Patent Publication Number: US-11381319-B2

Title: Underwater wireless optical communication unit and system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application of International Application No. PCT/NL2018/050467, which was filed on Jul. 11, 2018, which claims priority to Netherlands Application Number 2019224 filed on Jul. 11, 2017, of which is incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The invention relates to an underwater wireless optical communication unit, to a system for underwater communication comprising a plurality of such units, and to a distance determination method methods using such a system. Furthermore, the invention relates to a computer program product arranged to perform the method, and a computer readable medium comprising such a computer program. 
     BACKGROUND ART 
     Various underwater monitoring devices are known, with sensors for observing structures that are installed underwater for a long time on the seafloor or ocean floor. Subsea monitoring devices should preferably be self-powered, energy efficient, and able to function autonomously for a prolonged time, in order to reduce required deployment and collecting operations to a minimum. In deep sea environments with typical ocean floor depths in the order of several kilometers, the sensors must additionally be able to withstand pressures of several hundred bars. 
     International patent publication WO2016/068715A1 describes underwater positioning systems configured to provide position information for a remotely operable vehicle (ROV). One system is formed by underwater beacons, each including an imaging device that observes the surroundings of the beacon. This imaging device is configured to detect light sources on the ROV, and to determine direction data representing a direction or change in direction of the ROV light sources with respect to the imaging device. The beacon acquires scaling information by observing a known scaling element that carries light sources at predetermined distances apart. Acquired positioning information is communicated by the beacon to the ROV via an acoustic transponder. 
     It would be desirable to provide a wireless communication unit that can be deployed underwater together with similar units for a prolonged time, to form a versatile system that enables various underwater monitoring tasks with improved accuracy. 
     SUMMARY OF INVENTION 
     Therefore, according to a first aspect, there is provided an underwater wireless optical communication (UWOC) unit for underwater deployment on or in a submerged earth layer or a submerged structure. The UWOC unit is configured for wireless optical communication in an underwater environment, and comprises an optical transmitter, an anidolic optical receiver, and a processor. The optical transmitter is configured to transmit data by emitting an optical signal into the surroundings of the UWOC unit. The optical receiver includes an optical detector, which is omnidirectionally sensitive and configured to receive a further optical signal approaching substantially along a first azimuthal plane that is orthogonal to a nominal axis extending through the UWOC unit. The processor is communicatively coupled to the optical receiver, and configured to process received further optical signals. 
     Acoustic communication channels provided by an underwater environment (e.g. the sea or ocean) are band-limited, and acoustic signals propagate through the fluid medium at relatively low speeds, resulting in high data latency. Propagating acoustic signals may also be subject to severe multipath effects, and the acoustic transmission properties of the fluid medium can vary significantly in time. In contrast, the small carrier wavelengths associated with optical data signals allow construction of small UWOC units and communication components with high resolution (e.g. a factor 10,000 compared to acoustics), low latency, and fast update rates. 
     The term “omnidirectionally sensitive” is used herein to refer to reception of signals approaching from any or all directions with a substantial component along an azimuthal plane around the UWOC unit. The term “azimuthal plane” is used herein to generally refer to a plane orthogonal to an axial direction on which the UWOC unit is centered. The reception sensitivity may decrease with elevation angle above or below the first azimuthal plane. The UWOC unit is capable of detecting with high sensitivity (reception gain) optical signals originating from directions substantially along this azimuthal plane, compared to signals from directions with a substantial elevation angle above or below this azimuthal plane. This elevational sensitivity reduces the likelihood that light from sources located significantly above or below the UWOC device interferes with optical communication channels established between the transmitters and receivers of cooperating UWOC units. 
     A plurality of the proposed UWOC units may be deployed at various positions on or in a submerged earth layer or a submerged structure, and employed as an underwater optical communication network for sharing or distributing various telemetry data to other units or underwater vehicles in the vicinity. 
     In embodiments wherein the UWOC unit further comprises an omnidirectional photogrammetric camera for acquiring image data within a wide field of view around the UWOC unit, the elevational sensitivity range for the optical receiver may be made substantially overlapping with or even equal to the elevational FOV range of the camera. This ensures that light signals from the optical transmitter may be received by the optical receiver and simultaneously imaged by the camera of another unit within range. For instance, for a system of such UWOC units that are deployed underwater to monitor displacement of components (e.g. wellheads and manifold) in a subsea oil extraction system, elevational sensitivity for the optical receiver and the camera of the unit may cover a minimum elevational range of −20° to +30°. 
     In an embodiment, the optical transmitter is configured to omnidirectionally emit the optical signal substantially along a second azimuthal plane, which is substantially parallel with the first azimuthal plane. 
     Also the emission of optical signals may have an omnidirectional gain profile centered on an azimuthal plane around the UWOC unit. The second azimuthal plane for optical emission may extend substantially parallel with the first azimuthal plane, and preferably at a non-zero axial distance therefrom. Omnidirectional transmission ensures that a unit is able to receive an optical signal of another unit in its vicinity, irrespective of its relative axial orientation on the submerged earth layer or structure. The omnidirectional emission gain profile may be restricted to a limited elevational range centered on the second azimuthal plane around the UWOC unit, and decrease with increasing (absolute) elevation angle above or below this azimuthal plane. 
     According to embodiments, the processor of the UWOC unit is configured to determine an indication of an inter-unit distance between the first UWOC unit and a second UWOC unit, on the basis of sending with the optical transmitter an optical interrogation signal to the second unit, and receiving with the optical receiver an optical response signal from the second unit ( 30   b ). 
     Optical transmitters and receivers of deployed units may be used to autonomously derive inter-unit ranging data, based on exchange of optical signals. In turn, such ranging information may for instance be used to determine scale characteristics for the network of deployed UWOC units. For UWOC units that also include an underwater camera for acquiring image data of light sources in the vicinity, any direction angle data acquired with such camera may be efficiently combined with the inter-unit ranging data, to confer three-dimensional positioning capabilities (e.g. range, azimuth, and elevation) to the UWOC unit. 
     In further embodiments, the UWOC unit is configured to send with the optical transmitter an optical response signal to a second UWOC unit, upon receiving an optical interrogation signal from the second unit. The UWOC unit may then be configured to measure a roundtrip time (RTT) between sending the optical interrogation signal to the UWOC second unit, and receiving the optical response signal from the second unit. 
     Alternatively or in addition, the UWOC unit may be configured to determine the inter-unit distance by comparing phase and frequency characteristics of the clocks of the first and second UWOC units, after exchange of optical signals with predetermined carrier wave characteristics. 
     In embodiments, the optical detector comprises a silicon photomultiplier (SiPM) sensor. 
     Due to considerable photon detection efficiency, high detection gain, and fast response times achievable with a SiPM sensor, a SiPM sensor may be effectively employed for wireless optical communication in an underwater environment. The low driving voltage requirement renders the SiPM sensor particularly suitable for long-term underwater deployment. At considerable water depths (e.g. depths greater than 500 meters) there is no ambient light disturbance due to sunlight left, so the environment is completely dark all the time. The superior sensitivity, response, and gain characteristics of SiPMs render this sensor particularly suitable for subsea light detection applications. UWOC units that have inter-unit ranging capability may particularly benefit from detector implementations that employ such a (fast responding) SiPM sensor. 
