Abstract:
A system (10) and method for matching a set of detected points (26) to a reference constellation (20). A programmable processor (14) receives detected coordinate locations from a sensor (16) and reference coordinate locations from a reference storage device (12). The processor (14) performs a coordinate transformation from the reference coordinate frame to the detected coordinate frame and contructs a table of possible matches. Potential matches are found from the possible matches which have their difference vectors below a predefined threshold. These possible matches are then placed in a match matrix histogram (30) which is further refined to eliminate all but the best matches. The final matching then may be used to find a particular aimpoint (24) in the detected constellation (26).

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to systems and methods for pattern matching and, more particularly, to a system and method for matching a set of detected points to a set of reference points in real time. 
     2. Discussion 
     Pattern, or constellation, matching problems arise in a variety of settings. One example of a pattern matching problem is in radar seeker systems in which detected points in a scene form a known pattern or constellation of points. The problem then is to match a new set of detected points to the known constellation. Similar problems exist with infrared seeker systems and also in systems for recognizing patterns in visual images. In a typical pattern matching problem, a known pattern or set of points are established. An attempt is then made to match a new set of data to the reference pattern. One conventional approach to solving pattern matching problems involves complex scene recognition algorithms. These algorithms are typically very time consuming to develop, often require special processors, and may not be fast enough for real-time application. Other approaches to pattern matching use a human at some point in the loop. This creates problems in speed, reliability, etc. 
     Thus, it would be desirable to provide a system for constellation matching which can be implemented in real time. In addition, it would be desirable to have a pattern matcher which minimizes the amount of processing necessary and avoids the need for special purpose hardware. This would permit the use of conventional processors already installed in existing systems or vehicles. 
     SUMMARY OF THE INVENTION 
     Pursuant to the present invention, a system and method is provided for matching a set of detected points to a reference point pattern. This system includes a programmable processor for storing the coordinates of reference points from the reference pattern. These reference coordinates are defined in a reference frame. In addition, the processor stores the coordinates of the detected points. The detected coordinates are defined in a detected coordinate frame. Next, the points in the reference coordinate frame are transformed to the detected coordinate frame using an apriori coordinate transform. Vector tables of each possible ordered pair of reference points and detected points are then created. A list of pairs of matched reference and detected vectors is constructed. The criteria for matching vectors is that the magnitude of the difference vectors between pairs of reference and detected vectors is less than a predetermined threshold. 
     A match matrix histogram is then created which contains bins for counting each Detection--Reference point correspondence generated by relating the Heads and Tails of the matched Detection--Reference vector pairs. Finally, matches are selected in the matrix by eliminating less probable matches based upon the number of counts in the bins and the geometrical consistency of the implied point matches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to one skilled in the art, by reading the following specification and by reference to the drawings in which: 
     FIG. 1 is a block diagram of the overall constellation matching system in accordance with the present invention; 
     FIG. 2 is a diagram of a set of reference points forming a reference constellation; 
     FIG. 3 is a diagram of a set of detected points to be matched with the reference points shown in FIG. 2; 
     FIG. 4 is a first pass of a match matrix histogram utilized by the constellation matcher in FIG. 1; 
     FIG. 5 is a table of matched vectors in accordance with the present invention; 
     FIG. 6 is a refined version of the match matrix histogram shown in FIG. 4; 
     FIG. 7 is a diagram of the computation of median offsets in accordance with the present invention; 
     FIG. 8 is a diagram of the use of median offsets to check row MAX matches for consistency; 
     FIG. 9 is a flowchart of the initialization routine in accordance with the present invention; 
     FIG. 10 is a flowchart of the detection routine in accordance with the present invention; 
     FIG. 11A-B are flowcharts of the matching routine in accordance with the present invention; 
     FIG. 12 is a flowchart of the update routine in accordance with the present invention; 
     FIG. 13A is a drawing of reference and detected points for the real-time example in accordance with the present invention; 
     FIG. 13B is a drawing of the coordinate system for transforming input frame to REF frame. 
