Aerospace and Electronic Systems Magazine December 2017 - 69

Frazer
region (range 850-1,300 km and 0.1-0.7 Hz Doppler) and in the
region (range 1,100-1,500 km and -0.4-0 Hz Doppler). While no
targets are present, it is typical that surface vessels could be within
the range-Doppler regions identified as contaminated by ionospheric clutter.
Mode-selective processing has been applied to create four additional range-Doppler maps S rp (τ ,ν ) for p = 1,..., 4 (see a Figures
13b-e). These have been processed using the range-dependent classical beamformer described in the previous Mode-Selective Radar
MIMO System Section where in each case the beamformer applied
is selected to achieve mode-selection.
In the following, we retain the notation 1E for the one-hop Emode path and 1F2l for the one-hop F2 low-ray path. The plots in
Figures 13b-e correspond to the 1E-1E, 1E-1F2l, 1F2l-1E, and 1F2l1F2l mode combinations.
Figure 13b shows the backscattered clutter for the mode-selected 1E-1E path. The land and ocean clutter show a concentrated
land-sea boundary at a range of 1,010 km. Clutter returns from
grating lobes in the spatially under-sampled transmit aperture are
apparent at (0.4-0.7 Hz and 920-1,000 km). The grating lobe
clutter is due directly to the under-sampled transmit array and the
chosen 1E steer direction on transmit. Apart from the grating lobe
clutter this range-Doppler map is typical of that attained with a
conventional OTHR for the case of single mode ionospheric propagation and is a significant improvement on Figure 13a.
Figure 13c shows the backscattered clutter corresponding to the
mixed mode-selected case of 1E-1F2l. Ionospheric Doppler shift as a
function of range is present on the 1F2l path. The 1E-1F2l case retains
the grating lobe clutter due to the under-sampled transmit array with
transmit steer direction directed at the 1E path. This clutter is not apparent on the second of the mixed mode cases since the receive array
has very different sidelobe properties than the transmit array. The
concentrated land-sea boundary is at 1,090 km range compared with
Figure 13b due to the additional path length of the 1F2l path through
the ionosphere to the land-sea boundary versus the 1E path.
The case in Figure 13e shows the backscattered clutter corresponding to the mode-selected 1F2l-1F2l path. The land-sea boundary
is at further range than before (at 1,170 km range) due to the two-way
longer 1F2l path. The ionospheric Doppler shift varies with range.
Target detectability is improved in each of the four mode-selection cases compared with the classical OTHR case of Figure 13a.
A key point in a MS-OTHR is that all four range-Doppler maps
are created simultaneously from a single coherent radar measurement and then subsequently processed independently to generate a
single joint-mode track solution (for targets only and not shown in
this clutter only example). Propagation mode diversity is a major
advantage and we note that propagation temporal fading effects
occur differently across modes so that MS-OTHR has improved
robustness to ionospheric fading.

CONCLUSION
This article reports the first demonstration of mode selectivity on
transmit for a one-way ionospherically propagated signal using an
adaptive transmit array with sufficient aperture. We have shown
using adaptivity on transmit that mode-selection/rejection can be
DECEMBER 2017

achieved on representative ionospheric propagation paths by controlling the illumination. It is possible to preserve a chosen mode
and reject all other modes and this can be applied for each mode simultaneously. Mode-selection beamformer solutions generated at
one range are ineffective for mode-selection/rejection at different
ranges. Effective mode-selection/rejection beamformer solutions
need to be range dependent. Mode-selection beamformer solutions generated at one azimuth are ineffective for mode-selection/
rejection at different azimuth but with comparable range. Effective
mode-selection/rejection beamformer solutions vary with azimuth
and interact with the sidelobe characteristic of the transmit antenna. This is an important issue in the case of distributed scatterer
returns such as earth return clutter in OTHR.
The temporal stability of a particular mode-selection/rejection
beamformer solution is poor and fixed mode-rejection beamformer
solutions have reduced performance. Adaptive beamformer solutions are required for the radar signals at the time of transmission
to sustain beamformer performance in the presence of the dynamic
ionosphere. The mode take-off elevation angle determined using
OIS data compares well with direction-of-departure estimates
determined using the MISO transmit array. Measurement of the
practical limit of aperture resolution for the array used agrees well
with the directly calculated Rayleigh limit and the chosen aperture
of 1,200 m has proven sufficient for mode-selection/rejection as
required for a Mode Selective OTHR.
Demonstration of the energy budget penalty associated with
MIMO is supported by a comparison of the respective MISO
and SISO energy budgets. Minor reconfiguration of our test configuration allowed us to confirm previous results showing modeselectivity on receive is achievable provided the array aperture is
sufficient.
The Mode Selection Experiment has generated numerous results and provided essential insight into the design of a Mode-Selective OTHR that we have constructed and provide initial results.
We have shown that a Mode-Selective OTHR can be realized
using MIMO transmit techniques and a skew-fire OTHR architecture. Total surveillance coverage area is reduced with the skewfire design compared to classical OTHR coverage however this
coverage area is still substantial. A limited mode-selective system
has been built and tested and the results have shown that effective
mode-selection can be achieved.

ACKNOWLEDGMENTS
The author thanks his research coworkers and the staff of HighFrequency Radar Branch, in the National Security and Intelligence,
Surveillance and Reconnaissance Division, of the Defence Science
and Technology Group, Department of Defence, Australia, for the
custom equipment and the installation, operation and deconstruction of the experiment.

REFERENCES
[1]

IEEE A&E SYSTEMS MAGAZINE

Brookner, E. MIMO radar demystified and where it makes sense to
use. In Proceedings of the 2014 IEEE Radar Conference, May 2014,
0411-0416.

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