Aerospace and Electronic Systems Magazine December 2017 - 22

Design of a Network of Skywave Over-the-Horizon Radars

Figure 2.

Example of one-way ray paths for 10 MHz rays calculated by 2D numerical ray-tracing for the New Zealand radar during daytime, summer, solar maximum conditions. Note the different ionospheric propagation modes and the ionospheric penetration by the high elevation angle rays.

The notional potential coverage region for each radar has an
azimuth extent of 90° and range of 1,000 to 3,000 km. OTHR
generally has a large possible coverage, subject to ionospheric
conditions, so this particular network arrangement will have some
capability against most of the eastern half of Australia. However,
the instantaneous actual coverage is moderated by time and spacevarying propagation and the radar sensitivity.

RADAR MODELLING
The performance of radar systems, such as OTHR, is governed
by factors including the physical radar equipment, such as transmitter power and transmit and receive antenna gain, the selection
of signal and data processing algorithms, the waveform scattering properties of the target and any unwanted scatterers (called
clutter), and the physical operating environment governing radar
signal propagation support and level of external noise encountered by the radar receiving system. OTHR relies on target motion
and the Doppler effect to separate radar returns of moving targets
from the backscatter from stationary land or the slowly moving
sea surface. The typical range and azimuthal resolution cell size in
OTHR is many tens of square kilometres surrounding the target.
Earth return ground clutter is usually more than 50 dB larger than
the return from an aircraft. In some cases, usually associated with
particular ionospheric conditions, clutter may become spread in
Doppler and extend into the non-zero Doppler detection space. In
our study, we use a simplified clutter model, as described shortly.
For this case study, we consider a set of models for the radar,
environment, target, and values for operational parameters. These
are all intended to be examples to allow us to demonstrate the radar
network design methodology. We intend the reader to be able to
use their own models or parameters, as appropriate, to answer their
own performance analysis questions using the metrics developed
in the follow sections.
With OTHR, the available radar transmit power and transmit
and receive antenna gain are fixed for a given installation; the
signal and data processing algorithms are specified and generally
fixed (although may adapt dynamically to the propagation, clutter,
and noise environment); and target scattering properties are either
20

pre-measured, modelled, or unknown for any target that may be
detected. The radar signal propagation and external noise environment is highly varying, and any model-based predictor of OTHR
performance must incorporate this variability, although as noted,
we do assume the receiver system to be externally noise limited.
We model each of these aspects independently in our approach.
A model of the environment in which an OTHR operates requires contributing models of the propagation of the transmitted
signal to the target and clutter sources and return to the radar receiving system. It also requires knowledge of the external noise
environment at the radar receiving location and a measure of the
Earth backscatter from the region surrounding the target.
In our case study, we choose the transmit antenna to be a linear
array of eight log-periodic dipole curtain elements, with an interelement spacing of 6 m. We specify the individual per-element
power-amplifiers to be capable of up to 5 kW of peak power perelement. The receiving array is designed to be a linear array of 64
doublet elements, with an interelement spacing of 6.4 m and the
doublet design detailed in [4]. Doublets, in this case, are defined to
be two monopole antennas with one phase-shifted by π radians and
then combined to gain directivity perpendicular to the length of the
linear array and provide a null in the reverse (non-transmit) direction. Both arrays are designed to operate in the bandwidth of 13 to
26 MHz. This frequency limitation reduces the design cost of the
radar as transmit operation at lower frequencies typically requires
an additional separate transmit array of size, and hence cost, scaled
to the lower frequency range of operation. All arrays are directed
to have array boresight maxima directed toward Sydney (SYD),
Australia. They are modelled by using a method-of-moments electromagnetic (EM) solver (Numerical Electromagnetics Code-4
[5]). We note that we do not require a model for the internal noise
of the receivers because, for well-designed high-frequency (HF)
radar receiver systems, the system internal noise will be lower than
the external noise impacting the receiver.
Ionospheric propagation is modelled by using radio wave raytracing methods applied to the International Reference Ionosphere
(IRI) [6]. In our work, we use the ray-tracing toolbox (PHaRLAP)
[7], developed by one of the authors. Figure 2 displays an example
of one-way ray paths calculated by using PHaRLAP's two-dimen-

IEEE A&E SYSTEMS MAGAZINE

DECEMBER 2017



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