Patent Publication Number: US-2013239863-A1

Title: Tethered tow body, communications apparatus and system

Description:
FIELD OF THE INVENTION 
     The invention relates generally to communications apparatuses, and in particular to a tethered communications apparatus that provides submerged vehicles with communications to the outside world. 
     BACKGROUND 
     Submerged vehicles, such as unmanned underwater vehicles (UUVs), are used in a variety of military applications, for example, surveillance, reconnaissance, navigation, and defense. When these vehicles are submerged, however, navigation and communication are difficult. Inertial navigation systems, such as gyroscopes or other computer and motion sensors that track position, orientation and velocity can be used, but these systems are subject to drift the longer they remain below the water surface. Highly accurate global positioning system (GPS) navigation systems and high-bandwidth radio frequency (RF) communications links are not directly available to submerged vehicles due to the rapid attenuation of radio frequency energy by water. Thus, submerged vehicles are limited to communicating with low bandwidth acoustics or wiring back to another vessel or shore platform. 
     Prior art communications devices for submerged vehicles, such as the device disclosed in U.S. Pat. No. 5,379,034, rely primarily on buoyancy to float an antenna to the water surface. The tow angle β of a tethered cable, calculated as the angle between the cable and the direction the submerged vehicle is traveling, is affected by the speed of the submerged vehicle. The faster the vehicle travels, the smaller the tow angle β, resulting in the tethered cable being pulled straight back and the communications device never reaching the water surface. The slower the submerged vehicle travels, the larger the tow angle β, resulting in the tethered cable drifting straight up and the communications device drifting to the surface. Prior art devices that rely primarily on buoyancy require the submerged vehicle to be stationary or to be traveling at significantly reduced speed in, order for the antenna to drift to the surface. Thus, submerged vehicles using these prior art devices cannot simultaneously communicate and travel at operational speed. Other prior art systems, such as those disclosed in U.S. Pat. Nos. 3,972,046 and 7,448,339, rely on an intermediary float tethered to an underwater vehicle and a surface float having an antenna. These prior art systems operate at very limited speed ranges because the surface floats would be pulled underwater at all but the slowest speeds. Additionally, the intermediary floats of these prior art systems are towed underwater, thereby increasing the probability of entanglement and drag when deployed. Still other prior art arrangements, including the antenna arrangement disclosed in U.S. Pat. No. 6,058,874, do not provide for conformal stowage in which a tethered communications device can be stowed within and be quickly deployed from an underwater vehicle, thereby, minimizing drag and the likelihood of vehicle entanglement during operation. 
     Accordingly, there is a need and desire for an efficiently deployable tethered communications apparatus and system for providing submerged vehicles with bi-directional, high data rate communications to a nearby vessel or shore platform as well as GPS coordinate data for precise navigation while traveling at operational speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a UUV system in accordance with an embodiment described herein. 
         FIG. 2  is a diagram of a communications apparatus in accordance with an embodiment described herein. 
         FIG. 3  is a partial internal view of a communications apparatus in accordance with an embodiment described herein. 
         FIGS. 4A and 4B  are respectively a front view diagram and a bottom view diagram of a tow body in accordance with an embodiment described herein. 
         FIGS. 5A and 5B  are respectively a front view diagram and a bottom view diagram of a tow body in accordance with another embodiment described herein. 
         FIG. 6  is a schematic diagram of an electronics assembly of a communications apparatus in accordance with an embodiment described herein. 
         FIG. 7A  is a diagram of a reeling assembly in accordance with an embodiment described herein. 
         FIG. 7B  is a diagram of a reeling assembly mounted inside a UUV system in accordance with an embodiment described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and illustrate specific embodiments that may be practiced. In the drawings, like reference numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice them, and it is to be understood that structural and logical changes may be made. Sequences of steps are not limited to those set forth herein and may be changed or reordered, with the exception of steps necessarily occurring in a certain order. 
