Abstract:
A multirotor mobile buoy combining MR-VTOL capability with environmentally hardened electronics, exchangeable sensor suites, and a solar recharge system and providing sensing in aquatic environments. The multirotor mobile buoy provides for the detection, classification and location of underwater objects using self-contained electronics, and repositions with aerial means using a plurality of rotors. The multirotor mobile buoy additionally incorporates solar panels for recharging of on-board batteries enabling the flight and other functions, and comprises a buoyant assembly and extended tether in order to promote stability in dynamic, open ocean environments. The multirotor mobile buoy may be employed singly or as a swarm of underwater detection platforms, and may utilize its positioning ability to optimize the effectiveness of sonobuoy systems arrayed as a distributed sensor field.

Description:
RELATION TO OTHER APPLICATIONS 
     This patent application claims priority from nonprovisional patent application Ser. No. 14/246,611 filed Apr. 7, 2014, which is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     One or more embodiments relates to a sonobuoy for the detection of underwater objects in a manner incorporating self-contained electronics, a plurality of on-board rotors for aerial positioning, and solar panels for solar energy harvesting and energy-independent operation. 
     BACKGROUND 
     Sonobuoys are expendable underwater listening devices typically employed for the detection, classification and location of underwater objects. Although models and capabilities vary, sonobuoys generally include a float, radio transmitter, a battery and a hydrophone attached to a wire for detecting sound under water. In its simplest form, the sonobuoy is a compact, self-contained package of electronics designed to be enter the water, separate into an underwater acoustic sensor and an on-the-surface radio transmitter, and relay the underwater acoustic signals it detects to some external system, where the radio frequency (RF) transmission is received and processed to detect and locate a detected object. 
     Sonobuoys generally lack the means for effective self-positioning, and current- and wind-induced drift of sonobuoys can have a detrimental impact on the effectiveness of sonobuoy systems arrayed as a distributed sensor field. Not only can the sonobuoys drift away from the area-of-interest, but buoy movement relative to other buoys in the field can reduce field integrity by creating coverage gaps in the field or clustering too many buoys in one area. Despite the significant advances made in the underwater detections, the most significant gaps still exist in the areas of responsive mobility and timely communication of acquired contacts; in short, it takes too long to communicate the positively detected signatures to external systems. Low bandwidth and operational range of communication links, and low mobility of the undersurface autonomous platforms within the existing onboard energy budgets are the key constraining factors of the current state of the art of sonobuoy technology. 
     Provided here is a multirotor mobile buoy providing for the detection, classification and location of underwater objects using self-contained electronics, and able to reposition by aerial means using a plurality of rotors. The multirotor mobile buoy additionally incorporates solar panels for recharging of on-board batteries enabling the flight and other functions, and comprises a buoyant assembly and extended tether in order to promote stability in dynamic, open ocean environments. The multirotor mobile buoy may be employed singly or as a swarm of underwater detection platforms, and may utilize its positioning ability to optimize the effectiveness of sonobuoy systems arrayed as a distributed sensor field. 
     These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims. 
     SUMMARY 
     The disclosure provides a multirotor mobile buoy combining MR-VTOL capability with environmentally hardened electronics, exchangeable sensor suites, and a solar recharge system in order to provide long endurance sensing in aquatic environments. The multirotor mobile buoy may perform sensing below the thermocline layer while riding on ocean currents, and utilizes a plurality of VTOL-oriented rotors when required for rapid aerial relocation. The multirotor mobile buoy provides INS/GPS capability and sensing and signal processing capabilities for detection of underwater objects, as well as solar energy harvesting for energy-independent operation. The multirotor mobile buoy may be deployed as a plurality of multiple cooperative vehicles in a particular operational area with heterogeneous sensors and may, for example, distribute themselves to guarantee a required node-density for a given desired coverage. 
     The multirotor mobile buoy comprises a central frame comprising a central housing, a plurality of frame support members extending from the central housing, a plurality of rotor support members. A buoyant assembly couples to the central frame and is arranged to promote stability in wave environments. A rotor couples each rotor support member, and the rotor support members, central housing, and buoyant assembly are coupled such that the rotating mast, hub, and rotor blades of the rotor are above the water and oriented to direct a thrust in a direction toward the surface of the water. 
