System and method for extending GPS to divers and underwater vehicles

A navigation system extends satellite navigation to divers, underwater vehicles, and surface vessels. The navigation system comprises a location reference unit and a plurality of sub-surface beacon units. The location reference unit includes a receiver to receive navigation signals from earth-orbiting satellites and/or an inertial navigation system. The location reference unit further includes processing circuits to communicate with to sub-surface beacon units and to transmit location information to said sub-surface beacon units, and a sonar transmitter to transmit location information to the sub-surface beacon units. The beacon units include processing circuits to determine the location of the beacon unit based on location information received from the location reference unit, and a sonar transceiver to receive location information from the location reference unit and to transmit location information to a diver unit to provide navigation assistance to the diver unit.

BACKGROUND OF THE INVENTION

The present invention relates generally to underwater navigation and, more particularly, to a method and apparatus for extending GPS navigation to divers and underwater vehicles.

Satellite-based positioning systems, such as the Global Positioning System (GPS), provide the ability to accurately determine location virtually almost anywhere on the Earth's surface. The GPS comprises 24 earth-orbiting satellites located in 6 orbital planes. Each earth-orbiting satellite carries an atomic clock and continuously broadcasts radio signals indicating its current time and location. A receiver located on the Earth's surface can receive these radio signals and determine its distance from the satellites based on the time of arrival of the signals. By receiving signals from four satellites, an Earth-based receiver can determine its location by triangulation.

GPS signals do not propagate underwater. Consequently, divers and underwater vehicles beneath the water's surface are not able utilize these GPS signals to accurately navigate between two points. A number of systems have been proposed for extending GPS to underwater divers and vehicles. For example, U.S. Pat. Nos. 6,701,252 to Brown and U.S. Pat. No. 6,657,585 to Kucik disclose a floating buoy that is connected by a tether to a diver or underwater vehicle. The floating buoy carries a GPS antennas and/or receiver and conveys signals via the tether to the diver or underwater vehicle. This solution is limited in utility by the need for a tether connecting the underwater diver or vehicle to the floating buoy. U.S. Pat. No. 5,119,341 to Youngberg discloses a system for extending GPS to divers and vehicles beneath the water's surface using buoys that float freely on the surface. The floating buoys can receive signals from GPS satellites and can communicate underwater users using acoustic signals. However, the floating buoys do not stay in place, but instead drift on the surface of the water. Further, floating buoys are subject to easy detection and thus are not suitable for covert operations.

SUMMARY OF THE INVENTION

The present invention provides an underwater navigation system that effectively extends GPS to underwater users and devices. The present invention could also be used to provide navigation signals to surface moving vessels. The underwater navigation system comprises a plurality of sub-surface beacon units that are designed to sink to the ocean bottom and at least one location reference unit that is designed to initialize the beacon units with their location. The beacon units and the location reference unit are positioned in an area where navigation assistance is needed. The beacon units sink immediately to the ocean bottom. In one embodiment, the location reference unit remains partially above the surface to receive GPS signals from GPS satellites and determines its location using the received GPS signals. In another embodiment, the location reference unit is entirely submerged beneath the surface and determines its location using an inertial navigation system, Doppler system, or other similar system. Nonetheless, in either embodiment, the location reference unit remains at least partially below the surface to communicate with the sub-surface beacon units via acoustic signals. After its location is fixed, the location reference unit transmits its location to the sub-surface beacon units. In another embodiment, the location reference unit sequentially calculates the position offset of each beacon unit within range, combines the calculated offset with the location reference unit's latitude, longitude and rotational orientation (e.g. relative to magnetic north) and transmits the fully calculated beacon unit position to each beacon respectively. The beacon units can determine their location based on the signals received from the location reference unit and/or signals received from other beacon units. With their location established, the beacon units may then provide navigation assistance to divers, underwater craft, or surface vessels without the aid of the location reference unit. After a predetermined period of time, then, or after the location reference unit receives a position confirmation from a predetermined number of beacon units, the location reference unit is removed to avoid surface exposure. If the location reference unit comprises a buoy, for example, it may sink below the surface. If the location reference unit comprises a surface watercraft, a helicopter, or a submarine, however, it may simply transport itself to an area where it is unexposed. Because only the sub-surface beacon units remain in the area where navigation assistance is needed, the underwater navigation system may provide navigation assistance to users or devices without surface exposure.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings,FIG. 1illustrates a non-limiting example of an underwater navigation system10to provide navigation assistance to a diver, underwater vehicle, or surface vessel, which are referred to herein generically as guided units. The underwater navigation system10comprises at least one location reference unit20and a plurality of beacon units50located beneath the water's surface. The location reference unit20and the sub-surface beacon units50communicate using acoustic signals. The location reference unit20includes a sonar transceiver28for such communication. In the embodiment shown inFIG. 1, the location reference unit20also communicates with earth orbiting satellites12using navigational signals. Navigational signals, such as GPS signals provided from GPS satellites, permit the location reference unit20to determine its location. Land-based navigation systems, such as DGPS, could also be used in place of space-based satellites. The location reference unit20includes a receiver26for receiving these navigational signals from the earth orbiting satellites12.

