Simultaneous beams in large phased radar arrays

Summary form only given. Modern radar systems, especially those devoted to surveillance and defense mission, are able to accomplish different functions, change their operating modes and adapt the achievable performance according to the conditions and the peculiar environmental features (number and types of threats/tracks), in order to provide the optimal capabilities to cope/deal with. Those systems are, hence, demanded for high performance and high flexibility, allowing to save resources from not primary functions to be exploited to boost performances (resolution, search volume, detection range, renewal time, track refresh rates and initialization, etc.) of other proper functions according to the current needs of operation. Naturally, the required flexibility has significant impacts on the radar systems and involves the design of the overall components/sub-systems of the radar chain, and accordingly also the antenna front-end. Multi Functional Radars systems essentially exploits Active Electronically Scanned Antennas (A-ESA) with a large number of Transmit Receive Module (TRM) - from several hundreds to thousands - where phase and amplitude control of each TRM excitation enables the steering of the beam in the scan domain. A significant improvement of the antenna flexibility can be achieved by means of a (full) Digital Beam Forming (DBF) scheme, where the output of each TRM is digitized and the numerical outputs are properly combined in order to simultaneously generate the desired antenna patterns, such as in the case of multiple simultaneous beams (at the same reference frequency) covering different pointing directions to allowing a faster scan of the surveillance volume. More generally, a full level of digitalization at element level provides the highest level of degrees of freedom enabling, in principle, simultaneous generation of any pattern supported by the antenna array aperture, hence providing to the radar system a number of advanced spatial processing features such as adaptive digital beam forming, adaptive angle estimation, space-time processing against clutter. Unfortunately, DBF scheme, when implemented at element level requires a significant increase of HW components and of complexity of the radar architecture, since the number of receivers and, accordingly the number of A/D converters, explodes from few units (analog beam forming) up to hundreds or even several thousand (as in performance military radars). This leads typically to unacceptable solutions due to the HW costs of the overall architecture. In addition, it is also worth noting that the highest degree of freedom provided by the full level of digitization may turn out to be unnecessary since many practical applications require a limited re-configurability, i.e. for instance a side lobe cancellation or a beam pointing scan only into an assigned and limited angular region. A trade off-solution which introduces a controlled degree of flexibility, keeping under control the cost and the complexity of the radar HW architecture, is to adopt a DBF layer over-posed to the conventional analog beam forming, where the array elements are grouped into sub-arrays, whose outputs are combined together by means of analog beam forming networks (BFNs) and digitized. Then digital data are properly combined by a processing HW to produce the simultaneous desired beams. Sub-array architecture allows, indeed, to reduce the number of digitizers in the order of 5%-10% of the number of elements (e.g. 40-200 digitisers with a 2000 element array), introducing a proper degree of flexibility and spatial re-configurability. Naturally, determining the optimal sub-array layout and, thus, the digital sampling of the radar antenna outputs (DAR - Digital Array Radar) ensuring an adequate sampling to achieve acceptable low sidelobes (for simultaneous beams) and beam correction is crucial in order to provide an architectural solution satisfying the performance specification while keeping the number of digital channel acceptable. Different strategies have been proposed in the open literature and explored in real radar equipments. This paper focuses on new approaches for clustering very large arrays of interest for civil and military applications, taking into account HW constraints, costs and weights. In particular the proposed tools and algorithms have been adopted to design the S Band and X Band innovative radar front-end.