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Integration of IR Sensor Clutter Rejection Techniques with Pixel Cluster Frame Manipulation Allison Floyd, Sabino Gadaleta, Dan Macumber, and Aubrey Poore Numerica Corporation, P.O. Box 271246, Fort Collins, CO 80527 ABSTRACT Track initiation in dense clutter can result in severe algorithm runtime performance degradation, particularly when using advanced tracking algorithms such as the Multiple-Frame Assignment (MFA) tracker. This is due to the exponential growth in the number of initiation hypotheses to be considered as the initiation window length increases. However, longer track initiation windows produce significantly improved track association. In balancing the need for robust track initiation with real-world runtime constraints, several possible approaches might be considered. This paper discusses basic single and multiple-sensor infrared clutter rejection techniques, and then goes on to discuss integration of those techniques with a full measurement preprocessing stage suitable for use with pixel cluster decomposition and group tracking frameworks. Clutter rejection processing inherently overlaps the track initiation function; in both cases, candidate measurement sequences (arcs) are developed that then undergo some form of batch estimation. In considering clutter rejection at the same time as pixel processing, we note that uncertainty exists in the validity of the measurement (whether or not the measurement is of a clutter point or a true target), in the measurement state (position and intensity), and in the degree of resolution (whether a measurement represents one underlying object, or multiple). An integrated clutter rejection and pixel processing subsystem must take into account all of these processes in generating an accurate sequence of measurement frames, while minimizing the amount of unrejected clutter. We present a mechanism for combining clutter rejection with focal plane processing, and provide simulation results showing the impact of clutter processing on the runtime and tracking performance of a typical space-based infrared tracking system. Keywords: Infrared Sensor Surveillance, Clutter Rejection, Pixel (Clump) Cluster Tracking, Pixel-Cluster Decomposition, Image Processing 1. INTRODUCTION We consider the problem of geosynchronous IR satellites observing multiple ballistic missile launch events in dense clutter. Most of the techniques described here are extensible to LEO IR satellites performing below-the-horizon and Earth limb observation; above-the-horizon viewing will tend not to exhibit the same dense clutter problem, and is therefore not addressed in this discussion. The multitarget tracking software we utilize as a reference is Numerica’s Multi-Frame Assignment (MFA) Tracker, 1 although the problem at hand will be present in any multiple frame or multiple hypothesis tracking system. The MFA Tracker typically uses a six-frame window for track initiation on IR angle-only measurements. While this process does result in high association accuracy, even in the presence of multiple target tracks initiating near-simultaneously, its runtime performance suffers in the presence of dense clutter. Accordingly, we have developed a preprocessor system that merges elements of clutter rejection, pixel cluster decomposition, and group clustering to synthesize revised measurement frames which mitigate the runtime bottleneck of track initiation. This preprocessor is intended to reside in the mission data processing sequence between signal processing and tracking; its input and output can both be specified as measurement frames with auxiliary data (Figure 1). This processing reduces the number of measurements in a frame that the tracking system must consider for both track initiation and association while running in a seperate process space, reducing the computational demands on the tracking system and improving runtime performance. Clutter rejection techniques can be loosely divided into kinematic and feature-aided components. Kinematic clutter rejection relies on the ability to identify dynamic characteristics of targets of interest (persistence, particular ranges of Further author information: (Send correspondence to A.F or A.P.) A.F.: E-mail: [email protected], Telephone: (970) 419 8343 x15 A.P.: E-mail: [email protected], Telephone: (970) 419 8343 x20 Mission Data Processing Sensor Track States Image Image Processing Raw Measurement Frame Clutter Rejection Cluster Processing Edited Measurement Frame Tracker Figure 1: Mission Data Processing Architecture. velocity and acceleration) that are unlikely to be shared by clutter returns. These properties are dependent on the scene phenomenology as well as the target type. The kinematic clutter rejection process is thus analogous to