Patent Publication Number: US-2021172742-A1

Title: Underwater navigation

Description:
The present invention relates to an underwater navigation system. In particular, the invention relates to the use of underwater electromagnetic propagation to determine a receivers position relative to a node or distributed assembly of nodes. 
     BACKGROUND 
     Underwater navigation has typically been accomplished using inertial navigation or acoustic nodes. Acoustic systems are degraded by noise and interference from a number of sources. They are also subject to multi-path effects and in some environments are virtually unusable. Inertial navigation systems are complex, bulky, high cost, accumulate inaccuracy over time and require knowledge of an initial reference point. 
     U.S. Pat. No. 6,865,139 describes a sub-sea navigation system that uses electromagnetic transmission. This has a plurality of antennas located at known positions on a sub-sea structure. Each antenna is electrically coupled and includes a cathodic protection anode. Signals emitted by the antennas are used by a detection means on a sub-sea vehicle to allow the vehicle to navigate relative to the sub-sea structure. The detection means uses a measure of the electric field of the emitted signals in order to determine the position of the vehicle. A problem with the system of U.S. Pat. No. 6,865,139 is that the signals emitted by the electrically coupled antennas are subject to high near field attenuation and the receive antennas have low efficiency. This reduces the range over which position can be determined and limits the applicability of the system. Also, there is little information provided on how exactly the position is determined using the measure of electric field. 
     U.S. Pat. No. 8,315,560 describes a navigation system which uses electromagnetic transmission with multiple nodes providing transmission signals enabling cross reference of the multiple signals to determine the position of the, for example, a passing AUV. However, this system depends on the AUV being within range of multiple nodes at a given time. 
     High quality Ins are expensive and unable to accommodate many days of operation without correction. Acoustic repeaters produce a:foot-print ̆ that is readily detected and are unable to operate for more than 1-2 years between battery swaps. Passive sonar devices require the use of a sonar which is detectable. Further, passive devices are unable to support 2-way data transmissions. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided an underwater navigation system comprising: a transmitter for transmitting an electromagnetic signal, a receiver for receiving an electromagnetic signal from the transmitter, and means for determining the position of the receiver relative to the transmitter using the received electromagnetic signal. 
     In far field electromagnetic propagation, the relationship between the electric and magnetic field is determined by the transmission media ̆s characteristic impedance. An electrically coupled antenna launches a predominantly electric field that transitions to the characteristic impedance over an area known as the near field. 
     Underwater attenuation is largely due to the effect of conduction on the electric field. Since electrically coupled antennas produce a higher E-field component, in the near field the radiated signal experiences higher attenuation. The same performance issues apply to a receive antenna. Magnetic coupled antennas do not suffer from these problems and so are more efficient under water than electrically coupled antennas. 
     Using an electrically insulated antenna provides further advantages. This is because for a non-insulated electrically coupled antenna, there is a direct conduction path between it and the dissipative water. This leads to dissipation as the signal propagates along the antenna even before the electromagnetic signal is launched. Providing an insulated antenna reduces this effect. 
     The means for determining the position of the receiver may be operable to determine the distance of the receiver from the transmitter and/or the direction of the receiver relative to the transmitter. These may be determined using signal strength at the receiver and/or the direction of signal propagation at the receiver and/or the time taken for a signal to travel between the transmitter and the receiver. 
     The field strength may be used to determine proximity based on strength of received signal for a given transmitter power and propagation characteristics. To this end, means are provided for measuring the strength or magnitude of the received signals. 
     F or most applications calculation of the receivers range to the transmitter can be based on a typical physical model of the underwater environment. This model could be improved by measurement of attenuation using a comparison of signal strength between multiple antennas with known relative positions within the navigating station. 
     The direction of signal propagation may be determined by alignment of a highly directional antenna or based on comparison of the field strength received by several antennas distributed in space. In the latter case, because of the high attenuation per metre experienced in water, a local loss gradient vector can be established by comparison of field strength measured from the multiple antennas. Attenuation will be measurable within the dimensions of a typical mobile vehicle. 
     The transmitter and receiver may be operable to simultaneously provide a communication links. 
     According to another aspect of the present invention, there is provided an underwater navigation system comprising: a transmitter for transmitting an electromagnetic signal, a navigation station having receiving means for receiving a signal from the transmitter, and determining means for determining the position of the station using signals received by the receiving means at three or more different positions. 