     The multirotor mobile buoy further comprises solar cells coupled to the frame support members such that a sun-facing side of the solar cell is above the water when the buoyant assembly floats on water. Additionally, a battery is electrically connected to each solar cell and each rotor to enable aerial flight. A tether couples to the central housing and deploys a sensor configured to detect underwater objects. 
     The novel apparatus and principles of operation are further discussed in the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a view of an embodiment of the multirotor mobile buoy. 
         FIG. 1B  illustrates another view of an embodiment of the multirotor mobile buoy. 
         FIG. 2  illustrates an orientation of the buoyant assembly and plurality of rotors. 
         FIG. 3A  illustrates a view of the multirotor mobile buoy and deployed sensor. 
         FIG. 3B  illustrates another view of the multirotor mobile buoy and deployed sensor. 
         FIG. 4  illustrates an embodiment of a computing system for the multirotor mobile buoy. 
         FIG. 5  illustrates a method for the detection of underwater objects using a plurality of multirotor mobile buoys. 
     
    
    
     Embodiments in accordance with the invention are further described herein with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is provided to enable any person skilled in the art to use the invention and sets forth the best mode contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the principles of the present invention are defined herein specifically to provide a multirotor mobile buoy capable of underwater detections and repositioning utilizing Multi-Rotor Vertical Take Off and Landing (MR-VTOL) capability. 
     The disclosure provides a novel autonomous vehicle combining MR-VTOL capability with environmentally hardened electronics, exchangeable sensor suites, and a solar recharge system in order to provide long endurance sensing in aquatic environments. The vehicle performs sensing below the thermocline layer while riding on ocean currents, and flying in the air when required for rapid relocation or local communication with similar cooperative nodes. The vehicle instrumentation typically combines INS/GPS and aircraft control, data processing, and the command and control (C2) capabilities of a VTOL aircraft with the sensing and signal processing capabilities of an underwater robot with a vector sensor suspended below the thermocline, as well as solar energy harvesting for energy-independent operation. The multirotor mobile buoy may be deployed as a plurality of multiple cooperative vehicles in a particular operational area with heterogeneous sensors and may, for example, distribute themselves to guarantee a required node-density for a given desired coverage. 
     A particular embodiment of the multirotor mobile buoy is represented at  FIGS. 1A and 1B  and indicated generally as  100 , with views in accordance with the axes shown The multirotor mobile buoy is generally represented at  100  and incorporates a central frame comprising a central housing  101 , a plurality of frame members such as  102 ,  103 ,  104 , and  105 , and a plurality of rotor support members such as  106 ,  107 ,  108 , and  109 . Typically central housing  101  is a water tight enclosure surrounding an internal volume. The plurality of frame members  102 ,  103 ,  104 , and  105  extend from central housing  101  in a generally radial fashion, as illustrated. The plurality of rotor support members  106 ,  107 ,  108 , and  109  are coupled either to the central housing, one of the frame support members, or some combination thereof so that the rotor frame behaves as a substantially rigid collection. For example at  FIG. 1A , every rotor support member  106 ,  107 ,  108 , or  109  is coupled to central housing  101  by fixed attachment and extends from central housing  101  in such a manner that the rotor support member is interposed between a first frame member and a second frame member. Additionally, a buoyant assembly indicated by  115   a ,  115   b , and  115   e  is coupled to the central frame, as illustrated at  FIG. 1B . The buoyant assembly is typically fixably attached to the rotor frame and is configured to float on water. For example,  FIG. 1B  illustrates buoyant assembly members  115   a ,  115   b , and  115   c  floating on water surface S. 