the gating process within a target tracker; measurements which do not pass gating tests with any other measurement over a requisite number of frames are assumed to be false alarms, i.e. clutter, and are removed from the output measurement frame. In order for a preprocessor to be able to make use of the full range of gating tools, it must have the ability to accumulate frames over time, and it must also receive feedback in the form of current track states. Feature-aided clutter rejection incorporates a priori knowledge of expected target feature information; in this case, inband infrared intensity information. For many systems, clutter returns that can be differentiated from target returns on the basis of single- or multiple-frame intensity information will have been removed as part of signal processing. Accordingly, we do not assume that clutter returns can be immediately removed on the basis of feature data. However, if a model can be generated of the expected time variation in feature data for target returns, it is possible to utilize feature data as part of a gating distance metric, thus further restricting the number of measurement arcs under consideration. Clutter rejection and more generalized gating techniques are discussed in Section 3. Basic point-to-point clutter rejection suffices when the clutter is sparse enough that the average clutter point does not tend to gate with any other clutter point or true target, for a sufficient set of gates. Spatially correlated clutter resulting from landmass edges, cloud formations, and the terminator may be intractable using these techniques, however. These regions of dense clutter require a different approach. We apply our group cluster tracking technology 2 to the problem of tracking temporally persistent, spatially correlated clutter regions. We discuss the application of group cluster tracking to the clutter environment in Section 4.2. The final component of our complexity reduction preprocessor is not itself aimed at clutter rejection, but rather at merged measurement processing. The infrared tracking problem from geosynchronous altitude is predominantly a boost phase tracking problem, and thus does not tend to have the same level of closely-spaced object challenge as the midcourse tracking problem. However, due to the long range from target to sensor, it is still possible for one or more observers of a scene to perceive multiple launch events as a single pixel cluster. While it is not necessary to address the pixel cluster tracking problem at the same time as clutter rejection, the introduction of feedback data into the preprocessor as part of the clutter rejection process suggests that some additional time can be saved by simultaneously addressing the track-to-measurement multiassignment problem and applying pixel-cluster decomposition to those measurements which have been identified as candidate CSOs. In this fashion, the preprocessor handles all of the mechanics required to take a raw measurement frame and convert it to a reduced-clutter frame in which the remaining clutter measurements have been clustered to reduce processor load, and the hypothesized target measurements have been reassessed to minimize CSO conditions. We discuss the implications of pixel cluster decomposition on the GEO IR tracking problem in Section 4.1. 2. MATHEMATICAL PROBLEM FORMULATION Before describing the preprocessing algorithms, we discuss the underlying modeling assumptions for clutter and target characteristics. For this section, we adopt the following notation: u, v: horizontal and vertical axes in the focal plane coordinate system ρ: distance from a point target location to the current focal plane I(ρ): Intensity (W/cm2 ) at a specified distance from a point target location σpsf : Radial (two-dimensional) standard deviation of the optical point spread function (PSF) I0 : Radiant intensity of the point target at the aperture (W/cm 2 ) ci : Random clutter point i mij : Markov clutter point i at time j 2.1. Clutter Model Scene clutter for an EO sensor can be modeled as a superposition of a random component, a spatially uncorrelated persistent (Markov) component, and a spatially correlated (specular reflection point and edge effects) component. The first component, if not dense compared to the average true target frame to frame motion, can be readily addressed via clutter rejection processing. The second component may be addressed via clutter rejection if sufficient data exists to differentiate target motion from stationary clutter. The third component, typically, must be handled separately (see Section 2.2). We describe the random and Markov clutter components as follows: ci ∈ U(< [−π/2, π/2], [0, 2π] >) mij = mi + νj , mi ∈ U(< [−π/2, π/2], [0, 2π] >), νj ∈ N (0, R) (1) for a number of random and Markov clutter points i determined by a clutter density estimate, and Markov time sequence j determined by the Markov transition probability for persistent clutter. Both the random and the Markov clutter components are assumed to be uniformly distributed in latitude and longitude at a reference altitude (U(< [−π/2, π/2], [0, 2π] >)); the Markov clutter, rather than being exactly fixed with respect to time, is normally distributed with variance corresponding to the measurement noise R (N (0, R)). 