Real-time ship position error compensation system and method

A system for measuring the position of an asset is provided. The system includes a first plurality of global positioning system (GPS) sensors arranged on a first asset, and a reference position source arranged on the first asset. One or more computer processors is provided in communication with a memory and is configured to measure position data associated with the first asset from the outputs of the first plurality of GPS sensors, and calculate a measurement offset between the measured position data associated with the first asset and position data obtained from the reference position source. From these measurements, position data measurements obtained from the plurality of GPS sensors of the first asset may be corrected using the calculated measurement offset. Likewise, position data measurements obtained from a plurality of GPS sensors of a second asset, located remotely from the first asset, are also corrected using the calculated measurement offset.

FIELD OF THE INVENTION

The present invention relates to systems and methods for generating real-time normalized position data for use among multiple remote marine platforms.

BACKGROUND

Accurate tracking of targets (e.g. radar tracking of airborne targets) among multiple floating platforms or vessels at sea, as well as sharing tracking data among multiple floating platforms and land-based systems, requires increasingly advanced technical innovations to provide corrections to internal ship flexure and relative earth coordinates. Positioning systems according to the prior art typically attempt to acquire satellite tracking data over a period of hours, determine errors, and apply appropriate corrections. However, these processes ignore any shifts in static flexure or any current dynamic flexure errors of the platforms or vessels being tracked. These types of bias changes may be the result of many factors, including temperature distortions (e.g. from the sun), which can be observed in a matter of minutes after maneuvering a ship, or in mere seconds in the case of dynamic flexure events. Accordingly, hours of data acquisition would be insufficient to accomplish an accurate correction solution without some additional augmentation. Moreover, these systems inherently introduce errors as a result of inaccuracies in their measuring devices.

Referring generally toFIG. 1, there is shown an exaggerated illustration of the above-described errors induced by current GPS-based positioning systems employed on vessels100,101,102. These errors include, for example, deviations in perceived True North and actual True North bearings, as well as deviations in perceived location versus an actual or true location. In addition, a ship can distort such that one structural section can be out of phase in roll, pitch or yaw with respect to another structural section. The errors may create significant performance reductions when affected position data is utilized by on-ship systems. For example, a ship's on-board radar system may magnify these errors when attempting to track distant airborne targets. Further still, these inaccuracies may result in increased difficulties performing operations among multiple ships. By way of example, exchanging radar tracking or track data of an airborne target between multiple platforms may be detrimentally affected by the discrepancies between the relative reference positions of each platform and its influence on a perceived measured flight path. This may result in the creation of one or more sets of phantom tracks in gridlocking processes resulting from these discrepancies.

As will be understood by one of ordinary skill in the art, the term gridlocking is generally used to describe a process of receiving track data from multiple source locations and correlating jointly held targets to build a composite track, larger in volume compared to that achievable by a single source. One issue is that track data from multiple sensors on one platform can have time varying biases from each sensor, but gridlocking techniques apply a single correction for data from one platform. This can create multiples of a single target or even delete a valid target from consideration in the composite track picture.

Alternative systems and methods to determine local flexure corrections, roll and pitch values, as well as for determining relative True North for use by multiple platforms are desired.

SUMMARY

In one embodiment of the present disclosure, a system for measuring the position of an asset is provided. The system includes a first plurality of global positioning system (GPS) sensors (e.g. two sensors) arranged on a first asset, and a reference position source arranged on the first asset. One or more computer processors is provided in communication with a memory and is configured to measure position data associated with the first asset from the outputs of the first plurality of GPS sensors, and calculate a measurement offset between the measured position data associated with the first asset and position data obtained from the reference position source. Position data measurements obtained from the plurality of GPS sensors of the first asset may be corrected using the calculated measurement offset. Likewise, position data measurements obtained from a plurality of GPS sensors of a second asset, located remotely from the first asset, are also corrected using the calculated measurement offset.