     The problem of providing a submerged vehicle with above-the-surface communications to a nearby vessel, shore platform, or satellite while traveling at operating speed is solved by an efficiently deployable tethered tow body having a hydrodynamic and buoyant hull body and incorporating a lift-generating wing that provides hydrodynamic lift to efficiently lift the tow body containing antennas and other communications devices to the surface. The tow body allows for stable operation during underwater tow, surface tow, and transitions between underwater tow and surface tow. 
     Disclosed embodiments include communications apparatuses encompassing the principles of the tethered tow body, as well as various underwater systems that incorporate a tethered tow body or communications apparatus for establishing communications with a nearby vessel, shore platform, or satellite. 
     The invention may be used to particular advantage in the context of submerged vehicles. Therefore, the following example embodiments are disclosed in the context of UUV systems. However, it will be appreciated that those skilled in the art will be able to incorporate the invention into numerous other alternative systems that, while not shown or described herein, embody the principles of the invention. 
       FIG. 1  shows an underwater vehicle system  100  in accordance with an embodiment described herein. UUV  170  may be, for example, a modified ANT Glider Eyak  01  developed by Alaska Native Technologies, LLC or a modified Remus  600  developed by Hydroid, Inc. UUV  170  is modified to integrate with a communications apparatus  110  having a tether  130  connected on one end to a reeling assembly  150  within UUV  170  and on the other end to tow body  120 . UUV  170  has propulsor  180  at the aft end and a tow body stowage area  160  cut out on the top surface of UUV  170 . The tow body stowage area  160  has a length and width equal to the length and width of tow body  120 , and a depth sufficient for tow body  120  to fit entirely within UUV  170 . 
     In accordance with an advantageous feature of this disclosed embodiment, tow body  120  is deployed from the tow body stowage area  160  of UUV  170 , thus, enabling UUV  170  to repeatedly establish communications with the outside world in a quick and efficient manner. Communications apparatus  110 , comprising hydrodynamic tow body  120  and tether  130  connecting tow body  120  to reeling assembly  150 , can be completely stowed inside the tow body stowage area  160  to achieve seamless integration within UUV  170 . Communications apparatus  110  is positively buoyant enabling it to float to the surface using hydrostatic force when UUV  170  is stationary. If desired, vehicle guidance docking plates can be installed in the tow body stowage area  160  to help align tow body  120  inside UUV  170 . Seamless integration of communications apparatus  110  has the effect of minimizing drag and minimizing the possibility of entanglement as UUV  170  moves underwater. The communications apparatus  110  and reeling assembly  150  are made so that they are collectively neutrally buoyant and, therefore, will not impact the depth control of UUV  170  when stowed or deployed. 
     The present inventors have discovered that tow bodies that combine a lift-generating wing and a stable body structure achieve good hydrodynamic performance. Therefore, in accordance with the embodiments described herein, tow body  120  has a lifting wing mounted on top of a tow body structure and, optionally, at least one side float arranged on either side of the body structure for providing buoyancy at the outer edges of lifting wing and to stabilize tow body  120  during underwater tow. 
     In accordance with an advantageous feature of the disclosed embodiment, tow body  120  is hydrodynamically clean in that it is designed to minimize drag during underwater tow, to achieve good hydrodynamic performance during surface tow, and to transition stably between underwater tow and surface tow. Tow body  120  is able to smoothly transition from underwater tow to being towed at least partially above the surface during communication. Additionally, tow body  120  is able to smoothly transition from surface tow to being towed below the surface during retrieval. 
       FIG. 2  is a diagram of a communications apparatus  110  in accordance with an embodiment described herein. Communications apparatus  110  has a hydrodynamic tow body  120  with a mounted antenna  250  and a tether  130  attaching tow body  120  to reeling assembly  150 . Tether  130  is comprised of tow cable  230  and bridles  270 . 
