On the Use of Terrain
|Stephen M. Kogon, E. Jeff Holder 1 ,
and Douglas B. Williams 2
MIT Lincoln Laboratory
244 Wood Street
Lexington, MA 02173-9108
Tech Research Institute
2 Georgia Institute of Technology, School of ECE
Abstract The presence of jammers within the mainbeam of a radar receiver presents a particularly challenging adaptive processing problem. In some instances, the interference can still be mitigated via null steering, provided the target and the jamming source do not share the same azimuth angle. However, when the angular separation of the target and the jammer becomes too small, the two are essentially spatially colocated. In this case, spatial nulling techniques will cancel the target in their attempt to suppress the jammer. The current approach to this problem is a technique known as "burn-through," which exploits the temporal coherency of target returns by integrating over long periods of time. This coherent integration increases the target signal-to-noise ratio, while the jammer does not experience this gain since it lacks temporal correlation. However, large pulse lengths increase the occupancy of the radar resource period and, as a result, decrease the number of targets the radar can track. In addition, in airborne systems the maximum pulse repetition frequency the radar is able to support is reduced, creating possible velocity ambiguities. We propose the use of space-time adaptive processing (STAP) methods to combat the mainbeam jammer problem. Typically, the jammer transmitter has a broad transmit beam with high sidelobe levels. As a result, the jammer signal is transmitted over large angular regions which can produce terrain reflections. These potential multipath signals are incident on the radar receiver and can be exploited to achieve mainbeam jammer suppression while maintaining gain on the target. Even modest suppression decreases the pulse length required for burn-through. The proposed strategy is the converse of that used for terrain scattered interference (TSI) mitigation, which uses the jammer direct-path signal to remove TSI. Target extraction results with a mainbeam jammer are presented with experimental data collected as part of the DARPA/Navy Mountaintop program.
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