Abstract:
In accordance with the present invention, at least three geosynchronous satellites are employed in combination, at respective known positions above a terrestrial water surface to locate an underwater terminal (vehicle). Each satellite includes a light source, and each has a controller for activating its respective light source to simultaneously transmit a light pulse, to a predetermined cell area on the terrestrial water surface, at a precisely scheduled time, t 0 , for receipt by the underwater terminal. A computer at the terminal then evaluates the respective light pulse arrival times, t 1, 2 &amp; 3 , to determine the location of the underwater terminal.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/175,040, filed Jun. 12, 2015. The entire contents of Application Ser. No. 62/175,040 are hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains generally to systems and methods for locating objects relative to a terrestrial surface. More particularly, the present invention pertains to systems and methods for locating underwater objects (vehicles). The present invention is particularly, but not exclusively, useful for systems and methods which employ pulsed light beams that are transmitted from geosynchronous satellites for passive use by an underwater vehicle to determine an exact position (location) of the underwater vehicle. 
       BACKGROUND OF THE INVENTION 
       [0003]    Accurately and precisely locating a communications terminal (e.g. a vehicle) can be necessary for a variety of reasons. As is well known, the task of doing this can be difficult. It becomes even more difficult when the communications terminal is located underwater (e.g. in an undersea environment). Typically, present day systems for precisely locating objects rely primarily on radio waves such as are employed by Global Positioning Systems (GPS). Radio waves, however, are essentially impenetrable into an undersea environment. 
         [0004]    Unlike radio waves, under certain circumstances and conditions, light waves can penetrate and propagate through water; at least to some extent. Of particular importance is the demonstrated ability of light to penetrate into seawater when the light has a wavelength in the so-called blue-green seawater window (e.g. λ≈450-500 nm). Importantly, there are indications that light with a wavelength λ≈455 nm can penetrate seawater to a depth d in excess of 40 m. With this in mind, the possibility of replacing radio waves with light waves for the purpose of passively locating an underwater terminal deserves consideration. 
         [0005]    An important consideration is that the daylight operation of an optical Global Positioning System (oGPS) must necessarily account for the solar background, which can be substantial and significantly reduce the Signal-to-Noise Ratio (SNR). For very high noise environments such as solar background, it is therefore necessary to eliminate as much noise as possible. To do this, several types of optical filters exist which only transmit light over a very narrow wavelength bandwidth, such as Lyot filters and atomic line filters. For example, U.S. Pat. No. 5,731,585, which issued to Menders et al. on Mar. 24, 1998, for an invention entitled “Voigt Filter,” discloses a kind of atomic line filter which can operate at the 455 nm cesium resonance wavelength. 
         [0006]    In light of the above, it is an object of the present invention to provide a system and method wherein light pulses are radiated from a transmitter in near space (e.g. outside the atmosphere) to a terminal (receiver) within a liquid medium (e.g. undersea), for use at a terminal to accurately and precisely determine the terminal&#39;s position in the liquid medium. Another object of the present invention is to provide a system and method for passively locating an underwater terminal wherein light pulses are simultaneously transmitted from different geosynchronous satellites onto a same cell area of a terrestrial water surface, for a use of light pulse arrival times at the underwater terminal to establish the underwater terminal&#39;s terrestrial location. Still another object of the present invention is to provide a system and method for passively locating an underwater terminal using light pulses which is simple to use, relatively easy to implement, and comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with the present invention, a system for locating a mobile underwater terminal requires the use of at least three geosynchronous satellites. A same type transmitter assembly is located on each one of the satellites, and the three transmitter assemblies are operationally employed in combination. 
         [0008]    In structural detail, each transmitter assembly comprises a universal clock (e.g. an atomic clock); a light source (i.e. a pulsed laser unit); and a controller. In this combination, the controller of each transmitter assembly is synchronized by its universal clock with the other transmitter assemblies to activate its light source at a precisely scheduled time t 0 . Thus, three different light pulses are generated which are simultaneously transmitted at the same time t 0  from a respective transmitter assembly. The present invention, however, envisions the transmission of light pulses from different transmitter assemblies with differences in transmission times being as much as 100 msec. These differences in times of transmission must be known. In any event, the light pulses are directed to shine onto a same predetermined cell area on a terrestrial water surface. The consequence here is that each light pulse will have a respective arrival time t n  at the underwater terminal. In general the subscript n for t n  is an integer indicative of the satellite from which the light pulse is received. For a number N=3 geosynchronous satellites, n ranges from 1 to 3. 