In another embodiment of the present disclosure, a method of measuring the position of an asset is provided. The method includes the steps of measuring position data associated with a first asset from outputs of a first plurality of global positioning system (GPS) sensors arranged on a first asset, and measuring reference position data of the first asset with a reference position source of the first asset. From these measurements, a measurement offset between the measured position data associated with the first asset and the reference position data obtained from the reference position source is calculated. Using this measurement offset, measured position data obtained from the plurality of GPS sensors of the first asset may be corrected. Likewise, position data measurements obtained from a plurality of GPS sensors of a second asset, located remotely from the first asset, may also be corrected using the calculated measurement offset.

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements found in communications and positioning systems, such as GPS-based systems. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. The disclosure herein is directed to all such variations and modifications known to those skilled in the art.

Embodiments of the present disclosure include systems and methods for improving the accuracy of measuring a position of an asset, such as a marine ship or a vessel, and for improving the performance of on-ship systems reliant on these position measurements. Embodiments also include systems and methods for generating real-time differential position and orientation data on single marine platforms that describes the gross structural distortion of the platform for purposes of correcting measured target track data. According to one method, a platform, ship or land-based location is designated as a master positional reference. Measured positions or measured position references of a selected group of neighboring secondary platforms or ships may be normalized to this master position reference in order to improve on-ship operations, such as aircraft tracking, as well as to improve the accuracy and efficiency of data sharing between these platforms. The accuracy of positional measurements of the platforms may also be improved by performing platform flexure measurements, as well as by accounting for local pitch and roll.

A system according to embodiments of the present disclosure may include a matrix of Differential Global Positioning Sensor (DGPS) units (e.g. 4 or 6), such as Carrier-Phase Differential Global Positioning Sensors (CDGPS) arranged in a high fidelity configuration on a platform or ship designated as a “master” or reference platform. In one embodiment, these sensors may be located on long baselines of the platform (e.g. along baselines of a ship) in an approximately rectangular formation. One CDGPS unit may be designated as a local reference for the platform. Position data measured by this master platform may be used to normalize position data measured from one or more secondary platforms or ships located in geographic proximity with the master reference platform. In one embodiment, the master reference platform would comprise the most stable platform, such as an aircraft carrier, if a ground platform is not available.

These embodiments provide a basis for secondary ships or platforms to estimate “True North” relative to their centerline by determining errors or offsets in measured positions. Embodiments may also measure, for example, any flexural twist of a ship or platform, and use this data to further correct measured position data. In other embodiments, the CDGPS units on each ship will be augmented by a differential accelerometer or other inclination measuring system to monitor angular tilt differences between locations on ship and fill in gaps to any loss of DGPS. Information will be filtered to take advantage of expected ship sinusoidal motions. The use of a shared relative True North between each of the platforms may improve cooperative efforts between these platforms, in addition to correcting ship navigation parameters. For example, in the context of an airborne target being tracked by multiple platforms, the use of a normalized True North among platforms reduces tracking errors as data is transmitted between the platforms, improving gridlocking operations.

Referring generally toFIG. 2, a system105according to an embodiment of the present disclosure includes an exemplary master platform or ship110and an exemplary secondary platform or ship140. Master ship110includes a master ship DGPS system, comprising, for example, a matrix of 4 to 6 (or more) CDGPS sensors112. Referring generally toFIGS. 3A and 3B, andFIGS. 4A and 4B, sensors112may be located along baselines of an exemplary ship, as will be set forth in greater detail below. Sensors112are operative to precisely measure position data at each of the illustrated points, and compare these measurements to a perceived position obtained from a reference position source117, such as an on-board GPS system for determining a measurement offset. Position data is measured in real time, providing the ability to create time-filtered position estimates. A differential accelerometer or inclination system122may also be provided, and used to measure ship motion (e.g. sinusoidal motion) corresponding in time to the of the CDGPS measurements. Accelerometer/inclination system122may monitor angular tilt differences between various locations on ship and fill in gaps to any loss of CDGPS data. This information may also be filtered to take advantage of expected or measured ship sinusoidal motions. See, for example,FIG. 10, illustrating exemplary sinusoidal motion115of a vessel extrapolated from historical position and/or acceleration measurements.