2.2. Group Cluster Model While the group clustering techniques were originally designed to address dense target environments, they can also be applied to spatially correlated persistent clutter. We expect spatially correlated clutter to fall into two general categories: clutter present in a Gaussian distribution around a central point (e.g., the specular reflection point), and clutter present in a Gaussian distribution around some edge phenomenon. We assume that these edges can be described as one or more quadrics of the form a1 u2 + a2 uv + a3 v 2 + b1 u + b2 v + c = 0 or, in vector notation T T x Ax + b x + c = 0, x = u v (2) , expressed in focal plane coordinates. 2.3. CSO Model The unresolved CSO problems addressed in Section 1 are the result of finite focal plane resolution, imaging blur due to optics and jitter, atmospheric turbluence blur, and thermal, shot and background noise. In general, the optical point spread function (PSF) is not Gaussian; however, for many applications a Gaussian approximation suffices. The two-dimensional Gaussian approximation to the optical PSF is then given by 2 I0 − 2σρ psf 2 e . I(ρ) = 2 2πσpsf (3) The σpsf resulting from optical blur is typically measured experimentally for a given telescope. We obtain pixel intensity values by integrating Equation (3) over the pixel dimensions. If the target is δ x and δy from the edge of the pixel, then for i = 1 : n: i − n+1 i − n+1 2 − δx 2 − δx X(i) = erf − 0.5 · erf σx σx (4) n+1 n+1 i − 2 − δy i − 2 − δy Y (i) = erf − 0.5 · erf σy σy Given a target point intensity at the aperture of I 0 , the blur submatrix is then: Iblurred (i, j) = X(i) · Y (j) · I0 (5) The individual submatrices for each point target are accumulated into the final image sum. Multiple targets present within a small region of space results in overlapping blur matrices, which can lead to merged measurements after signal and image processing. 3 3. CLUTTER REJECTION APPROACH The clutter rejection approach involves subjecting successive frames of data to a sequence of gates, which produce arc suggestions to which various kinematic tests can be applied. Output frames contain arc suggestions which pass the tests. 3.1. Gating Given a sequence of input frames, we utilize a gating strategy composed of several layers of gates applied in the order of increasing processing time. The basic series involves a cell (or bin) gate, followed by a pair velocity gate, and concluding with a standard filter gate 4 ; we do not utilize higher-order measurement gates (three point gates) for this application because the typical measurement noise from a GEO observer makes direction estimation from a small number of measurements fragile. We have developed a cell and pair gate for infrared observations that functions regardless of whether the successive frames are from a single observer or from multiple observers, as described below. 3.2. Altitude Intersection Gate (Mono or Stereo View) The gate developed in this section differs from previously developed gates in that it is effective for either stereo or mono view satellite data. This is possible because this gate transforms a measured line of sight vector ˆ1eci from the satellite at t1 to the target at t1 into a mean possible line of sight vector ˆ2eci from the satellite at t2 to the target at t1 , as seen in Figure 2. In order to precisely define the vector ˆ2eci the position of the target must be known, which it is not because the range r1 to the target pointed at by ˆ1eci is unknown. However, this range is bounded by two cases; r 1surf ace when the target is at its minimum possible altitude r surf ace and r1altitude when the target is at its maximum possible altitude raltitude . These two bounding ranges may be solved by the following equations Xs1eci + r1surf ace ˆ1eci = rsurf ace Xs1 + r1altitude ˆ1 = raltitude eci eci which have the solutions T T 2 (Xs1 Xs1eci + rsurf ˆ )2 − Xs1 ace eci 1eci eci T T 2 ± (Xs1 Xs1eci + raltitude ˆ )2 − Xs1 eci 1eci eci T r1surf ace = −Xs1 ˆ ± eci 1eci T r1altitude = −Xs1 ˆ eci 1eci When solving any quadratic equation there are some special cases which must be accounted for. First, each range equation has two solutions; one being the intersection of the line of sight vector with the desired Earth radius closest to X s2 ι 2min ι1 X s1 Xs1+ r1min ι 1 ι 