Abstract:
A device and method for communicating from an underwater vehicle by  surfag an antenna in a towed buoy without surfacing the underwater vehicle. The underwater vehicle has control circuitry and sensors allowing the control circuitry to receive data from sensors measuring velocity, acceleration, distance to bottom, depth of vehicle, and ambient acoustic noise. The buoy is towed behind the vehicle on a tether having insulated wires therethrough which link the antenna to the vehicle. The buoy is positively buoyant.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and use by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to an apparatus and a method for using radio communication from a submerged underwater vehicle and more particularly to an apparatus and method for briefly surfacing an antenna towed behind an underwater vehicle. 
     (2) Description of the Prior Art 
     The characteristics of many radio frequencies do not allow communication through water from a submerged underwater vehicle. Extremely low frequency radio bands can be used for submarine communications; however, these bands are limited by the antenna structure required and the bandwidth necessary to transmit information. For this reason, submarines or other submerged craft must either surface or deploy an antenna above the ocean communicate using conventional radio frequency bands. Stealth the primary asset of submerged craft, and it is often impractical for the submerged craft to surface to communicate. Thus it is well known in the art to deploy a positively buoyant tethered buoy to carry an antenna to the surface. 
     Surfacing and deploying a buoy are impractical for small underwater craft such as underwater vehicles and long range mobile mines. Nevertheless, these craft must surface on occasion to obtain navigational information from the Global Positioning System (GPS) and to communicate with the crafts&#39; base of operations. In order to determine its coordinates, the underwater vehicle receives a code from a GPS satellite which the vehicle uses to determine its coordinates. Depending on the azimuth of the satellite, Global Positioning System data can be obtained in about one minute; however, the underwater device is subject to detection while it is above the surface. Furthermore, surfacing and submerging the underwater vehicle requires that the underwater device contain a large variable buoyancy system to allow the communications portion of the vehicle to break the surface. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general purpose and object of the present invention to provide an apparatus and method for radio frequency communication by an underwater vehicle. 
     It is a further object that such apparatus and method allow the underwater vehicle to remain below the surface while the vehicle is in communication. 
     Another object is that such apparatus and method be applicable to a small underwater vehicle communicating with a station above the surface or obtaining global positioning information. 
     These objects are accomplished with the present invention by providing a system comprising an underwater vehicle, a tow cable and a towed antenna buoy. The underwater vehicle has control circuitry and sensors allowing the vehicle to surface the towed buoy without surfacing the vehicle and thereby exposing the vehicle to detection. The control circuitry receives data from sensors measuring velocity, acceleration, distance to bottom, depth of vehicle, and ambient acoustic noise. The buoy is towed behind the vehicle on a stress bearing tether cable having insulated wires passing therethrough which link the buoy antenna to the vehicle. The buoy is positively buoyant and the underwater vehicle can be either positively or negatively buoyant. 
     If the underwater vehicle is positively buoyant, the vehicle first climbs to a preassigned depth. The vehicle reduces its speed to the minimum possible. At this minimum speed the tethered buoy is allowed to drift to the surface. Once the buoy is at the surface the antenna can communicate with a base or receive information from a GPS satellite. Meanwhile, the vehicle is floating toward the surface. When the vehicle reaches a minimum depth, it turns on its propulsor and dives to avoid surfacing. The tether provided is selected to be icing enough to allow the buoy to acquire a Global Positioning System fix or communicate with the base of operations before the vehicle floats to the minimum depth. 
     If the underwater vehicle is negatively buoyant, the vehicle first climbs to a preassigned depth. The vehicle reduces its speed to the minimum possible. At this minimum speed the tethered buoy drifts to the surface. At the surface the buoy obtains coordinates from the Global Positioning System or communicates with the vehicle&#39;s base. Meanwhile, the vehicle is slowly sinking. When the sinking vehicle reaches the length of the tether, the buoy is pulled beneath the surface. The tether cable is selected to be long enough to allow the vehicle time to communicate before pulling the buoy under the surface. The vehicle then turns on its propulsor and continues with its mission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
     FIG. 1 shows a partially cutaway pictorial view of the underwater vehicle with the towed antenna buoy of the current invention; 
     FIG. 2A shows a pictorial view of the first stage of the method of surfacing the towed antenna buoy when the underwater vehicle is positively buoyant; 
     FIG. 2B shows a pictorial view of the second stage of the method of surfacing the towed antenna buoy when the underwater vehicle is positively buoyant; 
     FIG. 2C shows a pictorial view of the third stage of the method of surfacing the towed antenna buoy when the underwater vehicle is positively buoyant; 
     FIG. 2D shows a pictorial view of the fourth stage of the method of surfacing the towed antenna buoy when the underwater vehicle is positively buoyant; 
     FIG. 3A shows the method of surfacing the towed antenna buoy when the underwater vehicle is negatively buoyant; 
     FIG. 3B shows a pictorial view of the second stage of the method of surfacing the towed antenna buoy when the underwater vehicle is negatively buoyant; and 
     FIG. 3C shows a pictorial view of the third stage of the method of surfacing the towed antenna buoy when the underwater vehicle is negatively buoyant. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 there is shown a cutaway pictorial view of a typical small underwater vehicle 10 with an antenna buoy 12 towed on a tether 14. Tether 14 has electrically conductive cables running the length thereof to allow vehicle 10 to communicate with antenna buoy 12. Underwater vehicle 10 is controlled by control circuitry 16 comprising a microprocessor and memory components. The procedures for operating, navigating and surfacing vehicle 10 are stored in these memory components. Control circuitry 16 is electrically linked to a navigational computer 18 and various instruments 20. Some of the instruments present aboard underwater vehicle 10 are accelerometers for determining acceleration in linear, lateral or vertical directions; doppler sonar for sensing bottom depth and vehicle speed relative to the bottom; a pressure gauge for sensing vehicle depth; and listening devices for detecting the presence of other craft. Control circuitry 16 is electrically connected to control a propulsor 22 and fins 24 to guide vehicle 10. 