     In embodiments, the optical detector defines a sensor surface. The optical signal receiver may comprise first reflector optics, which is adapted to receive the further optical signal approaching substantially along the first azimuthal plane, and to reflect the further optical signal towards and onto the sensor surface. 
     In a further embodiment, the sensor surface is substantially planar. The first reflector optics may then include a conical mirror with a top directed towards the optical detector. This conical mirror may be centered on an axis of revolution that extends through the sensor region and is substantially parallel with the nominal axis. 
     Alternatively, the first reflector optics may include a plurality of conical mirrors arranged rotationally symmetric around the unit axis, each mirror being centered on an axis of revolution that extends through the sensor region and is at a non-zero distance from and substantially parallel with the nominal axis. 
     The proposed reflector optics with one or more conical mirrors provides omnidirectional azimuthal detection sensitivity to the optical detector, while allowing robust alignment of the detector optics inside the unit&#39;s housing. The elevational sensitivity of the optical signal receiver can be adjusted by changing the maximum diameter of the conical mirror (i.e. the maximal radial extent of the mirror relative to its axis of revolution). 
     The one or more conical mirrors may each define an inverse parabolic conical mirror surface. The term “inverse parabolic conical surface” is used herein to refer to a surface of revolution formed from a parabolic segment described by x=a·y 2 +b, wherein the y-coordinate is associated with the axis of revolution. (In the case of a mirror with a downwards top, a&gt;0; x&gt;0; and y&gt;0). 
     In further embodiments, the UWOC unit comprises a housing including an optically transparent body formed by a solid of revolution, which is centered on the unit axis and has an outer surface with a convex curvature along radial and axial directions and a center of curvature located on the unit axis. The first reflector optics may be accommodated inside this transparent body and located substantially at the center of curvature of the convex body surface. 
     The proposed body provides a pressure-resistant and optically transparent housing, in which the signal receiver can be reliably accommodated and attributed an omnidirectional azimuthal view. The curved solid of revolution may for instance have a (truncated) spherical outer surface. 
     In further embodiments, the SiPM sensor includes sensor elements and a frontend circuit. The frontend circuit includes a voltage source for providing the sensor elements with a reversed bias voltage, and a low-pass filter that is provided between and electrically connected to the sensor elements and the voltage source, and which is configured to suppress or even eliminate frequency components of 100 hertz and above in/from the bias voltage. The low-pass filter may for instance be formed by an RC-filter. 
     By applying a sufficient reverse bias voltage across the elements of the SiPM sensor (e.g. about 24 volts), the sensor elements are capable to generate self-sustaining avalanche currents upon absorption/detection of a photon. The output signal of the sensor may suffer from intermodulation distortion effects, though, which may be caused by non-linear response of the sensor to a changing bias voltage resulting from simultaneous detection of the target light signal and other external light sources with different frequency characteristics, such as pulse width modulated (PWM) dimmable LED sources on other objects in the unit&#39;s vicinity (e.g. an ROV or UAV with LED spotlights). Typical PWM LED driving frequencies range from 100 hertz up to values in the order of hundreds of kilohertz, for instance up to 500 kHz. Intermodulation distortion in the SiPM caused by the bias circuit may be reduced by using the low-pass filter, which decouples the SiPM bias voltage over a wide frequency bandwidth, e.g. of 100 kHz and above, and keeps the bias voltage as constant as possible. 
     Alternatively or in addition, the optical transmitter may be configured to form the optical signal by modulating a carrier wave with a frequency of at least 500 kilohertz using a modulation with a predetermined maximum bitrate. The SiPM sensor may then include a frontend circuit with an analogue-to-digital converter (ADC), and a parallel resonant band-pass filter. The band-pass filter is provided between and electrically connected to the sensor elements and the ADC, and is configured to pass only a frequency band corresponding with the carrier wave and the maximum bitrate. The band-pass filter may for instance be formed by a parallel L/C filter. 
     The optical communication signals from the units may for instance be formed by modulating data signals via binary phase shift keying (BPSK) or differential phase shift keying (DPSK) with a predetermined maximum bitrate onto a high frequency carrier wave with a frequency of at least 500 kHz, for instance of about 800 kHz. Even if the data rate in the wireless optical communication channel is relatively low, e.g. in the order corresponding with a 25 kHz bitrate, it may still be beneficial to use a carrier wave at a frequency that is considerably higher than (e.g. at least 500 kHz) switching frequencies of external artificial light sources. This allows spectral bands associated with the desired communication signal to be clearly separable from spectral bands associated with other external light sources, via bandpass filtering of the SiPM output signal, to improve analog signal-to-noise performance. Due to the high frequency subcarrier of the communication signals, a parallel LC filter may be used to pre-filter the output signal of the sensor elements of the SiPM sensor, before it is digitized by the ADC. The bandpass filter is preferably tuned to pass the entire band containing the communication channel for the maximum data rate that it is designed for. 
     According to a second aspect, and in accordance with advantages and effects described herein above with reference to the first aspect, there is provided an UWOC system for optical communication in an underwater environment, and comprising at least a first and a second UWOC unit according to the first aspect. The first and second optical transmitters of the respective UWOC units are each configured to omnidirectionally emit a respective optical signal. The first and second optical receivers of the respective UWOC units are each omnidirectionally sensitive and configured to receive the respective optical signal emitted by the second or first optical transmitters respectively. 
     In an embodiment, the first UWOC unit is configured to determine an indication of an inter-unit distance between the first UWOC unit and the second UWOC unit, on the basis of sending with the optical transmitter an optical interrogation signal to the second UWOC unit, and receiving with the optical receiver an optical response signal from the second UWOC unit. 
     The first UWOC unit may for instance be configured to derive a TOF for an optical signal travelling between the first UWOC unit and the second UWOC unit, on the basis of measuring RTT with the first UWOC unit via sending the optical interrogation signal to the second UWOC unit and receiving the optical response signal from the second UWOC unit. In this case, the second UWOC unit is configured to send the optical response signal to the first UWOC unit upon receiving the optical interrogation signal from the first UWOC unit, and the processor of the first UWOC unit is configured to determine the inter-unit distance on the basis of the TOF. Such distance determination requires knowledge of a propagation velocity of the optical signal through the intervening liquid, which may be based on a predetermined model and/or on measurements. Initial knowledge of various time biases (e.g. component time delays) may also be provided in advance. 