A measurement processor114may be responsive to the outputs of these measuring devices for storing recorded position and/or acceleration data into a memory device116. Measurement processor114may also be configured to calculate pitch, roll, and heading of ship110, as well as calculate hull flexure, or twist, using this measured data. Measurements of pitch, roll, heading, and flexure may also be stored into memory device116. A position correction processor118may be responsive to measured ship motion and flexure, as well as position data taken from CDGPS sensors112and reference position source117, to generate reference correction data or a measurement offset. This reference correction data may account for local pitch, roll and flexure measurements, as well as any other errors induced by the CDGPS sensors. Errors can principally be detected by monitoring relative distances between sensors112on ship110and determining if those relative distances exceed reasonable structural tolerances (without catastrophic failure).

Local reference correction data may be utilized by other on-ship systems124, for example, guidance and navigation systems, communications systems, radar or sonar systems, and the like. For example, this reference correction data or measurement offset may serve as a basis for determining the relative position of tracked airborne targets when utilized by the master ship's radar system(s). Master ship110further comprises a transceiver120forming part of a communications system for transmitting and receiving data between a plurality of platforms, including transmitting the above-described reference correction data. While the master platform110has been described herein as a ship, it should be understood that such a master reference may comprise a ground station (where available within a given region), or other suitably stable platform.

As described above, according to embodiments of the present disclosure, reference correction data generated by master ship110may be used by one or more secondary platforms or ships within a given geographical area. More specifically, other platforms, ships or vessels may be “assigned” or provided this reference correction data for use by their own on-ship systems. Still referring toFIG. 2, an exemplary secondary ship140is provided and may comprise a communications system, including a transceiver150responsive to received reference correction data transmitted from master ship110via transceiver120.

Transceiver150may be configured to receive position data obtained from master ship110and store this data onto a memory device146for further processing. For example, as described above with respect to master ship110, secondary platform or ship140may also comprise a matrix of, by way of non-limiting example only, four, six or more CDGPS sensors142for measuring its own local time-varying position with respect to a reference position obtained by a reference position source (e.g. a GPS system)119. A measurement processor144may be provided, and responsive to the outputs of sensors142, reference position source119, and differential accelerometer/inclination system152, for generating position data for ship140, and storing it in memory146. As set forth above, a differential accelerometer/inclination system152may be used to measure ship motion (e.g. sinusoidal motion) corresponding in time to the of the CDGPS measurements. Accelerometer/inclination system152may monitor angular tilt differences between various locations on ship and fill in gaps to any loss of CDGPS data.

Measurement processor144may also be configured to measure pitch, roll, and heading of ship140, as well as measure hull flexure, or twist, using this data. Measurements of pitch, roll, heading, and flexure may also be stored into memory146. A position correction processor148may be responsive to measured ship motion and flexure, as well as position data taken from CDGPS sensors142, to generate local reference correction data. This reference correction data may be corrected for local pitch, roll and flexure measurements, as well as any other errors induced by CDGPS sensors142. Position correction processor148may be further configured to compare the reference correction data of ship140generated by sensors142, with the master reference correction data received from master ship110, for determining any offset or error occurring therebetween. This comparison may be used to, for example, correct a local estimated True North to the True North of the master reference. Any deviations or offsets between the reference correction data and the local reference correction data may also be stored in memory146.

Position correction processor148may be further configured to correct the measured local position data for a given measured error. By way of example only, position correction processor148may be operative to calculate an offset or error between a measured True North and the master reference True North. From this measured offset, position data may be corrected (i.e. normalized to the master reference True North). Referring generally toFIG. 5, two exemplary measured points B and R, as well as an estimated local relative True North, for secondary ship140are shown. Referring generally toFIG. 6, point B may comprise the measured point of a CDGPS sensor designated as a local reference for the secondary platform. A calculated offset or error ΔBmay be applied to the position data corresponding to point B, to generate a corrected or true point B. Correction processor148may be used to apply the corrected offset ΔBto the remaining sensors of the secondary platform (i.e. to the sensor measuring illustrated point R, for similar offset correction. Likewise, the master True North reference may be adopted for use by the system of secondary ship140, as illustrated inFIG. 7. This corrected or normalized position data may be used by on-ship systems154, such as a radar system of ship140.FIG. 8generally illustrates the sharing of master reference data generated by a designated master ship110between secondary platforms or ships140, as set forth in detail above.

