SYSTEM AND METHOD FOR SELF-CONTAINED HIGH-PRECISION NAVIGATION

Abstract

A self-contained, high precision navigation method and system for a mobile vehicle comprising an active coherent imaging sensor array with multiple receivers that observes the surrounding environment and a digital processing component that processes the received signals to form interferometric images and determine the precise three-dimensional location and three-dimensional orientation of the vehicle within that environment.

Claims

1. A system for determining positioning and orientation within an environment, comprising: a two-dimensional transmitter/receiver array including a transmitter and at least three receivers, said transmitter directing energy at an environment, said environment reflecting energy back to said at least three receivers, said at least three receivers capturing and digitizing said reflected energy; and a digital processing unit in communication with said two-dimensional array, said digital processing unit creating a coherent image from the digitized reflected energy captured by each of said three receivers and comparing said created coherent images on a pairwise basis to produce interferograms.

2. The system of claim 1, wherein one interferogram is produced for each pair-wise combination of said at least three receivers.

3. The system of claim 2, wherein said digital processing unit provides an expected system position and orientation based on forward projection, wherein said digital processing unit uses said interferograms to produce an estimate of the error in system position and orientation relative to the expected system position and orientation, and wherein the estimates of position and orientation error are applied to the expected position and orientation to precisely update the system position and orientation within the environment.

4. The system of claim 1, wherein said system is mounted on a mobile vehicle, said steps of directing energy, capturing reflected energy, transmitting digitized reflected energy, creating images and comparing images to create interferograms being repeated as a position of said vehicle changes.

5. The system of claim 3, wherein said system is mounted on a mobile vehicle, said steps of directing energy, capturing reflected energy, transmitting digitized reflected energy, creating images and comparing images to create interferograms being repeated as a position of said vehicle changes.

6. The system of claim 1, further comprising: an inertial measuring unit positioned in fixed position relative to said two-dimensional array and in communication with said digital processing unit, said inertial measuring unit detecting acceleration and rotation rate information relating to the expected position and orientation of the system and transmitting said information to the digital processing unit.

7. The system of claim 6, wherein one interferogram is produced for each pair-wise combination of said at least three receivers.

8. The system of claim 7, wherein said digital processing unit uses said interferograms to produce an estimate of the error in system position and orientation relative to the system position and orientation reported by said inertial measurement unit, and wherein said estimates of position and orientation error are applied to the said position and orientation reported by the inertial measurement unit to precisely update the system position and orientation within the environment.

9. The system of claim 6, wherein said system is mounted on a mobile vehicle, said steps of directing energy, capturing reflected energy, transmitting digitized reflected energy, creating images and comparing images to create interferograms being repeated as a position of said vehicle changes.

10. The system of claim 8, wherein said system is mounted on a mobile vehicle, said steps of directing energy, capturing reflected energy, transmitting digitized reflected energy, creating images and comparing images to create interferograms being repeated as a position of said vehicle changes.

11. A method of determining positioning and orientation within an environment, comprising: providing a two-dimensional transmitter/receiver array including a transmitter and at least three receivers and a digital processing unit in communication with said two-dimensional array; directing energy at an environment using said transmitter capturing a reflection of said energy reflected by said environment using said at least three receivers, said at least three receivers capturing and digitizing said reflected energy; transmitting said digitized reflected energy to said digital processing unit; creating a coherent image from the digitized reflected energy captured by each of said three receivers and comparing said created coherent images on a pairwise basis to produce interferograms.

12. The method of claim 11, wherein said digital processing unit produces one interferogram for each pair-wise combination of said at least three receivers.

13. The method of claim 12, wherein said digital processing unit provides an expected system position and orientation based on forward projection, wherein said digital processing unit uses said interferograms to produce an estimate of the error in position and orientation of said two-dimensional array relative to the expected position and orientation of said two-dimensional array, and wherein the estimates of position and orientation error are applied to the said expected position and orientation to precisely update the position and orientation of said two-dimensional array within the environment.

14. The method of claim 11, wherein said two-dimensional array is mounted on a mobile vehicle, said steps of directing energy, capturing reflected energy, transmitting digitized reflected energy, creating images and comparing images to create interferograms being repeated as a position of said vehicle changes.

15. The method of claim 13, wherein said two-dimensional array is mounted on a mobile vehicle, said steps of directing energy, capturing reflected energy, transmitting digitized reflected energy, creating images and comparing images to create interferograms being repeated as a position of said vehicle changes.

