Aerospace and Electronic Systems Magazine March 2018 - 9

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conventional SIMO system, which has been demonstrated by the
coastal HFSWR systems in [16] and [17]. The clutter spectrum
will be almost the same as the SIMO result shown in Figure 5b but
with better spatial resolution.
Figure 5c presents a comparison of optimal signal-to-interference and noise ratio (SINR) loss for a target at the array boresight
detected by the monostatic shipborne HFSWR under SIMO and
MIMO configurations. The notch width of the MIMO system is
narrower than that of the SIMO system, which means it will be
easier for the MIMO shipborne HFSWR to detect a target whose
Doppler is close to sea clutter Doppler. In addition, the target angular resolution and localization accuracy will be improved when
using the MIMO configuration.

BISTATIC COASTAL TRANSMITTER AND SHIPBORNE
RECEIVER

Figure 6.

Bistatic HFSWR coastal transmitter and shipborne receiver as SIMO.
(a) System configuration. (b) Clutter spectrum. (c) SINR loss.

target discrimination performance (i.e., the ability to separate target Doppler and sea clutter Doppler) of HFSWR, a long antenna
array is desired. Unfortunately for the shipborne system, the array
aperture is strictly limited by the physical length of the ship platform. One solution to increase the array aperture size is to place
two transmitting antennas at the fore and aft of the ship platform
to form a MIMO system, as depicted in Figure 5a. In this way,
the effective array aperture size can be doubled compared to the
MARCH 2018

Next, we consider a more complicated configuration, in which the
transmitter is at the coast but the receiver is installed on a moving
ship platform. We still assume the transmitter contains one antenna and the receiver contains an eight-element ULA, which is a
bistatic SIMO configuration, as shown in Figure 6a. We arbitrarily
choose a moving direction for the shipborne receiver, e.g., 45°
w.r.t. the x-axis. Figure 6b illustrates the clutter spectrum. In this
configuration, because the transmitter is stationary and the receiver moving direction overlaps the array axis, the clutter spectrum
will always be simple (like the monostatic case shown in Figure
5b, but with different curvature), no matter which direction the
receiver is moving toward. The SINR loss, for the assumed target
at (100, 80 km), shown in Figure 6c only has two deep notches,
which is also similar to Figure 5c. Both have a good usable Doppler space fraction (UDSF) for target detection. However, in this
SIMO configuration, only a large ship platform can be used to
accommodate the long receiving array system, which is not favorable in practical applications.
Alternatively, we can choose to set up a stationary long transmitting array on the coast and a single receiving antenna on the
ship platform, as plotted in Figure 7a. This MISO configuration
is the concept proposed in [12]. The advantage is that a small boat
carrying a simple and cheap HF receiving antenna will be capable
of detecting targets beyond the line of sight. For this configuration, some preliminary analyses on sea clutter distribution were
presented in [20]. Unfortunately, the study did not include the effects of array axis orientation. In a general configuration in which
the receiver's velocity direction is not aligned with the transmitting
array axis, a complicated sea clutter distribution will be produced.
Here, it is assumed that the transmitting array consists of eight
antenna elements along the y-axis and the receiver consists of a
single antenna and still moves toward 45° w.r.t. the x-axis. Figure
7b illustrates the clutter spectrum. Compared to the SIMO case of
Figure 6b, it can be seen that two more clutter ridges were generated because of misalignment of the receiver moving direction and
transmitting array orientation. This effect is equivalent to that in
the forward-looking configuration, in which the clutters from the
array frontlobes and backlobes have different Doppler frequencies,
while in the previous SIMO (side looking) case, the clutters from

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Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine March 2018