Aerospace and Electronic Systems Magazine December 2017 - 23

Francis, Cervera, and Frazer
sional (2D) numerical ray-tracing engine. It is beyond the scope
of this article to discuss the physics describing the effect of the
morphology of the ionosphere on the ray paths; however, we note
that there is often more than one propagation mode to a particular
ground range. In general, for a quasi-monostatic system, if there
are N propagation modes to a target, then there are a total of N 2
modes that are available to return energy to the receiver. Each of
these modes will have an associated group (or radar) range, elevation angle at the receive array, and power loss.
To characterise the signal propagation properties of each radar
in the proposed network, we calculate propagation look-up tables
over a range of environmental conditions: this includes the equinoxes and solstices at low, medium, and high solar activity levels.
The Zurich smoothed sunspot number (SSN) is a measure of the
solar activity [2] and used in the IRI model. For this case study, we
present results using low and medium solar activity of SSNs 20
and 70, respectively. Ionospheric propagation effects, such as ray
focusing [2] and HF radio wave absorption, are included. The ionospheric absorption model is from George and Bradley [8], which
varies across solar activity, season, time of day, spatial location,
and the angle of the ray path taken.
The look-up tables take the form of predicted received power
from a 1-W radiator, assuming a 1-second coherent integration
time (CIT) and a target with 1-m2 radar cross section (RCS). The
tables are parameterised by ground range and radar operating
frequency, with array gain included. As noted previously, there
may be several propagation modes to a particular ground range;
only the strongest mode for each ground range is retained for the
construction of the tables. Figure 3 displays graphically propagation tables for the New Zealand radar over a range of ionospheric
conditions.
This figure demonstrates the significant impact ionospheric
conditions have on the frequency variability of OTHR propagation. The ionosphere is weakest at pre-dawn during winter at solar
minimum, where in the top right of Figure 3, there is no propagation support at all for frequencies above 10 MHz. Propagation
modelling (not shown here) indicates that an OTHR at this location would have to operate at frequencies near 5 MHz during these
conditions. This would require a dual-band transmit array design
that would increase the complexity and cost of the system, as mentioned earlier.
Anticipated target signal-to-noise ratio (SNR) is calculated
from the propagation tables by scaling the power by the radar transmit power and CIT, using a frequency- and aspect-sensitive target
RCS model, and a location-, time-, and frequency-dependent background noise model. We choose the frequency that maximises the
return power over the radar processing area. This frequency is typically close to the "leading edge," where skip focusing occurs [2].
The frequency choice is additionally restricted to be slightly less
than the leading-edge frequency. This is to emulate how OTHR are
operated in practice, where operating frequencies too close to the
leading edge leave the radar susceptible to losing propagation support if the ionospheric conditions change rapidly, such as at times
around the dusk terminator or due to ionospheric disturbances.
In this article, the RCS is characterised by using analytic estimates of a commercial airliner over different aspect angles [9]
DECEMBER 2017

justified using EM modelling [5]. For the noise estimate, we used a
rural ITU median model for the background noise at the radar sites
[10], although more sophisticated models can be used if desired
[11].
We assume a simple ground clutter model with uniform backscatter coefficient σo (Earth RCS per unit area of Earth illuminated), where the clutter is confined to a Doppler band corresponding
to no more than ±25 knots. This is a valid model for an undisturbed
ionosphere and the assumption that operators correctly select the
radar operating frequency. If the target radial speed is less than 25
kn, then we assume it has been obscured by the ground clutter and
will not be detected. In any given OTHR location, there are likely
to be periods of Doppler-spread clutter that will reduce our estimates of system performance; however, we consider that to be a
second-order issue for most cases within our task of overall OTHR
network design. Investigating the consequences of, and provision
for mitigation methods required for, Doppler-spread clutter is location dependent. For example, the impact of Doppler-spread clutter can be ignored at first-order for an OTHR network located at
mid-lattitude, although it will dominate performance estimates if
located close to the southern or northern Aurora.
The most important parameter for OTHR tasking is the operating carrier frequency of the radio waves transmitted [3]. In a surveillance mission, we can split the area of regard into a set of radar
"tiles" [12]. The tiles are the range and azimuth extent processed
by the radar in each observation, as modern HF radars are fully
digital coherent phased array radars. For each tile, we determine
the optimal carrier frequency that maximises the energy received
by propagation from the tile. This is an optimisation over SNR.
One may extend the frequency optimisation to include directional
noise and an external large signal environment using the models
in [11], [13].
Typical OTHR waveform parameters are specified to determine additional gain and loss through the radar signal processing.
We assume the use of a linear frequency modulated continuous
waveform with a CIT of 4 seconds, noting that up to roughly 2
minutes is propagation coherent [1]. An additional signal processing loss of 10 dB is included as representative of losses incurred
by tapering [1] used on directional transmission tapers, clutter windowing in Doppler, target windowing in range, and spatial rejection windowing in azimuth.
In summary, the assumed the base-system for each radar in
the network has transmitter power of 40 kW, an eight log-periodic
dipole transmit array, and 64-element doublet receiver array. Subsequently, we shall consider radar configurations with increased
sensitivity. In these cases, we assume that higher radar sensitivity is achieved by employing one or more of increased transmitter
power, a higher gain transmitter array containing more elements,
or a higher gain receiver array containing additional antenna elements. Note that these changes may have follow on performance
effects requiring performing array modelling again, and the cost of
each of these options may not be equal.
We combine all the parameters so far into an estimate of SNR
of the target in the detection stage of the radar system, as described
in Equation (1). This equation is presented in log-scale, as the traditional parametric radar equation does not simply scale when ad-

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