1. Field of the Invention
This invention relates to tracking multiple targets using passive or active sensors. Particularly, it relates to identifying each target in groups where the targets are closely clustered. More particularly, it relates to closely spaced targets which diverge, converge, merge, or split and which may be traveling at varying speeds.
2. Background
To identify targets between observation times (referred to as scans), the targets observed (hits) must be correctly correlated between scans. Multiple target tracking systems are well known in the art and described in the literature, e.g., xe2x80x9cMultiple-Target Tracking with Radar Applicationsxe2x80x9d by Samuel S. Blackman (Airtech House, Inc. 1986).
Kalman or batch filters are used as predictor functions to provide accurate tracking parameters. Such predictors, however, fail when the objects being tracked are closely clustered. Furthermore, the accuracy of such predictors depends on the accuracy in position determination. To track the paths of individual targets, it is necessary to determine the continuity of the individual trajectories from one observation time period to the next. When multiple targets are tightly grouped, the predictor functions cannot provide a sufficiently accurate projection due to inadequate target separation and to noise on the measurements used by the predictor. The time interval between observations may be long and the motion of clusters is fast enough that the error in prediction of the target""s positions is greater than the separation of the targets in the cluster.
The track determination is more difficult when the locations of the objects are observed for only short time periods separated by long intervals, especially when the objects are moving at a rapid rate.
Object identification in the prior art is directed to recognizing objects moving against a stationary background or to identifying a bounded object of a given shape. These techniques are not applicable to situations where multiple targets are being tracked and the targets can interchange apparent relative positions.
In accordance with the invention, the tracks of closely spaced objects are maintained by locating the objects in two successive patterns according to a coordinate system. Identifiers are assigned to the objects depending on their location in each pattern. The objects in the successive patterns are then correlated by a procedure that adjusts their position parameters within the patterns relative to a modified origin such as the centroid of their locations. The adjusted position parameters within the second pattern are transformed relative to a displacement of their centroid. The displacement is determined from the differences between all pairs of locations in both patterns. A figure of merit can be derived to aid matching the objects from one pattern to the next.