     Alternatively, the UWOC units are configured to determine the inter-unit distance by deriving phase and frequency differences between the clocks of the first and second UWOC units, via exchange of optical interrogation and response signals with predetermined carrier waves and comparison of received signals with internal clocks. The first unit may transmit an optical interrogation signal including a carrier wave, which may be received by the second unit. The second unit compares the interrogation signal with the frequency and phase characteristics of its own clock for generating an optical carrier wave, derives phase differences between the signal and its clock, and stores the results. Optionally, the second unit may also derive frequency differences between the received signal and its clock, and store the results for future transmission or comparison purposes. The second unit transmits its clock back to the first unit via an optical response signal. The first unit similarly compares the received response signal with its own clock, to measure phase and frequency differences between the clocks of the units. In addition, the second unit transmits the stored phase (and possibly frequency) differences to the first unit. The first unit may then resolve the inter-unit distance with an ambiguity of one clock period. This ambiguity may be resolved by modulating the clock with additional information. The optional determination and transmission of frequency differences by the second unit and receipt thereof by the first unit allows the first unit to determine an average frequency difference (e.g. to reduce measurement noise) and/or to detect and possibly compensate for potential frequency drift over time. 
     According to a third aspect, there is provided a method for using the UWOC system according to the second aspect. The method comprises:
         deploying first and second UWOC units underwater at non-coinciding first and second positions on or along a submerged surface or structure, followed by:   emitting an optical interrogation signal with a first optical signal transmitter of the first UWOC unit;   receiving the optical interrogation signal with a second optical signal receiver of the second UWOC unit;   emitting an optical response signal with a second optical signal transmitter of the second UWOC unit after receiving the optical interrogation signal;   receiving the optical response signal with the first optical signal receiver of the first UWOC unit, and   determining an indication of an inter-unit distance between the first and second positions, based on at least the optical response signal from the second UWOC unit.       

     In one embodiment, the method comprises:
         determining a TOF for an optical signal (e.g. the interrogation signal and/or response signal) travelling between the first and second UWOC units, based on a time difference between the emission of the optical interrogation signal and the receipt of the optical response signal, and   determining the indication of inter-unit distance between the first and second positions from the TOF.       

     The first UWOC unit may transmit an optical pulse or optical pulsed carrier as interrogation signal to the second unit at time T0. The second unit receives the signal at T0+ΔTab (i.e. TOF), and promptly returns a similar optical response signal at T0+ΔTab+ΔTb (fixed time offset) to the first unit. The first unit may then receive this response signal at T0+2·ΔTab+ΔTb. From the elapsed RTT=2·ΔTab+ΔTb measured by the first unit, the TOF between the units can be derived. Based on advance knowledge of (average/modelled/measured) light propagation speed in the intervening liquid and of system time delays, the unit&#39;s processor may derive the inter-unit distance from the TOF. 
     In alternative embodiments, indications of inter-unit distances may be determined by measuring phase differences Δφ and frequency differences Δω between internal clocks and received optical signals of distinct UWOC units, via method steps described herein. 
     According to a further aspect, there is provided a computer program product configured to provide instructions to carry out a method according to at least one the abovementioned aspects, when loaded on a computer arrangement. 
     In yet a further aspect, there is provided a computer readable medium, comprising such a computer program product. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts. In the drawings, like numerals designate like elements. Multiple instances of an element may each include separate letters appended to the reference number. For example, two instances of a particular element “ 20 ” may be labeled as “ 20   a ” and “ 20   b ”. The reference number may be used without an appended letter (e.g. “ 20 ”) to generally refer to an unspecified instance or to all instances of that element, while the reference number will include an appended letter (e.g. “ 20   a ”) to refer to a specific instance of the element. 
         FIG. 1  schematically shows an embodiment of an observation system, deployed underwater on submerged structures and supporting surface; 
         FIG. 2  presents a perspective view of an embodiment of an observation unit, deployed underwater on an submerged surface; 
         FIG. 3  shows a side view of an upper portion of the observation unit from  FIG. 2 ; 
         FIG. 4  shows a side view of a medial portion of the observation unit from  FIG. 2 ; 
         FIG. 5  presents a perspective view of another embodiment of an observation unit, deployed underwater on an submerged surface; 
         FIG. 6  illustrates a schematic diagram for a frontend circuit of an optical detector, according to an embodiment; 
         FIG. 7  illustrates a deployed observation system and a method for structure displacement monitoring according to embodiments, and 
         FIG. 8  illustrates a deployed observation system and a method for enhancing position information for an underwater vehicle according to embodiments. 
     
    
    
     The figures are meant for illustrative purposes only, and do not serve as restriction of the scope or the protection as laid down by the claims. 
     DESCRIPTION OF EMBODIMENTS 
     The following is a description of certain embodiments of the invention, given by way of example only and with reference to the figures. 
     In the next figures, various coordinate systems will be used to describe spatial characteristics and relations for exemplary embodiments of the observation unit and system. The “unit axis” A refers herein to a nominal axis through an observation unit, and on which an imaging device of the unit is centered. The “axial direction” Z is used herein to refer to the direction along this unit axis A. The term “radial direction” R refers herein to the directions that point radially away from the unit axis A (i.e. perpendicular to the axial direction Z), and which lie in a transversal plane for which a surface normal vector points along the axial direction Z. The “angular direction” (or “azimuthal direction”)  1  corresponds to a unit-vector that initiates at a local radial position, and which points anti-clock-wise along an (infinitesimal) angle of rotation about the unit axis A, and perpendicular to both the (local) radial and axial directions R, Z. Any radial-angular plane transverse to the axial direction Z is referred to herein as an “azimuthal plane” P. 
     The term “surface” is used herein to generally refer to a two-dimensional parametric surface region, which may have either an entirely or piece-wise flat shape (e.g. a plane or polygonal surface), a curved shape (e.g. cylindrical, spherical, parabolic surface, etc.), a recessed shape (e.g. stepped or undulated surface), or a more complex shape. The term “plane” is used herein to refer to a flat surface defined by three non-coinciding points. 
     It should be understood that the directional definitions and preferred orientations presented herein merely serve to elucidate geometrical relations for specific embodiments. The concepts of the invention discussed herein are not limited to these directional definitions and preferred orientations. Similarly, directional terms in the specification and claims, such as “top,” “bottom,” “left,” “right,” “up,” “down,” “upper,” “lower,” “proximal,” “distal” and the like, are used herein solely to indicate relative directions and are not otherwise intended to limit the scope of the invention or claims. 
       FIG. 1  schematically shows a perspective view of an exemplary observation system  20  deployed underwater. The system  20  includes a plurality of observation units  30   a ,  30   b ,  30   c ,  30   d , which are all immersed in a body of water  10 , and are positioned at respective positions Qa, Qb, Qc on submerged structures  14 ,  16 . The submerged structures  14 ,  16  are arranged across a submerged surface  13 , which forms a water-soil interface between the above-situated body of water  10  and the earth layer  12  below. In this example, the submerged surface  13  forms the surface of a portion of a seabed  12 , and the submerged structures  14 ,  16  form part of a subsea oil extraction system, which includes several wellheads  14  and at least one production manifold  16  that is connected to the wellheads  14  via jumpers  15 . The surface  13  of the seabed  12  is typically not perfectly planar, but has local height variations with respect to a vertical direction Z (corresponding with gravity). 