     In the example embodiment of  FIG. 2 , tow body structure  210  is multi-sectional with an elongated center hull body  235 , an aft section  240  and a fore section  245 . Bulkheads are optionally placed at both ends of center hull body  235  to separate center hull body  235  from aft section  240  and fore section  245 . 
     Lifting wing  200  is mounted on top of center hull body  235  to provide hydrodynamic lift for lifting an underwater tow body  120  to at least partially above the water surface. Lifting wing  200  is at least as long as the length of tow body structure  210  and is wider than the width of tow body structure  210 , preferably, not greater than its length. The width of lifting wing  200 , however, is constrained by the width of UUV  170 . According to the example embodiment of  FIG. 2 , lifting wing  200  curves outward, forming a convex surface. Preferably, lifting wing  200  also has a convex fore end, which reduces drag as tow body  120  is pulled through water. 
     According to the example embodiment of  FIG. 2 , center hull body  235  has a cylindrical shape while the aft section  240  and fore section  245  are cone shaped. Aft section  240  and fore section  245  of tow body structure  210  have convex surfaces and are seamlessly integrated with center hull body  235 . Preferably, aft section  240  is slightly longer than fore section  245 . Vent holes  260  are used for cooling an electronics assembly located inside the center hull body  235 . 
     Tow body structure  210  of the disclosed embodiment is made of polycarbonate, however, tow body structure  210  can be made of any other non-metallic material having positive buoyancy, such as, for example, carbonfiber, plastic, and fiberglass. The outer hull of tow body structure  210  is preferably coated with a fiberglass resin or polyester coating to provide a low drag surface. 
     Vertical stabilizer  255  extends from the bottom of tow body structure  210 , preferably the bottom of aft cone  240 , to keep tow body  120  substantially parallel with the water surface. If desired, vertical stabilizer  255  is mounted to tow body structure  210  through a keel slot  265  built on the underside of aft cone  240 . In an advantageous feature of this embodiment, vertical stabilizer  255  is retractable during stowage to minimize the size of tow body stowage area  160  within UUV  170 . Vertical stabilizer  255  can be made retractable using a spring or tether  130  can be used to extend vertical stabilizer  255  during deployment of tow body  120 . Upon retrieval, vertical stabilizer  255  will be forced inside aft cone  240  by the rear edge of tow body stowage area  160 . 
     According to the example embodiment of  FIG. 2 , communications apparatus  110  can provide UUV  170  with high-bandwidth RF communications link and GPS coordinate data. Antenna  250  is a 802.11 antenna providing bi-directional, high speed data rate of at least 1 Mbps at a distance of at least 1 km. Antenna  250  is preferably small for taking up the least amount of space in UUV  170  and for being less likely to be noticed when deployed above the surface. Antenna  250  should also be omnidirectional to allow it to change position relative to a remote receiver. 
     Antenna  250  should be as vertical as possible during surface tow so as to provide optimum communications to a nearby vessel or shore platform. In the disclosed embodiment, antenna  250  is spring mounted to lifting wing  200  to keep antenna  250  substantially upright during surface tow. Antenna  250  is preferably positioned to pivot slightly to the rear of tow body  120  to reduce the possibility of breakage if tow body  120  encounters an obstacle during tow. According to another advantageous feature of this embodiment, antenna  250  folds down during retrieval and stowage to reduce drag. It will be appreciated by those skilled in the art that an electro-mechanical device can be used to raise and fold the spring mounted antenna  250 . Alternatively, a gimbaled antenna mount can be used to maintain correct antenna position. Those skilled in the art will appreciate that numerous other ways can be devised to keep antenna  250  substantially vertical during surface tow. 
       FIG. 3  is a partial internal view of communications apparatus  110  in accordance with an embodiment described herein. Center hull body  235  is at least partially hollow. Aft bulkhead  310  separates aft section  240  from center hull body  235  and creates a watertight enclosure inside hull body  235  for storage of electronics assembly  320 . If desired, tow body structure  210  can optionally include a fore bulkhead that separates fore section  245  from center hull body  235 . Particular embodiments may optionally fill the inside of hollow hull body  235 , aft section  240 , and fore section  245  with foam  550  to achieve positive buoyancy. Fore section  245  has a convex surface with a V-shaped upper edge  540  for deflecting water as tow body  120  is towed on a water surface. 