16. The method of claim 11, further comprising: providing an inertial measuring unit positioned in fixed position relative to said two-dimensional array and in communication with said digital processing unit, said inertial measuring unit detecting acceleration and rotation rate information relating to the expected position and orientation of the system and transmitting said information to the digital processing unit.

17. The method of claim 16, wherein one interferogram is produced for each pair-wise combination of said at least three receivers.

18. The method of claim 17, wherein said digital processing unit uses said interferograms to produce an estimate of the error in position and orientation relative to the position and orientation reported by said inertial measurement unit, and wherein said estimates of position and orientation error are applied to the said position and orientation reported by the inertial measurement unit to precisely update the position and orientation of the two-dimensional array within the environment.

19. The method of claim 16, wherein said two-dimensional array is mounted on a mobile vehicle, said steps of directing energy, capturing reflected energy, transmitting digitized reflected energy, creating images and comparing images to create interferograms being repeated as a position of said vehicle changes.

20. The method of claim 18, wherein said two-dimensional array is mounted on a mobile vehicle, said steps of directing energy, capturing reflected energy, transmitting digitized reflected energy, creating images and comparing images to create interferograms being repeated as a position of said vehicle changes.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0020] In the drawings which illustrate the best mode presently contemplated for carrying out the present invention:

[0021] FIG. 1 is a schematic illustration of a self-contained high-precision navigation system and method in accordance with an embodiment of the present disclosure;

[0022] FIG. 2 is a schematic illustration of the relationship between the reflected energy images captured by each receiver for pairwise computation of interferograms; and

[0023] FIG. 3 is a schematic illustration of the manner in which the position errors of the receive channels determined from the interferograms are employed to estimate roll, pitch and yaw information of the vehicle.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Now referring to the drawings, a method and system of self-contained high-precision navigation comprising an active coherent imaging sensor array with multiple receivers is shown and generally illustrated in the figures. The method and system employ a two-dimensional transmitter/receiver array that observes the surrounding environment and a digital processing component that processes the received signals and produces interferometric images to determine the precise three-dimensional location and three-dimensional orientation of the system within that environment. In an exemplary embodiment, the system employs a two-dimensional array of at least three receivers and at least one transmitter that are directed towards the surrounding environment. The two-dimensional array is connected to a digital processing unit. In a preferred method of operation, an inertial measurement unit (IMU) is also connected to the digital processing unit.

[0025] Referring now to FIG. 1, in an exemplary embodiment the system 10 generally comprises a two-dimensional array 12 of at least three receivers 14a, 14b, 14c and at least one transmitter 16. The fixed two-dimensional array 12 is mounted or supported in a manner such that the array 12 is directed towards the surrounding environment 18. Additionally, the two-dimensional array 12 is connected to a digital processing unit 20. An inertial measurement unit (IMU) 22 is also connected to the digital processing unit 20 in a position that is fixed relative to the array 12.

[0026] The system 10 in its most general operation transmits acoustic or electro-magnetic energy 24, via the transmitter 16, toward the environment 18. The environment 18 reflects the transmitted energy wherein the reflected energy 26 is coherently received at at least three receivers 14a, 14b, 14c that are arrayed in the two-dimensional array 12. As the mobile vehicle upon which the system 10 is mounted moves, the transmitter 16 continues to transmit energy 24, and the receivers 14a, 14b, 14c continue to receive the reflected energy 26. The reflected energy 26 from the environment 18 is captured and digitized at the receivers 14a, 14b, 14c and sent to the digital processing unit 20.

[0027] Additionally, in some embodiments of the invention, the IMU 22 detects acceleration and rotation rate information relating to the position and orientation of the vehicle mounted array 10 and transmits this information to the digital processing unit 20 as the vehicle moves. The accuracy of the IMU measurements can be coarse in quality. Using the information from the receivers 14a, 14b, 14c and the IMU 22, the digital processing unit 20 creates coherent snapshot images of the environment 18 from the digitized reflected energy 26 collected at the receivers 14a, 14b, 14c as modified using the acceleration and rotation rate data from the IMU 22. One image is created for each of the receivers. The created images are compared to one another on a pair-wise basis and coherently interfered to produce interferograms. One interferogram is produced for each pair-wise combination of receivers 14a, 14b, 14c. The interferograms created from the reflected energy 26 detected from the environment 18 are then used to produce an estimate of the error in vehicle position and orientation relative to the vehicle position and orientation reported by the IMU 22. The errors are then used to precisely update the vehicle's location and orientation within the environment 18. The precision and accuracy of the localization computed is a small fraction (˜ 1/100) of the transmit signal's wavelength which enables micron-level positioning and milli-degree orientation and achieves improved localization performance over other approaches.

