Aerospace and Electronic Systems Magazine December 2017 - 20

Feature Article:

DOI. No. 10.1109/MAES.2017.170056

Performance Prediction for Design of a Network of
Skywave Over-the-Horizon Radars
David B. Francis, Manuel A. Cervera, Gordon J. Frazer, Australian Department of
Defence, Edinburgh, South Australia, Australia

INTRODUCTION
Skywave over-the-horizon radar (OTHR) is long-range beyondhorizon radar technology that is presently employed by several
nations for wide-area surveillance of aircraft and maritime vessels [1]. This radar class uses propagation via the ionosphere [2]
to achieve radar energy propagation to and from the target region.
The ionosphere is driven by varying solar interaction with the
Earth, and this variability significantly influences the operational
performance of a skywave OTHR. Considerable care in selecting
radar-operating parameters is required to achieve the best performance.
Australia has a network of three skywave radars known as the
Jindalee Operational Radar Network [3]. These three radars have
a degree of overlapping coverage and are operated jointly in a coordinated manner as a single radar network, with individual radar
employed as appropriate, to achieve the overall mission objectives.
Consider, for example, a mission with the goal of detecting and
tracking aircraft in a particular geographical region for several
days. The varying ionosphere will require judicious dynamic selection and operation of one or more of the three available radars
to achieve the mission objective in the presence of the changing
ionosphere.
Skywave OTHR is typically a bistatic system with separate
transmitter and receiver subsystems sited 100 to 200 km apart.
The separation is small compared with the radar to target range of
1,000 to 3,000 km. The radio-frequency isolation resulting from
this physical separation allows radar waveform transmission to be
continuous without overloading the receiver; hence, it maximises target detectability. The transmit and receiver subsystems are
asymmetric with completely different transmit and receive antenna
array designs. The spatial resolution of the transmit subsystem is
typically one twentieth of that of the receiver subsystem (so the
Authors' current address: D. B. Francis, M. A. Cervera, G. J.
Frazer, Defence Science and Technology Group, Australian
Department of Defence, Third Avenue, Edinburgh, South Australia 5111 Australia, E-mail: (david.francis@dsto.defence.gov.
au). M. A. Cervera is also at the School of Physical Sciences,
University of Adelaide, Adelaide, Australia.
Manuscript received March 2, 2017, revised September 18,
2017, and ready for publication September 22, 2017.
Review handled by D. O'Hagan.
0885/8985/17/$26.00 © 2017 Crown
18

transmit array is approximately one twentieth the physical size of
the receiver array). The radar operates by directing a transmitter
beam via the ionosphere to produce an illumination footprint on
the Earth's surface at some subregion of the total potential coverage area. Multiple simultaneous receiver beams with higher spatial
resolution cover the transmitter footprint. A footprint is several
hundred kilometres by several hundred kilometres in area. The
radar mission and the ionospheric propagation conditions govern
the footprint location within the area of total potential coverage.
Depending on the type of target, the radar will coherently measure
within the illuminated footprint between 1 and 60 seconds. The radar will then switch the illumination footprint to some other region
of total potential coverage, again based on the radar mission and
propagation conditions. To establish and sustain tracks on detected
targets, it is important that radar measurement of a particular target be updated, so a given illumination footprint will regularly be
revisited. The radar revisit strategy depends on the target detectability and the target dynamics. For highly manoeuvring targets,
the radar operator may configure the system to stare at a single
footprint continuously. However, this will decrease the proportion
of total potential coverage area for a given radar resource.
Operating a network of OTHR in this manner involves many
considerations. These include the underlying concept of operations, ionospheric conditions, the particular level of operator experience, the number of radars and their location, the individual radar
sensitivities, instrumental fidelity, and so on. When considering
new surveillance applications, the question naturally arises as to
how one might design a new OTHR network. The system designer
has many factors that will influence their design selections. Significant design degrees of freedom are manifold. They include the operational mission and definition of success, anticipated ionospheric
conditions, the number of radars and their physical locations, individual radar sensitivity and the operating parameter space of each
radar, the network coordination strategy, and the total system cost.
In this article, we explore this question and propose a methodology for designing a network of skywave OTHR. We call this
the radar network design methodology. We are also interested in
OTHR networks of many individual OTHR compared with current
systems with only a few radars. In this new approach, the individual radars have lower sensitivity than existing systems, and hence
reduced cost, but are located as required to provide range and
aspect diversity to the target region of interest. We suspect such
netted-diverse-compact OTHR networks will achieve comparable

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DECEMBER 2017

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