2max Xs1+ r1max ι 1 raltitude rsu rfac e Figure 2: Intersection of satellite line of sight vector with rsurf ace and raltitude the satellite and the other being the more distant intersection. In this problem we select the closest intersection which is written T T T 2 ˆ Xs1eci + rsurf (6) ˆ )2 − Xs1 r1surf ace = −Xs1eci 1eci − (Xs1 ace eci 1eci eci T T T 2 r1altitude = −Xs1 Xs1eci + raltitude (7) ˆ − (Xs1 ˆ )2 − Xs1 eci 1eci eci 1eci eci The second condition to be careful of when solving these equations is that the value under the square root may be negative, giving an imaginary solution for the range to the target. When solving for r 1altitude this condition would mean that the given line of sight vector does not intersect the maximum altitude and thus cannot represent a real target. When solving for r1surf ace this means that a given line of sight vector does not intersect the Earth and this case should not be ignored. To ensure that true associations are not gated out, in the case that a line of sight does not intersect the Earth the second position considered corresponds to the further intersection with the maximum altitude T ˆ1 + (X T ˆ1 )2 − X T Xs1 + r2 (8) r1surf ace = −Xs1 eci eci s1eci eci s1eci altitude eci Once the maximum and minimum range to the target along ˆ1eci are known then the maximum and minimum line of sight vectors to the possible target positions are r2surf ace = Xs1eci + r1surf ace ˆ1eci − Xs2eci Xs1eci + r1surf ace ˆ1eci − Xs2eci ace = ˆsurf 2eci r2surf ace r2altitude = Xs1eci + r1altitude ˆ1eci − Xs2eci Xs1eci + r1altitude ˆ1eci − Xs2eci = ˆaltitude 2eci r2altitude ace All other possible target line of sight measurements lie on the plane between ˆsurf and ˆaltitude . The mean of these 2eci 2eci values is then ˆsurf ace + ˆaltitude 2eci ˆ2eci = 2eci 2 and the maximum error in ˆ2eci is ˆ2eci = ace − ˆaltitude ˆsurf 2eci 2eci 2 Then, if the target’s maximum change in position is V max |t| the difference between a line of sight vector at t 2 and the projection of the line of sight vector from t 1 is bounded by ˆ2eci − ˆ2eci ≤ ˆ2eci + Vmax |t| min(r2surf ace , r2altitude ) (9) ˆ we have a bound on the difference If instead of the true line of sight vector we have a measured line of sight vector , ˆ between 2eci and 2eci given by the measurement noise ν: ν2max = νmax By a geometrical argument we see that the maximum difference between 2eci and ˆ2eci due to errors in 1eci is bounded by r1surf ace r1altitude ν1max ≤ max |atan tan(νmax ) |, |atan tan(νmax ) | r2surf ace r2altitude r1surf ace r1altitude ≤ max νmax , νmax r2surf ace r2altitude and the small angle assumption for ν 1max gives 2eci ∼ ˆ2eci So that Vmax |t| min(r2surf ace , r2altitude ) Vmax |t| + ν1max + ν2max + min(r2surf ace , r2altitude ) 2eci + ν1max − 2eci − ν2max ≤ 2eci + 2eci − 2eci ≤ 2eci (10) (11) For any vector (1i − 2i )2 + (1j − 2j )2 ≤ (1i − 2i )2 + (1j − 2j )2 + (1k − 2k )2 Therefore, the bound developed in Equation (10) holds for comparisons between only the i and j components of 2eci and 2eci . Therefore, it is valid to rotate 2eci and 2eci into satellite 2’s local sensor frame at t 2 and perform either the radial gate Vmax |t| + ν1max + ν2max (iT 2eci − iT 2eci )2 + (j T 2eci − j T 2eci )2 ≤ 2eci + (12) min(r2surf ace , r2altitude ) or the cuboid gate Vmax |t| + ν1max + ν2max min(r2surf ace , r2altitude ) Vmax |t| + ν1max + ν2max + min(r2surf ace , r2altitude ) |iT 2eci − iT 2eci | ≤ 2eci + (13) |j T 2eci − j T 2eci | ≤ 2eci (14) 3.3. Arc Generation All sequences that pass cell and pair gates are considered for further analysis. Those measurements which also pass a filter gate with an existing track need no further investigation; they are immediately added to an output frame. Sequences which do not correspond to an existing track must be considered for track initiation. The system under consideration is assumed to have a known minimum probability of detection for true targets. Based on this probability of detection, we assume that for any N frames of data, true targets will be detected at least M times. As additional frames are considered, the successive pairs of observations are assembled into arcs. An arc consists of a sequence of points from consecutive frames. The points can either be measurements, or a marker representing the fact that no measurement passed the pair gate with the arc in the frame. On receipt of a new frame, the clutter rejection filter applies its gating sequence to the measurements in the incoming frame; if the last point of an arc passes gating with the new measurement, that measurement is added to the arc. If more than one measurement gates to the arc, a new arc is added to the arc set with the second matched measurement. If the measurement does not pass the proximity test with any of the existing arcs, a new arc is added containing only the new measurement. At the end of frame processing, arcs which have not been updated within the last N frames are deleted. Figure 3 depicts this process for a sequence of N frames. As an optimization, the interface between the clutter rejection preprocessor and the tracker can include the full arc set, rather than just the measurements in the current output frame. This prevents the tracker from re-running gating, as it already posesses the full set of arc information. Measurement arcs not corresponding to an existing track are run through track initiation. Measurements which gate with an existing track are candidates for track update or spawning. 