[0028] The same process can be applied to a system without an IMU 22 using forward projection of previous navigation solutions to provide estimates of the vehicle's expected position and orientation within the environment 18 for use in the interferogram-based corrections, as long as the kinematic dynamics are low. For situations of high kinematic dynamics, the same process used with the IMU 22 can be applied to a system without an IMU 22 as long as there is an alternative means to provide some level of position and orientation estimates for use in the interferogram-based corrections.

[0029] The system 10 of the present disclosure can be used in any environment 18 where objects and surfaces within the environment 18 reflect enough energy to be received by the receivers 14a, 14b, 14c, and a sufficient number of objects and surfaces distributed within that environment are stationary at the scale of the time it takes for one receiver within the array to move to the location of another receiver within the array. The system 10 can be used at short distances such as inside a building or in a tunnel as well as for far distances such as from a satellite in orbit around the Earth or another planet. With appropriate scaling of the strength of the transmit energy and sensitivity of the receivers, the system 10 can be deployed for use in conjunction within a broad range of environments and vehicles. It should be appreciated by one skilled in the art that an exhaustive list of environments and vehicles need not be listed as the system 10 is self-contained and intended to operate in a standalone manner thereby being unlimited in its deployment relative to vehicles and environments, except as specifically stated herein.

[0030] Turning now to FIG. 2, images are formed from the digitized reflected energy 26 using the synthetic aperture technique known as backprojection. This technique directly incorporates the locations of each receive phase-center represented by (x.sub.i) and the assumed locations of the objects and surfaces from which the reflected energy 26 is gathered from the environment 18. Backprojection calculates the signal received at a phase-center for the i.sup.th pulse as a function of range, r, denoted as s.sub.i(r), and is formulated as:

I.sub.z=Σ.sub.i=1.sup.Ns.sub.i(∥x.sub.i−z∥).Math.e.sup.j4π∥x.sup.i.sup.−z∥/λ.

wherein:

[0031] I.sub.z—represents the environmental response,

[0032] s.sub.i(r)—represents signal received at a phase-center,

r=∥x.sub.i−z∥,

[0033] x.sub.i—represents the time-evolving phase center location in three-dimensional space,

[0034] z—represents a pixel being observed in the environment, and

[0035] λ—represents the wavelength of the propagating energy.

[0036] To determine the complex environment response, I.sub.z, the signal from range location r=∥x.sub.i−z∥ is summed over a set of N pulses, and the collection of these responses for the p.sup.th receive phase-center over all pixel locations constitutes the coherent image for that receive phase-center, denoted as f.sub.p.

[0037] Once the coherent images have been formed for all the receive phase-centers, interferograms are computed between all pairwise combinations of phase-centers. Each interferogram is computed using the complex cross correlation coefficient for a pair of coherent images f.sub.p and f.sub.q:

[00001] $? = \frac{{.Math.}_{m = 1}^{M} f_{p} (m) ? (m)}{\sqrt{{.Math.}_{m = 1}^{M} {.Math. f_{p} (m) .Math.}^{2} .Math. {.Math.}_{m = 1}^{M} {.Math. f_{q} (m) .Math.}^{2}}},$ $? indicates text missing or illegible when filed$

where ′ indicates complex conjugation. This coefficient is computed across the image over a neighborhood of M pixel locations where M is chosen based on the desired signal-to-noise ratio in the resulting interferometric phase.

[0038] The interferometric phase for the pq.sup.th phase-center pair at pixel location z, denoted as ϕ.sub.p,q,z, is described precisely with the following equation:

[00002] $ϕ_{pq, z} \frac{λ}{4 π} = [ε_{p} .Math. ? - ε_{q} .Math. ? + δ_{z} {\hat{h}}_{z} .Math. (? - ?)]$ $? indicates text missing or illegible when filed$

where ε.sub.p and ε.sub.q are the position errors associated with the p.sup.th and q.sup.th phase-centers respectively. The unit vectors {circumflex over (r)}.sub.p,z and {circumflex over (r)}.sub.q,z point from the p.sup.th and q.sup.th phase-centers to the pixel location z. The unit vector ĥ.sub.z is the height direction at pixel location z and, finally, δ.sub.ε represents the height error at this pixel location.