         [0009]    For the present invention, light pulses will preferably be generated at 1 kHz and will have a pulse duration of less than 20 ns. Also, a cell area will be approximately 400 km 2 . Further, as intended for the present invention the cell area can be moved along a predetermined path (e.g. a raster type pattern) over a vast water surface (e.g. the Atlantic Ocean), and repeated within a relatively short time cycle. 
         [0010]    As implied above, a receiver will be located with the underwater terminal at an underwater depth d below the terrestrial water surface. Periodically, the receiver will receive an N number of light pulses, from an N number of geosynchronous satellites at respective arrival times t n  where n ranges from 1 to N. A computer, also located with the underwater terminal, is connected with the receiver. Its purpose is to evaluate the arrival times t n  from the respective transmitter assemblies, together with the depth d of the underwater terminal, and to thereby determine a terrestrial location for the underwater terminal. In detail, this calculation will preferably be accomplished by first calculating a plurality of differences Δt between different arrival times t n . Using well know geometric and mathematical techniques, each Δt can then be used to define a curve which is approximately hyperbolic containing the receiver. Further, an intersection of two different hyperbolic curves, plus the depth of the receiver, can then be used to establish the position of the receiver (underwater terminal). 
         [0011]    For a preferred embodiment of the present invention, an atomic line filter is included within the receiver at the underwater terminal to prevent solar background from obscuring the light pulses. In detail, the atomic line filter will include an x-polarizer; a y-polarizer; and a narrowband atomic vapor cell within a magnetic field. Functionally, the x-polarizer and y-polarizer serve to block all out-of-band light (e.g. solar background light) from passing through the atomic line filter assembly. The atomic vapor cell in the magnetic field (which in the preferred embodiment uses cesium vapor) serves to rotate the polarization of the signal pulses at 455 nm (received from the geosynchronous satellites) by 90° so that they can pass through the y-polarizer onto a detector. The increased signal-to-noise ratio afforded by the atomic line filter allows the detector to discriminate the signal pulses from the solar background light and measure the respective pulse arrival times t n  for evaluation by the computer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0013]      FIG. 1  is a schematic presentation of an optical Global Positioning System (oGPS) in accordance with the present invention; 
           [0014]      FIG. 2  depicts travel time lines for light pulses simultaneously transmitted from different geosynchronous satellites at a time t 0 , and their respective time of arrival t n , at an underwater terminal; 
           [0015]      FIG. 3  is a two-dimensional presentation showing the intersection of hyperbolic curves (i.e. at the location of the underwater terminal), wherein the hyperbolic curves are each calculated to include the underwater terminal and are based on a difference between selected arrival times t n  shown in  FIG. 2 ; and 
           [0016]      FIG. 4  is a schematic presentation of the operating principle for an atomic line filter as incorporated into the system of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring initially to  FIG. 1  a system for passively locating an underwater object in accordance with the present invention is shown and is generally designated  10 . As shown, the system  10  includes a plurality of satellites  12 , of which the satellites  12   a,    12   b  and  12   c  shown in  FIG. 1  are only exemplary. As intended for the present invention, the satellites  12  are preferably geosynchronous. Other type satellites, however, are also envisioned for use with the present invention (e.g. medium Earth orbit satellites).  FIG. 1  also shows that the system  10  includes an underwater vehicle  14  which, for disclosure purposes, is located at a depth d below the surface  16  of the sea  18 . 
         [0018]    The essential components of the system  10  which are located onboard a satellite  12  are grouped together in a transmitter assembly  20 . It is to be appreciated that different satellites  12  (e.g. satellites  12   a,    12   b  and  12   c ) will each have a dedicated transmitter assembly  20  onboard. Moreover, each transmitter assembly  20  will be essentially the same as every other transmitter assembly  20  in the system  10 . 
         [0019]    It is also shown in  FIG. 1  that the essential components of system  10  which are onboard the underwater vehicle  14  are grouped together in a receiver assembly  22 . As intended for the present invention, the receiver assembly  22  operates with the plurality of transmitter assemblies  20  in a one-way transmission mode. Stated differently, each transmitter assembly  20  in the plurality will only transmit, and the receiver assembly  22  will only receive. Thus, the underwater vehicle  14  has a passive ability to determine its location in the sea  18 . 