     FIG. 14 is an input reference point table in accordance with the present invention; 
     FIG. 15 is a reference point table derived from the input reference point table in FIG. 14; 
     FIG. 16 is an input reference aimpoint table in accordance with the present invention; 
     FIG. 17 is a reference aimpoint table derived from the input reference aimpoint table of FIG. 16; 
     FIGS. 18A and 18B are a series of tables illustrating the Detection Point/Vector/Quantization Coordinate Definition; 
     FIG. 19 is a reference vector table in accordance with the present invention; 
     FIG. 20 is a quantization pointer table in accordance with the present invention; 
     FIG. 21 is a detection points linked list in accordance with the present invention; 
     FIG. 22 is a DET/REF vector match linked list in accordance with the present invention; 
     FIG. 23 is a potential match matrix in accordance with the present invention; 
     FIG. 24 is a refined potential match matrix derived from the potential match matrix in FIG. 23; 
     FIG. 25 shows refined potential match matrix row statistics and REF/DET offset vectors table in accordance with the present invention; 
     FIG. 26 shows a final point match table in accordance with the present invention; and 
     FIG. 27 is an estimated DET frame aimpoint table in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The basic function of the constellation matching system, in accordance with the present invention, is to locate a pattern of reference points within an image of detected points and then to use the correspondence information to locate a specific point or points of interest in the detection image. Referring now to FIG. 1, the overall scheme of the constellation matching system 10 is shown. A reference point storage means 12 contains coordinates for the reference points in a well defined coordinate system. In addition, the coordinates of the specific point of interest are also supplied to the reference point storage means 12. The coordinates of the reference points and the specific point (aimpoint) are transmitted to a constellation matching processor 14. 
     Overall, the general approach of the present invention is to proceed in four basic steps: Initialization, Detection, Matching and Update. During the Initialization period the constellation matching processor 14 will perform all computations that can be performed prior to the knowledge of the detection points. A sensor 16 will then provide coordinates of detected points (in a detection coordinate system) to the constellation matching processor 14. The sensor 16 may be gathering radar, IR, or other information to generate detected point coordinates. 
     All computations that can be made based on knowledge of a single detection point, will be made by the constellation matching processor 14 during an ongoing Detection period as time permits. Then all remaining computations based upon the entire detection image will be made during a periodic Matching period as detection points are sequentially added to the detection image. Next, purging of &#34;extinct&#34; detection point data and reinitialization will be performed after each unsuccessful matching attempt during an Update period. Once a final match has been derived by the constellation matching processor 14 the coordinates of the matched points will be used to determine the location of the specific aimpoint in the detected image. The coordinates of the aimpoint may then be transmitted to a control system 18 which may be, for example, a missile guidance system or other means of utilizing the aimpoint information. 
     While the present invention is useful in many general cases, the preferred embodiment, described below, will be presented for the case where the reference and detected images are two-dimensional and involve only minimal differences in viewing perspective (rotation and warping) or scaling. This a simple but useful model for many real situations. 
     1. Initialization 
     Referring now to FIG. 2, a simple constellation 20 consisting of four reference points 22 is shown. An aim point 24 is located in a given, fixed, known location with respect to the orientation of the constellation 20. Coordinates for each of the points 22 and as well as the aimpoint 24 are given in a particular reference coordinate system. FIG. 3 shows a set 26 of six detected points 28. The detected points 28 are defined by coordinates in a detected reference frame. An apriori encounter geometry is specified; thus, the angle of bearing of the detected coordinate frame is known with respect to the reference coordinate frame. An appropriate coordinate transformation is established and the supplied reference coordinates are converted to the apriori detection coordinate frame. These points are referred to as the REF coordinates of the reference points. A North, East orientation for X, Y with an arbitrary origin is used. Also, the apriori encounter bearing (angle relative to North) will set the internal X axis orientation. 
