Aerospace and Electronic Systems Magazine December 2017 - 60

MIMO Methods Applied in Over-the-Horizon Radar

Figure 6.

End aspect photograph showing the 12 element minimum redundancy
array used in the Mode Selection Experiment. The photo is taken from
the southern end of the array looking north. The white equipment shelter
just visible on the horizon contained the transmitter equipment and the
central control system for the experiment. It was located mid-way along
the array.

staggered waveform set can be clearly recognized. The waveform
parameters were f b = 20KHz, f wrf = 4Hz, and Kt = 12.
The transmitter array at Coondambo comprised a 12-element
minimum redundancy array with a total aperture of 1200 m (endfire bearing 359.5° T). The array geometry is configured to provide
elevation discrimination in the direction of the receiver locations.
This aperture corresponded to our estimate of the likely spatial
resolution required for effective mode selection in elevation. The
minimum redundancy elemental unit spacing was {1, 2, 3, 7, 7, 7,
7, 7, 4, 4, 1} spanning 50 units with a unit element spacing of 24
m. The transmitter system could transmit up to 100 W power per
element and the array is shown in Figure 6. In this experiment, we
used element-space MISO where each member of the waveform
set (cardinality Kt = 12) was transmitted via a separate element in
the transmit array. Separate antennas were provided for OIS transmission and for a receiver to allow operating frequency selection
advice. The array is designed to operate over a frequency range of
8-12 MHz which was the anticipated range of ionospheric propagation support for the chosen down-range receiver locations during
December 2009 and March 2010 (noting that ionospheric propagation conditions changed slightly during the three month interval
between the experimental campaigns).
The choice of a minimum redundancy array (MRA) for the
transmit array was made in order to maximize the spatial aperture
of the transmit system given the equipment that we had available.
Based on the typical propagation path take-off elevation angles anticipated for our propagation geometry, and our desire for cost reasons to use an Earth conformal array, rather than a vertical array, we
estimated that an aperture in excess of 1000 m would be required.
Typically, MRA are not used in OTHR. To imagine why, consider an OTHR receive situation. The predominant radar backscatter
received by the radar is from Earth return clutter that is spatially
continuous and related to the illumination footprint of the OTHR
transmitter, the Earth backscatter radar cross-section, and propagation losses. Spatially discrete target radar returns are far weaker
than clutter returns and are separated from the clutter by differing
58

Doppler. With a MRA, and in particular the poor sidelobe performance achieved by a MRA, Doppler-spread clutter from almost any
direction may reduce the detectability of a target that is actually well
separated from the clutter in the same general direction as the target,
and that would remain separated with a low sidelobe filled array.
MRA are effective for a limited number of spatially discrete
signals or sources but deliver uncertain performance against a large
number of spatially discrete sources or spatially continuous sources such as clutter return. Additionally, the overall gain of a MRA
compared with a filled array of the same total aperture size is lower
by the ratio of the respective number of elements in each array.
In the one-way case employed in MSE we knew in advance
that there would be a finite and small number of discrete propagation paths from the transmitter to each receiver corresponding to the
ionospheric modes present at the time. This number would typically
be between two and six and was not expected to ever exceed Kt =
12 and could be determined in real time from the OIS. We also had
prior experience indicating that, while each potential propagation
path would likely have differing time-delay (range) and Doppler
shift and spread, the spatial signature of each of these various propagation paths would be close to spatially discrete. The transmit system had sufficient total radiated power so this was not a constraint,
meaning that the limitations of a MRA would not be of consequence
in this experiment. We acknowledge, though, that the MRA configuration is not expected to be appropriate for a radar design for the
two-way backscatter case that we were ultimately pursuing.

MODE SELECTION EXPERIMENT RESULTS
MODE SELECTIVITY ON TRANSMIT
A snapshot of typical results demonstrating mode selectivity on
transmit is shown in Figures 7a-c. The first figure shows the rangeDoppler map for a single waveform channel. This is the response
expected for a conventional SISO system measured over a one-way
propagation path, and shows four modes that have been identified
from the accompanying OIS ionogram record to be 1E (one-hop
propagation via E-layer), 1F21 (one-hop propagation via F2-layer
low-ray), 1F2h-o (one-hop propagation via F2-layer ordinary highray), and 1F2h-x (one-hop propagation via F2-layer extraordinary
high-ray) [45].
We present two examples of mode selectivity in Figure 8. In
the first example, we have selected the 1F2h-o (third mode out in
range) to preserve and our goal is to reject all other propagation
modes. In Figure 7b we see that all three unwanted modes have
been rejected to the level of the noise floor using the MVDR modeselective beamformer. The training data used in the MVDR beamformer comprised the range-Doppler cells in the range-Doppler
data for each waveform that contained the three unwanted modes.
The beamformer weights are such that unit gain is applied in the
direction-of-departure of the wanted mode and beamformer nulls
have been applied to the directions-of-departure of the unwanted
mode. The background noise in the range-Doppler map sets the
rejection level for the unwanted modes.
In the second example shown in Figure 7c, the 1F2l mode
(second mode out in range) is preserved while the remain-

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DECEMBER 2017

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