Aerospace and Electronic Systems Magazine December 2017 - 56

MIMO Methods Applied in Over-the-Horizon Radar

Figure 1.

Figure 2.

minutes apart, during which time ionospheric propagation conditions at this operating frequency had changed. In the former, a ship
target is easily detected at midrange, well separated from the ocean
clutter, while in the latter, the ionospheric motion due the changed
ionospheric propagation conditions leave the target completely obscured by spread-Doppler Earth return clutter.
The key difference between the measurements shown in Figures
1 and 2 is that in the first figure the propagation path from radarto-target and return is a single ionospheric propagation path (the Elayer) with low Doppler-spread and so the target and clutter are well
separated in Doppler. The ionosphere has introduced little distortion
and a radar operator would consider these to be excellent propagation conditions for detecting surface vessels. In the second figure,
an additional propagation path via the F-layer is present, and this
path has significant Doppler shift (known as ionospheric Doppler
shift) and Doppler-spread which has spread the ocean clutter return
in Doppler by an amount comparable to the Doppler of the target and
so the target is obscured by clutter and cannot be detected.
An obvious solution to this so-called slow-target in disturbed
clutter problem is to design a radar that separates the various propagation paths when multi-path conditions exist such as shown in
Figure 2. In such a radar, it is expected then, that at least one of the
separated propagation paths will enable satisfactory target detection free from clutter.

OTHR are typically bistatic (or, quasi-bistatic) with separate
transmitter and receiver systems. In a multilayered ionosphere
consisting of both an E-layer and F-layer [45] there can be four
possible propagations paths between radar and target (E-E, F-F,
and the mixed cases E-F and F-E). More significantly, and due to
the use of reflection via the ionosphere, each of these paths has
a particular and different elevation geometry for each range in
the range-gate of the radar, and hence a different elevation spatial selectivity requirement in order to isolate the various propagation paths. In addition, propagation mode separation cannot be
achieved using spatial selectivity (i.e. elevation beamforming) at
only the receiver or the transmitter system alone, but requires a
joint transmit-receive solution. For example, for the case of a Doppler disturbed F-layer, the E-F, F-E, and F-F propagation paths will
be corrupted, with the E-E path being the remaining satisfactory
path. Yet, E-E and E-F have very similar elevation response at the
transmitter and E-E and F-E at the receiver, so both the transmitter
and receiver will require E/F elevation selectivity jointly, and this
differs for each range cell in the radar range gate. A range-dependent multibeam transmit beamformer can only be achieved with
MIMO techniques. This is shown diagrammatically for one-way
paths in Figure 3. The diagram shows the propagation geometry
for a simplified two-layer ionosphere, with layers labelled E and
F, and for propagation to two different radar ranges, labelled 1 and

Range-Doppler (RD) map showing ocean clutter returns due to Bragg
scatter at all ranges and a slow-moving target at approximately midrange at
higher Doppler than the clutter return (red corresponds to higher radar return energy with greater than 50 dB dynamic range shown in the RD map).

Range-Doppler map showing ocean clutter returns due to Bragg scatter
at all ranges measured via an ionosphere with Doppler-spread slightly
greater than the target Doppler.

Figure 3.

The propagation geometry for a
simplified two-layer ionosphere
and for propagation to two different radar ranges.

54

IEEE A&E SYSTEMS MAGAZINE

DECEMBER 2017



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