The beacon units50and location reference unit20are positioned in an area where navigation assistance is needed and within sufficient proximity of one another for communicating. The beacon units50are designed to sink immediately to the floor of a body of water, such as the ocean. The location reference unit20is designed such that its receiver26, if present, remains above the surface to receive navigational signals from the earth orbiting satellites12and its sonar transceiver28remains below the surface to communicate with the beacon units50. As illustrated below in alternative embodiments of the underwater navigation system, this may be accomplished in various ways depending on the implementation of the location reference unit20. Regardless, the location reference unit20determines its location using the received navigational signals. After its location is fixed, the location reference unit20transmits its location to each of the beacon units50. The beacon units50can then determine their location based on signals received from one or more location reference units20. In some embodiments, the beacon units50may determine their location based on signals received from a single location reference unit20. In other embodiments, the beacon units50may receive signals from multiple location reference units20. The beacon units50may also exchange information between themselves to further refine position calculations. After initializing beacon units50within its communication range, a location reference unit20may change its function to become a beacon unit50, by sinking entirely below the water surface and then itself being initialized to its location on or near the sea floor by one or more subsequently deployed location reference units20.

For example,FIG. 2illustrates the main components of the location reference unit20. The location reference unit20comprises processing circuits22, memory24, a GPS receiver26, a sonar transceiver28, and a compass30to determine the rotational orientation of the location reference unit20. If the location reference unit20also functions as a beacon unit50, the location reference unit20may further include a pressure sensor32to determine its depth below the surface of the water. The processing circuits22control overall operation of the location reference unit20, process position signals received by the GPS receiver26, and generate information and control signals to be transmitted to the beacon units50via sonar transceiver28. Processing circuits22also process information and control signals received from the beacon units50. Memory24stores program instructions and data needed for operation. Sonar transceiver28is used for underwater communications with beacon units50. Location reference units20could also use the sonar transceivers28or radio transceivers (not shown) to communicate with other location reference units20. In some embodiments, the location reference unit20may further include a relay34activated by the processing circuits22to sink or destroy the location reference unit20after the beacon units50have determined their locations.

FIG. 3illustrates the main components of the beacon unit50. Beacon unit50comprises processing circuits52, memory54, a sonar transceiver56, and pressure sensor58. Processing circuits52control the overall operation of the beacon unit50, and process information and control signals received from the location reference unit20, other beacon units50, and/or diver units100. Memory54stores program instructions and data needed for operation. Sonar transceiver56is used for underwater communications with the location reference unit(s)20, other beacon units50, and/or diver units100. The pressure sensor58is used to determine the depth of the beacon unit50, which may be used in some position calculations. In some embodiments, the beacon unit may further include a compass60to determine the rotational orientation of the beacon unit50.