     Next, a set of vectors is established for each possible ordered pair of reference points. Vectors go from one point to another, and each point pair is nominally used twice. In order to improve throughput, &#34;very long&#34; vectors oriented predominantly &#34;backward&#34; may be eliminated since the corresponding detection vectors could not match them anyway due to the sequential manner in which they are formed. For example, REF vectors with a negative X component exceeding a predetermined magnitude need not be retained. 
     Next, the storage depth (along the X axis) required to encompass the reference image is computed and the &#34;extinction&#34; criteria are established for detection points. Then a potential match threshold is established which will define counts in a potential match histogram based upon the number of reference points supplied and the apriori probability of point detection. 
     Referring now to FIG. 4, a potential match matrix 30 is constructed with all of its bins set to 0. The rows 32 in the matrix correspond to detected (DET) points and columns 34 in the matrix 30 correspond to REF points. Other miscellaneous constants, tables and variables which require Initialization are established and initialized during this period. Also, data needed for aimpoint estimation which can be pre-computed for all outcomes of the matching process can be made and stored at this time. 
     2. Detection 
     As each new detection point is encountered, a new row 32 in the match matrix 30 is selected from a list of available free rows and the row is designated as &#34;in use&#34;. A set of vectors are then established for each possible pair of detected points. Vectors go from each of the old &#34;non-extinct&#34; points to the new point. As each new set of DET vectors is generated, a vector match is attempted between each DET vector and each REF vector. A match between two vectors occurs when the length of their difference vector is less than a predetermined threshold. In order to reduce throughput, a preliminary screening of match attempts is accomplished using a look-up procedure. During the Initialization period, the DET vector space is partitioned into a predetermined number of rectangular regions. Then for each such region, a list of REF vectors which could possibly match any DET vector falling in that region is stored in a table. Then during the Detection period, only a small number of potential REF to DET vector matches need be attempted. Additionally, by testing X and Y separately with a slightly looser criteria and then only testing the length for the few potential matches surviving their initial screening, one can substantially reduce the required throughput. A list of current vector matches is augmented with the new matches. 
     Referring now to FIG. 5, this list 36 of vector matches is shown. To illustrate, the first entry in the list of vector matches indicates that the vector beginning at detected point 1 and ending with detected point 2 in FIG. 3 has been found to match with the vector beginning with reference point 1 and ending with reference point 3 shown in FIG. 2. From visual inspection it can be seen that these vectors are roughly the same length and direction. 
     Each vector match yields two corresponding point matches thus, the first entry in the vector match list 36 will yield a match between detected point number 2 and reference point number 3 and also between detected point number 1 and reference point number 1. The match matrix 30 in FIG. 4 will accordingly have its bins incremented to tabulate these results. Thus, a 1 appears at the intersection of the row for detected point number 2 and the column for reference point number 3. Also, a 1 is placed in the bin at the intersection of the row for detected point number 1 and the column for reference point number 1. In FIG. 4 the histogram matrix 30 shows the results of tabulating all matches found in the vector match table 36. It can be seen in the match matrix 30 that for certain detection and reference points there were no matches and that for some there were a maximum of three matches. 
     3. Matching 
     The match matrix 30 is now scanned to determine the maximum value contained (MAX). As discussed above, during the Initialization period, a potential match threshold was established. For example, as shown in FIG. 4, this threshold is established to be 2.25. Now, the MAX, which is equal to 3, is compared to the match threshold. If this MAX is less then the potential match threshold, then the matching process terminates for this attempt without finding the reference pattern. Otherwise a bin retention threshold is established and the process continues below. As shown in FIG. 4, the MAX is greater than the threshold and the bin retention threshold (REQ) is calculated as half the MAX, or 1.5. 