     Multirotor mobile buoy  100  additionally comprises a plurality of rotors such as the rotor generally indicated by  110 , in order to provide for flight capability of multirotor mobile buoy  100 . Multirotor mobile buoy  100  indicates four rotors, however any desired number of rotors may be utilized as a design choice. Rotor  110  comprises motor  111 , rotating mast  112 , hub  113 , and a plurality of rotor blades  114 , where motor  111  is coupled to an generates rotation of rotating mast  112 , and rotating mast  112  is fixably attached to hub  113 , such that rotation of motor  111  generates subsequent rotation of the rotor blades  114  in some plane of rotation. Such rotor systems are well known in the art. See e.g., Federal Aviation Administration,  Helicopter Flying Handbook  (FAA-H-8083-21A) (2012), among others. Each rotor is coupled to at least one of the rotor support members  106 - 109 . Additionally, the each of the rotor support members  106 - 109  are coupled to central housing  101  and/or at least one of the frame support members  102 - 105 , and the buoyant assembly  115   a ,  115   b , and  115   c  is coupled to the rotor frame, such that the combination of the coupling maintains the plurality of rotor blades  114  both above the water surface S and in a position to direct thrust toward the water surface S when the buoyant assembly floats on water. This is additionally illustrated at  FIGS. 1A and 1B , where rotor  110  is fixably attached to rotor support member  109  and rotor support member  109  is fixably attached to central housing  101  such that rotor blades  114  are above water surface S and oriented to direct thrust generally in a negative direction of the y-axis when the buoyant assembly  115   a ,  115   b , and  115   c  floats on the water surface S. In a particular embodiment, the rotor blades have a propeller pitch such a rotation of the blade around the tip-path P generates a displacement in the positive direction of the y-axes of  FIGS. 1A and 1B , and generally away from water surface S. 
     Multirotor mobile buoy  100  additionally comprises a plurality of solar cells such as  116 ,  117 ,  118 , and  119 . Each solar cell is coupled to at least one of the frame support members  102 - 105  and comprises a sun-facing side, such as sun-facing side  120  of solar cell  116 , sun-facing side  121  of solar cell  117 , and sun-facing side  122  of solar cell  119  at  FIG. 1B . As is understood, solar cells are electrical devices intended to concert the energy of light into electricity by the photoelectric effect and are well known in the art. See e.g. Wurfel, P.,  Physics of Solar Cells: From Basic Principles to Advanced Concepts , (2009), among others. The solar cells are coupled to the frame members, the frame members extend from the central housing, and the buoyant assembly is coupled to the rotor frame such that the sun-facing sides of the solar cells are above the water when the buoyant assembly floats on water. For example, at  FIG. 1B , solar panel  117  is coupled to frame support  103  and frame support  103  extends from central housing  101  in such a manner that sun-facing surface  121  of solar panel  117  is above the water surface S when the buoyant assembly  115   a ,  115   b , and  115   c  floats on the water surface S. 
     Multirotor mobile buoy  100  further comprises a battery  123  electrically connected to the solar cells and electrically connected to each motor in the plurality of rotors, in order that battery  123  may be charged from the solar cells and deliver power to the rotors to facilitate buoy mobility. In some embodiments, battery  123  is housed within the internal volume of central housing  101 , as illustrated. In addition to solar panels  116 ,  117 ,  118 , and  119 , power may be generated by other mechanisms. For instance, a fan (not shown) extending above multirotor mobile buoy  100  from any desired attachment location may capture wind energy to charge battery  123 . 
     Multirotor mobile buoy  100  further comprises a tether  124  extending generally from central housing  101  to a sensor  125  configured to detect underwater objects. Sensor  125  is configured to sink in water such that tether  124  generally hangs beneath the rotor frame when when the buoyant assembly floats on water. Multirotor mobile buoy  100  typically comprises some means for deploying tether  124 , such as a winch which provides for reel-out and/or reel-in when desired, or a latching mechanism which may release tether  124  and allow tether  124  to extend from gravitational forces on sensor  125 . Once deployed, it is generally not required to stow tether  124  prior to multirotor mobile buoy  100  engaging in flight. In some embodiments, tether  124  further comprises motion dampening unit  127 . In a typical embodiment, multirotor mobile buoy additionally comprises a computing system  130  where the computing system is configured to receive data from sensor  125 . In a further embodiment, computing system  130  receives data via a communication cable extending between computing system  127  and sensor  125 . The communication cable may be an integral part of tether  124  or otherwise independent. Tether  124  is generally a flexible cable of extended length to allow sensing below thermoclines as the rotor frame of multirotor mobile buoy  100  remains above the water surface. 