     By using signals received at a plurality of different receiver positions, the position of the navigation station can be determined relative to a single transmitter. This reduces the number of nodes required and allows applications where location of an isolated object is required rather than the distributed objects required by a multiple transmit antenna system. 
     The receiving means may comprise three or more spatially separated receivers. In this case, the determining means may be operable to determine the position of the station using a signal from each of the receivers. A n advantage of this is that the measurements can be taken simultaneously. 
     The receiving means may include a single antenna. To determine the position of the station, the antenna would be moved to three or more different measurement positions. This could be done either by moving the station or by moving the antenna. In this latter case, the antenna could be provided at the end of a rotating arm. In any case, an inertial navigation system could provide accurate short-term knowledge of the relative position of successive measurements as the vehicle moves through the water. Position relative to the node may then be determined using standard trigonometry. 
     The means for determining may be operable to determine the directional position of the receiver using the magnitude of the field at the plurality of receiver positions. 
     The means for determining the position may be operable to determine proximity to the transmitter using the magnitude of the signal received at one or more receiver positions. 
     The transmitter may include an electrically insulated magnetic coupled antenna for transmitting an electromagnetic signal. 
     The receiver may include an electrically insulated magnetic coupled antenna for receiving an electromagnetic signal from the transmitter. 
     According to another aspect of the present invention, there is provided an underwater navigation system comprising: a transmitter for transmitting an electromagnetic signal, a receiver for receiving a signal from the transmitter, and means for determining the position of the receiver using the magnitude of the received signal. 
     According to yet another aspect of the present invention, there is provided a system for determining underwater electromagnetic signal propagation direction comprising at least one receiver for receiving the electromagnetic signal and determining means for determining the direction of propagation using field strength at three or more receiver locations. Three or more receiver antennas may be provided and the determining means may be operable to determine the direction of propagation by comparison of the field strength received at each antenna. 
     Alternatively, a single receiver antenna may be provided and the determining means may be operable to determine the direction of propagation by comparison of the field strength received at three or more different receiver locations. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which: 
         FIG. 1  is a block diagram of an underwater navigation system; 
         FIG. 2  is a schematic diagram of a section of deployed system according to the present invention; 
         FIG. 3  is a schematic diagram of a section of deployed system according to the present invention; 
         FIG. 4  is a block diagram of an underwater transmitter for use in the underwater navigation system of  FIG. 1 ; 
         FIG. 5  is a block diagram of an underwater receiver for use in the navigation system of  FIG. 1 ; 
         FIG. 6  is a diagrammatic representation of a magnetically coupled solenoid antenna in a waterproof enclosure for use in the transmitter of  FIG. 2  and the receiver of  FIG. 3 ; 
         FIG. 7  is a diagrammatic representation of another direction finding technique, and 
         FIG. 8  is a diagrammatic representation of the field pattern produced by a magnetically coupled solenoid antenna. 
     
    
    
       FIG. 1  shows a mobile navigating station  10  that is operable to navigate around an underwater environment using magnetic radiation transmitted between one or more fixed position nodes  12  and at least one receiver  14  carried on the mobile station  10 . Due to the short range nature of underwater electromagnetic propagation, if a signal is detectable above a given threshold the receivers  14  position is known to be in close proximity to the transmitting node  12 . To allow the navigating station  10  to differentiate between individual sources in a multi-node environment, the transmitting nodes  12  may produce an identifying signal. For example, each transmitter  12  may broadcast on a different frequency. Alternatively, each transmitter  12  may encode some form of identifying modulation. 
     With reference to  FIG. 2 , the nodes  12  may be deployed on a seabed  15 , which is formed of sedimentary layers  5  and solid rock strata  3 . Mobile nodes, which may be transceivers but in this case are receivers  14  can be deployed on divers, AUV s, tethers to the seabed or on land, are operable to receive data from fixed nodes  12 . Data transmission can follow paths through water, the water air interface, ground or air to be received by the node  14  deployed, in this case, on land. 
     Electromagnetic signal transmission by nodes  12  provides a secure, persistent and low-cost and low-power method of providing location, navigation and communications with a water based vehicle, in this case an AUV  10  operating over a wide area within the ocean. In this embodiment, and with reference to  FIG. 3 , the network  11  of nodes  12  comprises wireless nodes  12  spaced at 20-25 km intervals on the seabed  15  which can be configured to act as ‘wireless white lines’  17 . AUV  10  is provided with a INS (Inertial Navigation System) integrated within a processor mechanism with a communication unit which uses electromagnetic signal transmission to transmit a wake-up signal to seabed nodes  12 . When the AUV  10  passes within 250 m of a node, the node is wakened and then transmits ID and location information. Magnetic field strength data is used to correct the INS within the AUV  10 . 