     Next, the match matrix 30 is scanned again and any bin count which falls below the established threshold has its corresponding bin in a new refined potential match matrix 38 shown in FIG. 6, set to -1 (all other bins are set to 0). This flags the matrix refinement process to exclude &#34;weak&#34; vector matches which would normally increment this bin. Thus, each of the bins in FIG. 4 having only a 1, do not meet the 1.5 retention threshold and are underlined. Likewise, in the vector match table 36 in FIG. 5, these matches are identified under the column labeled &#34;drop&#34;. 
     The refined potential match matrix 38 is completed by scanning the list of current vector matches and incrementing the two corresponding bins only if neither bin to be incremented is currently negative. This eliminates the underlined bins in the match matrix 30 and may reduce the counts in other bins as well in the refined potential match matrix. It should be noted that the BIN&#39;S do not get reduced directly. They do get reduced indirectly via the refinement process. Since 2 BIN&#39;S are eliminated for each vector match that is eliminated, other BIN&#39;S can be reduced also. Next, the refined potential match matrix 38 is scanned to determine the maximum value (MAX) contained. The maximum value for each row plus the sum of the row entries is also determined for this scan. The column index for the maximum bin in each row is then determined. If more than one entry is at the row maximum value, the column index is set to minus one. 
     Next, a list of &#34;high confidence&#34; point matches is generated from the revised potential point match matrix 38 based on the first criteria which applies from the selection below: 
     1. If any row in the matrix 38 has a single non-zero value and that value is equal to the matrix maximum (MAX) then select all such point matches in the matrix. There could be up to one of these per row. 
     2. If any row in the matrix 38 has only one value equal to the matrix maximum (MAX) then select all such point matches in the matrix. There could be up to one of these per row. 
     3. Select all point matches in the matrix 38 whose bin value is equal to the matrix maximum (MAX). 
     The appropriate preliminary REF to DET transformation calculations are performed at this point to prepare for the final point match selection process. Note that for now, a median offset vector (REF to DET) is established from the set derived from the high confidence point matches generated above. X and Y medians are determined separately. It will be appreciated that while the above coordinate transformation converted the reference points to the correct bearing angle of the detected point frame, there still will be an offset between the reference and detected points. For example, FIG. 7 shows that the high confidence matches (2, 2) (3, 3) and (6, 4) represented by the solid circled bins in the refined matrix 38 have their median offsets X, Y computed. 
     The final point matches are selected by transforming each potential REF match point to the DET frame using the preliminary transformations defined above. That is, the median offset vector is added to REF point coordinates to estimate their expected DET frame values. The corresponding DET match point is checked for spatial consistency (the length of the difference vector lies within a predetermined tolerance). The potential matches to be tested during this final path are those whose non-zero bin values are equal to their row maximums. A list of final point matches and a final match count is then generated. This process is illustrated in FIG. 8. 
     If the number of final point matches is insufficient, then the process terminates. Otherwise the reference pattern is said to be found and the process proceeds to the aimpoint location process described below. 
     The appropriate REF to DET transformation calculations are performed using the final point match pairs. A more sophisticated transformation is used at this stage to account for any minor perspective distortions and small differences in scale and rotation. The transformation is then used to locate the specific aimpoint in the DET image. 
     4. Update 
     Detected points which have become extinct (X coordinates less than predetermined minimum) must be eliminated. This is accomplished by decrementing the appropriate bins in the permanent potential match matrix which arose from the extinct point then, by removing the corresponding vectors which contain the extinct point as an end point from the vector match list of current detection points. Further, the rows of the matrix and the point and vector lists should be appropriately compressed. The actual compression may be by chain pointer modification. The vacated rows should contain all zeros. 
     FIGS. 9 through 12 are flowcharts of the above-described process. In particular, the Initialization period is summarized in the flowchart in FIG. 9. The Detection process is summarized in the flowchart in FIG. 10. The Matching Process is summarized in FIGS. 11a-11b and the Update Process is summarized in the flowchart in FIG. 12. 