     In some embodiments, multirotor mobile buoy  100  additionally comprises an external support member  128  fixably attached to one or more frame members. External support member  128  may generally surround the plurality of rotors in a manner to provide protection as the buoyant assembly generates floatation or otherwise, and/or provide additional support for the plurality of solar panels, or be utilized for other functions to enhance the operation of multirotor mobile buoy  100 . 
     As discussed, multirotor mobile buoy  100  integrates a number of technologies for propulsion, energy harvesting and storage, signal processing, and communication all into a single platform for tracking underwater objects. Since multirotor mobile buoy  100  may operate in dynamic ocean environments, multirotor mobile buoy  100  also incorporates positive buoyancy and a high degree of stability. Toward this end, the pendulum-like nature of tether  124  and sensor  125  assists in self-righting behavior. In addition and as discussed, the buoyant assembly is coupled to the rotor frame in such a manner, as the buoyant assembly floats on water, the rotor blades are both above the water surface and oriented to direct thrust toward the water surface, and the sun-facing sides of the solar cells are above the water surface. To facilitate this arrangement in a particular embodiment, the central housing has a vertical axis and the buoyant assembly generates buoyant forces which are substantially parallel to the vertical axis, and each plurality of rotor blades has a plane of rotation substantially perpendicular to the vertical axis. For example,  FIG. 2  illustrates selected components of multirotor mobile buoy  200  specifically indicating central housing  201 , frame support member  202  supporting solar panel  216 , frame support member  203  supporting solar panel  217 , and frame support member  205  supporting solar panel  219 . Central housing  201  is positioned similarly to central housing  101  of  FIG. 1A  as indicated by hidden lines, and frame support members  202 ,  203 , and  205  extend from central housing  201 . Multirotor mobile buoy  200  further comprises a fourth frame support member (not shown) analogous to frame support member  104  of  FIG. 1A . Additionally, multirotor mobile buoy  200  comprises a buoyant assembly indicated by  215   a ,  215   b , and  215   c , where  215   a ,  215   b , and  215   c  are coupled to frame support members  202 ,  203 , and  205  respectively. As illustrated, as the buoyant assembly floats on water surface S, a vertical axis V extends through central housing  201  in an orientation parallel to the y-axis indicated as buoyant assembly components  215   a ,  215   b , and  215   c  generate distributed buoyant forces represented by B 1 , B 2 , and B 3  in directions parallel to vertical axis V. Additionally, rotor blades  214  are above water surface S and have a plane of rotation P substantially perpendicular to vertical axis V. Here, “substantially perpendicular” means that a first direction vector lies within the plane of rotation P and a second direction vector is parallel to vertical axis V, and the angle between the first direction vector and the second direction vector is at least 75 degrees, preferably at least 80 degrees, and more preferably at least 85 degrees. In another embodiment, rotor blades  214  have a propeller pitch such a rotation of rotor blades  214  generates a displacement substantially parallel to the vertical axis V. Here, “substantially parallel” means that a first direction vector is parallel to the displacement generated and a second direction vector is parallel to vertical axis V, and the angle between the first direction vector and the second direction vector is less than 15 degrees, preferably less than 10 degrees, and more preferably less than 5 degrees. 
