IEEE - Aerospace and Electronic Systems - January 2023 - 39

substantial upgrade program that will extend the
network's operational life to beyond 2040.
OTHR development in Australia is well captured in
several documents including [1], [2], [3], [4], [5]. The
motivation for this article is to enhance the visibility
of the Australian OTHR program within the AESS community
and to reflect on what is one of Australia's most
impressive technological feats relevant to celebrating the
Society's 50th anniversary. This article is structured as
follows. The " Fundamental Principles " section reviews
the operational concept and key characteristics of OTHR
systems. The " Historical Perspective " section provides a
brief chronological account of OTHR development in
Australia. The " Jindalee Operational Radar Network " section
discusses existing and planned technology upgrades
in the three most recent phases ofJORN. The " Reflections
and Prospects " section shares insights on the evolution of
OTHR in Australia and future opportunities.
FUNDAMENTAL PRINCIPLES
Skywave OTHR operates in the HF band (3-30 MHz) and
exploits signal reflection from the ionosphere over a twoway
propagation path to detect and track air or surface
targets at ranges an order ofmagnitude greater than is possible
with a ground-based microwave radar. This section briefly
reviews the fundamental principles, operational characteristics,
and indicative capabilities ofa nominal OTHR system. A
more detailed coverage of OTHR systems may be found in
[6] and [7].
OPERATIONAL CONCEPT
Figure 1 illustrates the OTHR principle of operation. A
transmitter radiates the HF radar signal as a directional
beam toward the ionosphere at oblique incidence. At one
or more heights in the ionosphere, the signal is reflected
and propagates downward to illuminate the Earth's surface
and airspace above it well beyond the horizon. This downJANUARY
2023
Figure 1.
OTHR principle of operation illustrated for E-layer propagation.
The range coverage of 1000-3000 km includes F-layer
propagation.
IEEE A&E SYSTEMS MAGAZINE
39
looking geometry eliminates shadow regions such that targets
cannot exploit mountains or valleys to deliberately
avoid illumination.
Targets illuminated by the transmitter scatter the incident
signal and a small fraction of each target echo propagates
back to a highly sensitive receiver. The receiver
acquires target echoes superimposed with clutter backscattered
from the Earth's surface, interference, and noise.
Signal processing is used to detect target echoes against
powerful disturbance signals and to estimate target parameters
including range, azimuth, and relative velocity.
Doppler processing is essential in OTHR to discriminate
moving target echoes that compete for detection with
much stronger clutter returns. A real-time air and surface
surveillance picture of confirmed target tracks on a geographic
display is the final OTHR output.
SKYWAVE PROPAGATION
The ionosphere is a region of the Earth's upper atmosphere
where the density of naturally occurring plasma
IEEE - Aerospace and Electronic Systems - January 2023

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