Aerospace and Electronic Systems Magazine December 2017 - 55

Defence Research and Development, Canada, detail an experimental investigation into the use of MIMO techniques for the mitigation of Auroral clutter for a proposed northern looking skywave
OTHR based in Canada [29], [36]. These are initial results and part
of an ongoing investigation with further results anticipated over
the next few years.
Previous work by this article's author and his coworkers [24],
[26], [27], [37]-[41], and the references therein, has addressed
a range of issues relevant to designing a Mode-Selective OTHR
based on MIMO methods. Taken together, the results thus far have
comprehensively demonstrated that MIMO radar techniques are
applicable for clutter mitigation in the HF radar context.

APPLICATION IN OTHR
There are two major applications for MIMO methods in OTHR.
The first is for the problem of slow-moving target detection in
typical midlatitude ionospheric conditions. This addresses the reduction of clutter during periods of multipath propagation where
each path may have differing ionospheric Doppler shift and spread.
MIMO solves this problem by enabling the radar to generate multiple, simultaneous elevation-range-dependent transmit beams,
possibly adaptive, and these form the basis of propagation mode
separation which in turn reduces clutter and hence improves radar
detection performance. The second application is concerned with
reducing poor radar performance due to anomalous clutter caused
by disturbed ionospheric conditions, such as for OTHR operating
in the southern or northern auroral regions. It typically involves
adaptive joint azimuthal and elevation transmit-receive beamforming.
The author and coworkers have been addressing the former
problem and have succeeded in demonstrating a working ModeSelective Radar [42]. In this article, we also wish to report an
earlier series of one-way skywave multiple-input single-output
experiments, in addition to the two-way backscatter experiments,
that we conducted as we built toward a full skywave backscatter
Mode-Selective Radar design. These proved to be hugely informative in our subsequent work.
Although the article is concerned with the slow-moving target
detection in typical midlatitude ionospheric conditions situation,
the methods and experience reported is relevant to the anomalous
clutter caused by disturbed ionospheric conditions case, with both
applications sufficiently similar the techniques discussed are relevant in either situation.
DECEMBER 2017

SKYWAVE RADAR
OTHR uses propagation via the Earth's ionosphere to achieve
beyond line-of-sight horizon radar coverage [43], [44]. The ionosphere is a Solar driven multilayered ionized media spanning a region between approximately 100 km and 400 km above the Earth's
surface [45]. The particular state of the ionosphere at any moment
depends on factors, such as the time-of-day, season-of-year, the
eleven-year solar cycle, and contributions such as solar flares due
to coronal mass-ejections, and so forth. The consequence is that the
ionosphere is a dynamic medium that can complicate the detection
of slow-moving targets when used as the radar signal propagation
path.
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. This unwanted
backscatter is called clutter. Land returns appear as clutter at zero
Doppler, since the land is stationary, while ocean returns appear
as clutter across significant Doppler space generated by the interaction between the impinging electromagnetic wave of the radar
signal and the hydrodynamic waves from the motion of the ocean
surface, although ocean clutter is predominantly concentrated in
narrow Doppler bands at the first-order Bragg resonance frequencies. Clutter from land, at zero Doppler, and clutter from the ocean
at the Bragg resonance Doppler frequencies, are almost always far
stronger radar returns than the return from a target. Clutter to target
ratios greater than 50 dB are typical. This means that any spreading
in Doppler of the land or ocean clutter due to the propagation path
will almost certainly obscure any slow-moving targets.
In the case of slow-moving surface vessels, such as land vehicles and ocean-going ships, the motion of the ionosphere and
that of the target are of similar order and targets can become lost
in clutter that is spread in Doppler due to motion of the dynamic
ionosphere. In practice, not all possible radar-to-target and return
propagation paths are necessarily subject to the same ionospheric
motion and hence reduced target detectability. Figures 1 and 2 together show a contrasting example of good and poor ionospheric
multi-path conditions. In the former case, the propagation is via
the E-layer only (so single layer) which characteristically has low
Doppler spread and both Bragg ocean clutter and a target are apparent. However, in the latter case propagation is via both the E
and F-layer and where the F-layer has significant Doppler-spread
causing spread-Doppler Earth return clutter that now obscures the
target. The radar measurements were recorded approximately 30

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine December 2017