When the beacon units50are initially deployed, the beacon units50must determine their position after sinking to the ocean floor. While dropping to the ocean floor, ocean currents may carry the beacon units50a substantial distance from the original drop zone. Therefore, it is not sufficient for the beacon units50to determine their location before sinking. As earlier noted, the beacon units50determine their location after reaching the ocean floor by exchanging messages with one or more location reference units20. Once the location of a beacon unit50is known, the beacon unit50can also serve as a location reference unit20for other beacon units50. Two basic approaches may be taken to determine the location of the beacon units50.

The first approach is based on the calculation of the distance from the beacon unit50to multiple location reference units20. At least three location reference units20are needed to accurately determine the location of the beacon unit50using this method. Each location reference unit20determines its location based on receipt of GPS signals and transmits its location to the beacon unit50. The distance of the beacon unit50from each location reference unit20is also determined. The distance may be determined by the location reference units20and transmitted to the beacon units50, or may be determined by the beacon units50themselves. Based on the distance of the beacon unit50from the location reference units20and the location of the location reference units20, the beacon unit50can determine its location accurately by triangulation. Because the beacon units50may be assumed to be below the surface of the water, a hence below the location reference units20, the beacon units50can accurately determine their location based on its distance from three location reference units20and its depth. The depth of the beacon unit50may be determined based on a pressure sensor. Alternatively, one or more of the location reference units20could calculate the position of the beacon unit50based on distance measurements and transmit the location to the beacon unit50.

The second approach requires only one location reference unit20. In the second approach, the location reference unit20determines its location based on receipt of GPS signals and transmits its location to the beacon unit50. The beacon unit50includes a compass to determine the rotational orientation of the beacon units50. The beacon unit50determines the direction and distance to the location reference unit20. With knowledge of the location reference unit's location and the vector extending between the location reference unit20and the beacon unit50, the beacon unit50can accurately determine its location. Alternatively, the location reference unit20could include a compass and compute the location of the beacon unit50and transmit the computed location to the beacon unit50.

Various techniques can be used for determining the distance between the location reference unit20and beacon unit50. Three exemplary methods for determining distance are described below. These three methods are referred to herein as the time of arrival method, the time of travel method, and the dual tone method. Those skilled in the art will appreciate that the present invention is not limited to the methods enumerated herein and that other methods may be used for determining distance.

The time of arrival method requires clock synchronization between the location reference unit20and beacon unit50. In this method, the beacon unit50sends a message to the location reference unit20requesting the location reference unit20to transmit a response message at a time known to the beacon unit50. The request or response message may specify the transmit time, or the transmit time may be specified by a protocol. For example, the protocol may specify that the location reference unit20transmit a response message only when the m least significant bits of the location reference unit's clock are all 0. Because the clocks are synchronized, the beacon unit50can use the time of arrival of the signal to compute the distance to the location reference unit20. Using the m least significant bits as a time of transmission reference is advantageous, however, it results in aliasing of the distance measurement. For example, the m least significant bits will be the same if the message is sent at exactly 2:05 and 2:06 PM. The ambiguity can be resolved by repeating the message and using the time of travel or dual tone methods described below.

The time of travel method does not require clock synchronization. In this method, the beacon unit50sends a message to the location reference unit20. Upon receipt of the message by the location reference unit20, the location reference unit20generates and sends a reply message to the beacon unit50. The reply message includes a delay value indicating the delay between the time the first message was received at the location reference unit20and the time that the reply message was sent. The beacon unit50may use the round trip time and the turnaround delay to compute the distance to the location reference unit20.