3.4. Target Dynamics If the system is expected to perceive target motion sufficient to dominate the measurement noise, an additional dynamics test can be applied to arcs which pass the M of N test. As previously noted, it is not typical for target motion to be guaranteed perceptible within the usual N frames; accordingly, application of a motion test requires accumulation of arbitrary-length arcs. Once the target motion exhibits characteristics noticeably different from stationary clutter, the entire sequence of measurements can be submitted to track initiation. This process can dramatically reduce the initiation of tracks on persistent clutter, at the expense of lag in track initiation on true targets. 4. PIXEL CLUSTER AND GROUP CLUSTER GENERATION 4.1. Pixel Cluster Decomposition The pixel cluster decomposition algorithm we utilize takes feedback from stereo tracking to assist in detecting and resolving CSO measurements. This algorithm has been discussed in detail elsewhere 5 ; in this context we note that the primary advantage to performing pixel cluster decomposition as part of the preprocessing stage is that all of the necessary feedback and assignment information has already been computed, and therefore pixel cluster decomposition is a trivial additional step. This step reduces duplication of effort between tracker and preprocessor. 4.2. Group Cluster Tracking Group cluster tracking involves the replacement of a large number of measurements with some much smaller number of characteristic measurements using one of several possible algorithms. Group clusters may be formed from any set of measurements which gate with one another. 6 Our previous work on group cluster tracking 2 has predominantly utilized Expectation-Maximization (EM) clustering with an assumption of Gaussian density functions. While this process will adequately represent specular reflection point clutter, cloud and landmass edge clutter 7 exhibits characteristics that are more consistent with distributions around a polynomial function. Fuzzy clustering schemes are more adroit at extracting clusters exhibiting these characteristics. 8 In particular, we tailor the Fuzzy C Quadric Shells (FCQS) algorithm to this problem. The algorithm can be summarized as follows: • Hypothesize clusters j ∈ 1, .., M • Optimize cluster states – Given measurements x i = [ui , vi ], i ∈ 1, .., N C4 C3 C1 T1 M=3 N=4 T1 C2 First frame: Target and two clutter points with search neighborhood for M of N test. Arcs: [C1] [T1] [C2] At end of frame processing, the arcs are shifted: [C1 0] [T1 0] [C2 0] Second frame: Search neighborhood for clutter points does not contain a measurement. Search frame for true target contains a measurement. [C1 0 0] [C1 0] [T1 T1 0] [T1 T1] Arcs: [C2 0] Shifted: [C2 0 0] [C3 0] [C3] [C4 0] [C4] C5 C7 T1 T1 T2 T2 C6 C8 Third frame. Second target appears. [C1 0 0 0] [C1 0 0] [T1 T1 T1 0] [T1 T1 T1] [C2 0 0 0] [C2 0 0] Arcs: [C3 0] Shifted: [C3 0 0] [C4 0 0] [C4 0] [T1 T1 T2 0] [T1 T1 T2] [C5 0] [C5] [C6 0] [C6] Fourth frame. Second target passes proximity test. After frame shifting, first frame clutter points expire. [C1 0 0 0] Arcs: [T1 T1 T1 T1] [C2 0 0 0] [C3 0 0] [C4 0 0] [T1 T1 T2 T2] [C5 0] [C6 0] [C7] [C8] [T1 T1 T1 0] [C3 0 0 0] [C4 0 0 0] [T1 T2 T2 0] Shifted: [C5 0 0] [C6 0 0] [C7 0] [C8 0] Figure 3: Example showing the clutter rejection filter methodology. N M 2 wij (xT Ax + bT x + c) M – subject to the constraints j=1 wij = 1, i = 1, .., N, wij ∈ [0..1] – Minimize i=1 j=1 • Iterate over range of possible cluster values to minimize residual Each measurement is then assigned to its maximal weight cluster. The cluster centroid and extent are computed by taking the covariance-weighted mean and second moment of the measurements in the cluster. This centroid and extent are then reported as the measurement to be sent to tracking. Some care must be taken to ensure that low-probability cluster associations, which may represent true targets emerging from a dense clutter region, are not collected in the cluster. Table 1: Statistics showing the performance of the clutter rejection filter. Sensor Sensor 1 Sensor 2 