     The observation units  30  include watertight enclosures, and can be deployed in submerged environments for a long term (e.g. years). Once deployed, the units  30  are configured to establish communication channels between pairs of units  30  to form a meshed network. The units  30  are also configured to acquire image data of each other and of the surroundings at desired periods and update rates, and to exchange acquired data via the communication channels. The units  30  are interchangeably referred to herein as observation units or as underwater wireless optical communication (UWOC) units. 
     Although four observation units  30  are illustrated in  FIG. 1 , it should be understood that a greater or smaller number of units can be employed. An increased number of deployed units in the meshed network arrangement allows a larger and/or denser spatial coverage, and may provide increased measurement redundancy which may be exploited to improve measurement accuracy and reliability of the system  20 . 
       FIG. 2  presents a perspective view of an exemplary observation/UWOC unit  30 , which is part of the system  20  shown in  FIG. 1 , and which is deployed underwater on the seafloor  13 . 
     The observation unit  30  comprises a housing  32 , which accommodates various sensors  38 ,  40 ,  42  and other electronic components  36 ,  44 ,  46 ,  48  in a watertight and pressure resistant manner. The housing  32  is at a lower distal portion  56  coupled to a base  34 . The base  34  defines a support structure for the housing  32 , and accommodates a power supply  48 , which is electrically coupled to the sensors  38 ,  40 ,  42  and the other electronic components  36 ,  44 ,  46  to provide required electrical power. The base  34  further includes a support arrangement, which in this example is a tripod leg structure on a lower side, and which is adapted to support the base  34  and underwater observation unit  30  relative to the seabed  12  or structure  14 ,  16 . In this example, the power supply  48  is formed by a replaceable seawater battery, which is known per se. The base  34  is selectively detachable from the housing  32 , to allow the battery  48  to be replaced. 
     The housing  32  of the unit  30  includes an optically transparent medial portion  50 ,  51  with an optical communication device  35  inside, a component casing  52 , and a transparent dome  54  with an optical imaging device  40  on an upper side of the housing  32 . The medial portion  50 ,  51 , the component casing  52 , and the dome  54  jointly form an elongated body that extends along a central unit axis A. In this example, the unit  30  is essentially rotationally symmetric about the unit axis A. The medial portion  50 ,  51 , the component casing  52 , and the transparent dome  54  are essentially continuously rotationally symmetric about unit axis A, whereas other unit components have discrete rotational symmetries about axis A (e.g. the base  34  has three-fold symmetry, and the communication device  35  has two-fold symmetry). 
     In a deployed state of the unit  30 , the unit axis A is preferably directed with a substantial component normal to the (macroscopic) orientation of the supporting submerged surface  13  or structure  14 ,  16 , to allow the optical communication device  35  and the imaging device  40  a largest possible unobstructed FOV. Furthermore, the deployed observation units  30  project with at least the medial portion  50 ,  51 , the component casing  52 , and the transparent dome  54  above the surface  13  of the seabed  12 . This allows the imaging device  40  of one unit  30  to observe the unit&#39;s surroundings and to provide the optical communication device  35  a line of sight to communication devices  35  of one or more other units  30  in the vicinity. The achievable visual and/or optical communication range between units  30  deployed underwater may be in the order of several hundreds of meters. In this example, the units  30  are relatively small; A height ΔZu of the housing  32  (from  56  to the top of dome  54 ) along the axis A is several tens of centimeters e.g. about 25 centimeters, and diameter Øu of the housing  32  transverse to the axis A is about 10 centimeters. 
     The component casing  52  forms a pressure resistant shell, which consists essentially of Titanium and defines an internal chamber for accommodating an orientation sensor  42 , a processor  44 , and a memory unit  46 . Titanium is a strong, light, and corrosion-resistant metal. In addition, the thermal expansion coefficient of Titanium can advantageously be selected to approach or even match the thermal expansion coefficient of particular types of glass that may be used for forming the dome  54  and/or the medial portions  50 ,  51 , to reduce differential thermal stress between these parts (and potential negative optical effects) under varying temperature conditions. 
     The medial portion  50 ,  51  is formed by a first medial portion  50  and a second medial portion  51 , which are stacked along and centered on the unit axis A, and which accommodate distinct functional parts of the optical communication device  35 . The communication device  35  includes an optical signal transmitter  36 , and an optical signal receiver  38  of the anidolic (non-imaging) type. 
     The optical signal transmitter  36  includes a plurality of light sources ( 70 , see  FIG. 4 ), and is configured to transmit an optical data signal via light that is emitted by the light sources  70 , through the second medial portion  51 , and into the body of water  10  surrounding the unit  30 . 
     The imaging device  40  is formed by a photogrammetric camera  40  with an ultra-wide field of view (UW-FOV), which is configured to acquire image data of objects located in the vicinity of the unit  30 . The camera  40  is configured to detect and acquire image data of other light sources in the vicinity of the unit  70 . 
     In this example, the camera FOV faces away from the housing  32  and upwards along the unit axis A, to ensure that portions of the housing  32  are not within the camera FOV when the observation unit  30  is deployed. In particular, the camera FOV faces away from the optical signal transmitter  36  of the optical communication device  35 . 
     The orientation sensor  42  is configured to acquire attitude data for the unit  30 , by determining at least a pitch and a roll of the underwater imaging device  40  relative to the surface  13  or structure  14 ,  16  on/in which the unit  30  is deployed. 
     The processor  44  and memory unit  46  are communicatively coupled with the orientation sensor  42 , to receive and store the attitude data acquired by the orientation sensor  42 . The processor and memory units  44 ,  46  are also coupled with the camera  40 , to receive and store image data acquired by the camera  40 . Furthermore, the processor and memory units  44 ,  46  are coupled to the communication device  35 . 
     A cable router tube  53  is provided through the medial part  50 ,  51  of the housing  32  and along the unit axis A. The processor and memory units  44 ,  46  are electrically connected with the communication device  35 , via signal conduits. In addition, the communication device  35  and other sensors  40 ,  42  and electronic components  44 ,  46  are electrically coupled to and powered by the power supply  48  via power conduits. In addition, a data port (not shown) may be provided in the base  34  or at an underside of the housing  32 , which is electrically connected to the processor and memory units  44 ,  46  via further signal conduits. This data port may be configured for downloading measurement data and/or uploading configuration data (e.g. for upgrading firmware) once the unit is recovered from the seafloor  13 . The tube  53  accommodates the various conduits and prevents optical masking of the signal transmitter and receiver  36 ,  38 . 
     The processor  44  is configured to receive the image data from the camera  40 , and to determine positional data of the second light source relative to the camera  40 . The memory unit  46  is configured for storing the positional data with timestamps, to form a dataset of time-dependent positional data. The communication device  35  is configured to transmit the positional data to other underwater observation units  30   b ,  30   c ,  30   d , a nearby underwater vehicle  18 , and/or an underwater processing station. 