The dual tone method uses the fact that acoustic signals transmitted at different frequencies will attenuate at different rates. In this method, the beacon unit50sends a message to the location reference unit20requesting the location reference unit20to send a dual tone signal. In response, the location reference unit20transmits a dual tone signal comprising two distinct tones. The distinct tones may be transmitted with equal power. Alternatively, the distinct tones can be transmitted at different power levels if the power levels, or the power ratio, are known. The power in each tone will attenuate as a known function of the distance traveled. With knowledge of the attenuation rate for each tone component, the beacon unit50can compute distance to the location reference unit20based on the difference in the received power of the tone components.

Those skilled in the art will appreciate that the operations of the beacon unit50and location reference unit20in the distance calculation could be reversed. That is, the location reference unit20could compute the distance to the beacon unit50and transmit the distance to the beacon unit50.

To determine the direction to the location reference unit20, the sonar transceiver for the beacon unit50comprises an array of sonar transducers. Assuming that the rate of travel of a signal in water is known, the beacon unit50can compute the direction to the location reference unit20based on the time difference of arrival of a signal transmitted by the location reference unit20at each of the sonar transducers. In a preferred embodiment, three (3) receiver transducers are positioned such that one transducer lies along a line perpendicular to a line drawn between the other two transducers. If the beacon unit50is positioned above the sea floor such that divers or underwater vehicles could operate below the beacon unit, then four (4) transducers are used, with the fourth transducer placed out of the plane defined by the first three transducers.

During deployment of the beacon units50, the location reference unit20acts as a master unit and controls communication with the beacon units50. The beacon units50sink to the ocean floor and remain silent until activated by the location reference unit20. The location reference unit20may activate the beacon units50one at a time by sending an activation code to the beacon unit50. Alternatively, the beacon units50could initiate communication with the location reference unit20after reaching the floor of the ocean. After waking, the beacon unit50determines its location as previously described by exchanging signals with the location reference unit20. Once the beacon unit50has determined its position, the beacon unit50may send a confirmation message to the location reference unit20. The confirmation message indicates to the location reference unit20that the beacon unit50has determined its location. The location reference unit20can then send a de-activation code to the beacon unit50so that the beacon unit50returns to a sleep mode. In sleep mode, the transmitter of the beacon unit50is turned off, but the beacon unit50periodically activates the receiver to monitor for incoming messages. The beacon unit50will remain in an inactive mode or sleep mode until it receives an activation code. In some embodiments, the beacon units50may be programmed to wake at a predetermined time to avoid the need to transmit an activation code to wake the beacon unit50.

After a predetermined period of time or after receiving a confirmation message from a predetermined set of beacon units50, the location reference unit20is removed from or leaves the area to avoid detection and thereby permit covert underwater navigation in the area. The means by which the location reference unit20is removed may depend on the specific implementation and design of the location reference unit20. In some embodiments, the location reference unit20may sink or destroy itself after the beacon units50determine their location. In other embodiments, the location reference unit20may comprise a surface vessel, aircraft, or underwater vehicle that can leave the vicinity of the beacon units50once the beacon units50are deployed and their location determined.

In the embodiment illustrated inFIG. 1, for example, the location reference unit20is implemented as a surface unit comprising a surface buoy. The surface buoy and beacon units50are both dropped into the ocean, such as by aircraft. The beacon units50sink to the ocean bottom while the surface buoy floats on the surface long enough for the sunken beacon units50to determine their locations. Once the beacon units50have determined their locations, the surface buoy may sink or self-destruct. Alternatively, the surface buoys can be removed by a surface vessel, aircraft, or underwater vehicle.

To sink or destroy the surface buoy, the location reference unit20may further include a relay34. The relay34is actuated by the processing circuits22to sink or destroy the surface buoy after a predetermined period of time or after a certain number of the beacon units50have established their locations. The processing circuits22may, for example, generate a control signal that causes the surface buoy to sink or self destruct. In one embodiment, the control signal activates a relay34that controls a mechanical system on the surface buoy. For example, activation of the relay34may cause floatation devices attached to the surface buoy to detach, thus allowing the surface buoy to sink. In another embodiment, the relay34may open a valve, causing a ballast tank to flood with water. The particular mechanism employed to effectuate the sinking or destruction of the surface buoy is not a material aspect of the invention. Any known methods for sinking or destroying the surface buoy can be used. In some embodiments, the location reference unit20may switch to a beacon unit mode upon sinking and thereafter function as any other beacon unit50.