     In accordance with an advantageous feature of the disclosed embodiment, the watertight chamber of center hull body  235  preferably encloses all electronics required for communications apparatus  110  except for antenna  250 . Communications apparatus  110  may be rapidly integrated with many different types of UUV systems since UUV systems need only be able to send and receive data over standard Ethernet connection using standard internet protocol (IP) network protocols. 
     Heat sink plate  300  is preferably composed of aluminum and welded perpendicularly to aft bulkhead  310 . Electronics assembly  320  is mounted on both sides of heat sink plate  300 . Electronics assembly  320  is connected to 802.11 antenna  250  and a watertight connector  330  for tow cable  230 . Alternatively, electronics assembly  320  may be potted inside hull body  235 . 
     The present inventors have discovered that high signal attenuation, increased power Consumption, and difficulty in detecting when an antenna has reached the surface result from locating only the 802.11 and GPS antennas on tow body  120  such that the two antennas are connected to radio receivers onboard UUV  170  via a RF coaxial cable. Therefore, UUV  170 , preferably, incorporates a power over Ethernet module that co-locates radio electronics and antennas for both 802.11 and GPS frequency bands. Co-location of the radio electronics and antennas allows for a thin tow cable to be used for communications apparatus  110  and minimizes signal attenuation from the use of tow cable  230 . 
     Tow cable  230  transfers both power and data between tow body electronics assembly  320  and UUV  170 . The present inventors have found that using a coaxial cable to send RF signals to a surface antenna would significantly increase the overall weight of communications apparatus  110 . At low operational speeds, tow body  120  would be unable to lift a heavy cable, thereby increasing the likelihood of entanglement and significantly reducing the operational range of UUV  170 . Thus, tow cable  230  is preferably a fiber optic cable. Using a polypropylene jacket, fiber optic cable  230  can be made slightly buoyant, thereby, reducing the possibility of cable entanglement. If UUV  170  is stationary, a buoyant fiber optic cable  230  can reach the surface if the deployed cable scope is greater than the depth. 
       FIGS. 4A and 4B  are respectively a front view diagram and a bottom view diagram of an alternative embodiment of tow body  120  having a hydrodynamic boat hull shaped body structure  410 . An optional stabilizing side float  420  and at least one bridle attachment bar  220  each having at least one bridle attachment point are mounted onto a lifting wing  200  on either side of hull body  410 . Lifting wing  200  is centered on and mounted on top of hull body  410 . Those skilled in the art will appreciate that electronic assembly  320  can also be mounted inside boat hull shaped body structure  410 . 
     Another alternative embodiment of tow body  120  is illustrated in  FIGS. 5A and 5B , which respectively depicts front and bottom views of tow body  120  having a hydrodynamic submarine shaped body structure  510 . It will be appreciated by those skilled in the art that tow body  120  can have other alternative hydrodynamic and buoyant tow body structures. 
     While the embodiment of  FIG. 3  is described with regard to multi-sectional tow body  120  of  FIG. 2 , it will be appreciated by those skilled in the art that the tow bodies disclosed in  FIGS. 4A ,  5 A, and other hydrodynamic tow bodies may be appropriately modified to embody the principles of the invention described herein. 
       FIG. 6  is a schematic diagram of electronics assembly  320  in accordance with an embodiment described herein. Electronics assembly  320  contains an embedded processor  650  that relays data to and from UUV  170  via fiber optic cable  230 . Embedded processor  650  contains an onboard 802.11 radio receiver chip  660 , RS232-level serial interface  670  for GPS connectivity, 10/100 Ethernet LAN port  680  for tow cable  230 , digital input/output  690 , and sufficient CPU and memory for routing data at up to 54 Mbps between the Ethernet LAN port and the Wi-Fi interface of antenna  250 . Antenna  250  is connected to 802.11 transceiver  660  onboard embedded processor  650 . In addition to the 802.11 and GPS antennas, embedded processor  650  can be configured to capture other types of data, such as, for example, images with an onboard camera. Electronics assembly  320  also includes a float switch  610  connected to the digital input/output  690  of embedded processor  650 , a DC power converter  630 , and an Ethernet to fiber optic converter  640 . 