     Additionally, as has been indicated, in some embodiments, the buoyant assembly provides an outrigger-style stability by utilizing a plurality of pontoon-type flotation devices coupled to the frame support members, such as pontoons  215   a ,  215   b , and  215   c  coupled to frame support members  202 ,  203 , and  205  respectively. Typically, a multirotor mobile buoy such as that shown at  FIG. 2  would additionally incorporate a pontoon coupled to each support member. For example, at  FIG. 1A , in certain embodiments, the buoyant assembly might incorporate a pontoon coupled to each of frame support member  102 ,  103 ,  104 , and  105 . Such an arrangement promotes stability via distributed buoyancy and by taking advantage of the weighted water plane area for righting moments, and increases stiffness during roll. In a particular embodiment, stability is promoted by providing pontoons sufficient to provide buoyant forces to mitigate pontoon effects and tendencies toward capsize when lateral forces are applied. In some embodiments, the multirotor mobile buoy has a total combined weight and each pontoon in a plurality of pontoons has a displacement volume of water when completely submerged, and the weight of water having a volume equal to the displacement volume is at least 70%, preferably at least 80%, and more preferably greater than the total combined weight of the apparatus. 
     It is understood that the buoyant assembly illustrated is exemplary only. The buoyant assembly may take any form sufficient provide buoyancy in a manner which maintains the relationship of other components as described. 
     As discussed, the tether is generally a flexible cable of extended length to allow sensing below thermoclines as the central frame of the multirotor mobile buoy generally remains above the water surface. In a particular embodiment illustrated at  FIGS. 3A and 3B , a reference diameter D is perpendicular to the vertical axis V and surrounds the central frame and the buoyant assembly of multirotor mobile buoy  300 , and the tether  324  has a length L from the central housing to the sensor  325  with the length L at least 3 times, preferably at least 10 times, and more preferably at least 15 times greater than the reference diameter D. In addition to providing for deeper sensor deployments, the length of tether  324  and mass of sensor  325  may also act to shift the center of gravity of multirotor mobile buoy  300  closer to or below the center of buoyancy of the buoyant apparatus to promote increased stability. See e.g., C. B Barrass and D. R. Derret,  Ship Stability for Master and Mates  (2012), among others. For example, at  FIG. 3B  where multirotor mobile buoy is subject to the gravity vector G while the buoyant assembly floats on water and defines a center of buoyancy, sensor  324  may be extended using tether  324  to some length L, where the length L and the mass of submerged sensor  324  combine to influence the center of gravity for the multirotor mobile buoy such that the center of gravity is below the center of buoyancy, where here “below” means displaced in the direction of the gravity vector G. Such an arrangement may significantly enhance stability in environments subject to surface wave action, such as an open ocean environment. Correspondingly, in a particular embodiment, the length of the tether and the mass of the sensor comprising the multirotor mobile buoy is sufficient to generate a center of gravity of the multirotor mobile buoy below the center of buoyancy of the multirotor mobile buoy, when multirotor mobile buoy is subject to a gravity vector and while the buoyant assembly floats on water and defines a center of buoyancy. In another embodiment, the resultant gravity force of the multirotor mobile buoy acting through the center of gravity has a first direction vector and the resultant buoyant force of the buoyant assembly acting through the center of buoyancy has a second direction vector, and the first and second direction vector intersect a circular area perpendicular to the vertical axis V and having a diameter of less than 10% of the reference diameter D. 
     In certain embodiments multirotor mobile buoy  100  comprises computing system  130 , and in some embodiments an onboard global positioning system and inertial navigation system (GPS/INS) provides position information for multirotor mobile buoy  100 . The GPS/INS in concert with computing system  130  may provide autopilot functionality that positions multirotor mobile buoy  100  in accordance with provided instructions. For example, a plurality of multirotor mobile buoys may be positioned in a geographic grid in any desired two dimensional pattern and spacing. As an example  FIG. 4  is a block diagram of a computing system  430  configured to control a multirotor mobile buoy, according to an embodiment of the present invention. Computing system  430  includes a bus  435  or other communication mechanism for communicating information, and processor(s)  440  coupled to bus  435  for processing information. Processors  440  may be any type of general or specific purpose processor, including a central processing unit (“CPU”) or application specific integrated circuit (“ASIC”). Processor(s)  440  may also have multiple processing cores, and at least some of the cores may be configured to perform specific functions. Computing system  430  further includes a memory  445  for storing information and instructions to be executed by processor(s)  440 . Memory  445  can be comprised of any combination of random access memory (“RAM”), read only memory (“ROM”), flash memory, cache, static storage such a magnetic or optical disk, or any other types of non-transitory computer-readable media or combinations thereof. Additionally, computing system  430  includes a communication device  450 , such as a transceiver and antenna, to wirelessly provide access to a communications network. 
