Design Of Decoupled Tracking Filter With Platform Motion Compensation For Airborne Surveillance Radar

This paper discusses about the design of tracking filter for airborne multifunction active phased array long range radars. The radar under consideration has surveillance as primary function, where "track-while-scan (TWS) mode" of tracking is performed. In addition to surveillance, the radar has to perform closed loop tracking on priority targets of interest. Long range surveillance radars generally are designed to have narrow azimuth beam and wide elevation beam to obtain required height coverage with single bar. The measurement accuracies in elevation axis would be generally 10 times poorer compared to azimuth axis. This poses a problem to tracking system which uses Cartesian co-ordinates for modeling target kinematics. The higher error in any one of measurement dimension gets coupled to the other measurement axes resulting in inaccurate tracking. In tracking literature various decoupled tracking schemes are proposed to avoid the coupling of errors. This approach produces the desired results if there is no measurement transformations involved between different coordinate systems which leads to coupling of measurement axes. For the airborne target tracking system, co-ordinate transformations between different frames of reference is unavoidable. Own platform movement (translational and rotational) has to be fully compensated for optimal tracking filter. This necessitates use of 3D transformations resulting in coupling of errors between measurement axes. Hence decoupling the tracker design which would yield favorable results as suggested earlier does not provide relief in this case. The elevation measurement being not so accurate owing to broad beam, its coupling with other radar measurements during transformations disturbs the tracking accuracies leading to track deviations and inaccurate beam positioning for target under dedicated mode of tracking. Further, in search on move ground based radars, due to the multipath phenomena the elevation measurement sometimes depicts larger inaccuracies leading to inaccurate tracking. This paper proposes a scheme to address the tracking problems for both the above cases with weak coupling between axes at the output of transformation. The performance proposed method is compared with standard filter relying on coupled coordinate transformation for platform motion compensation (PMC). The results show that the proposed technique greatly mitigates the problem of widely different inaccuracies of radar measurements yet enabling compensation of own platform motion for all six degrees of freedom.