Cylindrical phased arrays are an attractive aperture for radar applications due to their steering-angle independent gain, beamwidth, polarization, sidelobe levels, and reflection coefficient. Moreover, the ease with which they form omnidirectional and wide–sector–covering transmit beams facilitates ubiquitous radar – allowing the ability to simultaneously see everywhere. The reduced scan time of ubiquitous radar, however, is coupled with an increased dwell time to maintain constant range.
Small platforms play a pivotal role in the modern Navy by performing the forward sensing required to extend the eyes and ears of larger platforms. Efficient surveillance radar is a necessity, enabling these platforms to sense their environment. Initial studies investigating the optimal configuration for a multi-faced array-based surveillance radar focused on horizon surveillance and used the relative horizon search time as the defining metric. In these studies, it was assumed that transmit/receive (T/R) modules were distributed equally throughout the array. The benefits of simultaneously using multiple array faces were considered, but no mention was made of the benefits obtained when using multiple faces coherently.
The initial studies concluded that the optimal number of faces is three – a finding reinforced by a later study on volume surveillance. However, a subsequent study that included a redefined representation of scan loss and the impact of clutter fill pulses concluded that the reduction in scan positions offered by a four-face system proved optimal.
The concept of ubiquitous radar is to look everywhere, all the time – a concept that is seen as a way of providing the persistent awareness desired by small Navy platforms. In ubiquitous radar systems, a broad transmit beam illuminates a large surveillance volume that is then tiled with many simultaneous, contiguous, high-gain receive beams. This continuous illumination allows increased integration time to make up for the decrease in sensitivity resulting from a low-gain transmit beam. This radar concept is now realizable thanks to advances in digital beamforming (DBF).
The concept of ubiquitous radar was initially discussed using planar arrays. However, key limitations are presented by planar arrays that limit the effectiveness of this radar technique. Synthesizing broad transmit patterns without severely limiting effective radiated power (ERP) is a challenging problem with planar arrays. Techniques have been developed, but even those cover an angular sector of only moderate width.
Circular/cylindrical apertures overcome many of the challenges presented by planar arrays to a ubiquitous radar system. Transmitting with omnidirectional patterns – or broad, sector–covering patterns – is straightforward with appropriately designed circular apertures. Directional radiation patterns may also be formed and steered over a full 360 without significant changes to pattern shape, sidelobe structure, or polarization. The width of the sector and beamwidth of directional beams are reconfigured with ease, thus providing variation on the effective size of the transmit/receive faces. These advantages make cylindrical apertures preferable, in principle, over planar arrays for applications requiring 360 visibility.
Despite these obvious advantages, radar systems have been slow to adopt circular apertures and instead accept the limitations offered by the more traditional aperture of a planar array. One reason for this is the ease of design. Planar array design is a straightforward design process often consisting of unit-cell simulations and array factor synthesis. The design process for circular arrays is less simple and straightforward. The geometry of the circular array gives each element a unique pointing direction, which makes each embedded element pattern unique. This further complicates the design of the array by making the synthesis of array tapers and the steering of directional beams more challenging.
In recent years, increased interest in the benefits offered by circular/cylindrical apertures has led to increased research and development to mitigate the limitations in their design, implementation, and characterization. The limitations in pattern-synthesis techniques have become less of a roadblock as algorithms have been developed for both transmit and receive operation. Phase-only pattern synthesis techniques exist to form omnidirectional transmit patterns with multiple nulls to minimize radiation in desired directions while still maximizing transmit power. On receive, directional patterns with custom sidelobe constraints may be formed and steered over a full 360 in azimuth with no significant degradation to pattern characteristics.
This work was done by W. Mark Dorsey for the Naval Research Laboratory. For more information, download the Technical Support Package (free white paper) below. NRL-0079
This Brief includes a Technical Support Package (TSP).
Cylindrical Array Radar: Comparison to Multi-Faced Systems for Horizon Surveillance and Application to Ubiquitous Radar
(reference AD1110196) is currently available for download from the TSP library.
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