     Non-transitory computer-readable media may be any available media that can be accessed by processor(s)  440  and may include both volatile and non-volatile media, removable and non-removable media, and communication media. Communication media may include computer-readable instructions, data structures, program modules or other transport mechanism and includes any information delivery media. Memory  445  stores software modules that provide functionality when executed by processor(s)  440 . The modules include an operating system  455  for computing system  430 . The modules further include a detection and control module  460  that is configured to detect underwater objects and control operation of the buoy. Computing system  430  may include one or more additional functional modules  465  that include additional functionality. Power management circuitry  470  is responsible for accumulation of energy from solar panels and its distribution to the onboard instrumentation and the propulsion that enables aerial mobility (e.g., rotors) in this embodiment. GPS/INS  475  provides position information for computing system  430  in this embodiment. 
     One skilled in the art will appreciate that a “system” could be embodied as an embedded system, a personal computer, a server, a console, a personal digital assistant (“PDA”), a cell phone, a tablet computing device, or any other suitable computing device, or combination of devices, capable of providing processing functionality for the buoy. Presenting the above-described functions as being performed by a “system” is not intended to limit the scope of the present invention in any way, but is intended to provide one example of many embodiments of the present invention. Indeed, methods, systems and apparatuses disclosed herein may be implemented in localized and distributed forms consistent with computing technology, including cloud computing systems. 
     It should be noted that some of the system features described in this specification have been presented as modules, in order to more particularly emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom very large scale integration (“VLSI”) circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices, graphics processing units, or the like. 
     A module may also be at least partially implemented in software for execution by various types of processors. An identified unit of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions that may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module. Further, modules may be stored on a computer-readable medium, which may be, for instance, a hard disk drive, flash device, RAM, tape, or any other such medium used to store data. Indeed, a module of executable code could be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network. 
     The multirotor mobile buoy disclosed provides a platform that is highly controllable, agile, and simple to maintain. Further, wireless mesh communication and the minimal production cost of multicopters may facilitate the use of multiple quadrotors, which opens the door to cooperative and swarm behaviors over a broad spectrum of missions. They have relatively high efficiency for vertical takeoff and landing (VTOL) platforms and can precisely control their attitude with motor speed changes. The brushless motors in each rotor have essentially no wear parts and are tolerant of wet environments. This makes them highly reliable with only four moving parts when in a quad-rotor configuration. Further, a plurality of multirotor mobile buoys can be employed as a distributed sensor network. The use of dispersed sensors in this manner generates great operational flexibility, since they can be arranged and configured to maximize the group capability. Track of an underwater object can be maintained using a single sensor, but the position error is greatly reduced by increasing the number of platforms sharing their individual measurements. This improves the accuracy of a final position estimate. Additionally, multirotor mobile buoy&#39;s position above the waterline facilitates air-based communication. 
     The use of multirotor mobile buoys also allows for rapid repositioning at speeds higher than those typically seen in underwater platforms. This helps to ensure greater continuous coverage of the target being tracked and facilitates maintaining closer proximity to the moving target. The multirotor mobile buoy may travel through a series of flight and drifting segments. This provides minimization of energy expenditure via computationally feasible path planning that utilizes a priori given or in-situ measured ocean currents. The approach accounts for the fact that, due to the constrained battery capacity, the multirotor mobile buoys may primarily drift while conducting their surveillance mission, but may fly for periodic repositioning, obstacle avoidance, and inter-node communication. Therefore, with known currents and the desired final position of each vehicle, the onboard path planning algorithm will find a path that minimizes the total energy expended. See U.S. patent application Ser. No. 14/246,611 filed Apr. 7, 2014, filed by Jones et al.; see also Chase H. Dilliard, “Energy-efficient underwater surveillance by means of hybrid aquacopters,” (M.S.thesis, Naval Postgraduate School, 2014). 