         [0020]    Each transmitter assembly  20  includes a universal clock  24  which will provide its respective geosynchronous satellite  12  with the exact same time. Preferably, the universal clock  24  is an atomic clock of a type well known in the pertinent art. Also included in each transmitter assembly  20  is a controller  26  and a pulsed laser unit  28 . On the other hand, the receiver assembly  22  onboard the underwater vehicle  14  includes a receiver  30  and a computer  34 , and the receiver  30  further comprises an atomic line filter  32  and an optical pulse detector  33 . The receiver  30  is designed to measure the respective arrival times of the optical pulses from the transmitter assemblies  20 . 
         [0021]    For an operational overview of the system  10 ,  FIG. 1  indicates that each geosynchronous satellite  12   a,    12   b,  and  12   c,  will transmit a respective light beam  36   a,    36   b,  or  36   c  onto a same cell area  38  that is located on the surface  16  of sea  18 . As envisioned for the present invention, the cell area  38  will be approximately 400 km 2 . Further, each light beam  36   a - c  will be pulsed at 1 kHz with light pulses having a pulse duration of less than 20 ns. Preferably, light pulses in the light beams  36   a - c  are in the blue-green seawater window with a wavelength λ≈455 nm matching a cesium atomic line filter. Under typical operation, each light beam will be scanned to a different cell area  38  on the terrestrial water surface  16  after each pulse. 
         [0022]    Operationally, all satellites  12  will each be at a known position above the surface  16  of sea  18 . As positioned, an N number of satellites  12  will transmit an N number of light pulses in their respective light beams  36  onto a predetermined cell area  38  of the water surface  16 , at a precisely scheduled time t 0 . As noted above, the transmit time t 0  may be the same for each satellite  12  (i.e. simultaneous) or they may have known differences. In any event, the pulses transmitted in the light beams  36  at time t 0  will then have respective arrival times t n  at the underwater vehicle  14  where n ranges from 1 to N. Importantly, the relationships between the transmit time t 0 , will be known for all satellites  12 , and the various arrival times t n  at the underwater vehicle  14  will be different from each satellite  12 . An example for simultaneous times t 0  is set forth in  FIG. 2 . As shown, the subscripts used for the arrival times indicate the satellite  12  from which the particular pulse was transmitted. For instance, t 1  indicates the arrival time at underwater vehicle  14  of a light pulse that was transmitted at time t 0  from a first satellite  12  (e.g. satellite  12   a ). Accordingly, t 2  is the light pulse arrival time from a second satellite  12  (e.g. satellite  12   b ) et seq. Although disclosure here indicates the possibility of an N number of satellites  12 , and a respective number of arrival times t n , it is to be appreciated that only three satellites  12  are required for the present invention. Further,  FIG. 2  indicates that differences in arrival times (e.g. Δt 1-2  and Δt 2-3 ) are required for calculations. 
         [0023]    An operation of the present invention essentially involves evaluating each arrival time t n  together with the depth d of the underwater vehicle  14  to determine a terrestrial location for the underwater vehicle  14 . Sequentially, this determination requires first calculating a plurality of differences Δt between different arrival times t n  (e.g. Δt 1-2  and Δt 2-3 ). By measuring the differences in arrival times rather than the actual arrival times, an atomic clock is not needed on the underwater vehicle. Mathematically it can be shown that each Δt, together with a measure of the depth d of the underwater vehicle  14 , will define a hyperboloidal surface in three dimensions. Thus, Δt 1-2  and Δt 2-3  will each define such a surface. Further, each hyperboloidal surface will include a hyperbola such as hyperbolas  40  and  42  shown in  FIG. 3 . More specifically, the hyperbola  40  is generated using Δt 1-2  and the hyperbola  42  is generated using Δt 2-3 . Recall, both hyperbolas  40  and  42  will also contain information regarding the depth d of the underwater vehicle  14 . Thus, still referring to  FIG. 3 , it then follows that the intersection of hyperbolas  40  and  42  will establish the position of the underwater vehicle  14 . In general, there may be two intersection points which would indicate a location for the underwater vehicle  14 , but which are separated by a very large distance. The computer  34  in the underwater vehicle  14  can determine the correct intersection point to use by knowing its approximate position ahead of time. 