       FIG. 3  shows a schematic side view of the upper portion of the observation unit  30  from  FIG. 2 , including the transparent dome  54  with camera  40 . The camera  40  is accommodated inside the dome  54 , and includes a digital imaging sensor  41  and a fish-eye lens  58  for receiving and refracting light from the surroundings and projecting the light onto the sensor  41 . The imaging sensor  41  includes a two-dimensional (2D) array of pixels. 
     The transparent dome  54  is formed as a hyper-hemispherical shell of optically transparent material, which is sufficiently rigid to resist considerable external pressures associated with underwater deployment without significant deformation. The camera  40  is positioned with its fish-eye lens  58  substantially coinciding with a nominal center of curvature Cd of the dome  54 . The spherical portion of the dome  54  extends over an azimuthal range of 360° and an elevational range that at least equals the elevational coverage ΔΘv of the camera&#39;s FOV. 
     The fish-eye lens  58  confers an omnidirectional UW-FOV to the camera  40 . In this example, the camera FOV covers 360° in the azimuthal plane P φd . The camera FOV has an elevational coverage ΔΘv of −20° to 90° relative to the azimuthal plane P φd . The resulting UW-FOV allows instantaneous imaging of a large portion of the unit&#39;s surroundings. The UW-FOV covers a relatively narrow elevational range around the azimuthal plane R φd , in which other units  30  are expected to be located, but also larger elevational angles corresponding with an upwards region in which underwater vehicles  18  (e.g. an ROV or UAV) are expected to move around. 
     The fish-eye lens  58  has a focal length that is slightly larger than the distance to the sensor  41 , so that a focal point F of an imaged light signal  83  from a point-like light source  72  will be located slightly behind the imaging sensor  41 . Such a point-like light source  72  may for example correspond to the optical signal  80  from an optical signal transmitter  36  of a visible but remote other unit  30 . Light received from a point-like light source  70  will thus be projected slightly out of focus onto the imaging sensor  41 , to generate an image blob that extends over multiple adjacent pixels of the array. 
       FIG. 4  shows a schematic side view of the transparent medial portion  50 ,  51  of the UWOC unit  30  from  FIG. 2 . The medial portion  50 ,  51  is formed by a first medial housing portion  50  and a second medial housing portion  51 . Exemplary materials for the medial portions  50 ,  51  are glasses or acrylic glasses. The medial housing portions  51 ,  52  are formed as truncated spheres that consist essentially of pressure-resistant and optically transparent material, and which are mutually stacked and coaxially arranged around the unit axis A. Each medial housing portion  51 ,  52  has an outer surface with a convex curvature along radial and axial directions R, Z, and an associated center of curvature Ct, Cr located on the unit axis A. 
     The optical signal receiver  38  is of an anidolic type. The optical signal receiver  38  is accommodated in the first medial housing portion  50 , and includes a detector  62  with SiPM sensors  64   a ,  64   b  and first reflector optics  60   a ,  60   b.    
     The first reflector optics  60   a ,  60   b  are positioned with volumetric center substantially coinciding with a nominal center of curvature Ct of the first medial housing portion  50 . The first reflector optics  60  includes an inverse parabolic conical reflector  60   a ,  60   b  for each associated SiPM  64   a ,  64   b . Each reflector  60  is adapted to reflect incoming light signals  83  that radially approach with a substantial component along a first azimuthal plane R φr , and to project the reflected light onto the associated SiPM  64   a ,  64   b . The reflectors  60  are arranged inside the first medial portion  50  with two-fold (180°) rotational symmetry around the unit axis A. The axis of revolution Ba, Bb of each reflector  60  is parallel with the unit axis A, and the (possibly truncated) top of each mirror is directed toward the associated SiPM  64   a ,  64   b.    
     The optical signal transmitter  36  is accommodated in the second medial housing portion  51 , and includes light sources  70  and second reflector optics  61 . The light sources  70  are formed by LED units that are arranged in a regular azimuthal distribution around the unit axis A, and which are adapted to emit light with wavelengths substantially in an optical range of 300 nanometers to 600 nanometers. 
     The second reflector optics  61  are formed by another parabolic conical reflector  61 , which is adapted to reflect optical signals  80  emitted by the LEDs  70  outwards, with a substantial component along a second azimuthal plane R φt  that is parallel with the first azimuthal plane R φr . The second reflector optics  61  are positioned with its volumetric center substantially coinciding with a nominal center of curvature Cr of the second medial housing portion  51 . 
     The processor  44  and memory unit  46  are communicatively coupled with the optical signal receiver  38  via the cable router tube  53 , to receive and store data acquired by the SiPMs  64   a ,  64   b . The processor unit  44  is also communicatively coupled with the optical signal transmitter  36  via the cable router tube  53 , to control the emission of optical signals  80  by the LEDs  70 . 
       FIG. 5  shows a schematic perspective view of one of the conical reflectors  60   a ,  60   b  in the optical signal receiver  38 . The parabolic conical reflector  60  is formed as an inverse parabolic cone with an outer surface of optically reflective material, and with an annular focal region that coincides with the planar sensor surface Ps of the associated SiPM  64   a ,  64   b . The conical mirror  60  is centered on an axis of revolution B, which extends through the sensor region Ps of the underlying SiPM  64 , and is substantially parallel with the axis A of the UWOC unit  30 . The truncated top of the mirror  60  faces the SiPM  64 . 
     A parabolic cone allows all light rays from a particular radial direction (i.e. particular value for the azimuth angle) to be projected onto a single focal point of the parabola.  FIG. 5  shows radial parallel light rays of an incoming optical signal  83 , and reflection thereof by the reflector  60  towards this focal point and onto the sensor surface Ps of the SiPM  64 . 
     The parabolic conical reflector  61  of the optical signal transmitter  36  has a similar shape and will reflect optical signals  80  emitted by the LEDs  70  radially outwards. This reflector  61  is centered on an axis of revolution which essentially coincides with the axis A of the UWOC unit  30 . 
       FIG. 6  illustrates a schematic diagram for a frontend circuit  63  of an exemplary SiPM-based optical detector  62 . The SiPM detector  62  includes a matrix of reverse biased Geiger Mode avalanche photodiodes (APD), which are connected in parallel between a common cathode and a common anode, and which are collectively indicated in  FIG. 6  by reference numeral  65 . The frontend circuit  63  includes a voltage source  67 , a transistor Q 1 , an ADC  68 , and various passive electric components R 1 , R 2 , R 3 , C 1 , C 2 , C 3 , L 1 . 
     The voltage source  67  is configured to provide the APD elements  65  with a (reversed) bias voltage Vb. The gain of a SiPM element  65  (in the order of 10 6 ) is highly dependent on the bias voltage Vb across the SiPM elements  65 . The bias voltage Vb is formed by a sum of a breakdown voltage Vbd and an overvoltage Vo (e.g. around 3V). The breakdown voltage Vbd is a minimum reverse bias voltage that is needed to induce self-sustaining avalanche multiplication in an APD element  65  upon absorption/detection of a photon (e.g. around 24V). 