Clutter Rejected 388000 (99.95%) 387925 (99.93%) Clutter Passed 194 269 Target Rejected 40 (12.08 %) 60 (17.65 %) Target Passed 291 280 5. RESULTS This subsection presents simulation results on a simulated ballistic missile scenario in the presence of Earth background clutter. The scenario contains two missile launches observed by two GEO satellites using a notional unclassified sensor design. The clutter rejection algorithm component, in this scenario, has been tuned to aggressively reject stationary clutter by requiring perceptible motion over the system noise. Since this is a simulated scenario, it is possible to determine when the clutter rejection algorithm rejects or passes a clutter observation. Table 1 shows the rejection statistics for the clutter rejection algorithm. With this set of tuning parameters, the clutter rejection algorithm does an excellent job removing most of the clutter, at the expense of also removing some of the early target observations. From the viewing geometry present in this scenario, one of the observers perceives the two targets as a single CSO measurement for a significant fraction of the scenario. As a result, the pixel cluster decomposition algorithm component is required in order to provide effective stereo track accuracy. For this scenario, image data was not available; when this occurs, the pixel cluster decomposition algorithm is designed to utilize a measurement multi-assignment strategy. Figure 4 shows the x coordinate of the estimated tracks in ECEF with and without the pixel cluster decomposition algorithm. A total of nine tracks are observed. Two of the tracks are tracking the truth objects. One track is a persistent multiview clutter track, i.e., a persistent track on clutter measurements viewed by both sensors. This track is on the cluster of measurements at the solar specular reflection point. The other six tracks are clutter tracks formed on data reported by a single sensor. The nature of each track is indicated in Figure 4. The tracks on the truth targets are more accurate when using the pixel cluster decomposition algorithm. This can be seen in the RMSE metric shown in Figure 5. X Coordinate of Target Trajectories 6 7 x 10 6 x 10 6 Truth target tracks Truth target tracks 5 5 4 4 x [ECEF, m] x [ECEF, m] X Coordinate of Target Trajectories 6 7 3 2 Multi−sensor clutter track Single sensor clutter tracks 1 3 2 0 0 −1 −1 −2 Stage 1 Stage 1 Stage 2 Multi−sensor clutter track −2 Stage 3 Single sensor clutter tracks 1 Stage 1 Stage 1 Burnout −3 0 20 40 60 80 100 120 140 160 180 0 Time (Seconds) (a) Stage 2 Stage 3 Burnout −3 20 40 60 80 100 120 140 160 180 Time (Seconds) (b) Figure 4. Scenario B1 with clutter. x coordinate of estimated tracks in ECEF (a) without pixel cluster decomposition and (b) with pixel cluster decomposition. The data for this scenario contained approximately 2000 clutter points per frame for 194 seconds at an update rate of one frame per second). This scenario was run 10 times and the average time to process this data was approximately 196 seconds.∗ ∗ All tests were performed on a dual processor Intel Xenon 3.0GHz with 8GBs of memory running a 2.69 Mandrake Linux Kernel. RMSE Position Error (Target 1) 5 10 RMSE Position Error (Target 2) 4 10 With Multi−Assignment Without Multi−Assignment 4 RMSE Pos [m] (log−scale) RMSE Pos [m] (log−scale) With Multi−Assignment Without Multi−Assignment 10 3 10 2 10 Stage 1 1 10 50 Stage 1 Burnout Stage 2 3 10 Stage 3 Stage 1 2 100 Time (Seconds) 10 150 (a) 50 Stage 1 Burnout Stage 3 Stage 2 100 Time (Seconds) 150 (b) Figure 5. Scenario B1 with clutter. (a) RMSE metric for truth target 1. The red curve is obtained using pixel cluster decomposition. The black-dashed curve is obtained without the pixel cluster decomposition method. (b) RMSE metric for truth target 2. The red curve is obtained using pixel cluster decomposition. The black-dashed curve is obtained without the pixel cluster decomposition method. 6. CONCLUSION We present an algorithm architecture for integrating clutter mitigation techniques with measurement preprocessing using feedback from stereo tracking. This mechanism allows a multi-frame assignment algorithm to operate in realtime despite the presence of dense clutter, while maintaining good track accuracy in CSO conditions. The preprocessor architecture, while optimized for use with the Numerica MFA tracker, has been developed so as to be useful with any desired downstream tracking algorithm. ACKNOWLEDGMENTS This work was supported in part by Lockheed Martin. REFERENCES 1. A. B. Poore, S. Lu, and B. J. Suchomel, “Data association using multiple frame assignments,” in Handbook of Multisensor Data Fusion, CRC Press LLC, 2001. 2. S. Gadaleta, M. Klusman, A. B. Poore, and B. J. 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