As illustrated inFIG. 4, however, the location reference unit may comprise a surface vessel. In such instances, the surface vessel may simply ship into the area where underwater navigation assistance is needed, drop the beacon units50, transmit location information received from receiver26to the beacon units50via sonar transceiver28, and ship out of the area after the beacon units50determine their location. Because the surface watercraft can transport itself to an area where it is unexposed, easy detection of the underwater navigation system10on the surface is minimized without sinking or destroying the location reference unit20.

Likewise, the location reference unit20may comprise a helicopter, as illustrated inFIG. 5. Again, re-positioning of the location reference unit20by sinking or destroying it is likely undesirable and unnecessary. Rather, the helicopter may fly into the area where underwater navigation assistance is needed, drop the beacon units50, transmit location information received from receiver26to the beacon units50via dipping sonar transceiver28into the water, and fly out of the area after the beacon units50determine their location. As the helicopter can transport itself to an area where it is unexposed, easy detection of the underwater navigation system10on the surface is minimized without sinking or destroying the location reference unit20.

The embodiments illustrated thus far inFIGS. 1-5have all relied upon communication with earth orbiting satellites12or terrestrial navigation systems at the area where navigation assistance is needed to determine the location of the location reference unit20. Such communication permits very accurate localization, but requires surface exposure in the area for at least the amount of time necessary for a number of the beacon units50to determine their location.

FIG. 6illustrates an alternative embodiment of an underwater navigation system10. In this embodiment, the location reference unit20comprises an underwater vehicle which remains below the surface to communicate location information as described above to the beacon units50. The location reference unit20may determine its location while it is on the surface of the water. While the location reference unit20is submerged, it may use an inertial navigation system40(FIG. 7) to keep track of its location.

The inertial navigation system40may, for example, measure the linear and angular acceleration applied to the location reference unit20since being at the starting location. Such internal measurements do not rely on navigational signals receivable only at the surface and may be made while the location reference unit20is entirely submerged. The only necessary exposure of the location reference unit20to the surface, therefore, occurs when determining the starting location with navigational signals.

Using an inertial navigation system40, then, the location reference unit20may determine a starting location by receiving navigational signals at an area other than where underwater navigation assistance is needed. Also, the starting location may be the known coordinates of a port of call. The location reference unit20may thereafter submerge and travel to the area where underwater navigation assistance is needed. While traveling beneath the surface, the inertial navigation system40tracks the location of the location reference unit20. Upon reaching the area where underwater navigation assistance is needed, the location reference unit20transmits location information determined by the inertial navigation system40to the beacon units50. Because the inertial navigation system40determines the location of the location reference unit20without relying on navigational signals received at the area where underwater navigation assistance is needed, surface exposure in this area is eliminated.

The main components of the location reference unit20utilizing an inertial navigation system40are illustrated inFIG. 7. The location reference unit20comprises processing circuits22, memory24, a sonar transceiver28, and an inertial navigation system40. Analogous to the embodiment described inFIG. 2, the processing circuits22control overall operation of the location reference unit20, process position signals received by the inertial navigation system40, and generate information and control signals to be transmitted to the beacon units50via sonar transceiver28. The processing circuits22also process information and control signals received from the beacon units50. The memory24stores program instructions and data needed for operation. The sonar transceiver28is used for underwater communications with beacon units50. The compass30is used to determine the rotational orientation of the underwater vehicle and the pressure sensor32is used to determine its depth below the surface of the water. The inertial navigation system40tracks the current location of the underwater vehicle from a known location.

