Patent Application
20080129599
This invention relates generally to target locators, and more specifically, to methods and systems for locating targets using digital elevation model survey points.
A target locator is typically used to remotely locate a target by measuring a range and a direction (e.g., azimuth and elevation angles) to the target. The location of the target, for example, in coordinates, is then computed based on the GPS coordinates of the position of the target locator and the range and direction. The target location is then utilized by a command and control center to guide surveillance or a weapon system to the computed location of the target.
In one known system, the target location process utilizes gyro-compassing techniques coupled with a laser range finder to obtain an absolute direction and range to the target. However, this target locator system is only suitable for large explosive weapon systems because there are some inaccuracies in the range and direction measurements. These inaccuracies result in a circular error probability (CEP) of approximately 80 meters. For lower cost and smaller explosive weapon systems, the existing target locator system does not provide the necessary target location accuracies. For these smaller explosive weapons systems, a CEP of about five meters at ranges of about five kilometers is desired.
The existing system using absolute target measurement techniques along with the gyro-compassing mechanization is not capable of meeting these higher accuracy requirements. Therefore, a different target locator mechanization is needed to meet the higher accuracies desired.
In one aspect, a method for determining a position of a target using digital terrain elevation data survey points is provided. The method comprises selecting at least two surveyed reference points from the digital terrain elevation data, and determining a location of the target locator with respect to the digital terrain elevation data. The method also comprises referencing the location of the target to the digital terrain elevation data, measuring a position of the target locator, and translating a difference between the determined location and the measured position of the target locator to the referenced location of the target.
In another aspect, a target location system is provided that comprises a digital terrain elevation data and a system processor comprising a user interface. The digital terrain elevation data comprises a plurality of surveyed points and is communicatively coupled to the system processor. The system processor is configured to allow a user to select at least two of the surveyed points as reference points, and further programmed to determine a location of the target location system and a target with respect to the digital terrain elevation data based on the selected reference points.
In still another aspect, a processor for determining a position of a target is provided. The processor forms a portion of a target locating system and is programmed to receive data relating to at least two surveyed reference points from a digital terrain elevation data, receive data relating to a global position of the target locating system, receive data relating to an angle to the surveyed reference points and the target with respect to a magnetic field, and receive data relating to a range to the target and a range to each surveyed reference point. The processor determines a location of the target locating system and with respect to the digital terrain elevation data utilizing the data relating to the range and angle to each surveyed reference point, and determines a position of the target utilizing the data relating to the range and angle to each surveyed reference point and a translation of the location of the target locating system to the received position of the target locating system.
By using relative sensing with respect to a fixed reference or surveyed point and a more accurate azimuth and elevation sensor mechanization, the accuracy of a target locator can be improved as much as a factor of ten and can provide position accuracies of five meters at ranges of five kilometers. Such accuracies allows low cost, small, explosive weapon systems to be used effectively against targets.
Relative sensing is accomplished by establishing a reference survey point that is located in less-hostile areas.
In one embodiment incorporating relative sensing, it is assumed that either the same GPS receiver or another GPS receiver with similar error characteristics is used and the measurement time between the two surveyed points (xp, yp, zp and xrt, yrt, zrt) is small, that is, satellite positions are similar. As a result, the errors at both these locations are related and therefore, most of the GPS errors are canceled which results in relative position of the reference target and the measurement location being very accurate.
Assuming positions xp, yp, zp and xrt, yrt, zrt are accurately “surveyed” using GPS, then the exact range, Rrt, between these two points is computed to establish range truth. Using a laser rangefinder, range, Rrt, is measured and compared against a range truth. A laser rangefinder bias error is determined and used as an offset when an actual target range, Rt, is measured. As a result, the target range can be measured very accurately to within one-half meter.
For making azimuth and elevation measurements, in one embodiment, rather than a magnetic compass sensor, a non-contact, high resolution anisotropic magneto-resistive (AMR) sensor is utilized to measure angular position. One particular AMR sensor is capable of measuring the angle direction of a magnetic field from a self-contained magnet with less than 0.05° resolution.
The advantages of measuring field direction versus field strength (i.e. like a magnetic compass) include: insensitivity to the temperature coefficient of the magnet, less sensitivity to shock and vibration, and the ability to withstand large variations in the gap between the sensor and magnet. Such magnets are typically located on a stationary tripod section and the AMR sensor is aligned and then rotated with the optical sites and the laser rangefinder.
The field strength from the magnet at the sensor is 100 times the strength of the earth field and as a result, is more stable and less susceptible to perturbations from outside environments. Magnetic field direction is not critical since relative angular positions rather than absolute positions are being measured. As a result, there is minimal calibration of the AMR sensor mechanization in the field. Output is from a Wheatstone bridge that permits balanced output signals for noise immunity. A low offset amplifier and high resolution delta-sigma converter (i.e. analog to digital converter) is utilized to meet a desired accuracy of ±0.05°.
