Aerospace and Electronic Systems Magazine December 2017 - 6

Feature Article:

DOI. No. 10.1109/MAES.2017.170062

OTH Radar Phenomenology: Signal Interpretation and
Target Characterization at HF
Stuart Anderson, University of Adelaide, Adelaide, Australia

INTRODUCTION
Implicitly or explicitly, the extraction of information from radar
echoes is based on a physical model of the interaction between
the radar system and the target, the study of which is sometimes
referred to as radar phenomenology [1]. It follows that the detail and fidelity of the physical model determine the prospective
observables, the accuracy of measurement, and the fidelity of
interpretation. Of course, there is little to be gained from employing a highly sophisticated model if the radar measurements
have neither the dimensionality nor the dynamic range to support estimation of the parameters invoked. Contrariwise, there
is much to be lost if valuable information encoded in the target
echoes is overlooked because the signal analysis and interpretation procedures fail to undertake the corresponding decoding
processes.
In the case of over-the-horizon (OTH) radars operating in
the HF band, it is more often than not the case that sins of the
latter type prevail. This should come as no surprise. The present
generation of HF radars can call on technologies that dramatically surpass those available in the 1970s and 1980s, when many
of the best known OTH radars were designed and the associated
interpretative models formulated [2]. These include processors
with extraordinary speed and capacity, signal generators with ultra-low phase noise, precise remote timing and synchronization,
analog-to-digital converters able to digitize the entire HF band
at the antenna outputs, yielding far in excess of 100 dB dynamic
range, high-resolution colour displays and user interface tools,
advanced signal processing algorithms, robust broadband wireless communications networks, detailed geographic information
systems with a wide variety of overlay information, exotic new
materials that can sidestep classical performance limits associated with the electromagnetics of conventional conductors and
dielectrics, and powerful, versatile computational electromagnetics codes. To give just two examples of the scale of the improvements, the data processing rate of the Australian Jindalee
skywave radar near Alice Springs increased by 5 orders of magAuthor's address: University of Adelaide, Physics, North Terrace, Adelaide, SA 5000, Australia. E-mail: (stuart.anderson@
adelaide.edu.au).
Manuscript received March 22, 2017, revised July 9, 2017, and
ready for publication July 10, 2017.
Review handled by D. O'Hagan.
0885/8985/17/$26.00 © 2017 IEEE
4

nitude between 1978 and 2003, while the achievable subclutter
visibility increased by a comparable factor by 2015.
To take full advantage of this symphony of technological advances, the radar system and its operators must be equipped with
the means to detect, isolate, and interpret subtle features in the radar
echoes. This requirement typically manifests itself as a family of
coupled estimation or inverse problems. But, prior to any attempt
to implement such a means, the phenomenology must be explored
by building and experimenting with physical models that reveal
the signatures of prospective observables and quantify the retrievable information. Of course, some performance metrics are direct
beneficiaries of the improvements in dynamic range and computer
speed, without refining the underlying physical models, but they
are the exceptions.
The pursuit of higher fidelity in phenomenological models may
serve several purposes:
i. to improve performance in existing radar missions by understanding the factors that limit performance and developing
ameliorative procedures in design, processing, or operation
ii. to reduce vulnerability to hostile countermeasures of various
types
iii. to establish new capabilities which draw on information hitherto inaccessible or ignored
iv. to honour the spirit of scientific inquiry
We illustrate each of these in later sections, but to make the point
here, consider the following extract from a letter written some
years ago by the Commanding Officer of a major Western OTH
radar to the Chief Scientist of the civilian defense laboratory responsible for providing scientific support to his radar:
"... Notwithstanding, ship detection and tracking using the OTH radar remains a proven scientific fact. Dr X from your laboratory routinely
displays the unique capacity to not only detect,
but also track, potentially any type of ship, under almost all conceivable environmental conditions - a feat which has not been equalled by
any Air Force personnel, or indeed, other scientists."
The explanation? Dr X possessed an uncommon combination of
expert knowledge in ionospheric physics, radiowave propagation, electromagnetic scattering, and oceanography, and had also

IEEE A&E SYSTEMS MAGAZINE

DECEMBER 2017



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