While the exemplary embodiment illustrates only a single ship140, it should be understood that embodiments of the present disclosure relate to systems and methods for generating master reference position correction data for use by a plurality of ships or platforms operating in a given area. In this way, each of these platforms will be using shared, or normalized, corrected position data for use in each of their on-ship systems. As a result, improvements in the accuracies of these systems may be realized. Further, data shared between these systems, and corresponding systems of other platforms may be improved. In the case of airborne target tracking, for example, the use of normalized position data among a plurality of remotely-located radar systems improves these system's abilities to accurately communicate and pass tracking data between one another.

FIGS. 3A and 3Billustrate one exemplary advantageous mounting arrangement of four (4) CDGPS sensors142on ship140. As illustrated, sensors142may be arrange generally along a centerline of an exemplary vessel, as well as along a line running generally perpendicular to this centerline. In this way, sensors may be operative to estimate roll, pitch and ship heading relative to an estimated True North.

Similarly,FIGS. 4A and 4Billustrate another exemplary mounting arrangement of six (6) CDGPS sensors142on ship140. As shown, by arranging sensors142in a generally rectangular configuration, in addition to measuring roll, pitch, and heading relative to an estimated True North, relative flexural twist and bending of the ship's hull may also be estimated.

Referring generally toFIG. 9, an exemplary method200according to embodiments of the present disclosure is provided. Positional measurements may be made210via one or more GPS sensors on a designated master reference platform. Measurement correction data or a measurement offset may be calculated220based on past measurements and analyses, including those based on position data obtained by an on-board reference position source, such as a GPS, similar to fixed reference base stations that provide references within DGPS. This may also include analyzing on-ship geometries and augmented measurements. This master correction data is then transmitted230to each secondary platform.

Secondary platforms or ships may make local position measurements240, including corrected positional measurements based on established ship geometries. Corrections to this measured position data at respective secondary platforms are performed using the correction data received by the master reference platform, and offset or error values are calculated250. These calculated offset or error values may be applied to each of the sensors of the secondary platforms (or applied to a reference sensor of each platform which, in turn, applies the offsets to data generated by the remaining sensors) for correcting250measurements made thereby. In this way, position and True North data, relative True North estimations, pitch, roll and heading estimates, as well as flexure measurements as set forth above are normalized to the master reference data250. This normalized data is provided or transmitted260to other on-ship systems of each master and secondary platform for realizing the above-described benefits.

The embodiments are provided by way of example only, and other embodiments for implementing the methods described herein may be contemplated by one of skill in the pertinent art without departing from the intended scope of this disclosure. For example, systems, including the position correction and measurement processors, may include memory containing data (e.g. memory devices116,146ofFIG. 2), which may include instructions, the instructions when executed by a processor or multiple processors, cause the steps of a method for performing the measurements and measurement corrections set forth herein. Software may be embodied in a non-transitory machine readable medium upon which software instructions may be stored, the stored instructions when executed by a processor cause the processor to perform the steps of the methods described herein. Any suitable machine readable medium may be used, including but not limited to, magnetic or optical disks, for example CD-ROM, DVD-ROM, floppy disks and the like. Other media also fall within the intended scope of this disclosure, for example, dynamic random access memory (DRAM), random access memory (RAM), read-only memory (ROM) or flash memory may also be used. In one embodiment, the measurement and correction hardware and software can be implemented using any real time operating system software and associated DSP hardware, an FPGA, custom hardware and software, by way of non-limiting examples only. The processor type and software must be configured such that the timing of the system can be maintained.