Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Synthetic Aperture Sonar (SAS) <s> This paper introduces traditional techniques used on Submarine Optical Fiber Cable Detection firstly, and puts forward a new method — Synthetic Aperture Sonar (SAS) Detection — aiming at solving the shortages and limitations of traditional techniques. Then the elements and characteristics of the new method are expatiated and the practicability of it is analyzed. <s> BIB001 </s> Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Synthetic Aperture Sonar (SAS) <s> Introduction - Why Submarine Cables? Douglas Burnett, Tara Davenport, Robert Beckman PART I: BACKGROUND Chapter 1 - The Development of Submarine Cables Stewart Ash Chapter 2 - The Submarine Cable Industry: How Does it Work? Mick Green PART II: INTERNATIONAL LAW ON SUBMARINE CABLES Chapter 3 - Overview of the International Legal Regime Governing Submarine Cables Douglas Burnett, Tara Davenport, Robert Beckman PART III: CABLE OPERATIONS - LAW AND PRACTICE Chapter 4 - The Planning and Surveying of Submarine Cable Routes Graham Evans, Monique Page Chapter 5 - The Manufacture and Laying of Submarine Cables Keith Ford-Ramsden, Tara Davenport Chapter 6 - Submarine Cable Repair and Maintenance Keith Ford-Ramsden, Douglas Burnett Chapter 7 - The Relationship between Submarine Cables and the Marine Environment Lionel Carter Chapter 8 - Out-of-Service Submarine Cables Douglas Burnett PART IV: PROTECTING CABLESHIPS AND SUBMARINE CABLES Chapter 9 - Protecting Cableships Engaged in Cable Operations Mick Green, Douglas Burnett Chapter 10 - Submarine Cables and Natural Hazards Lionel Carter Chapter 11 - Protecting Submarine Cables from Competing Uses Bob Wargo, Tara Davenport Chapter 12 - Protecting Submarine Cables from Intentional Damage: The Security Gap Robert Beckman PART V: SPECIAL PURPOSE SUBMARINE CABLES Chapter 13 - Power Cables Malcolm Eccles, Joska Ferencz, Douglas Burnett Chapter 14 - Marine Scientific Research Cables Lionel Carter, Alfred H.A. Soons Chapter 15 - Military Cables J. Ashley Roach Chapter 16 - Submarine Cables and Offshore Energy Wayne Nielsen, Tara Davenport) PART VI: APPENDICES AND KEYWORD INDEX Appendix 1 - Timeline of the Submarine Cable Industry Appendix 2 - Overview of the Major Submarine System Suppliers (1850 -2012) Appendix 3 - Excerpts of Most Relevant Treaty Provisions Keyword Index <s> BIB002
Classical beam-forming has two main limitations: the spatial resolution is not homogeneous, and the resolution may become too low with increasing range. The synthetic aperture sonar (SAS) tries to address these limitations and improve the along-track resolution. The principle of SAS is to ensonify with several pings the same seabed area while the platform carrying the SAS is moving. The synthetic array is formed by the pings that have the image position within the beam-width. The coherent reorganization and combination of the signals produces a synthetic aperture image with improved along-track resolution compared to the conventional SSS. SAS can be applied both for acoustic imaging and bathymetry (lower resolution). The main limitation of SAS is that all data corrections have to be applied almost perfectly, otherwise the performance decrease will be high. High-resolution SAS is used for detecting mines [35] . It can also be used to detect underwater communication cables that lay on the seabed BIB001 . As happened for SSS, SAS can be used either for monitoring the status of an underwater cable or for surveying the underwater environment (including seabed) around the cable against objects / intruders that may harm the cables. Table 8 presents the main specifications of some identified commercial SASs. The sub-bottom profiler (SBP) operates in low frequencies. Its signal can penetrate the seabed and give an image of the upper seabed layers. Its working principle is not based on the backscatter (like the sonars used for seabed mapping, e.g., MBES). It is based on the reflection of a specular echo instead. SBP records echoes from the interfaces between the sediment layers below the seabed. At each interface, one part of the incident energy is reflected towards the surface, while the other part is transmitted to deeper layers. Since conventional SBPs do not always achieve good lateral resolution, a parametric array of SBPs is commonly used, especially for the detection of shallow buried objects (e.g., pipelines, cables, mines). Its operation is based on non-linear acoustic techniques for achieving a narrow directivity pattern at low frequency. This has two advantages: (a) wide bandwidth; and (b) frequency independent beam pattern. SBPs are used to map the sediments layers when determining the cable route and especially in the case where cable burial is required BIB002 . Focusing on this case of study, the surveillance of the underwater environment where the cable lies, SBPs can be used to detect anomalies/changes in the bottom properties affecting buried underwater communication cables  . Table 9 presents the main specifications of some identified commercial SBPs.