Like the embodiments described previously, after a predetermined period of time or after receiving a confirmation message from a predetermined set of beacon units50, the location reference unit20leaves the area where the beacon units50are deployed to prevent discovery. Because the submarine can transport itself to an area where it is unexposed, easy detection of the underwater navigation system10on the surface is eliminated without sinking or destroying the location reference unit20.

It should be noted, however, that while the embodiments ofFIGS. 6 and 7provide location information to beacon units50without surface exposure, the accuracy of such location information may be less than that provided by the embodiments ofFIGS. 1-5. Because the inertial navigation system40may be unable to account for drift in location not caused by applied linear or angular acceleration, the location information provided may be offset. This may be the case, for example, in areas having strong ocean currents. Such offset may be minimized by limiting the distance traveled between the starting location and the area where underwater navigation assistance is needed, thereby decreasing the accumulated drift. This, however, requires surface exposure closer to the area where underwater navigation assistance is needed. Utilization of these alternative embodiments, therefore, may depend on the particular requirements for surface exposure and accuracy of localization.

FIG. 8illustrates that after the beacon units50are deployed, the beacon units50can be used by divers or underwater vehicles to navigate covertly without any surface exposure. The beacon units50could also provide navigation assistance to surface vessels. The diver carries a diver unit100that communicates with the previously-deployed beacon units50. Each beacon unit50knows its location and can transmit its location to the diver unit100. The methods described above for determining the location of the beacon units50can also be employed by the diver units100to determine their location. In this case, the beacon units50provide the location reference for the diver unit100.

FIGS. 9 and 10illustrate an exemplary diver unit100. The diver unit100comprises a waterproof housing102mounted on a wristband104. The diver unit100includes an electronic display106, such as a liquid crystal display, and one or more input devices108. The exemplary embodiment shown inFIG. 9includes a scroll wheel110and a SEND/ENTER button112. Those skilled in the art will recognize that other input devices, such as a joystick controller, keypad or touchpad, could be used for user input. Additionally, the display106may comprise a touchscreen display to receive user input.

The border114of the display106includes a series of labels116that describe various functions of the diver unit100, e.g., “buddy,” “beacon,” etc. A function indicator118points to the currently selected function. InFIG. 9, the function indicator118indicates that the log function is selected. Additionally, display106may display other status indicators, such as power indicator120and alarm indicator122, to provide the user with status information. The function indicator118can be moved to select a function by rotating the scroll wheel110and pressing the “ENTER/SEND” button112. Selection of a function may change the operating mode of the diver unit100. Scroll wheel110and button112may perform differently depending on the current operating mode. For example, after a function is selected, the scroll wheel110could be used to scroll through menu options or lists presented on the display106.

In addition to status indicators, the display106is used to output useful information to the diver for viewing. In the exemplary embodiment, the display106can display a directional indicator124. As will be described in more detail below, the directional indicator124is used to indicate direction to a target and the current track for navigating under water. In the embodiment shown, the directional indicator124includes a first pointer126showing the current track, and a second pointer128showing the direction to the target. The display106may also display numeric and alphanumeric data to the diver. In the exemplary embodiment shown inFIG. 9, the display106is displaying the distance to a target (DIST) (420 meters), the estimated time en route (ETE) (6 min), the bearing (BRG) to the target (340 degrees), and the current track (40 degrees). Other information, such as the current depth, current time, current temperature and the current latitude and longitude, or any other useful information could also be displayed. These examples are not intended to be a comprehensive list of all information that can be displayed, but merely illustrative of the types of information that may be displayed.

FIG. 10is a functional block diagram illustrating the main components of the diver unit100. The main components comprise processing circuits150for processing data and controlling operation of the diver unit100, memory152for storing code and data used by the processing circuits150, a user interface154that includes the display106and user input devices108, and a communications interface156. The processing circuits150may comprise one or more programmable processors, which may be general purpose microprocessors, microcontrollers, digital signal processors, or a combination thereof. Memory152represents the entire hierarchy of memory within the diver unit100and may comprise discrete memory devices, or may comprise internal memory in one or more microprocessors. The communications interface156comprises a radio interface158for use above water, and a sonar transceiver60for underwater communications. The radio interface may comprise, for example, a conventional BLUETOOTH, 802.11b, or 802.11g interface. The diver unit100may further include a pressure sensor162to determine depth below the water's surface.