     To achieve a constant gain (linear operation), it is preferred to keep the bias voltage Vb constant. A maximum current through an APD element  65  should be limited, however, to avoid damaging of the element  65 . This may be achieved by providing resistor R 3  in series with the elements  65 . Using only resistor R 3  will cause a voltage across the APD element  65  to vary with the intensity of the light  83  it receives, and therefore cause the gain to vary as well. This causes non-linear amplification of a stream of photons associated with received light  83 , which are converted into an electrical current. 
     The optical transmitter  36  of the observation/UWOC unit  30  is configured to form an optical communication signal  80  by modulating a carrier wave with a frequency of at least 500 kHz, in this example of 800 kHz, using a modulation with a predetermined maximum data bitrate. If the elements  65  only receive photons from a modulated source of interest (in this case, the communication signal  80  from another unit  30 ), the non-linear amplification effect is less problematic. If, however the APD element  65  simultaneously receives light signals from other (unwanted) modulated sources, the non-linear response of the APD elements may cause intermodulation distortion (IMD) i.e. additional signal components at frequencies that are sums and differences of integer multiples of the original signal frequencies for the light of interest and the other sources. In a deep subsea environment, ambient sunlight is absent, but there may be artificial light sources (i.e. ROV lights), which are typically pulse width modulated. 
     The frontend circuit  63  therefore also includes a capacitor C 3 , to form with resistor R 3  a low-pass RC filter  69 . The RC-filter  69  is provided between and electrically connected to the sensor elements  65  and the voltage source  67 . In this example, C 3  is a polarized electrolytic capacitor. The RC-time constant of the filter  69  is selected to be larger than the expected PWM period. The RC-filter  69  is thus configured to attenuate time fluctuations in the bias voltage Vb, in order to keep Vb at/near an average value. The occurrence of intermodulation products with components within the frequency band associated with the carrier frequency and modulation bitrates of the optical communication signal can thus be reduced or even eliminated. 
     In use, a voltage V 1  on the cathode side of the APD elements  65  is kept constant through transistor Q 1 , and a base B of transistor Q 1  is connected to ground. Resistor R 2  is connected in parallel with the elements  65 , and allows voltage V 1  to be kept constant, even in the case that no light is received by the APD elements  65  (i.e. a current through elements  65  is almost zero). 
     The frontend circuit  63  further includes a parallel LC band-pass filter  66 , which includes in parallel, an inductor L 1 , a capacitor C 1 , as well as a resistor R 1  for tuning filter quality. The LC-filter  66  is provided between and electrically connected to the sensor element  65  and to the ADC  68 . This LC-filter  66  is tuned via selection of appropriate values for L 1 , C 1 , and R 1 , to pass only a frequency band corresponding with the carrier wave and the maximum bitrate of the optical communication signals  80  from the transmitters  36 . The subcarrier frequency on which the optical communication signal  80  is modulated, is chosen to be much higher (e.g. &gt;500 kHz) than the highest expected PWM frequency of ROV LED sources (and possibly also higher harmonic frequencies). The LC-filter  66  provides analogue pre-filtering to suppress any frequency components outside the band of interest. The required bandwidth for signal  80  around the subcarrier equals the data rate, so an exemplary bit rate of 25 kHz would require the LC filter  66  to be tuned to a band of approximately 787 kHz to 813 kHz. 
     Alternative resonators could be used (i.e. a quartz crystal or ceramic resonators) to achieve the above frequency filtering. For instance, an RF choking coil may be used to supply DC current to the collector of transistor Q 1 . A quartz crystal resonator might be of interest in very low bandwidth applications to improve the signal to ratio. 
     A plurality of the proposed units  30  from  FIGS. 2-4  can be deployed underwater to form an observation and monitoring system  20 .  FIG. 7  shows part of the exemplary system  20  in a deployed state, and illustrates a method for structure/asset displacement monitoring. The units  30   a ,  30   b ,  30   c ,  30   d ,  30   e  are configured to operate without external control, and to establish optical communication channels between pairs of units  30 . The resulting communication channels may form a meshed network, wherein the units  30  form network nodes that cooperate to perform one or several observation and monitoring functions. The displacement monitoring method is explained with reference to the exemplary units  30  from  FIG. 2-4 , but it should be understood that equivalent units may be used as an alternative or in addition to such units  30 . 
     In an initial deployment stage for the system  20 , the observation units  30   i  (i=a, b, c, . . . ) are placed at non-coinciding locations Qi on the submerged surface  12  or structures  14 ,  16 , such that each unit  30   i  is within visual and/or optical communication range with at least one other unit  30   j  (j=a, b, c, . . . ; j≠i). The relatively small units  30  may initially be placed by an underwater vehicle  18 , for instance a ROV  18 . The units  30  are deployed with inter-unit distances ΔRij between each pair of units  30   i ,  30   j  (e.g. ΔRab between units  30   a  and  30   b ). In a (quasi-static) displacement monitoring mode, inter-unit distances ΔRij of up to 200 meters or more may be achievable. 
     In order to conserve electrical power, the units  30  are configured to remain in a dormant mode for extended times, and to activate at predetermined time intervals and/or upon external request to perform measurements, to store measurement data, and/or to exchange measurement data. The processor  44  of each unit  30  is programmed with timing and/or external instruction protocols for activating the sensors  38 ,  40 ,  42  at predetermined periods and/or external request, and for storing the acquired data in the memory unit  44  and/or optically transmitting the acquired data to other units  30  in the network. 
     The signal transmitter  36   a  of a first observation unit  30   a  may emit light signals  80   a ,  81   a  (or  82   a ; not indicated in  FIG. 6 ). The emission of light may occur continuously, intermittently after predetermined time intervals, or upon request by the ROV  18  or another nearby underwater vehicle (e.g. an UAV). A portion of this light signal  80   a  may be received by other cameras (e.g.  40   b ) of nearby observation units (e.g. unit  30   b ), yielding image data for each unit  30  within visual range. Via initial calibration procedures, the pixel region where a received light signal hits the imaging sensor  41  of the camera  40  can be associated with a set of two angular coordinates (e.g. an azimuth angle ϕ and an elevation angle Θ, or direction cosine angles) relative to a local reference frame defined with respect to the camera  40 . 
     During imaging with the camera  40 , the orientation sensor  42  of each unit  30  acquires attitude data for this unit  30 , by detecting changes in at least pitch, and roll angles for the camera  40  relative to the surface  13  or structure  14 ,  16  on/in which the unit  30  resides. The processor  44  of each unit generates positional information with angular coordinates for the detected external light sources, on the basis of the acquired image data with the attitude data. The positional information is referenced with respect to a common coordinate frame and provided with a timestamp corresponding to the time of measurement. The resulting data with timestamp is locally stored in the memory unit  46 . The acquired image data and attitude data may also be separately stored in the memory unit  46 , for downloading and post-processing purposes. 
     In addition, each of the units  30  may be configured to send optical interrogation signals  81  to another unit  30  via its signal transmitter  36 , and to respond to an interrogation signal  81  received via the signal receiver  38  by emitting an optical response signal  82  via the signal transmitter  36 . The processor  44  of each unit  30  may then be configured to execute a ranging function between this unit  30  and a specific other of the surrounding units that is within optical range, by determining time of flight (TOF) between the emitted interrogation signal  81  and a received response signal  82 . The resulting TOF data may be stored in the memory unit  46  and/or transmitted via the communication device  35  to another unit  30 . 
