EP0583972A1

EP0583972A1 - Improvements in and relating to precision targeting

Info

Publication number: EP0583972A1
Application number: EP93306477A
Authority: EP
Inventors: Merrill J. Habbe
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1992-08-17
Filing date: 1993-08-17
Publication date: 1994-02-23

Abstract

An aircraft (1) with a SAR images a target area (5) and selects a target in the target area from a known first position (P1) whereat the GPS coordinates of the position are determined and from which the range and monopulse angles of the target are computed to establish the location of the target in three dimensions with associated errors. The aircraft then flies to a second position (P2) whereat the GPS coordinates of the second position are determined and obtains another high resolution image of the target. Again, the range and monopulse angles of the target are computed. Because of the accuracy of the GPS-aided INS, computer aiding to the pilot will make this second designation much easier. Since the position vector between the two known positions can be provided with accuracy to within a few inches due to the accuracy of the GPS receiver with carrier phase tracking, the position of the target is determined largely by the range measurement accuracy of the radar and the two position locations. Since this establishes the location of the target in a GPS coordinate system relative to the position of the aircraft, absolute position errors typically associated with GPS navigation are canceled. The GPS guided weapon, when supplied with the target location in GPS coordinates, is then guided to the target with the high precision available in clear weather conditions. Since the radar and GPS are both fully functional in adverse weather as well as clear weather, there is provided an all-weather solution to precision weapon delivery.

Description

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

This invention relates to a system and method for precision location of a target and precision delivery of a guided weapon to the located target under adverse as well as clear weather conditions.

BRIEF DESCRIPTION OF THE PRIOR ART

The art has long sought a system capable of delivering guided weapons to the computed target with extreme precision under adverse as well as clear weather conditions with accuracies equivalent to those achievable with clear weather sensors. At present, no such economically feasible system is known to exist.
Present precision guided weapon systems use forward looking infrared (FLIR) target acquisition sensors coupled with laser designators and infrared (IR) homing seekers that are capable of accuracies within a few meters. However, such weapon delivery techniques rely upon good weather conditions. Adverse weather weapon delivery relies upon radar to image and acquire targets. While imaging radars have been used under such adverse conditions to locate targets, only unguided bombs could be delivered to the located target. The accuracies achievable with this technique are far inferior to those achievable with the precision guided or smart weapons. The operational void stems from the lack of an all weather sensor in the weapon itself to guide it to the target and from the inability to precisely locate the target with on-board sensors in the aircraft.
While no prior art systems are known which provide all weather target location with the required precision, several conceptual systems are presently under development or consideration. Among these is the placement of a coherent radar seeker in the guidance section of the weapon that can image the target and generate steering commands to guide the weapon to the target. While this technique holds promise for achieving smart weapon accuracies, the cost of such a seeker would appear to prohibit its widespread use.
A further concept is the use of a Global Positioning System (GPS) receiver within the guidance section of the weapon for steering to the target coordinates supplied by some on-board sensor. While this technique will operate in adverse weather conditions, techniques to accurately determine the target coordinates in a GPS reference frame are lacking.
The most often discussed adverse weather technique for determining the target coordinates is with a high resolution radar using synthetic aperture radar (SAR) techniques, making range and monopulse measurements on the target and establishing the target location in GPS coordinates. While the range measurement is very accurate, the angle errors required to determine the position of the target in three dimensions are too inaccurate with radars on tactical aircraft. An angle error of one milliradian, one sigma, from distances of tens of kilometers causes target location errors of tens of meters.
Generation of a high resolution radar picture (image) of a target is a well established technique employed by many present day operational systems. This technique, in use since the early 1950s, is now used by tactical and strategic reconnaissance and weapon delivery systems. For example, the APG-70, which is used on the F-15 aircraft and the APQ-164, which is used on the B-1B bomber, have high resolution imaging capability and provide range measurements with accuracy of about one meter. These images approach photographic quality and are presented to the pilot and/or weapon systems operator (WSO) using high resolution displays in the cockpit. In the present art, the operator, when presented the high resolution radar picture, can recognize the intended target on the display and can place a cursor (crosshairs) on the intended target. The cursor location on the display designates to the weapons system computer the intended target whose location is to be determined. The computer then calculates the target position in a navigation reference system established by the aircraft inertial navigation system (INS).

