Aerospace & Defense Technology - December 2021 - 27

Tech Briefs
Cylindrical Array Radar: Comparison to Multi-Faced
Systems for Horizon Surveillance and Application to
Ubiquitous Radar
Cylindrical phased arrays are an attractive aperture for radar applications due to their steering-angle
independent gain, beamwidth, polarization, sidelobe levels, and reflection coefficient, as well as the
ease with which they form omnidirectional and wide-sector-covering transmit beams.
Naval Research Laboratory, Washington, DC
C
ylindrical 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.
y
R
φelem
x
φ
r
An N-element circular array of radius R. This diagram shows
the array elements (red) and far-field vector (green).
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 simul -
taneous, 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.
Aerospace & Defense Technology, December 2021 www.aerodefensetech.com
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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 unitcell
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, implementa27
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Aerospace & Defense Technology - December 2021

Table of Contents for the Digital Edition of Aerospace & Defense Technology - December 2021

Aerospace & Defense Technology - December 2021 - Intro
Aerospace & Defense Technology - December 2021 - Sponsor
Aerospace & Defense Technology - December 2021 - Cov1
Aerospace & Defense Technology - December 2021 - Cov2
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Aerospace & Defense Technology - December 2021 - Cov3
Aerospace & Defense Technology - December 2021 - Cov4
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