     A simple aimpoint estimation logic example for the Constellation Matcher 14 will now be reviewed. This case assumes unknown translation and negligible rotation or scale factors on either axis. The estimated coordinates of the aimpoint are found by the following transformation, where the centroids for all final matched pairs are computed as follows: ##EQU1## 
     Note that the above case is simple but is very dependent on assumptions. A more complex but also more tolerant approach will now be reviewed. This is for the case of Unknown Translation, Potentially Very Small Rotation (≦5°), No Y Scale Factor, and Potentially Small X Scale Factor (≦10). ##EQU2## 
     It will be appreciated that a more generalized transformation may also be used. For example, a least squares fit estimate of a transformation matrix that includes rotation and scale and shift (as well as perspective) may be employed. 
     Real-Time Implementation Example 
     An example of one implementation of the present invention for real-time constellation matching will now be discussed. It will be appreciated that this example is easily implemented in software on conventional computers to achieve rapid solutions to the matching problem. Referring now to FIG. 13A, the example shows five reference points and two aimpoints. FIG. 13B shows the rotation of the coordinates by a bearing angle of 90° after rotation. FIG. 14 indicates the coordinates of the input reference point table upon rotation, the coordinates become the reference point table coordinates indicated in FIG. 15. Likewise, FIG. 16 indicates the reference aimpoint table and FIG. 17, the coordinates of the reference aimpoint table upon rotation by 90°. 
     All definitions for terms used in the real-time implementation example in FIGS. 13-27 inclusive are as follows. 
     
         ______________________________________CONSTELLATION MATCHER DATA DEFINITION______________________________________Input REF Point Table:NRPTS =   Number of REF points     (3 ≦ NRPTS ≦ NREFMAX)XRIN(I) = X coordinate of Ith REF point (X = NORTH)YRIN(I) = Y coordinate of Ith REF point (Y = EAST)Input REF Aimpoint Table:NAPS =    Number of Aimpoints (1 ≦ NAPS ≦ NAPMAX)APIDX(I) =     Aimpoint designation index for Ith     Aimpoint(Table prioritized from highest (at zero) to lowest)          Reference point index; AP = REF pointAPIDX =         -1; otherwiseXAPIN(I) =     X coordinate of Ith Aimpoint (X = NORTH)YAPIN(I) =     y coordinate of Ith Aimpoint (Y = EAST)Values for XAPIN and YAPIN are supplied only ifAPIDX = -1Input Parameter: (See FIG 13B.)BEARING = Heading angle for encountering REF points(-180° ≦ BEARING &lt; 180°; Angle relative to NORTH)0° = NORTH, +90° = EAST, -90° = WEST, -180° =SOUTHθ = BEARING (from NORTH); ORIGIN arbitrary    The Transformation from Input Frame to    Apriori Internal REF Frame is given by ##STR1##Reference Point Table: 0 ≦ I &lt; NRPTSXREF(I) = X coordinate of Ith REF pointYREF(I) = Y coordinate of Ith REF pointReference Aimpoint Table: 0 ≦ I &gt; NAPSXAPREF(I) =      X coordinate of Ith reference APYAPREF(I) =      Y coordinate of Ith reference APReference Vector Table: 0 ≦ I &lt; (NRPTS) * (NRPTS-1)NRVECT =   Number of retained REF vectorsRPVHEAD(I) =      Index of REF point used as Head of      Ith REFRPVTAIL(I) =      Index of REF point used as Tail of Ith REF      vectorXRVECT(I) =      X component of Ith REF vectorYRVECT(I) =      Y component of Ith REF vector______________________________________ 
    
     Vectors which have XRVECT&lt;SVCUTOFF need not be retained in the table since they could never match a corresponding DET vector (HEAD=newest DET point; TAIL =Previous DET point) 
     
         ______________________________________Quantization Pointer Table: 0 ≦ I &lt; NQBINSPointer Table contains pointer to start of list andlength of list for each bin in the Matchable REF VectorsTableQTABPTR(I) =      Pointer to start of REF vector list for      Ith quantization BINQTABLEN(I) =      Length of REF vector list for Ith      quantization BINMatchable REF Vectors Table: 0 ≦ I &lt; NMRVTABNMRVTAB =  Number of entries in Matchable REF Vectors      TableRVPMIDX(I) =      REF vector potential match index for Ith      table entry______________________________________ 
    
     This table consists of consecutive lists of REF vector indices which could potentially match DET vectors falling in the corresponding quantization BIN. Lists for BINS are stacked contiguously in the table. 
