Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Periodic Surveillance Network (Periodic SN) <s> Due to their long endurance at sea, underwater gliders are envisioned to play a significant role in persistent maritime surveillance and monitoring. However, so far they have been mainly used for measuring sea water properties such as temperature, salinity and optical properties. CMRE is considering using gliders for different new missions such as passive acoustic monitoring, sea bottom and sea surface characterization as well as service as an underwater or surface node in a distributed heterogeneous autonomous vehicles network. This paper describes the effort made at CMRE for developing and assessing the new payloads necessary for fulfilling those new missions. Hence, a comprehensive description of acoustic, environmental characterization and communication glider payloads is given in the paper. Results obtained at-sea with the use of the different payloads are briefly mentioned and demonstrate great enhancement capability for the gliders. <s> BIB001 </s> Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Periodic Surveillance Network (Periodic SN) <s> Autonomous Underwater Vehicles (AUVs) have gained more popularity in recent years for military as well as civilian applications. One potential application of AUVs is for the purpose of undersea surveillance. As research into underwater surveillance using AUVs progresses, issues arise as to how an AUV acquires acts on, and shares information about the underwater battle space. These issues naturally touch on aspects of vehicle autonomy and underwater communications, and need to be resolved through a spiral development process that includes experiments at sea. This paper presents an implementation of swarm network communication for AUVs which is used to transfer the data, to communicate with one another to perform tasks as an intelligent group including surveillance. When a task identified from a seafloor, a single AUV could follow that task and report the whereabouts. Other vehicles in the swarm could track additional individuals, produce detailed maps of the area, detect AUV updates to a distant command post. As individual AUV leave the swarm to fulfill their assigned tasks, the swarm could autonomously reorient itself. This reduces the inspection duration and inspection cost for underwater vehicle. This paper has described the on-board signal processing including a navigation and network communication which were successfully implemented. The vehicle is designed and simulation is studied in computer analysis as per the required parameters and condition. The Simulation of swarm network communication and navigation multi-path trajectory is performed. <s> BIB002
A conclusion from the previous section (Section 4.2) is that there are several sensors that could be used for detecting multiple cable threats. A next step is the integration of these sensors with the platforms of Section 3.1 for building a surveillance network. The network could be sorted as: Periodic, if platforms move and perform surveys periodically (e.g., through USVs, AUVs and/or UUGs); or Permanent, if platforms are fixed and perform a constant survey of the underwater environment surrounding the communication cable. USVs (Table 2 ) have ranges from 22 to 5329 km. In many cases their range is greater than 1000 km, being sufficient to cover the EEZ. In the scenario described in this paper, docking stations should be available along the 13,000 km cable route for their operation along the whole cable route. USVs could carry most types of acoustic sonar, such as SBES, MBES, SSS, SAS, and OAS. USVs could also be equipped with ADCPs, cameras and oceanographic sensors. Nowadays, USVs can also support acoustic sensors like towed active and passive arrays and dipping sonars. Therefore, USVs constitute platforms that could be used in conjunction with almost any monitoring sensor for enhancing the security of underwater cables. In many cases (e.g., for anchors and fishing trawls), it is necessary to detect any potential threat when entering the monitoring zone and certainly before entering the warning zone (see Figure 7b ), as their impact could result in cable damage/failure. Therefore, it is important to monitor any suspect movements of surface vessels in the monitoring zone of the cables. It has to be noted that USVs, apart from underwater sensors, could be equipped with above-water sensors (their study is outside the scope of the current work), such as radars or electro-optical (EO)/infrared (IR) cameras capable of automatically detect, recognize and tracking surface threats (e.g., vessels). In addition, USVs could act as a first line of defense by performing inspection tasks on suspicious vessels. Nowadays, it is also possible to simultaneously command and control (C2) multiple USVs, and integrate them with tethered drones from a single C2 station  . AUVs (see Table 3 ) can be equipped with a large number of monitoring sensors, such as MBES, SSS, SBP, OAS, magnetometers, optical/cameras, and oceanographic sensors. Their main disadvantage is their limited range, which typically goes from 44 to 471 km (average 133 km). This means that their use should be mainly limited to the GCS or for specific-targeted operations in the EEZ or high seas. Docking stations could be available for AUV operations along the whole cable route. AUV advantage is that their operation depth is very high, reaching up to 6000 m. So AUVs could potentially follow bottom topography and survey the cable area at any depth. AUVs can be equipped with both downward and upward pointing sensors in order to capture big parts of the water column and reduce any dead zones. It is also possible to have swarms of AUVs for covering larger areas while performing various tasks. Each AUV can leave temporarily the swarm to fulfill their individual assigned task while the swarm could autonomously reorient itself BIB002 . Due to UUG long endurance at sea, UUGs are anticipated to be an important key player in networks involving maritime surveillance and monitoring. Their range is very high, starting from 3552 km and reaching up to 11,990 km. However, UUGs (see Table 4 ) are equipped mainly with Oceanographic sensors, i.e., payloads with very low energy consumption, which cannot be used for direct undersea surveillance. These sensors can sense anomalies that may indicate traces of an intruder but cannot directly detect the intruder. During the last few years, industrial efforts have been made to equip UUGs with additional systems like ADCPs or hydrophones (see Table 4 ) for passive acoustic monitoring (e.g., detecting marine mammals BIB001 ). Research work has also been made to develop towed arrays to be used by gliders to detect larger targets BIB001 . Table 4 includes a wave glider that operates on the sea surface. Based on its operation principle, it is a glider, but based on its operational field, it is a USV. Wave gliders can provide long endurance and be equipped with more sensors than UUGs. It has to be noted that UUGs, when operating in saw-tooth profile mode, cannot be used efficiently in conjunction with any acoustic system like SSS, SAS or MBES. It can be assumed that underwater cables are of similar nature to offshore oil and gas pipelines, although the cables operate in a much more difficult environment. Cable length is tenths of thousands of kilometers longer than that of pipelines, they are laid in much deeper waters, and they face much more unpredictable threats than pipelines. Furthermore, seemingly competing countries that may consider tape or harm each other's underwater cables, are in many cases cooperating for oil and gas exploitation. Nevertheless, some concepts related to pipeline maintenance may apply to the study announced in this document. Pipelines' maintenance is mainly preventive and much thought is given on constructing both durable and flexible systems in order to avoid regular short-time monitoring. The external inspection of pipelines involves ROVs and AUVs. This highlights that the concept for the near-future use of AUVs is to make them resident in the underwater environment for periods of months  . Whenever there is a need to monitor, inspect or detect something, then ROVs and AUVs can act. This is a concept that can be applicable in the case of monitoring underwater cables. AUVs can be placed in strategically located areas of the seabed along the cable path. Whenever, for example, a suspicious ship is detected by surface sensors inside the monitoring zone of the cable, then AUVs could be activated and sent to monitor the underwater area near the ship for identifying its underwater activity.