The diver units100use a request/response signaling scheme to communicate with beacon units50. The diver units100send a request message to either a beacon unit50or another diver unit100to initiate a transaction. Response messages are sent in reply to request messages. More than one response message may be sent in reply to a request message. The request message and all corresponding response message constitute a transaction. In the exemplary embodiment, the beacon units50do not send request messages, but only send response messages in reply to request messages from the diver units100. A multiple access schemes, such as frequency division multiple access, time division multiple access, or code division multiple access, may be used to enable communications between the beacon units50and multiple diver units100.

When a diver unit100needs to determine its location, the diver unit100sends a request message containing an activation code to a beacon unit50to initiate communications with the beacon unit50. The activation code may be encrypted to prevent unauthorized users from activating the beacon unit50. The beacon unit50may also authenticate diver units100via a pre-programmed list of diver unit (source) addresses or IDs that are permitted to obtain information from the beacon unit50. The diver unit100may initiate communications with more than one beacon unit50. After waking, the beacon unit50sends a reply message. The diver unit100can then send request messages to the beacon unit50to obtain navigation assistance. Exemplary uses of request messages include synchronizing clocks with a beacon unit50or establishing location. Once activated, the beacon unit50will remain awake until receipt of a deactivation code from the diver unit100, or until a predetermined period of time has elapsed without any communications. While in an active or wake state, the beacon units50will receive and respond to authorized request messages. Various techniques can be used to prevent the beacon unit50from responding to unauthorized requests. For example, the beacon unit50may require diver units100and other units sending requests to provide authentication before responding to the requests. Additionally, the beacon unit50may transmit a periodic signal while in the active state to enable diver units100to periodically update their location without the need to send an explicit request message to the beacon unit50. For example, the beacon unit50may periodically transmit a periodic message containing a time stamp indicating when the periodic message was transmitted. If the diver unit clock is synchronized with the beacon unit clock, the periodic message can be used by the diver unit100to determine distance to the beacon unit50. The periodic message could also be used to determine direction to the beacon unit50. If the diver unit clock is not synchronized, the periodic message could be a dual tone signal so that the diver unit100can determine distance by the received power of the tone frequencies.

In one exemplary embodiment, a time division multiple access scheme is used to enable a plurality of diver units100to communicate with the beacon units50using the same frequency. TDMA divides the communication spectrum into sequential time slots that are used to transmit and/or receive data. A device transmits and/or receives only in its assigned time slot(s). The set of non-repeating time slots constitutes a frame. Normally, a frame is a fixed-length. In the present invention, the frame is a variable length frame that accommodates propagation delays between diver units100and beacon units50.

If Chip Scale Atomic Clocks are included in the beacon unit50and diver units100, then clock synchronization is maintained over a usefully long time. In this case, the beacon units50can be programmed to periodically transmit its location. Passing divers or submarines can receive the signal and calculate their distance from the beacon unit50by the Time of Arrival method. This method allows divers and submarines to maintain stealth.

FIG. 11illustrates the frame structure of a TDMA frame accommodating n diver units100. The frame is divided into 2n slots of 20 msec duration each, where n is the number of diver units100. One diver unit100is designated as the master diver unit100and controls communications. The master unit may be selected, for example, by choosing the one with the highest or lowest unit ID. The master diver unit100assigns timeslots to the other diver units100. The first two time slots in the frame are reserved for the master diver unit100. Each diver unit100controls the use of its allocated time slots. A diver unit100may use its first timeslot to transmit a request message to a beacon unit50or other diver unit100, and its second timeslot to receive a response message from the beacon unit50or other diver unit100.