     
         __________________________________________________________________________Detection Points Linked List: 0 ≦ I &lt; NDETMAXNDPTS =     Number of DET points (0 ≦ NDPTS &lt; NDETMAX)DPLBEG =    Table index for first entry in Detection       Points Linked ListDPLEND =    Table index for last entry in Detection       Points Linked ListDPLNEXT(I) =       Table index for next entry in Detection       Points Linked List [-1 if last entry]DPLPREV(I) =       Table index for previous entry in       Detection Points Linked List [-1 if first       entry]DETTGT(I) = Target File Index for DET point stored at       table index IXDET(I) =   X coordinate of DET point stored at table       index IYDET(I) =   Y coordinate of DET point stored at table       index IDET/REF Vector Match Linked List: 0 ≦ I NDRVMMAXNDRVMATS =  Number of entries in DET/REF Vector Match       Linked List (Generate Error if # exceeds       NDRVMMAX)DRVLBEG =   Table index for first entry in DET/REF       Vector Match Linked ListDRVLEND =   Table index for last entry in DET/REF       Vector Match Linked ListDRVLNEXT(I) =       Table index for next entry in DET/REF       Vector Match Linked List [-1 if last       entry]DRVLPREV(I) =       Table index for previous entry in DET/REF       Vector Match Linked List [-1 if first       entry]BIDXHEAD(I) =       BIN index into Potential Match Matrix for       DET/REF pair comprising the vector HEAD of       the entry stored at the Table index IBIDXTAIL(I) =       BIN index into Potential Match Matrix for       DET/REF pair comprising the vector TAIL of       the entry stored at the Table index IFree Space Stack for Detection Point Linked List:DPLFREE(I) =       Index into Detection Points Linked List       table (used if I &lt; NDPTS; free if I ≧       NDPTS)Free Space Stack for DET/REF Vector Match Linked List:DRVLFREE(I) =       Index into DET/REF Vector Match Linked       List able (used if I &lt; NDRVMATS; free if       I ≧ NDRVMATS)Potential Match Matrix: 0 ≦ K &lt; NMATBINSMATBIN(K) = Histogram Count of number of occurrences       of the Kth DET/REF point pair in the       DET/REF vector match listRefined Potential Match Matrix: 0 ≦ K &lt; NMATBINSRMATBIN(K) =       Histogram count of number of occurrences       of the Kth DET/REF point pair in the       refined DET/REF vector match listRef to DET Offset Vectors Table: 0 ≦ I &lt; NRDOFFNRDOFF =    Number of REF to DET offset vectors       resulting from potential high confidence       matchesRDXOFF(I) = Ith x offset (REF to DET)RDYOFF(I) = Ith y offset (REF to DET)MEDXOFF =   Median x offset (REF to DET)MEDYOFF =   Median y offset (REF to DET)Final Point Match Table: 0 ≦ I &lt; NUMDMATNUMDMAT =   Number of final DET points matchedNUMRMAT =   Number of final REF Points matchedMDPIDX(I) = DET point index (Target File Index) for       Ith point matchMRPIDX(I) = REF point index for Ith point matchMDPX(I) =   x coordinate of DET point in Ith point       matchMDPY(I) =   y coordinate of DET point in Ith point       matchEstimated DET Frame Aimpoint Table: 0 ≦ I &lt; NAPSXAPDET(I) = X coordinate of Ith reference APYAPDET(I) = Y coordinate of Ith reference APVariables:FTDEST =    Estimated false target density (FTD) for       DET pointsMXTHRESH =  Maximum BIN value threshold for       establishing potential DET/REF matches       If BINMAX &lt; MXTHRESH: No matchBRTHRESH =  BIN retention threshold for second       matching pass       Kth BIN retained if MATBIN(k) ≧ BRTHRESHBINMAX =    Maximum count of all BINS in Potential       Match MatrixRBINMAX =   Maximum count of all BINS in Refined       Potential Match MatrixROWMAX(I) = Maximum count