Optical global positioning system

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, t0, for receipt by the underwater terminal. A computer at the terminal then evaluates the respective light pulse arrival times, t1, 2 & 3, to determine the location of the underwater terminal.

FIELD OF THE INVENTION

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

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.

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.

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.

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'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'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

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.

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 t0. Thus, three different light pulses are generated which are simultaneously transmitted at the same time t0from 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 tnat the underwater terminal. In general the subscript n for tnis 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.

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 km2. 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.

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 tnwhere 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 tnfrom 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 tn. 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).

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 tnfor evaluation by the computer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially toFIG. 1a system for passively locating an underwater object in accordance with the present invention is shown and is generally designated10. As shown, the system10includes a plurality of satellites12, of which the satellites12a,12band12cshown inFIG. 1are only exemplary. As intended for the present invention, the satellites12are preferably geosynchronous. Other type satellites, however, are also envisioned for use with the present invention (e.g. medium Earth orbit satellites).FIG. 1also shows that the system10includes an underwater vehicle14which, for disclosure purposes, is located at a depth d below the surface16of the sea18.

The essential components of the system10which are located onboard a satellite12are grouped together in a transmitter assembly20. It is to be appreciated that different satellites12(e.g. satellites12a,12band12c) will each have a dedicated transmitter assembly20onboard. Moreover, each transmitter assembly20will be essentially the same as every other transmitter assembly20in the system10.

It is also shown inFIG. 1that the essential components of system10which are onboard the underwater vehicle14are grouped together in a receiver assembly22. As intended for the present invention, the receiver assembly22operates with the plurality of transmitter assemblies20in a one-way transmission mode. Stated differently, each transmitter assembly20in the plurality will only transmit, and the receiver assembly22will only receive. Thus, the underwater vehicle14has a passive ability to determine its location in the sea18.

Each transmitter assembly20includes a universal clock24which will provide its respective geosynchronous satellite12with the exact same time. Preferably, the universal clock24is an atomic clock of a type well known in the pertinent art. Also included in each transmitter assembly20is a controller26and a pulsed laser unit28. On the other hand, the receiver assembly22onboard the underwater vehicle14includes a receiver30and a computer34, and the receiver30further comprises an atomic line filter32and an optical pulse detector33. The receiver30is designed to measure the respective arrival times of the optical pulses from the transmitter assemblies20.

For an operational overview of the system10,FIG. 1indicates that each geosynchronous satellite12a,12b, and12c, will transmit a respective light beam36a,36b, or36conto a same cell area38that is located on the surface16of sea18. As envisioned for the present invention, the cell area38will be approximately 400 km2. Further, each light beam36a-cwill be pulsed at 1 kHz with light pulses having a pulse duration of less than 20 ns. Preferably, light pulses in the light beams36a-care 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 area38on the terrestrial water surface16after each pulse.

Operationally, all satellites12will each be at a known position above the surface16of sea18. As positioned, an N number of satellites12will transmit an N number of light pulses in their respective light beams36onto a predetermined cell area38of the water surface16, at a precisely scheduled time t0. As noted above, the transmit time t0may be the same for each satellite12(i.e. simultaneous) or they may have known differences. In any event, the pulses transmitted in the light beams36at time t0will then have respective arrival times tnat the underwater vehicle14where n ranges from 1 to N. Importantly, the relationships between the transmit time t0, will be known for all satellites12, and the various arrival times tnat the underwater vehicle14will be different from each satellite12. An example for simultaneous times t0is set forth inFIG. 2. As shown, the subscripts used for the arrival times indicate the satellite12from which the particular pulse was transmitted. For instance, t1indicates the arrival time at underwater vehicle14of a light pulse that was transmitted at time t0from a first satellite12(e.g. satellite12a). Accordingly, t2is the light pulse arrival time from a second satellite12(e.g. satellite12b) et seq. Although disclosure here indicates the possibility of an N number of satellites12, and a respective number of arrival times tn, it is to be appreciated that only three satellites12are required for the present invention. Further,FIG. 2indicates that differences in arrival times (e.g. Δt1-2and Δt2-3) are required for calculations.

An operation of the present invention essentially involves evaluating each arrival time tntogether with the depth d of the underwater vehicle14to determine a terrestrial location for the underwater vehicle14. Sequentially, this determination requires first calculating a plurality of differences Δt between different arrival times tn(e.g. Δt1-2and Δt2-3). 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 vehicle14, will define a hyperboloidal surface in three dimensions. Thus, Δt1-2and Δt2-3will each define such a surface. Further, each hyperboloidal surface will include a hyperbola such as hyperbolas40and42shown inFIG. 3. More specifically, the hyperbola40is generated using Δt1-2and the hyperbola42is generated using Δt2-3. Recall, both hyperbolas40and42will also contain information regarding the depth d of the underwater vehicle14. Thus, still referring toFIG. 3, it then follows that the intersection of hyperbolas40and42will establish the position of the underwater vehicle14. In general, there may be two intersection points which would indicate a location for the underwater vehicle14, but which are separated by a very large distance. The computer34in the underwater vehicle14can determine the correct intersection point to use by knowing its approximate position ahead of time.

InFIG. 4, the functional characteristics of an atomic line filter32as employed for the system10are shown. Firstly, it will be appreciated that each pulse in a light beam36is essentially a signal44having a wavelength λ≈455 nm which matches the passband of the atomic line filter. As received by the receiver assembly22at the underwater vehicle14, the signal44will be obscured by noise46; most notably the solar background. The received signal44in this case, together with noise46, will be unpolarized light that is passed into the atomic line filter32. In sequence, an x-polarizer48is used to initially polarize the received signal44. Next, a vapor cell50in a magnetic field is used to rotate the signal polarization by 90° while leaving the polarization of the solar background (i.e. the obscuring noise46) unchanged. For the preferred operation at 455 nm, the vapor cell50contains cesium vapor. After its polarization is rotated through 90°, the signal44passes through a y-polarizer52, while the noise46, which is still polarized in the x-direction, is blocked by y-polarizer52. The result here is a signal44having the wavelength λ≈455 nm that has been filtered from the noise46, and can be detected by the detector33for use by the computer34for determining the exact location of the underwater vehicle14.

For clarity in the description of the preferred embodiment, the light pulses from the geosynchronous satellites12were all transmitted at the exact same time t0. In that embodiment, in order for the receiver30and computer34in the underwater vehicle14to determine which detected light pulse came from which satellite12, the computer34needs 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 satellites12emit pulses at different times t0, as long as the computer34in the underwater vehicle14has prior knowledge of the differences in the pulse transmission times.

Specifically, the transmission time for a light pulse from a geosynchronous satellite12to the surface of the ocean below it is on the order of 0.13 second. The differences in arrival times from the different satellites12(if they all transmit pulses at the same time t0) 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 satellite12(resulting in detected pulses at time tnat a given cell area38), the computer34can determine exactly which pulse was transmitted by which satellite12without 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 Δt1-2would be adjusted by subtracting 100 ms before performing the position calculation, Δt2-3would be adjusted by subtracting 100 ms, Δt1-3would be adjusted by subtracting 200 ms, etc.

It will be obvious to those skilled in the art that the pulsed laser transmitters28would not need to be placed on geosynchronous satellites12, but that the satellites12could 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 km2was 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.