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The ability to provide an accurate view of a region of interest is standard among existing tracking systems. The extension of this capability to include accurate projections of the targets to future times increases the complexity of the problem. However, this ability to predict future locations is an important problem due to the inherent time latency that is present from "sensor to shooter." Because of this, a major requirement of the tracking system is that it must be able to use available information to accurately predict future target locations. The focus of this paper is to describe an improved state estimation technique that incorporates road information by using a Variable Structure 1MM. Furthermore, this approach will identify and account for stopped or stationary moving targets. Finally, some preliminary performance results will be presented.

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instabilities especially in the residual modes where there is little or no stability margin. In a very simple and mathematically well-known example of a flexible system, we have illustrated the residual mode instability caused by spillover (Section V).Observation spillover can be virtually eliminated with a phase-locked loop prefilter (Section IV) and the closed-loop system poles then become the controlled poles, the estimator poles, and the residual poles (Section III) and no instabilities can occur. Allowances for the effect of control spillover on system response can often be made in the origrnal design [28]. When spillover is taken into account this control scheme has several advantages:1) Many modes can be controlled by only a few actuators and sensors.2) Actuators and sensors need not occur in pairs and they need not be at the same locations.3) The controller design requires only a knowledge of the elastic mode frequencies mode shapes at the locations of the actuators and sensors. ( l h s information can be approximated by existing computer programs for quite complicated dynamic structures.) 4) The system performance can be altered by changing parameters in the on-board control computer and this requires no modification of the physical structure (e.g., adding passive dampers); this may also make it possible to handle several different system configurations (e.g., control while assembling a structure in space).The active controller proposed here using a linear feedback control law, a state estimator, and a sensor prefilter is a straightforward method for controlling N modes of a flexible system and eliminating the disasterous effects of spillover from the residual modes. It uses well-known state variable control techniques which reveal the design tradeoffs available and it could be used as a starting point for a practical feedback controller for flexible systems, especially large spacecraft where it holds many advantages (e.g., 1221, 1301). Further investigation is necessary in two basic areas to bring these concepts to maturity: 1) Effect of dynamics of actuators, sensors, and prefilter (some preliminary work in [3 I]).2) Adaptive control, i.e., an estimator that adjusts its internal model parameters to better match those of the system it is attempting to follow (several adaptive methods available for model referenced systems, e.g., (321, [33D. REFERENCES 1 V. Larson, P. Likins, and E. Marsh "Optimal estimation and attitude control of a AES-13, pp. 3547, 1977.

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