     In swarm embodiments, individual multirotor mobile buoys may communicate with one another to establish the direction, speed, and depth of underwater objects. While active sonar may be used, many embodiments may use passive sensors and sensing techniques since these sensors tend to be smaller, lighter, and require less power. Passive sensors may also be capable of inferring size based on signal strength, but would likely not be capable of producing a sonar-like estimate of vehicle shape and size. Multirotor mobile buoys may also collectively move to track and match pace with a given object. Alternatively, in a group comprising multirotor mobile buoys, certain buoys may be capable of flying and others may not. Multirotor mobile buoys with a flying configuration may match pace with, and follow the underwater object and those without such a configuration may communicate with multirotor mobile buoys. Buoys that do not fly may be larger and have more advanced sensing equipment while flying multirotor mobile buoys may be designed to increase flying range and/or speed. In certain embodiments, multirotor mobile buoys may lack equipment other than that required for flight and communications. When buoys are specialized, they may detect different frequency ranges, use different sensing technologies, and or have any other equipment package as a matter of design choice. 
       FIG. 5  is a flowchart  500  illustrating a process for tracking underwater objects using a swarm of mobile buoys comprising multirotor mobile buoys, according to an embodiment of the present invention. The process begins with at least one buoy detecting an underwater object at  510 . The detecting buoy then communicates with other buoys to inform them that the object was detected at  520 . At least one buoy takes flight at  530 . In some embodiments, steps  520  and  530  may be performed in reverse. At least one flying buoy then communicates with external systems, such as satellites, ships, land stations, etc. to inform them that the object was detected at  540 . At least one flying buoy follows the detected object or repositions in response to the detection at  550 . As discussed above, in some embodiments, the buoy may not be able to match the pace and distance traveled by the detected object. In such embodiments, new buoys in the swarm may join in tracking the object when it comes within range. Also, buoys may stop detection when the object travels out of their range. In certain embodiments, buoys may not pursue the object, but may instead rely on the positioning of the swarm to handoff detection duties and collectively track the object. The swarm of buoys then tracks the object and communicates its position to the external system as the object travels at  560 . 
     In some embodiments, all of the buoys in the swarm on the surface that are within range can detect the presence of the target object, but they can only estimate position by comparing times of detection along with their own locations very accurately. Accordingly, at least one buoy may always be in the air to communicate with all of the buoys on the water within a certain range. All buoys may have GPS, which would allow sensory data to be accurately time-stamped. In certain embodiments, rather than having a buoy in the air at all times, all buoys may be in the water sensing. When the “event” is over, or after a certain period of time, one buoy may fly into the air to share data with external systems and allow for calculations to commence. The buoy in the air may not have to perform the calculations. Instead, the flying buoy may just allow for the sharing of time-stamped data among the buoys in range. This type of calculation is also known as time-delay on arrival (TDOA). 
     Thus, provided here is a multirotor mobile buoy combining MR-VTOL capability with environmentally hardened electronics, exchangeable sensor suites, and a solar recharge system in order to provide long endurance sensing in aquatic environments. The multirotor mobile buoy may perform sensing below the thermocline layer while riding on ocean currents, and utilizes a plurality of VTOL-oriented rotors when required for rapid aerial relocation. The multirotor mobile buoy provides INS/GPS capability and sensing and signal processing capabilities for detection of underwater objects, as well as solar energy harvesting for energy-independent operation. The multirotor mobile buoy may be deployed as a plurality of multiple cooperative vehicles in a particular operational area with heterogeneous sensors and may, for example, distribute themselves to guarantee a required node-density for a given desired coverage. 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention and it is not intended to be exhaustive or limit the invention to the precise form disclosed. Numerous modifications and alternative arrangements may be devised by those skilled in the art in light of the above teachings without departing from the spirit and scope of the present invention. It is intended that the scope of the invention be defined by the claims appended hereto. 
     In addition, the previously described versions of the present invention have many advantages, including but not limited to those described above. However, the invention does not require that all advantages and aspects be incorporated into every embodiment of the present invention. 
     All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.