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an all weather target locating and weapon delivery system and method which provides the high precision required. This is accomplished by synergistically combining synthetic aperture radar (SAR) measurements and relative measurements from a Global Positioning System (GPS) tightly coupled to an Inertial Navigation System (INS) to precisely locate a target in GPS coordinates. Once the target location is precisely established, a weapon with a GPS Guidance Package, an Inertial Navigation System or a combination of the two is initialized with the target coordinates. Using standard guidance and control techniques, the weapon then guides to the target. This technique is capable of yielding homing accuracies equivalent to those achievable with present day precision guided smart weapons.
Briefly, in operation, a high resolution SAR image of a target area is collected from a first location of the aircraft in space and presented to the pilot. The pilot designates his target on the image. The radar processor computes the location of the target using the radar measurements of range (R), azimuth angle (Θ) and elevation angle (φ) and resolves the target coordinates into a local GPS reference frame. The aircraft then flies to a second location and repeats the operation of generating the SAR image, designating the same target, and again computing the target location using the radar measurements of range, azimuth angle, and elevation angle. Using the two radar measurements and the vector difference between the two sampling positions in space, the target location, in a relative GPS frame of reference, is uniquely determined in the plane of measurements with accuracies of a few meters from a distance of up to about 50 kilometers.
A missile, glide bomb, or the like is then launched from the aircraft and guided to the target by a variety of means, including GPS guidance, inertial guidance or a combination of both. Targets can be located and targeted in adverse weather conditions as well as clear weather conditions since the target location technique employs a radar and a GPS receiver integrated with an inertial navigation system, all of which can function in clear and adverse weather conditions. Also, weapon guidance is provided by a GPS receiver, an INS or a combination of both, all of which can function in clear and adverse weather conditions. Weapon delivery accuracies of a few meters are achievable, equaling the precision weapon delivery of state of the art IR weapons with laser designation.
All of the information required to compute the target position is contained in a single set of radar measurements of range, azimuth angle, and elevation angle, in the absence of errors. However, measurement errors exist, so the computed target position is also in error. To reduce the error, an estimate of the target position is made using the radar measurements from each of a first aircraft position and a second aircraft position, given the GPS location of each aircraft position. When combined, these two measurement sets provide an estimate with smaller errors than that provided by either measurement set alone.
Techniques for combining the above described measurements are well known and are described in detail in textbooks and papers on estimation techniques. One common method, referred to as the least squares method, selects a target location estimate such that the sum of the squared error between the estimated coordinates of the selected target and the measured values is minimized. If some measurement sets are more accurate than others, they can be weighted more heavily in the minimization process, resulting in a weighted least squares solution.
The small target location error requires very accurate knowledge of the position vector between the two aircraft positions. The two aircraft positions along with the target position form a triangle in space. Each side of the triangle is a measured quantity. The two sides of the triangle that extend from the aircraft to the target are measured with accuracies of about 1 meter using the radar and the third side of the triangle is measured with an accuracy of a few centimeters by phase processing GPS signals from a set of GPS satellites. The accuracy of the third side measurement, using the phase processing technique (cycle counting), when combined with the accurate radar range measurements, is the innovation that gives this weapon delivery concept its precision.
The technique for measuring the position vector between the two aircraft positions adds a cycle counting feature to a GPS receiver that is tightly coupled to an INS. This is referred to in the literature as integrated navigation, or INAV. In INAV, GPS sensor data is integrated with data derived from inertial instruments. In such systems, if the output of the inertial unit is used to aid the GPS receiver tracking loops, very precise position and velocity information is achieved. Conventional GPS uses information primarily from the GPS transmitted code to unambiguously obtain position information. However, the GPS carrier frequency provides much more accurate relative position information, but it is ambiguous for relative position differences greater than its wavelength (19 centimeters in present practice). Accordingly, a unique approach uses counts of GPS carrier cycles as a precision tape measure to obtain centimeter level accuracies and also to resolve the ambiguity. With the tight coupling between the GPS receiver data and the inertial instrument data, the precision tape measure mode can be implemented in a moving platform.
The precision tape measure mode, to measure distances between two points in space, is a relative measurement which is not degraded by any absolute position error of the GPS, provided these absolute errors remain correlated during the measurement interval in time and space. A list of the correlated GPS errors and their respective correlation times or correlation distance are:

Source Correlation Time or Distance

Ionosphere/Troposphere 5-10 minutes

Control/Space Segment 15-30 minutes

Same Satellite Tracking 10-20 minutes

Allowable Separation 50-75 miles
As long as the two sets of radar measurements and the measurement of the aircraft sampling positions with respect to each other are taken within the above described spatial area and time constraints, the absolute errors in the measured position vector between the two aircraft positions cancel. The absolute errors, using conventional GPS receivers, are on the order of fifteen to thirty meters. While this accuracy is sufficient for point to point navigation in most applications, it is not sufficient for precision weapon delivery. Using the precision tape measure technique, the target location accuracy is established in a relative coordinate/frame of reference to a few centimeters.
Target location accuracies and weapon impact errors using the GPS-SAR precision targeting technique equal that of precision guided weapons at distances extending to 50 kilometers (KM). This distance is primarily determined by the range measurement accuracy which degrades as the range from the aircraft to the target increases. The range error is due to atmospheric uncertainties and the uncertainty of the speed of light through the atmosphere. The range extent is not regarded as a limitation of the concept because unpowered glide weapons have aerodynamic ranges well within this range. In addition, the accuracy of powered weapons from longer standoff ranges will only degrade slightly due to any increase in target location uncertainty.
The precise knowledge of the aircraft trajectory over a period of several minutes is obtained by the tape measure mode, which is achieved through tight coupling between a GPS receiver with a carrier cycle counting feature and an INS. While this technique is exploited in the GPS-SAR precision targeting concept, it can also be applied to other problems. In particular, it can be used to sense aircraft motion during the synthetic aperture generation time for a SAR.
The aircraft trajectory must be known to a fraction of a wavelength to focus a high resolution radar image during a coherent data collection interval. At X Band, where most of the operational SARs operate, the wavelength is approximately 0.1 foot (about 3 cm) and a synthetic aperture length can exceed several miles. The conventional approach is to measure the aircraft trajectory with a high quality INS. This approach limits the coherent data collection times because of uncorrected errors in inertial instrument biases and scale factors. The aircraft position is calculated by a double integration of the accelerometer outputs. Because of the double integration, trajectory estimation errors grow as the square of the coherent data collection time, limiting map resolution unless some form of automatic focusing of the image is employed. The GPS carrier cycle counting technique, when applied to this problem, effectively bounds the position error across the synthetic aperture to approximately an inch. This dramatically reduces the reliance upon autofocus techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic diagram of aircraft positioning relative to a target in accordance with the present invention; and
FIGURE 2 is a block diagram of a GPS carrier cycle processing system in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGURE 1, there is shown an aircraft at a first position, L₁, whereat the coordinates of the first position in a GPS are determined in standard manner from a set of at least four satellites, 3, of the GPS constellation. The aircraft has a synthetic aperture radar (SAR) which images the target area, 4, therefrom to generate a SAR image, a target within the image then being designated by a cursor. The range, R₁, and monopulse angles, φ₁ and Θ₁, from position L₁ to the target within the target area 4, are computed to establish the location of the target in GPS coordinates in three dimensions with associated errors in standard manner. The aircraft 1 then flies to a second position L₂. Using the GPS cycle count method, the vector position of the aircraft at position L₂ with respect to its previous position at L₁ is measured. At the position L₂, the SAR obtains another high resolution SAR image of the same target area 5 and locates the designated target therein. Coordinates of the second position in the GPS are again determined in three dimensions using the same satellite set 3 as discussed above. Again, the range, R₂, and the monopulse angles, φ₂ and Θ₂, from position L2 to the target within the target area 4 are computed to establish the location of the target in GPS coordinates in three dimensions with associated errors.
The precise location of the target is established at the aircraft position labeled L2 after collecting the radar measurements at points L₁ and L₂ and measuring the relative position difference between points L₁ and L₂ using the GPS cycle count method. The computations to calculate the target position are as follows:
Let the position vector of the target with respect to point L₁ be represented by the quantity P₁, the position vector of the target with respect to point L₂ be represented by the quantity P₂ and the vector difference between points L₁ and L₂ be represented by P₂ -P_1. Although the computations can be made in any arbitrary frame of reference, it is convenient to assume a locally level frame, such as North, East, and Down (NED). The vectors P₁ and P₂ are computed using the radar range and monopulse angles, taken in an antenna reference frame, and a transformation matrix (A) that relates the antenna frame with the local level frame. The vector P₁ is then a function, F₁, of the radar measurements and A expressed as $P₁=F₁(R₁, φ₁, Θ₁,A)$

