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Example of an adapted phased array antenna pattern for a main lobe jammer
The continued performance of a radar in the presence of jammers is an important factor in its overall capability.
Adaptive beamforming techniques provide the means to suppress jamming signals allowing targets to be detected that would otherwise have been completely obscured. Another significant problem in some (particularly airborne) applications is Doppler spread of clutter and this can also be suppressed adaptively. The ATC has significant experience in the development of algorithms and models as well as hardware architecture and sub-system design for radar adaptive signal processing.
Jammer Suppression
For many radars the main defence against jamming is provided by coherent processing gain and by the level of the antenna side-lobes. The jammer though has a significant power advantage (one way 1/r2 propagation loss compared with two way 1/r4 propagation loss). The solution, rather than relying on low sidelobes, is to place deep nulls in the direction of the jammers while maintaining gain in the desired look direction. To achieve this requires a multi-channel architecture with spatial control of the receive antenna pattern, provided by either auxiliary antennas or a phased array or a fully element digitised array. The adaptive algorithm can then process the data from the channels to provide a combined signal with the jammer suppressed.
Clutter Suppression
Clutter has an inherent Doppler width due to its own internal movement, e.g. for sea clutter or wind blown trees etc. This clutter is spread further by the antenna motion and by the platforms’ motion. So for a ship based radar the antenna rotation and the ships motion both contribute to further Doppler spreading of the clutter. This is a significant problem for airborne radar where the large own platform velocity causes wide Doppler spreading. This could in principle be compensated for explicitly but because of the dynamic nature of the situation a much more viable solution is provided by adaptive signal processing techniques.
Algorithms
A wide range of adaptive signal processing algorithms for both jamming and clutter suppression have been studied at the ATC. Different algorithms have been developed which are aimed at tackling a range of issues depend on the type of radar architecture. One of the most demanding conditions is for main-lobe jammer suppression. Being able to form a null in the main lobe while minimising disruption to the rest of the antenna pattern is often a requirement. Also, even quite slow antenna rotations can disrupt the forming of deep narrow antenna pattern nulls and algorithmic techniques have been developed to compensate for this. Different clutter environments can also significantly impact on the design of the algorithms.
Modelling
A modelling capability has been built up at the ATC to support radar design and development. The adaptive signal processing algorithm research requires as faithful as practical a modelling representation of the electromagnetic environment in which the antenna is situated. The level of detail is varied depending on which aspect of the algorithm is being studied. The environment primarily consists of jammers, targets and clutter, including ground reflections. It needs to allow for antenna and platform motion and rotations. The internal signal processing of the radar also needs to be modelled.
Experience
The ATC has been involved in the development of adaptive processing and phased arrays from the outset. It has participated in building several radars and demonstrators utilising adaptive processing as well as many theoretical and simulation based research projects for both company and international collaborative programmes.