of all BINS in Ith ROW of       Refined Potential Match MatrixROWSUM(I) = Sum of all positive counts in all BINS in       the Ith ROW of Refined Potential Match       Matrix       -1 BINS ignoredRMAXCOL(I) =       Column index for BIN containing Maximum       Count for Ith ROW of Refined Potential       Match Matrix       Set to -1 if more than one BIN in ROW is       at Max ValueMATHRESH =  Match threshold for final REF point       matchesMINTHR =    Absolute minimum value allowed for match       thresholdMATFLAG =   Match Flag (set if matched, reset       otherwise)XEXTINCT =  X value at beyond which old DET points       become extinct and are deleted from       further consideration in the matching       processCONSTANTS:NDETMAX =   Maximum number of DET points allowedNREFMAX =   Maximum number of REF points allowedNAPMAX =    Maximum number of REF aimpoints allowedDETDEPTH =  Storage Depth for DET points (X)DETWIDTH =  Storage Width for DET points (Y)SCANSIZE =  Width of a single detection scanXQSTEP =    REF vector quantization step size (X)YQSTEP =    REF vector quantization step size (Y)NQXSTEPS =  Number of quantization steps (in X) for       DET vector spaceNQYSTEPS =  Number of quantization steps (in Y) for       DET vector spaceNQBINS =    Number of quantization BINS for DET vector       spaceVCUTOOF =   Minimum negative X component allowed for       retention of REF vector (this must be at       least as large as the detection scan       width)XDVQOFF =   DET vector quantization offset (X) =       XVCUTOFF normallyYDVQOFF =   DET vector quantization offset (Y) =       DETWIDTH normallyNMRVTMAX =  Maximum number of Matchable REF Vector       Table entriesNDRVMMAX =  Maximum number of DET/REF Vector Match       Table entriesVMERRLIM =  Limit on allowable error for matching DET       and REF vectors (≈ √2 * PMERRLIM)PMERRLIM =  Limit on allowable error for matching DET       and REF points (combines point error and       rotation error and velocity/scale error)REGDIST =   Target Registration Distance (absolute       minimum distance between two DET points)NMATBINS =  Number of Potential Match Matrix BINS =       (NDETMAX * NREFMAX) normallyPDEST =     Apriori probability of detection (PD) for       REF pointsKMTHR =     Thresholding constant used in determining       MXTHRESHKBRTHR =    Thresholding constant used in determining       BRTHRESHKLOWFTD =   FTD breakpoint constant in determining       MATHRESHMINTHRLO =  Low value for MINTHRMINTHRHI =  High value for MINTHRKMATHR =    Thresholding constant used in determining       MATHRESHFormulas:MXTHRESH =  MAX {2, KMXTHR * (PDEST * NRPTS - 1)}BRTHRESH =  KBRTHR * BINMAXBIDXHEAD =  NREFMAX * DETTGT (for head point) +       RPVHEADBIDXTAIL =  NREFMAX * DETTGT (for tail point) +       RPVTAILMINTHR =    MINTHRLO if FTDEST ≦ KLOWFTD       MINTHRHI if FTDEST KLOWFTDMATHRESH =  MAX {KMATHR * NRPTS * PDEST,MINTHR}XEXTINCT =  XMSL + XOP - DETDEPTH       (where XMSL + XOP is the x coordinate at       center of DET scan)__________________________________________________________________________ NOTE: Need to define a CONST.sub.-- MATCHER.sub.-- DET.sub.-- POINT.sub.-- ACTIVE flag in each Target File (CMDPACT abbreviation). This flag is set when the target file is used as a DET point. Need to define a DETPTR in each Target File to point to DET Point Index i Table NDPTS is index of next unused entry in this table NDRVMATS is index of next unused entry in this table K = NREFMAX * I + J where I = DET point index and J = REF point index 2 BINS incremented for each vector match (one from HEADS, other from TAILS) Value = Count if nonnegative or exclusion flag if -1 Initially x/y pairs are aligned but then are sorted separately This number is theoretically very large, but in practice will not be very large. Value chosen somewhat arbitrarily to cover most anticipated cases but should be set empirically unless memory is no problem. ABSMAX = (NREFMAX) * (NREFMAX1) * (NDETMAX) * (NDETMAX1)/2 
    
