Aerospace and Electronic Systems Magazine December 2017 - 66

MIMO Methods Applied in Over-the-Horizon Radar
shows two ionospherically propagated paths with the same radar
range (i.e. same time delay) where one path propagates via the E
layer and one via the F layer. The take-off elevation angle for each
path is different and also differs for differing radar to target range.
Note that the location on the Earth surface (i.e. ground range) is
different for each propagation path with the same radar range.

EXPERIMENT DESCRIPTION
The Mode-Selective Radar experiment (MSR-I) involved testing
a limited capability MS-OTHR using an installation at the JORN
Laverton radar [52]. The test surveillance area was a region of
ocean approximately 500 km deep extending from the north-west
Australian coast out to sea between Pt. Hedland and Broome. The
distance between the two towns is approximately 500 km and the
coast is approximately 900 km from the experiment location. The
MSR-I radar configuration was limited in sensitivity and in steerdirection and overall coverage angle. The operating frequency
range was restricted to 11-13MHz and the MIMO radar subsystem
limited to a fixed K = 12 multiple-waveform configuration. The
radar used the existing JORN receive west-array and a purpose
built skew-fire transmit array installed at a site close to the current JORN transmit array. The skew-fire configuration is shown
in Figure 12.

Figure 12.

SKEW-FIRE ARRAY
There are potentially many antenna configurations that can support
the MS-OTHR concept [23]. In our experiment the azimuth and
elevation selectivity required was achieved using a new design we
have called a skew-fire array. This approach exploits the two-dimensional (2D) response of linear one-dimensional (1D) arrays and
the use of distinct transmit and receive arrays in (bistatic) OTHR. A
full description of the skew-fire array will be given in future work.
The experiment geometry (not to scale) showing the skew-fire
orientation of the transmitter array (109 elements total length 1,296
m axis heading 317°) and receiver array (480 elements total length
2,922 m axis heading 35°) is given in Figure 12. In that figure the
green line represents the coastline of North-West Australia. The
transmitter and receiver were approximately 100 km apart and it is
approximately 900 km from the receiver location to Pt. Hedland.
The shaded regions show that part of array coverage where coning
in the respective 1D arrays provides exploitable joint 2D selectivity in azimuth and elevation.

MODE-SELECTIVE RADAR MIMO SYSTEM
Conventional single waveform radar systems generate a scatterer
response function Sr(τ, ν) based on the waveform u(t) interaction
with the scatterer environment Sr(τ, ν) according to (convolution in
delay τ and Doppler ν)
S r (τ ,ν ) = S (τ ,ν ) ∗ χ u (τ ,ν )

(12)

where χu(τ, ν) is the Ambiguity Function of the waveform u(t) defined as
64

Skew-fire array configuration used in the Mode Selection Radar experiment.

χ u (τ ,ν ) =  u (t )u *(τ − t )e− j 2π f ν dt

(13)

The scatterer distribution S(τ, ν) encompasses all scatter sources
including (for the OTHR surface target case) ocean clutter, ionospheric clutter, and targets. It does not include contributions that
will ultimately enter the radar receiver from energetic sources that
are independent of the transmitted waveform.
The purpose of radar is to force
S (τ ,ν ) − S r (τ ,ν ) < e

(14)

for some small e so that the scatterer response function Sr well
approximates the scatterer distribution function S. It is the challenge of waveform design to ensure this is the case. In general, Sr
is also parameterized by direction (e.g. for the 1D azimuth only
case) so we can consider S r (τ ,ν ;φ ) although we ignore direction
dependence for the present.
In a practical radar, Sr(τ, ν) will include any noise and interference contributions external to the radar from energetic sources independent of the transmitted waveform, as well as radar instrument
imperfections such as system noise and nonlinear distortion (both
within the radar and between the radar and the external environment). It will be discretized and evaluated on a grid in delay and
Doppler to create range-Doppler maps (such as shown later in the
article). The range-Doppler map is then further processed to generate target detections according to some joint true target detection
and false target detection strategy.

IEEE A&E SYSTEMS MAGAZINE

DECEMBER 2017



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