     The node network  11  reduces the cost and complexity of AUV  10  navigation, allowing AUV  10  to act as data mules, interlinking smart wireless nodes  12  in, for example, creating and using Subsea Cloud Computing Networks. 
     In the event GPS is compromised, and above water navigation techniques are not useable, the node network  11  will provide an independent, reliable and persistent location and navigation system for underwater vehicles and, in littoral waters, for surface vehicles and low flying UAVs. 
     The underlying principles of operation of the node network  11  and nodes  12  includes low frequency electromagnetic transmission signals which attenuate in seawater at approximately 55 dB/wavelength; thus each node  12  has a transceiver  14  having a low frequency electromagnetic transmission transmitter produces a field that propagates up to 40 m through the water column and up to 250 m across the seabed. 
     The field strength emitted by the nodes  12  varies in a similar manner to Ordnance Survey contour lines and as a result, the field strength at any point is measurable and can thus be used to identify the location of the node  12  and of the AUV  10 . 
     When AUV  10  cuts through a field emitted by the node  12 , the AUV  14  is taking measurements of field strength using receiver within transceiver  14 , the location, orientation, speed, and direction of the vehicle can be determined 
     The performance of nodes  12  improves when embedded in the seabed  15  as the seabed presents a path of lower conductivity however, the nodes  12  may also be placed on the surface of the seabed  15 . It will be appreciated that a processor  38  within each node  12  may be provided with an algorithmic function to correct the transceiver signal strength for seabed properties. 
     In use, each node  12  is pre-programmed with a unique identification number and a GPS coordinate set at the time of deployment. 
     An integrated accelerometer can be provided within node  12  and be used to monitor movement of the node  12  during deployment and may be used to refine the accuracy of the resting location. The integrated accelerometer is also able to be used to determine the orientation of the integrated antenna within node  12   b  and provide information on the field orientation which is used to refine location information. 
     Antenna construction and antenna size have a material impact on system performance of the nodes  12 . For example, ferrite, loop and squids may be used as the antenna. Ferrite and loop technologies are mature. Ferrite sensors are compact but have a defined orientation. Loop antennas are generally larger than ferrite but are not orientation sensitive and more sensitive. Room temperature Squid is less mature and will require investigation but may prove to be more compact and more sensitive than either. 
     The seabed conductivity will impact the lateral propagation of magnetic fields. The system will incorporate a standard model used to determine field strength at a distance. The model may be based on a magnetic field modelling tool. The field propagation patterns may optionally be pre-adjusted by survey data providing further information on actual conductivity of the seabed. Seabed conductivity may optionally be refined using sensors incorporated in the node  12  as the seabed is not homogenous and thus transmission distance may be extended or foreshorted from an average. The pre-map system can provide data to account for this or alternatively transmission of signals at two different frequencies can be used to model the seabed materials. 
     Nodes  12  further incorporate a power source (not shown) in this case, a battery having battery management technology that allows seabed nodes  12  to run in Receive mode at a very low energy level (typically &lt;1 mW). Commercial off the shelf systems can, for example, run for up to 30 years on modest battery packs. 
     Nodes  12  have a low probability of intercept due to the use of electromagnetic signals for communication. To use nodes  12 , they must be activated by the correct frequency. They may optionally be configured to require two or more frequencies configured as a:lock and key ̆ to improve security. Tamperproof data erasing techniques may also be incorporated for further security. 
     The nodes  12  may be deployed by surface vehicle, aerial vehicle or underwater vehicle  10  at a time of choosing −  possibly many years in advance. Each node  12  may optionally incorporate a wirelessly triggered flotation device to allow recovery. 
     A navigation INS correction system is integrated with underwater vehicles  10 , surface or aerial vehicles to correct navigation during communication with the nodes  12 . In an AUV  10 , a:drop-in ̆module incorporating wireless communications systems. GPS correction information is streamed into the vehicle INS whilst the vehicle  10  is within the vicinity of the node  12 . 
     In use 2-way data communications link is set up whilst the vehicle is within range of the node. 
     The node transmits its unique ID, and GPS location to the passing vehicle  10 . The node can optionally transmit further information to the transiting vehicle, for example node health data, instructions for the vehicle  10 . The transiting vehicle  10  can optionally transmit information to the node, for example health data, mission information, software updates. 