and similarly $P₂=F₂(R₂, φ₂, Θ₂, A)$

There are then two measurements of P2, namely, $P₂=F₂(R₂, φ₂, Θ₂, A)$

and $P₂=F₁(R₁, φ₁, Θ₁, A)-(P₂-P₁).$
These two measurements are combined, for example, in a least squares sense to calculate an improved estimate of P2, called <P2>. <P2> represents the best estimate of the location of the target with respect to the NED frame of the aircraft at point 2. This estimate is then extrapolated using the INAV on-board the aircraft and provided to the weapon 2 at release. At release, the weapon knows its present position and the position of the target.
A computer associated with the navigation system of the weapon, which could be a GPS receiver only, an INS only, or a combined INAV, continues to calculate its present position relative to the target for the purpose of guiding the weapon to the target. Any of several guidance laws commonly used in homing weapons can be used, such as 1) proportional guidance, 2) pursuit guidance or 3) a combination of the two with trajectory shaping.
Proportional guidance is the technique where the angle rates of the line of sight between the weapon and the target are determined and acceleration commands are sent to the control section of the weapon to drive the angle rates to zero. This places the weapon on a collision course with a target, even if the target is moving.
Pursuit guidance is another technique whereby the line of sight angles to the target are computed, rather than the angle rates, and acceleration commands are sent to the control section to drive the angles to zero. This points the velocity vector of the weapon directly at the present position of the target.
The best choice of guidance law consistent with this invention is one that shapes the trajectory of the weapon to impact the target at a steep vertical angle. The reason for this is to minimize the miss distance at impact. The least accurate component of the position of the target is in the direction perpendicular to the plane containing the measurements. For most geometries, this will be in the near vertical direction. Trajectory shaping is a simple matter with this technique because the on-board computer of the weapon can calculate range-to-go, since it knows its continually updated position and the position of the target. Guidance sections of most currently operational seekers that employ electro-optical (EO) or infrared (IR) seekers have no range information and can only discern angles to the target.
Referring now to FIGURE 2, a brief description of the precision tape measure mode is provided. As shown in FIGURE 2, pseudo and delta range measurements from a GPS receiver with carrier phase tracking loops are combined with the position, velocity and orientation INS data of the aircraft in a common Kalman filter. The measurements from the GPS receiver are aided by code and carrier loop data. This forms a tightly coupled system that is optimum for calibrating the bias of the IMU and scale factor errors. It also allows for rate aiding of the code and carrier loops during aircraft dynamics. The Kalman filter calibrates and aligns the data received therein from the INS and GPS and provides calibration and alignment information to the INS. The INS than provides calibrated and aligned information to the weapon for guidance thereof to the target.
The GPS signal is modulated onto a 1.576 GHz, L band, carrier signal whose wavelength is 19 centimeters. Once a GPS receiver has established lock on a satellite signal, it can count the number of carrier cycles that have occurred in a finite interval of time. Since carrier tracking can be established under good signal conditions to an error of 5 degrees, one sigma, the accuracy of this count along the line of sight to a satellite is of the order of 0.26 centimeters. When resolved into a navigation frame of reference, this translates into a position increment of .84 centimeters, assuming a geometric dilution of precision of 3.14. The magnitude of this error is about 100 times less than that induced by code phase error, forming the basis of a highly accurate relative positioning system. However, this method suffers from a considerable degradation in accuracy if "cycle slips" occur. A cycle slip is equivalent to missing or dropping the counts of the number of carrier cycles. This can happen for many reasons, the primary reason being satellite obscuration, such as aircraft wing shading or electronic obstruction, such as the temporary loss of signal by sudden antenna motion. In such cases, there will be a miscount of the number of cycles and thus a corresponding position error. By combining inertial instrument data and GPS data in a tightly coupled system, these cycle slips can be detected and accounted for using the implementation shown in FIGURE 2.
Though the invention has been described with respect to a specific preferred embodiment thereof, many variations and modifications will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.

Claims

A precision targeting method which comprises the steps of:
(a) determining the position of a target using a GPS coordinate system;

(b) determining the position of said target from a second location different from said first location using said GPS coordinate system;

(c) determining the vector position between said first and second locations in GPS coordinates in response to signals from the GPS by a GPS cycle count; and

(d) determining the GPS coordinates of said target from the determinations of steps (a) through (c).
The method of claim 1, further including the step of guiding a weapon to said target in response to the determined GPS coordinates of said target.
The method of claim 1 or claim 2, wherein the step of determining the position of said target from said first location in GPS coordinates further includes the step of determining the GPS coordinates of said first location and providing a Synthetic Aperture Radar (SAR) determination of the position of said target from said first location and the step of determining the position of said target from said second location in GPS coordinates further includes the step of determining the GPS coordinates of said second location and providing a SAR determination of the range of said target from said second location.
A precision targeting system which comprises:
position determining apparatus to determine the position of a target from a first location in GPS coordinates; and
to determine the position of said target from a second position different from said first position in GPS coordinates;
vector position determining apparatus to determine the position between said first and second positions in GPS coordinates by a GPS cycle count in response to signals from the GPS; and
coordinate determining apparatus to determine the GPS coordinates of said target from the determinations of steps (a) and (b).
The system of claim 4, further including guiding apparatus to guide a weapon to said target in response to the determined GPS coordinates of said target.
The system of claim 4 or claim 5, wherein said apparatus for determining the position of a target from said first location in GPS coordinates further includes coordinate determining apparatus to determine the GPS coordinates of said first location and provide a SAR determination of the range of said target from said first location and said apparatus to determine the position of said target from said second location in GPS coordinates further includes coordinate determining apparatus to determine the GPS coordinates of said second location and provide a SAR determination of the position of said target from said second location.