In one operational scenario, a sight reticle is moved to align with the reference target. The angle between the magnetic field and the reference target is then measured (θmrt). The sight reticle is then moved to the target and the angle between the magnetic field and the target is measured (θmt). Subtracting one angle from the other results in an angle between the reference target and the actual target (θmt−θmrt). The angle θrt is calculated knowing the reference target position, and as a result, the target azimuth angle (θt) can be determined.
While the above described relative sensing method provides a great deal of accuracy, one of the problems associated with such a method is that a survey reference point has to be measured utilizing the GPS. Such a survey reference point can be several thousand meters away from the target locator position. This distance can result in time consuming measurements and also may put the person making the GPS measurements in danger while operating in a hostile environment, which is obviously undesirable.
The below described system and methods eliminate the need for a person to measure a survey reference point utilizing GPS while still providing a high resolution target location function and still incorporating a relative sensing mechanization.
Specifically,
Still referring to
To determine a location of the target 80, the target location is measured with reference to the map. First, the target range, Rt, is measured using the laser range finder, and azimuth angle to the target 80 is measured by measuring θMt and θMm2 and then determining θm2t according to θm2t=θMt−θMm2. Since points xm2, ym2, zm2 and xmp, ymp, zmp, are known θm2 can be determined which allows for θt to be determined according to θt=θm2t−θm2.
To measure the elevation angle of the target, the elevation sensor mechanization is utilized, for example, the orthogonal accelerometers in an inertial measurement sensor that senses gravity force vectors.
Target locator position is then measured using GPS. The difference between the GPS target locator position and the target locator position with respect to the map is then utilized to determine a translation correction from the map coordinate system to GPS coordinates. The target position is then translated into GPS coordinates using the translation correction. The result is that the target location is provided in either map coordinates or GPS coordinates. The weapon system is typically in GPS coordinates. Having both the target location and the weapon system in the same coordinate system provides relative positioning and therefore, minimizes target location errors (TLE).
The above described target location method utilizes accurate survey points from a survey map of dominant features. In at least one embodiment, survey points are measured by generating digital elevation models (DEMs) using high resolution preprocessed level IV or level V digital terrain elevation data (DTED) and corresponding maps with appropriate registration.
In one embodiment, system 100 includes a rotary platform 120 on which the above described components are mounted, and rotary platform 120 is attached to a stationary, adjustable tripod 122. All components of system 100 that utilize power are supplied that power from battery/power supply 124.
System 100 further includes several new technologies which enable such a target position solution. For example, a large capacity memory storage capability in smaller package sizes is available. In one embodiment, within a single small module (DEM survey map 112), more than 64 gigabytes can be stored, which allows for a large quantity of map survey point data to be included within. In addition, loss-less compression techniques enable even higher densities of data. With high resolution Level V digital terrain elevation data (DTED), more than 100,000 square miles can be stored on a small board housing 64 gigabytes of memory and using a 8× loss-less compression algorithm.
DTED is Department of Defense standard terrain model generated by NGA (National Geospatial Agency). Accurate precision strike needs prompted a requirement for higher resolution elevation data. For example, Level III (i.e. 10 meter accuracy) and Level IV (3 meter accuracy) DETD has been measured using optical and interferometric synthetic aperture radar (IFSAR) from air vehicles and satellites.
High resolution digital point position data base (DPPDB) from NGA is a set of controlled stereo images with support data covering nominally a one-degree rectangle (3600 nmi2). DPPDB provides for accurate three dimensional (3D) object measurement (i.e. Level V or 1 meter accurate elevation data) of cultural and object/terrain features for weapon system mission planning.
This high resolution DPPBD is typically used for navigation of weapon or aircraft systems such as a precision terrain aided navigation (PTAN) system. PTAN is an autonomous navigation aide that measures terrain features, correlates those terrain features to stored digital terrain elevation data (DTED) and provides precision air vehicle position. Since reference points for target location are stationary, this simplifies the application of geo-location survey maps using DPPBDs. Speed or time are not a major issues for this application. However, precision is still required to survey and map dominant object or terrain features so that high precision target location is achieved.
As described above, map resolution and accuracy of DTED data has been improved with the aid of optical and interferometric synthetic aperture radar. As such, digital maps can provide significantly better accuracy than GPS surveyed points, and digital map technology continues to improve. The above described methods and systems are capable of being integrated into next generation weapon systems. This integration provides a unique solution for determining a target position that allows relative sensing target location without having a person to travel to a reference point and take a GPS reference reading.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
This application claims priority to U.S. Provisional Application Ser. No. 60/708,577, filed Aug. 16, 2005, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60708577 | Aug 2005 | US |