     The node system  11  provides improvement over existing underwater vehicle INS systems by delivering significant extension in the distance AUV  10   s  can operate before resurfacing for INS correction. This dramatically reduces the cost of accurate navigation for AUV  10   s  by transferring the investment to infrastructure and provides resilient and reliable fall-back in event GPS unavailable. The network  11  provides persistent, non-deniable, waypoint nodes  20  to correct INSs and is deployable for up to 30 years on a single battery pack which operates in shallow and deep water and is unaffected by turbidity, biofouling, ambient noise which is deployed readily using existing hardware and the navigation system  11  is unaffected by EMP. 
     Systems nodes can be configured to be recovered using integrated wireless release trigger and deployments so then odes can be reused. The nodes  10  can be used to provide a low cost, covert method of monitoring critical infrastructure and a rapid and covert method of providing improved navigation in complex operating environments for AUV  10   s , diver, SDV and submarines. 
     Magnetic field pattern simulations will be incorporated within the nodes of system  11  and these will be used to estimate the field patterns produced by of loop and ferrite transmit antennas in a range of seabed conditions and pre-selected carrier frequencies. Algorithms which provide the accurate location, orientation, direction and speed of a vehicle by matching field strength data against pre-determined field patterns will also be incorporated within the nodes  12  and AUV  10 . 
     A user interface can be used prior to deployment to configure the network  11  and to configure each node  12  prior to deployment and subsequently to update software and to interrogate the network  11 . 
     Room temperature SQUID sensors are suitable for incorporation into the nodes  12  and processor electronics within nodes  12  will be pre-set with system design parameters with variables including system centre frequency, receive sensitivity, transmit power and wake circuit. 
     Each node  12  is, in this case, a single node incorporating electronics, battery and antenna that is sealed for life and designed for ease of deployment and recovery. 
     In one embodiment, the system design will, for example, enable navigation correction at 250 m from a node for an AUV  10  travelling at 5 m above the seabed with integrated INS correction. 
     Seabed conductivity variability leads to reduced operating range and system accuracy and this can be mitigated by local field characterisation using a fly-by AUV  10 . In areas of high turbulence or disturbance, the unpredictability of water currents requires nodes  12  to be placed at shorter intervals from one another. The integration of water current sensors in nodes  12  may provide improved information on local conditions to assist with refining vehicle way-point guidance 
     The system  11  has applications in a number of industries including:
         Oil &amp; Gas: using low cost AUV  10   s  to reduce the cost of asset integrity management;   Offshore wind, wave tidal: using low cost AUV  10   s  to reduce the cost of asset integrity management and environmental footprint monitoring;   Telecoms: using low cost AUV  10   s  to reduce the cost of monitoring submarine cables;   Environmental: using low cost AUV  10   s  to low the cost of building out wide area sensor networks to provide improved weather, oceanographic and climate change modelling;   Aquaculture: using low cost AUV  10   s  to reduce the cost of monitoring remote fish cages and environmental footprint monitoring sensors.       

     It will be appreciated that the system  11  will can improve location accuracy through increased use of environmental monitoring sensors; increase system intelligence through integration with Subsea CI oud Computing Network technology, incorporating Machine Learning; increase security through incorporation of Digital L edger technology. 
     A key feature of system  11  is the low cost of navigation nodes  12  and the long battery life. On-board diagnostics can be used to alert fly-by AUV  10   s  to anticipated problems providing time for replacement nodes  12  to be deployed. 
       FIG. 4  shows an example of a transmitter for node  12  for use in the mobile station  10  of  FIGS. 1, 2 and 3 . This has a data interface  16  that is connected to each of a processor  18  and a modulator  20 . The modulator  20  is provided to encode data onto carrier wave. At an output of the modulator  20  is a transmit amplifier  22 , which is connected to an underwater, electrically insulated magnetic coupled antenna  24 . In use, the processor  20  is operable to cause electromagnetic navigation signals to be transmitted at regular intervals or in response to an external signal, for example from the mobile station  10 . These magnetic signals can be received and used as a guide or navigational aid by any mobile station  10  in the vicinity. 