     As illustrated in  FIG. 7 , the first unit  30   a  emits an optical interrogation signal  81   a , at time T0. The second unit  30   b  receives the signal at T0+ΔTab(=TOF), and sends an optical response signal  82   b  at T0+ΔTab+ΔTb(=fixed time offset) back to the first unit. The first unit may then receive this response signal  82   b  at T0+2·ΔTab+ΔTb. The units  30   a - b  are assumed to be stationary when communicating with each other, so the transmit and receive paths are assumed to be equidistant and associated with identical propagation times (TOFs) ΔTab. The first unit  30   a  determines a RTT 2·ΔTab+ΔTb between transmission of the interrogation signal  81   a  and receipt of the response signal  82   b , to derive the TOF based on advance knowledge of the fixed time offset ΔTb. The processor  44  of the first unit  30   a  then determines an inter-unit distance ΔRab on the basis of the TOF and a pre-determined propagation speed for the light signals through the water  10 . 
     Determination of one or more inter-unit distances ΔRij between pairs of units  30   i ,  30   j  may for instance be executed during an initial system calibration stage, soon after the units  30  have been deployed underwater. Once determined, such inter-unit distances ΔRij may be used as scaling information for the entire system  20  of deployed units  30 . The angular positional data (e.g. azimuth, elevation, and inclination data) recorded by each unit  30  may then be supplemented with this scaling data, so that observations of light sources in the vicinity of the units  30  (e.g. ROV lights  19 ) can be mapped to full 3D positions. 
     Apart from the above, each unit  30  is configured to receive positional data of the other units  30  at predetermined times or upon request. The positional data is to be transmitted by each unit  30  via its communication device  35  to the other units  30 . The processor device  44  of one unit  30  is configured to merge positional data (including timestamps) received from the other units  30 , to form a merged dataset of time-dependent positional profiles for all observation units  30 , which is stored in the memory unit  46 . 
     The node positions can be computed from the recorded positional data (e.g. angular data and attitude data) and at least one known distance to determine the scale of the deployed system  20  (e.g. from one or more TOF-based inter-unit distances). The deployed system  20  can thus be used to accurately detect (e.g. sub-centimeter) relative motions (e.g. subsidence) between the deployed units  30 , and parts of the surface  13  and assets  14 ,  16  on which the units  30  are deployed, by retrieving the merged dataset and analyzing the time-variations in the positional data. The method may for instance be used to estimate mechanical stresses between two locations of a submerged object (e.g. wellheads  14  and manifolds  16 ), or of structures (e.g. jumpers  15 ) interconnecting such objects, to provide a timely indication of potential structure failure. 
     The underwater vehicle  18  may include a wireless optical communication device (not shown), which is configured to address any unit  30  and request for a transmission of positional data. Such a vehicle  18  may move within communication range of a selected unit  30 , and request the unit  30  for a transmission of network measurements. The addressed unit  30  may then upload its current merged dataset of time-dependent positional profiles to the vehicle  18 . 
     The system  20  may additionally include an acoustic modem (not shown), configured to upload positioning data to a vehicle at the surface of the sea  10 . Alternatively or in addition, one or more of the units  30  may be in signal communication via a wired connection with a nearby underwater data access point (also not shown). 
       FIG. 8  shows part of the exemplary observation system  20  from  FIGS. 1-5  in a deployed state, and illustrates a method for enhancing position information for an underwater vehicle  18  e.g. an ROV. In an initial calibration stage for the deployed system  20 , the observation units  30  may exchange interrogation and response signals  81 ,  82 , in order to determine inter-unit distances ΔRij via methods described herein above. 
     The ROV  18  may be fitted with a plurality of wireless optical communication devices  19 , each including an optical signal transceiver that is configured to emit light  73 , and to receive optical signals  80  from the signal transmitters  36  of nearby observation units  30 . 
     The ROV  18  may be configured to serve as a master node for the system  20 . This master node is configured to establish the common network time of the system  20 , and to synchronize time for all the units  30  in the network by broadcasting timing information via its optical communication device to the units  30   j . The master node is further configured to define tasks that individual units  30   j  need to execute per measurement cycle, and to transmit instructions to a specific unit  30   j  via communication device. Alternatively, one or several of the observation units  30  in the system  20  may be configured to function as the master node during different periods. 
     The underwater imaging devices  40   a ,  40   b  observation units  30   a ,  30   b  in visual range of the ROV lights  73  acquire image data of these lights  73 . The processor device  44   a ,  44   b  of each unit  30  may then determine positional data associated with of the ROV relative to the imaging device  40 , via methods described herein above. The communication device  35   a ,  35   b  of each unit  30  may then transmit this positional data to the ROV  18  upon request, via optical signals  80  that may be received by any or all of the ROV&#39;s communication devices  19  that have a line of sight to that unit  30 . Only optical signal  80   a  from unit  30   a  to ROV communication device  19   d  is shown in  FIG. 8  for clarity, but it should be understood that other units  30  may communicate positional information to any or all ROV devices  19 . 
     The system  20  may be kept deployed in dormant mode on the seafloor  13  and structures  14 ,  16  for a long time, but may be woken up by the ROV  18  (or another underwater vehicle) entering the site, and ordered to start tracking and broadcasting the 6DOF position of the ROV  18 . 
     Any or all units  30  in the system  20  can also be ordered by the ROV  18  to record images of the environment with the static cameras  40  with omnidirectional views. During such recording, the ROV  18  may project light (e.g. diffuse light or laser stripes) onto the otherwise dark scene. Full panoramic image data, or landmark features extracted from those images by the units  30 , may be transmitted together with positional reference data to the ROV  18  upon request. 
     The system  20  may be configured to operate in a single mode, and switch to another single mode upon request. The system  20  may also be configured to operate in multiple modes at the same time, and to de-activate one of the current modes and/or activate one or more other modes upon request by the ROV  18 . The system  20  may thus be efficiently used for different purposes and perform alternative measurements upon request, while saving energy when particular modes of operation are not desired. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. It will be apparent to the person skilled in the art that alternative and equivalent embodiments of the invention can be conceived and reduced to practice. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     In the above exemplary system, the UWOC units were configured to perform inter-unit ranging by determining TOF for optical signals travelling between the units. In alternative embodiments, the UWOC units  30  may be configured to perform inter-unit ranging by comparing carrier phase and frequency characteristics of the optical signals from the units.  FIG. 7  is used again to illustrate that the units  30  are initially deployed at respective positions (e.g. Qa, Qb) on the seafloor  13 , with initially unknown inter-unit distances ΔRij (e.g. distance ΔRab between units  30   a  and  30   b ). 
