A Coordinate Registration Technique for OTH Sky-wave Radars Based on 3D Ray-tracing and Sea-land Transitions

Target detection in Over-The-Horizon (OTH) radars is accomplished by tracking returns in slant range, Doppler and azimuth. Coordinate registration (CR) is the process of localizing the target by converting the slant coordinates to ground coordinates for all the frequencies used in transmission. The CR method uses a 3D ray-tracing algorithm which provides the ground range distance reached with a specific transmission frequency and elevation angle. The ray-tracing approach here adopted uses a 3D electron density model variable with height, latitude and longitude. The ray-tracing output is generally affected by errors due to numerical approximations of the 3D ionosphere model and discretization step used to integrate the differential equations of ray-tracing algorithm. Therefore the raw CR diagram suffers from an error which introduces as a consequence a degradation of target localization accuracy. Accordingly, we propose a coordinate registration technique for OTH sky-wave radars based on 3D ray-tracing that uses the sea-land transitions to mitigate the CR errors. The approach is based on the a priori knowledge of actual group delays relative to the sea-land transition within the area illuminated by the radar antenna beam. The method takes advantage of the geomorphological structure of the surveillance area. The errors introduced by the 3D ray-tracing software are then evaluated by using the actual group delay sat sea-land transitions.Afterwards, the estimated errors are used to correct the coarse CR diagram that was obtained straightforward from the ray-tracing output. Finally, the proposed correction method has been verified under the simplified assumption of a horizontally stratified ionosphere.