       FIG. 5  shows an example of a receiver for use in node  12  or the mobile station  14  of  FIGS. 1, 2 and 3 . This has an electrically insulated magnetic coupled antenna  26  adapted for underwater usage. This antenna  26  is operable to receive magnetic field signals from the transmitter antenna  24 . Connected to the antenna  26  is a tuned filter  28  that is in turn connected to a receive amplifier  30 . At the output of the amplifier  30  is a signal amplitude measurement module  32  that is coupled to a de-modulator  34  and a frequency synthesiser  36  that provides a Local Oscillator signal for down conversion of the modulated carrier. Connected to the de-modulator  34  is a processor  38  that is in turn connected to a data interface  40 . The data interface  40  is provided for transferring data from the receiver to a control or monitoring means, which may be located in the mobile device  10  or at another remote location. 
       FIG. 6  shows an example of an electrically insulated, magnetic coupled antenna that can be used in the transmitter  12  and receiver  14  of  FIGS. 2 and 3 . This has a high permeability ferrite core  42 . Wound round the core  42  are multiple loops of an insulated wire  44 . The number of turns of the wire  44  and length to diameter ratio of the core  42  can be selected depending on the application. However, for operation at 125 kHz, one thousand turns and a 10:1 length to diameter ratio is suitable. The antenna  24 ,  26  is connected to the relevant transmitter or receiver and is included in a waterproof housing  48 . Within the housing  38  the antenna may be surrounded by air or some other suitable insulator, for example, an impedance matched low conductivity medium such as distilled water. 
     In use, the receiver  14  is operable to receive signals from the transmitter  12  and use these to determine an indication of its own, relative position. In some circumstances, merely being in range of a transmitter  12  may provide enough information. However, if more detailed information is needed, such as the actual distance from the transmitter  12 , the receiver  14  may be operable to use the strength of the received signal. In this case, information would have to be stored on the expected strength of a received signal for a given transmitter power and propagation characteristics as a function of distance. For most applications calculation of the receivers range to the transmitter can be based on a typical physical model of the underwater environment (e.g. expected loss versus range). This model could be improved by measurement of attenuation using a comparison of signal strength between multiple receiver antennas with known relative position within the navigating station. 
       FIG. 7  shows another navigation system based on a mobile navigation AUV  10  and a transponder node  12 . In this case, the navigation AUV  10  includes a highly directional antenna  72 , such as a multiple turn solenoid wound around a ferrite rod. This type of antenna  72  generates a radiation pattern with a null point aligned to waves propagating along the axis of the rod as illustrated in  FIG. 8 . The antenna  72  could be rotated until a minimum is located in the received signal strength. As will be appreciated, a single antenna of this type would result in a 180 degree ambiguity. 
     This can be resolved by comparing the signal strength from a second antenna  74  located at some distance from the first to establish the approximate direction of the loss gradient. 
     For any of the radio transmitter and receiver navigation systems described above, the navigation node could remain in receive mode until it decodes a valid demand signal transmitted by the navigating station. A n advantage of this is that the nodes would remain covert. Also, this arrangement would reduce power consumption at remote node deployments thereby extending their operational life. Node and navigating stations both require transceivers for a system configuration of this type. 
     In another embodiment, location accuracy can be improved using data such as the vehicle height A off the seabed  15  using at least one of a pressure sensor and/or a sonar unit housed in AUV  10 . The integrated pressure sensor in AUV  10  is used to determine water depth. This information can be used to refine the accuracy of the final resting location of node  12  by cross-referencing with charts and/or specific survey data relating to the seabed topography. 
     In another embodiment, location accuracy can be improved using data such as the depth of the vehicle from the water surface for example, at least one of a pressure sensor and/or a sonar sensor unit or an ultrasound sensor unit housed in AUV  10 . The sensor in AUV  10  can be used to determine water depth. This information can be used to refine the accuracy of the final resting location of node  12  by cross-referencing with charts and/or specific survey data relating to the tide times and predicted tide depths. 
     In another embodiment, location accuracy can be improved using data such as the gravitational field, with, for example, a pressure sensor or gravitational field sensor housed in AUV  10 . The integrated sensor in AUV  10  is used to determine gravitational field at a specific location. This information can be used to refine the accuracy of the final resting location of node  12  by cross-referencing with charts and/or specific survey data relating to gravitational field patterns. 
     A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, whilst the systems and methods described are generally applicable to seawater, fresh water and any brackish composition in between, because relatively pure fresh water environments exhibit different electromagnetic propagation properties from saline, seawater, different operating conditions may be needed in different environments. Any optimisation required for specific saline constitutions will be obvious to any practitioner skilled in this area. Accordingly, the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.