation during the sea trials Generic Littoral Interoperable Network Technology 2011 and Exercise Proud Manta 2012, results which are also reported.

Bayesian Tracking in Underwater Wireless Sensor Networks With Port-Starboard Ambiguity

The typical 70 mm diameter towed array was developed for blue‐water detection at long range and low frequencies in the 1960s. Since then, there has been a need for towed arrays that are lighter and less expensive, especially since the maturing field of autonomous vehicles has expanded the potential of such arrays. The marriage of AUVs and lightweight towed arrays is a natural progression in the development of littoral autonomous sensing networks for applications such Anti‐Submarine Warfare, marine mammals, or ambient noise measurements. In August 2007, NURC began to design and build a new thin diameter (31 mm) high‐frequency (up to 20 kHz) nested towed array for ASW purposes. An engineering at‐sea trial of the array towed by OEX AUV was performed beginning of November 2007. The flow noise level of the array while towed and the potential influence of the AUV self‐noise on the acoustic array were also measured. This paper will first describe the array design, its acquisition system and its integration on the OEX AUV. Then, the results obtained from the data analysis are presented. It is shown that the SLITA array has performance that will make it easily fit requirements of the applications previously mentioned.

SLITA: A new slim towed array for AUV applications

This article deals with the detection of low-frequency noise sources and the estimation of their direction of arrivals using an acoustic vector sensor hosted on a buoyancy Slocum glider. The sensor used is a 3-D directional hydrophone capable of acquiring both the acoustic pressure and the components of the particle velocity vector. This article presents experimental results with data collected at sea and describes the signal processing chain, including detection, direction of arrival, and clustering. The vehicle is equipped with a modem and is supported by an ultrashort baseline (USBL) located on a mother ship or gateway to achieve the navigational accuracy required for correct target localization. The surface vessel localizes the glider via the USBL and then transmits position fixes, which are used by the underwater vehicle to reset the drift error of its navigation filter. The results discussed in this article demonstrate two important achievements, i.e., an underwater glider can produce accurate bearing estimates on a target and the feasibility of the integration of an acoustic modem and a USBL positioning system on a glider. The results obtained open up the possibility of using multiple autonomous sensorized gliders integrated in intelligent surveillance robotic networks.

Passive Acoustic Signal Processing at Low Frequency With a 3-D Acoustic Vector Sensor Hosted on a Buoyancy Glider

CMRE has been evaluating the potential of autonomous networked ASW using underwater vehicles through a program of sonar signal processing, underwater communications, navigation and robotic behavior developments and at-sea decision support tested through an ambitious seagoing experimental program and demonstrated during NATO ASW exercises. In this paper the various scientific aspects of the program are briefly outlined and some recent experimental results are shown.

Autonomous networked anti-submarine warfare research and development at CMRE

This paper presents the results of a two years project led and funded by Eni from 2008 to 2010 with the aim of supporting the development and experimentation of innovative technology for environmental monitoring. The problem addressed is the precise estimate of possible altimetric variations of the seabed through long-term monitoring. The selected methodology consists in the application of repeat-track interferometry to high-resolution, high-frequency sonar data collected from an AUV during repeated surveys of a seafloor area of interest. The paper describes the experimental measurements conducted at sea, the SAS and interferometry methodologies developed, and the results obtained on artificial objects sitting on the seabed. The quality of the achieved focusing is analyzed. The achieved repeat-pass SAS interferograms are shown and analyzed. The coherence along time of the particular kind of seabed (silty sand) characterizing the experimental area is presented and the utility of artificial reflectors for long-term SAS interferometry is discussed.

SAS multipass interferometry for monitoring seabed deformation using a high-frequency imaging sonar

For the first time at CMRE, an acoustic vector sensor, namely the 3D GeoSpectrum M20-040, has been integrated into one of CMRE Slocum buoyancy gliders. This sensor is able to provide directionality in the band from few Hz to 3 kHz, despite its limited size. The sensor beampattern was first measured in a water tank. The glider has been equipped for the first time also with an acoustic modem, able to communicate with a USBL system deployed from a mother ship or a gateway buoy. Through the USBL, a Command and Control Station can get R/T measurements of the glider position and receive its detection alarms during the underwater missions, without need for the glider to surface. First at-sea tests of navigation, underwater communications and acoustic data collection were conducted during REP18-Med trials (Palmaria Island, Italy, Oct. 2018). Evaluating the smoothness of the glider navigation is fundamental to understand whether a glider is suitable to conduct passive survey with a sensor sensitive to acceleration. The USBL position measurements were compared to the estimate achieved in post-processing by filtering the proprioceptive data collected on-board the glider, corrected with the available GPS fixes. The paper provides a detailed description of the system, along with the discussion of preliminary water-tank and at-sea results.

Stefano Biagini

Papers

SLITA: A new slim towed array for AUV applications

Passive Acoustic Signal Processing at Low Frequency With a 3-D Acoustic Vector Sensor Hosted on a Buoyancy Glider

Autonomous networked anti-submarine warfare research and development at CMRE

SAS multipass interferometry for monitoring seabed deformation using a high-frequency imaging sonar

A buoyancy glider equipped with a tri-dimensional acoustic vector sensor for real-time underwater passive acoustic monitoring at low frequency