     Buoy 12 is hydrodynamically shaped to allow it to be towed through the water with a minimum of drag. The rear end of buoy 12 is shaped to provide a slight amount of drag to prevent buoy 12 from passing vehicle 10 when vehicle 10 is decelerating. An antenna 26 is positioned at the back of buoy 12, the part of buoy 12 most remote from tether 14. Antenna 26 is in communication with a receiver 28 aboard vehicle 10 via tether 14. Tether 14 is attached to vehicle 10 by a tow skeg 30 to avoid fouling tether 14 in propulsor 18. Tow skeg 30 holds tether 14 away from propulsor 18 to prevent tether 14 from entangling in propulsor 18 during sharp turns. Tow skeg 30 has three support members 32 to provide lateral and vertical stability to skeg 30. 
     Overall buoy 12 is positively buoyant; however, front 34 of buoy 18 near tether 14 is slightly weighted to stabilize buoy 12 in the vertical direction while at the surface. Buoy 12 can be gas filled so that buoy 12 expands when subjected to lower external pressure at or near the surface and contracts at operating depth. A gas filled buoy 12 thus provides increased buoyancy when at the surface and reduced drag at operating depth 
     Control circuitry 16 can be programmed to communicate after any of several different criteria have been met. The vehicle 10 can surface buoy 12 to communicate after a set time period has elapsed, or vehicle 10 can also maneuver to obtain global positioning data if sensor readings are inconsistent with data stored in memory, e.g. sensor depth is different from the depth recorded in memory for a specific location. Vehicle 10 can be programmed to surface the buoy 12 to communicate detection or absence of detection of another craft after a preset time period. 
     Control circuitry 16 can be programmed to avoid a prescheduled communications maneuver if certain conditions are met. If instruments 20 detect excessive noise from the presence of other craft control circuitry 16 can abort the maneuver. Surfacing buoy 12 can also be aborted when the bottom is too shallow and vehicle 10 will either surface or ground out before radio communications have been completed. 
     FIG. 2A, 2B, 2C and 2D show the method of surfacing towed antenna buoy 12 when underwater vehicle 10 is positively buoyant. Referring now to FIG. 2A, when radio communication is required from a positively buoyant vehicle 10, vehicle 10 climbs to a preassigned depth 36 and reduces its speed to the minimum possible (i.e., dead slow or stopped). The speed reduction allows buoy 12 to float toward surface 38 instead of trailing vehicle 10. When buoy 12 breaks surface 38, it begins receiving or transmitting radio signals. While buoy 12 is drifting upward, vehicle 10 is also floating gradually toward surface 38 albeit at a slower rate than buoy 12. See FIG. 2B. By taking into account the ascent rate of vehicle 10, the length of tether 14 can be configured to allow buoy 12 to remain at surface 38 for enough time to communicate or obtain a global positioning system fix, approximately one minute. FIG. 2C displays when vehicle 10 passes a minimum depth 40, and vehicle 10 activates its propulsor 18 and dives to prevent vehicle 10 from surfacing. Thus, buoy 12 is pulled beneath surface 38. See FIG. 2D. 
     FIG. 3A, 3B and 3C show the method of surfacing towed antenna buoy 12 when underwater vehicle 10 is negatively buoyant. As with a positively buoyant vehicle (see FIG. 2A), FIG. 3A shows negatively buoyant vehicle 10 climbing to a preassigned depth 36. At this depth vehicle 10 reduces its speed to allow buoy 12 to rise to surface 38. Buoy 12 begins receiving or transmitting radio signals when buoy 12 breaks surface 38 as shown in FIG. 3B. Vehicle 10 slowly sinks while it is stationary. When vehicle 10 sinks to a critical depth 42 below reach of tether 14, buoy 12 is pulled beneath surface 38 as shown in FIG. 3C. Vehicle 10 can then turn on its propulsor continue movement. 
     The timing required for communicating or obtaining a GPS fix is dependent on the location of the other station with respect to the underwater vehicle. Because of surface conditions, communications are quicker when the other station is overhead than when the station is near the horizon. Accordingly, control circuitry should account for the timing of contact with the station by aborting the surfacing maneuver if contact is not obtained within a set time period or by executing the final phase of the maneuver as soon as communication has occurred. 
     The advantages of the present invention over the prior art are that an underwater vehicle can obtain a global positioning fix or communicate with a station without subjecting itself to detection. The underwater vehicle does not need a large variable buoyancy system to allow the vehicle to broach the surface nor does the vehicle need gas supplies to fill the variable buoyancy system. 
     What has thus been described is an apparatus and method allowing an underwater vehicle to engage in radio communications without surfacing itself. 
     Obviously many modifications and variations of the present invention may become apparent in light of the above teachings. For example: the antenna can be used to obtain other information from a control source as well as global positioning information; the underwater vehicle can be fitted with a winch to haul in the buoy once communication has been completed; the underwater vehicle can have vertical thrusters allowing it to hover; and the receiving circuitry can be embodied in the buoy rather than in the vehicle. 
     In light of the above, it is therefore understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.