[0039] Based on the above calculations an interferometric phase measurement is produced for each pixel location and each pairwise combination of receive phase-centers. These interferometric phase measurements constitute an overdetermined system of equations that is used to solve for the phase-center position errors and the pixel height errors using the method of least squares. Since the noise in the phase measurements is Gaussian, this approach is the Best Linear Unbiased Estimate (BLUE) of the phase-center position and pixel height errors. To maintain a unique solution, the first phase-center is assumed to have no errors, resulting in estimation of phase-center errors that are relative to the first.

[0040] The orientation and position error of the vehicle is then determined based on the estimated phase-center position errors. First, the expected phase-center positions, determined either from inertial measurements or forward propagation of prior navigation solutions, are corrected by the BLUE-estimated phase-center errors. The three-dimensional distance between the centroids of the expected and corrected phase center positions establishes the three-dimensional position error of the vehicle.

[0041] Turning now to FIG. 3, the Kabsch algorithm is used to estimate the roll, pitch, and yaw rotation angles between the expected phase-center positions and the corrected phase-center positions. The result is the three-dimensional orientation error of the vehicle. These estimated vehicle orientation errors are then used to update the vehicle's complete navigation solution.

[0042] This system and method can be used in any environment where the objects and surfaces within that environment reflect enough energy to be received by the receivers, and a sufficient number of objects and surfaces distributed within that environment are stationary at the scale of the time it takes one receiver within the array to move to the location of another receiver within the array. For example, the ocean surface is a moving environment, but electro-magnetic energy reflected from that surface is substantially the same as electro-magnetic energy reflected from that surface a short time later, when the trailing receiver views the same scene. With receivers arrayed appropriately to mitigate the impact of the motion, the system can operate within this type of moving environment. Similarly, the system can be used at short distances such as inside a building or in a tunnel and can also be used at far distances such as from a satellite in orbit around the Earth or another planet. With appropriate scaling of the strength of the transmit energy and sensitivity of the receivers, the invention can be used across a broad range of environments and vehicles.

[0043] In one of the preferred methods of operation, an IMU is part of the system and serves two functions. First, the IMU provides a means to initialize the navigation solution, e.g., initialize the orientation of the moving vehicle upon which the system is mounted. Second, the IMU provides a means to measure concurrent position and orientation that is used by the digital processing unit for image formation and is then updated with finer precision by the errors estimated from the collected interferograms. If an alternative means exists to initialize the navigation and an alternative means exists to estimate concurrent navigation information, the IMU can be removed from the system. In either case, the system provides a navigation solution relative to the initial position and orientation of the vehicle.

[0044] The feature of using coherent images from an array of active sensors to form interferograms of the environment that are then used to update the navigation solution is believed to be the novel and unique aspect of this disclosure. As such, the system employs an active radar or acoustic sensor array to coherently image the surrounding environment. The coherent imaging step and the processing of those coherent images provides orders-of-magnitude improved localization and orientation accuracy beyond existing technologies. For example, because the localization precision is on the order of a small fraction of the transmission wavelength, and wavelengths are typically centimeters or less, micron-level localization and milli-degree orientation are possible.

[0045] It should be appreciated that the disclosed system is a self-contained mechanism such that the operation is entirely under the control of the user and no coordination or communication with external support systems, like GPS, is required. This property enables robust operation in environments that block or otherwise interfere with external support systems. This allows the system to provide a continuous estimate of localization and orientation relative to the environment, provided the system maintains acoustic or electro-magnetic contact with the environment. These features ensure that the localization and orientation of the system relative to the environment is well maintained.

[0046] It can therefore be seen that the present disclosure provides a self-contained navigational system using multiple receivers in a two-dimensional array that creates pairwise interferograms from each of the receivers to precisely correct the position and orientation estimates based on inertial measurement data or based on forward propagation of previous navigation solutions. Further, the present disclosure provides a navigational method and system that creates a unique interferogram from multiple receivers within a two-dimensional array to precisely update the vehicle's location and orientation within the environment. For these reasons, the present disclosure is believed to represent a significant advancement in the art, which has substantial commercial merit.

[0047] While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims.

SYSTEM AND METHOD FOR SELF-CONTAINED HIGH-PRECISION NAVIGATION

Assignee

Inventors

Cpc classification

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G01S13/9023

PHYSICS

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G01S13/89

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G01S13/42

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International classification

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G01S13/90

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G01S13/42

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Abstract

Claims

Description