     The example embodiment of  FIG. 6  employs a Compulab CM-X270 computer-on-module board with a PXA270ARM processor to meet all of the above requirements, but other embedded processors that consume little power and space can be used. The Compulab CM-X270 board measures only 66×44×7 mm and consumes 2W at maximum processor load. 
     An integrated GPS antenna and receiver module  620  is connected to a RS232-level serial interface  670 . The integrated GPS antenna and receiver module  620  can be, for example, Mighty GPS&#39;s all-in-one BG-320RGT GPS module. The RS232-level serial interface  670  output is connected directly to the CM-X270 serial port of embedded processor  650 . Tow body structure  210  is made of a non-metallic material and, thus, will not interfere with satellite reception. 
     Embedded processor  650  preferably supports the open source embedded Linux operating system, but any other operating system supported by embedded processor  650  may be used. The operating system on embedded processor  650  runs at least three software modules that together provide the required functionality for communications apparatus  110 . 
     First, the disclosed embodiment includes network layer packet routing software to forward IP packets between UUV  170  and, for example, a remote surface receiver. The routing software should not buffer packets due to intermittent or slow wireless connections, for example, because buffering should be handled by a TCP control flow set up by UUV  170  or the remote surface receiver. 
     Second, embedded processor  650  includes a software module for, supporting GPS navigation or other similar type platforms as known in the art. This software module receives, parses and decodes serial GPS NMEA 0813 messages from integrated GPS antenna and receiver module  620 . The decoded GPS information would be collected and sent periodically to UUV  170  as, for example, a TCP, UDP, XML, or CORBA message through Ethernet LAN port  680 . 
     Third, embedded processor  650  includes a software module for supporting communications between UUV  170  and communications apparatus  110 . This software module sends status information to and receives command and control messages from UUV  170 . Status information from embedded processor  650  includes, for example, wireless signal strength, available wireless networks, status of float switch  610  and GPS receiver  620 , and other system information. Command messages from UUV  170  includes, for example, control over the transmit power, configured wireless network, encryption parameters, and other network and system configurations. 
     If desired, an optional bi-directional RF amplifier  600  can be added between antenna  250  and the onboard 802.11 radio receiver  620  to improve link reliability and boost transmit power. The disclosed embodiment uses a 2.4 GHz bi-directional RF amplifier, such as, for example, the 2400CAE 2.4 GHz bi-directional amplifier manufactured by RF Linx, which provides 1 W of transmit power and 20 dB of receive gain. Amplifier  600  is preferably mounted directly on heat sink plate  300  for improved heat dissipation. 
     In accordance with another illustrative feature of the disclosed embodiment, communications apparatus  110  has seawater cooling electronics capability. Referring to  FIG. 2 , vent holes  260  in aft cone  240  provide a constant supply of cooling water to heat sink plate  300 . Electronics assembly  320  is ventilated with cooling water entering through the vent holes  260  located on aft section  240  and exiting through keel slot  265  on the underside of aft section  240 . Alternatively, if electronics assembly  320  is potted inside hull body  235 , amplifier  600  should be mounted at the lowest point of tow body structure  210  so that seawater can be used for heat dissipation. 