     At time T0, the first unit  30   a  transmits an optical interrogation signal  81   a  with carrier wave characteristics S a (t)=ω a ·t+φ a  to the second unit  30   b  via its signal transmitter  36   a . Here, ω a  is the angular frequency and φ a  is the initial phase shift of carrier wave S a . The second unit  30   b  receives the interrogation signal  81   a  via its signal receiver  38   b , at time T1=T0+ΔTab(=TOF). The processor  44   b  of the second unit  30   b  may then compare the received interrogation signal  81   a  with the frequency and phase characteristics S b (t) of its own clock used for emitting an optical response signal. If these carrier wave characteristics are characterized by S b (t)=ω b ·t+φ b , then the resulting phase comparison data Δφ ba  may correspond to 
                 Δφ     b   ⁢   a       ⁡     (     T   ⁢   1     )       =           (       ω   b     -     ω   a       )     ·   T     ⁢           ⁢   1     +         Δ   ⁢   R   ⁢   a   ⁢   b       c   w       ·     ω   a       +     (       φ   b     -     φ   a       )             
with c w  an estimated, measured, or otherwise known propagation speed for the optical signal through the surrounding water  10 . In addition, the second unit  30   b  may derive frequency comparison data Δω ba =ω b −ω a . The phase comparison data Δφ ba  and possibly the frequency comparison data Δω ba  are stored by the second unit  30   b.  
 
     At a later time T0+ΔTab+ΔTb(=delay), the second unit  30   b  emits an optical response signal  82   b  via a carrier wave with characteristics S b (t), using its signal transmitter  36   b . The optical response signal  82   b  additionally includes or is accompanied by the phase comparison data Δφ ba , and may also include the frequency comparison data Δω ba . 
     The first unit  30   a  receives the response signal  82   b  via its signal receiver  38   a  at later time T2=T0+2·ΔTab+ΔTb, assuming that ΔTba=ΔTab. The processor  44   a  of the first unit  30   b  may then compare the received response signal  82   b  with the frequency and phase characteristics S a (t) of its own clock, via 
     
       
         
           
             
               
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     The first processor  44   a  may then perform a ranging function between the two unit  30   a ,  30   b , by deriving the inter-unit distance ΔRab from the phase difference measurements of the two units  30   a - b  via 
     
       
         
           
             
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     Any integer clock/wave period-based ambiguities in the above difference determination may be resolved by modulating the interrogation and response signals  81   a ,  82   b  with predetermined periodic data patterns having periods that significantly exceed the periods of the carrier waves. 
     Optional determination and transmission of frequency comparison data Δω ba  by the second unit  30   b  and receipt thereof by the first unit  30   a , allows the first unit  30   a  to first determine an average value for the frequency difference Δωab=ω a −ω b , or to detect and possibly compensate for potential frequency drift between the two measurement times T1 and T2. 
     In the above examples, the camera FOV had an azimuthal coverage Δϕv of 360° and an elevational coverage ΔΘv of −20° to +90°. Depending on the application and desired vertical observational range, the elevational coverage ΔΘv may be reduced to the range −20° to +30°, or may be increased to the range −50° to +90°. 
     The skilled person will appreciate that the component casing  52  may consist essentially of materials different than titanium. Other suitable materials are e.g. stainless steel, electrogalvanized steel, aluminum, or other sufficiently rigid materials that are corrosion-resistant or otherwise provided with an external coating of anti-corrosion material. 
     Also, the shape of the medial portions  51 ,  52  of the unit&#39;s housing  32  should not be considered limited to stacked truncated spheroids. Instead, a medial housing portion with a cylindrical shape, or another shape with rotational symmetry about the unit axis A, would be possible. 
     Furthermore, the power supply  48  was formed in the above examples as a replaceable seawater battery, but may alternatively be formed by other suitable water-compatible and pressure-resistant power supply arrangements. For instance, a pressure-tolerant non-rechargeable alkaline battery pack may be used in monitoring units for long-term deployment (e.g. long-term displacement monitoring mode), or rechargeable nickel-metal hydride (NiMh) batteries in a pressure housing may be used in monitoring units that are only deployed for a short period (e.g. in positioning mode). 
     Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
     The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. For instance, the image processing functionalities on the one hand, and the optical communication and network management tasks on the other hand, may be controlled by separate processor devices provided in the same underwater observation unit. 
     The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
     LIST OF REFERENCE SYMBOLS 
     
         
           10  body of water (e.g. seawater) 
           12  submerged earth layer (e.g. seafloor, ocean floor) 
           13  submerged earth surface 
           14  submerged structure (e.g. wellhead) 
           15  coupling conduit (e.g. jumper) 
           16  further submerged structure (e.g. manifold) 
           18  underwater vehicle (e.g. remotely operable vehicle, or unmanned autonomous vehicle) 
           19  vehicle light 
           20  underwater wireless optical communication (UWOC) system 
           22  underwater beacon unit 
           30  UWOC unit 
           32  housing 
           34  base 
           35  optical communication device 
           36  optical signal transmitter 
           38  optical signal receiver 
           40  underwater imaging device (e.g. photogrammetric camera) 
           41  imaging sensor 
           42  orientation sensor 
           44  processor 
           46  memory unit 
           48  power supply (e.g. battery) 
           50  first medial housing portion (e.g. first solid transparent dome) 
           51  second medial housing portion (e.g. second solid transparent dome) 
           52  component casing 
           53  cable router tube 
           54  transparent dome 
           56  distal housing portion 
           58  refractor optics (e.g. fish-eye lens) 
           60  first reflector optics (e.g. parabolic conical reflector) 
           61  second reflector optics (e.g. parabolic conical reflector) 
           62  optical detector 
           63  frontend circuit 
           64  Silicon photomultiplier (SiPM) 
           65  SiPM sensor element 
           66  parallel LC filter 
           67  bias voltage source 
           68  analog-to-digital converter (ADC) 
           69  low pass RC filter 
           70  light source (e.g. LED) 
           72  external light source 
           73  vehicle light signal (e.g. ROV LED) 
           80  light signal 
           81  optical interrogation signals 
           82  optical response signal 
           83  further light signal 
         X first direction (longitudinal direction) 
         Y second direction (transversal direction) 
         Z third direction (vertical direction/axial direction) 
         R radial direction 
         ϕ first angular direction (azimuthal direction) 
         Θ second angular direction (elevation direction) 
         Δϕv FOV azimuthal range 
         ΔΘv FOV elevation range 
         ΔZu unit height 
         Øu unit diameter 
         P φd  azimuthal plane (of camera dome) 
         P φt  azimuthal plane (of optical signal transmitter) 
         P φr  azimuthal plane (of optical signal receiver) 
         Ps sensor plane 
         i index for UWOC unit (i=a, b, c, . . . ) 
         j further index for UWOC unit (j=a, b, c, . . . ; j≠i) 
         Cd dome center 
         Ct first center of curvature (e.g. at/near optical signal transmitter) 
         Cr second center of curvature (e.g. at/near optical signal receiver) 
         Ai nominal unit axis (of unit i) 
         Qi unit position (of unit i) 
         ΔRij inter-unit distance (from unit i to j)