         [0024]    In  FIG. 4 , the functional characteristics of an atomic line filter  32  as employed for the system  10  are shown. Firstly, it will be appreciated that each pulse in a light beam  36  is essentially a signal  44  having a wavelength λ≈455 nm which matches the passband of the atomic line filter. As received by the receiver assembly  22  at the underwater vehicle  14 , the signal  44  will be obscured by noise  46 ; most notably the solar background. The received signal  44  in this case, together with noise  46 , will be unpolarized light that is passed into the atomic line filter  32 . In sequence, an x-polarizer  48  is used to initially polarize the received signal  44 . Next, a vapor cell  50  in a magnetic field is used to rotate the signal polarization by 90° while leaving the polarization of the solar background (i.e. the obscuring noise  46 ) unchanged. For the preferred operation at 455 nm, the vapor cell  50  contains cesium vapor. After its polarization is rotated through 90°, the signal  44  passes through a y-polarizer  52 , while the noise  46 , which is still polarized in the x-direction, is blocked by y-polarizer  52 . The result here is a signal  44  having the wavelength λ≈455 nm that has been filtered from the noise  46 , and can be detected by the detector  33  for use by the computer  34  for determining the exact location of the underwater vehicle  14 . 
         [0025]    For clarity in the description of the preferred embodiment, the light pulses from the geosynchronous satellites  12  were all transmitted at the exact same time t 0 . In that embodiment, in order for the receiver  30  and computer  34  in the underwater vehicle  14  to determine which detected light pulse came from which satellite  12 , the computer  34  needs to make use of further information about its approximate position. In some situations, especially when pulse arrival times are close together, there could be some ambiguity in this determination. In a second preferred embodiment of the present invention this ambiguity can be eliminated by having the geosynchronous satellites  12  emit pulses at different times t 0 , as long as the computer  34  in the underwater vehicle  14  has prior knowledge of the differences in the pulse transmission times. 
         [0026]    Specifically, the transmission time for a light pulse from a geosynchronous satellite  12  to the surface of the ocean below it is on the order of 0.13 second. The differences in arrival times from the different satellites  12  (if they all transmit pulses at the same time t 0 ) will typically be between 0 and 10 milliseconds. Therefore, if a known delay of exactly n*(100 ms) is added to the pulse transmission time for each satellite  12  (resulting in detected pulses at time t n  at a given cell area  38 ), the computer  34  can determine exactly which pulse was transmitted by which satellite  12  without ambiguity. The added time interval of 100 ms is short enough that a conventional quartz oscillator based timer is adequate for the time interval determination to the required accuracy. In performing the position calculation, the procedure described in the first preferred embodiment would simply need to be modified by subtracting out the known time delays. Specifically, the measured Δt 1-2  would be adjusted by subtracting 100 ms before performing the position calculation, Δt 2-3  would be adjusted by subtracting 100 ms, Δt 1-3  would be adjusted by subtracting 200 ms, etc. 
         [0027]    It will be obvious to those skilled in the art that the pulsed laser transmitters  28  would not need to be placed on geosynchronous satellites  12 , but that the satellites  12  could be in non-geosynchronous orbits, including Medium Earth Orbits such as those used by the current GPS satellites, Low Earth Orbits, or the transmitters could even be placed in aircraft, balloons, on mountaintops, etc. Furthermore, an optical GPS system could be built using laser transmitters at other wavelengths than 455 nm, and in particular doubled Nd:YAG lasers operating at 532 nm may be used. It is not necessary to use an atomic line filter of the type described, and other narrowband filters besides atomic line filters may prove useful. While an optical pulse duration of 20 ns has been described (and is currently achievable for a laser matched to a cesium atomic line filter operating at 455 nm), those skilled in the art will know that a shorter pulse is desirable, but that in any case the pulse length only affects the accuracy of the position measurement. The instantaneous coverage area of 400 km 2  was chosen to obtain reasonable water depth penetration with an available laser operating at 455 nm with a pulse repetition frequency of 1 kHz, while allowing for scanning a large area of the ocean in a reasonable time. This coverage area can obviously be traded off with available laser pulse energies and repetition rates to obtain different water penetration depths or ocean area coverage. Although the system has been described in terms of using three satellites and three optical pulses, it should be clear to one versed in the art that adding more satellites and measuring more pulse arrival time differences will serve to increase the obtainable position accuracy for the underwater vehicle. If a satellite based Optical Global Positioning System according to the present invention were to be built, it would also find other uses than for determining the position of underwater vehicles. For instance, such a system would also work above water. In addition, observing laser pulses from known satellite locations with a camera based system above the water would allow for three dimensional heading and attitude information to be obtained, analogous to what could be obtained with a star tracker, but with the advantage of also working during daytime. In addition, by sending multiple laser pulses to the same ocean location from one of the satellites, the system could be used for low data rate downlink communications to an underwater vehicle. 
         [0028]    While the particular Optical Global Positioning System as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.