       FIG. 7A  is a diagram of a reeling assembly  150  and  FIG. 7B  is a diagram of the reeling assembly  150  mounted inside UUV  170  in accordance with an embodiment described herein. Reeling assembly  150  includes a waterproof motor housing  700  enclosing a direct current (DC) motor with an attached spur gearbox (not shown), preferably having a 15:1 gear ratio, that is powered by a waterproof cable connected to a power supply and control switch in UUV  170 . Control switch directs the power to the motor to control reeling tow body  120  in and out of tow body stowage area  160 . Attached to the DC motor is a cable drum  710  large enough to accommodate the length of tether  130 . Cable drum  710  sits inside a reel frame. If desired, a level wind can be mounted on cable drum  710  to prevent tether  130  from jamming during reeling of tow body  120 . 
     Reeling assembly  150  provides tension for holding stowed tow body  120  inside UUV  170 . If desired, an inner cover  740  which conforms to the bottom of tow body  120  can be mounted over reeling assembly  150  to streamline the tow body stowage area  160  and, thereby reduce drag. A hole in the cover  740  serves as a fairlead in directing tether  130  onto the drum  710 . Once tow body  120  has reached the surface, float switch  610  of electronics assembly  320  is triggered to signal the DC motor to stop. High-speed communication to another vessel or shore platform and acquisition of GPS satellite data can then commence. 
     UUV  170  can provide all the power required to run electronics assembly  320  except for a small battery that runs a clock inside electronics assembly  320 . Fiber optic cable  230  preferably contains two 24 American Wire Gauge (AWG) conductors for transporting power to tow body  120  from UUV  170  and a fiber for transporting data. A single 24 gauge wire provides almost 7 W of power at 12 V. The present inventors found that electronics assembly  320  would require approximately 2 W to 12 W depending on the RF amplifier used. If needed, additional power can be obtained by using a DC-DC converter  630  to step down the transmitted voltage at tow body  120 . 
     Referring to  FIG. 1 , tow body  120  can be lifted to the surface within a UUV operational speed ranging from stationary to approximately 5 knots. After deploying to the water surface, tow body  120  should sit high on the water so that antenna  250  remains vertical and out of the water for better reception. Furthermore, tow body  120  must be stable at both planing and displacement speeds of up to approximately 5 knots for a prolonged period of time. The present inventors have discovered that the optimal attack angle α for tow body  120 , measured relative to the water surface, is approximately 10 to 20 degrees. Tow body  120  can be towed smoothly on the surface within this range for attack angle α. 
     Careful consideration must be given to selecting optimum location(s) to attach bridle(s)  270  to tow body  120  so that a sufficient lifting force is created to lift tow body  120  to the surface and the attack angle α is approximately 10 to 20 degrees when tow body  120  is pulled across the surface. The bridle attachment point(s) can be located on bridle attachment bars  220 , vertical stabilizer  255 , or at other locations including, for example, the tow body&#39;s  120  center of pressure and center of buoyancy. The present inventors have discovered that a two-point bridle attachment provided a stable configuration and low drag during underwater tow, surface tow, and transitions to and from the surface. The two bridle attachment points are located at the fore and aft ends of bridle attachment bar  220  extending from the bottom of tow body structure  210 . Alternatively, the aft end attachment point can be located on vertical stabilizer  255  below the center of buoyancy, as shown in  FIG. 2 . By locating an attachment point on vertical stabilizer  255 , bridle  270  can be used to extend vertical stabilizer  255  during deployment of tow body  120 . It will be appreciated by those skilled in the art that other bridle attachment configurations may be employed, such as, for example, a single point attachment near the middle of bridle attachment bar  220  extending from the bottom of tow body structure  210 , or a three point bridle attachment in which two attachment points are located on either fore corner of lifting wing  200  and a third attachment point is located on vertical stabilizer  255  below the center of buoyancy. 
     The foregoing merely illustrate the principles of the invention. Although the invention may be used to particular advantage in the context of submerged vehicles, those skilled in the art will be able to incorporate the invention into other non-vehicle systems, such as submerged platforms. It will thus be appreciated that those skilled in the art will be able to devise numerous alternative arrangements that, while not shown or described herein, embody the principles of the invention and thus are within its spirit and scope.