FIG. 11also illustrates the slot format. Within each time slot, there is a guard band at the beginning and end of the time slot. Those skilled in the art will appreciate that propagation delays may result in the transmission of one diver unit overlapping the time slot of another diver unit100. The guard bands are preferably wide enough to account for overlapping signals due to propagation delay. The remaining time within each time slot is used to transmit message data. In one embodiment of the invention, each time slot is used to transmit or receive one message. However, those skilled in the art will appreciate that multiple messages could be transmitted in a single time slot if the duration of messages is short relative to the slot period. Conversely, if the message length is greater than the slot period, a message could be segmented and transmitted over multiple slots.

As shown inFIG. 11, the diver units100transmit in a predetermined sequence. In this exemplary embodiment, only diver units100have assigned slots. Beacon units50transmit only in response to the diver units in the second slot after receiving a message from the diver unit100. In other embodiments, beacon units could also be assigned slots and initiate communications with diver units100. Before communications begin, the master diver unit100establishes the transmit order and notifies the other diver units100of their place in the sequence. During an initialization procedure, the master diver unit100transmits an initialization message to the other diver units100that identifies the preceding diver unit100in the sequence. The other diver units100acknowledge the initialization message. During normal operations, each diver unit100listens for the address of the preceding diver unit100, which can be determined by the source address element of the messages transmitted using slot1, and the source address or destination address of messages using slot2.

A problem may arise when an obstruction blocks signals between two diver units100that transmit consecutively. In this situation, the second diver unit100may not be able to “hear” communications from the first diver unit100. One consequence is that communications will stall because the second diver unit100will not transmit. In this situation, the master diver unit100may restart a transmit sequence if no signals are detected within a predetermined period of time. If the communications continue to stall, the master diver unit100can invoke the initialization procedure to change the transmit order. If changing the transmit order does not solve the problem, the diver unit100that is stalling the communications can be dropped from the transmit sequence.

Underwater communications between the diver units100and beacon units50occur at sonar frequencies. In the exemplary embodiment, the carrier frequency is 200 kilohertz and the bit period is 0.06 msecs (60 μsec), which equates to a data transmission rate of approximately 15.56 kbps. A message comprises a total of 112 bits plus 8 sync bits. Thus, it takes 7.2 msec to transmit the message. The 112 bit message contains 32 bits of user data. The other bits (command, source address, destination address, etc.) are “overhead.” Thus, the effective data rate is 32 bits/7.2 msec=417 bits per second, assuming zero inter-message gap, i.e. no “listen” intervals.

FIG. 13illustrates how data is modulated onto a carrier frequency.FIG. 13illustrates a clock signal, data signal, and modulation signal. The clock signal has a period of 60 μsec, which is equal to the bit period. The data signal represents binary data that is being transmitted. The modulation signal comprises a pulse train at the same frequency as the clock signal. The pulse width of a given pulse in the modulation signal is determined by the state of the data signal. When the data signal is low, a relatively short pulse is generated to indicate a “0” bit. Conversely, when the data signal is high, a relatively long pulse is generated to indicate a “1” bit. In the exemplary embodiment, the short pulses indicative of a 0 bit are 15 microseconds in duration and the long pulses indicative of a 1 bit are 45 microseconds. On/Off keying is used to modulate the carrier. The carrier is turned on when the modulation signal is high, and is turned off when the modulation signal is low. The receiver samples the signal 37 microseconds after detecting the rising edge of a pulse to detect the signal. In an alternative embodiment, a variable sampling time after detection of the rising edge may be used to compensate for variations in the clock rate at the transmitter and/or variations due to Doppler shift of the carrier frequency. More particularly, the receiving unit may measure the time between rising edges of the pulses in the received signal to determine the period and adjust the sampling time accordingly. The measurement of the clock period may be performed when the sync word is being transmitted. In one exemplary embodiment, the sampling time Tsis determined by multiplying the transmit clock period by 0.625.