     A Reference Vector Table is now created which contains all of the ordered pairs of reference points after eliminating those vectors having an XV less than XV cut-off where XV cut-off is equal to -75. In particular eight possible vectors were eliminated and 12 remain as shown in FIG. 19. 
     In this example, the storage depth for detection points (X) is DETDEPTH=900 for the Y axis DETWIDTH=1000 and the width of a single detect scan is SCANSIZE=75; XQSTEP=512, YQSTEP=512, NQXSTEPS=2 and NQYSTEPS=4. Also VMERRLIM=60, XDVQOFF=75 and YDVQOFF=1000. 
     FIG. 18 contains the Detection Point/Vector/Quantization coordinate definition tables as well as the constraints used in this example. The quantization pointer table and matchable reference vectors table are shown in FIG. 20. The purpose of quantization is to speed-up throughput. If all possible vector matches were tried, the number of trials would get very large due to the combinatorics. Since there are really very few vectors that are close to matching, the process can be streamlined by using quantization. Thus, all combinations of the reference points are used to make reference vectors and then they are divided up into quantized bins. Then for each quantization bin, a list of indices for all REF vectors which could possible match any DET vector corresponding to that bin is stored in a table. When a detected vector is determined, it is quantized and the system then tries to match only those REF vectors whose indices were stored in the corresponding quantization bin. Thus, the system knows which ones are even close to matching and the system does not have to look at all of them. In essence, the quantization pointer table provides a way of looking things up quickly. If enough memory were reserved to handle all possible matches, it would consume a great deal of memory storage space. 
     FIG. 21 shows the detection points linked list. FIG. 22 shows the DET/REF vector match linked list which is the set of all vector pairs that matched based on their difference vectors being small enough. FIG. 23 shows the potential match matrix where the bin MAX is 4, the MXTHRESH is 2.0625 and thus the BIN MAX is greater than the MXTHRESH. Also, the BRTHRESH=0.5x4 is equal to 2 thus, bins less than 2 are excluded to yield the refined potential match matrix shown in FIG. 24. FIG. 25 shows the refined potential match matrix row statistics which are used for active rows to determine where conflicts exist. FIG. 25 also shows the offset vector table. Finally, FIG. 26 is the final point match table indicating that five matches were found. To estimate the detection frame aimpoint coordinates in this simple case, the median REF to DET frame offsets are used. This yields the aimpoint coordinates shown in FIG. 27. 
     It should be recognized that the constellation matching system 10 can be used for matching a set of reference points with a second set of detected points in a wide variety of settings. Also, the present invention can be used in real-time in many applications. While the above description constitutes the preferred embodiments of the present invention, it will be appreciated that the invention is susceptible to modifications, variation and change without departing from the proper scope and fair meaning of the accompanying claims.