Aerospace and Electronic Systems Magazine March 2018 - 34

Experimental Analysis of a HF Hybrid Sky-Surface Wave Radar

Figure 4.

The geometric model of HFSSWR.

pulse compressed, and then we can get the amplitude of echoes as
a function of frequency and group delay over a one-way point-topoint circuit. The OIS transmitter is located in the radar transmitting
station, and the OIS receiver is located near the receiving array. The
ionospheric reflection point of the sounder is close to that of the
radar. From the perspective of signal processing, the most important
function of an OIS is to help to select frequencies so as to avoid the
negative influence arising from multimode propagation.

COORDINATE REGISTRATION
The coordinate registration is the process of converting tracks from
radar coordinates to geographic positions. This problem is more complicated under the HFSSWR. The geometric model of this radar is
shown in Figure 4, where the curvature of Earth is not considered for
simplicity. In Figure 4, L is the baseline length, Ra is the ground range
between the target and the receiver, θt is the angle between Ra and the
baseline, h is the virtual height of the ionospheric reflection point, and
D is the ground range between the transmitter and the target. According to the geometric relationships, the following two equations can be
obtained by the Pythagorean theorem and the law of cosines:
R12 = h 2 + ( D 2 )

2

D 2 = L2 + Ra2 − 2 LRa cosθ t

(1)
(2)

The range we can measure from the time delay is R = R1 + R2 + Ra.
The path R1 is the same as R2 under the assumption that the ionosphere
is not inclined. Combining the preceding equations, the positioning
relationship of the radar system can be obtained as follows [13]:
Ra =

R 2 − 4h 2 − L2
2 ( R − L cos θt )

(3)

If there are more receiving stations, the estimation accuracy of the
target location can be further improved.

FIRST-ORDER SEA CLUTTER FREQUENCY AND
BROADENING CHARACTERISTICS
The sea clutter for HFSSWR is more complicated than that for HF
skywave radar or HF surface wave radar. Because of the special
34

Figure 5.

The Bragg scattering geometry of bistatic radar. (a) Azimuthal plane. (b)
Elevation-angle plane.

system layout, the first-order sea clutter spectrum characteristics
are not only related to oceanic dynamics, beamwidth, and operating frequency but also influenced by grazing angle, bistatic angle,
and the ionosphere. The combined influences of various factors severely broaden the first-order sea clutter spectrum, which deteriorates the detection probability of low-velocity vessels. The study of
clutter characteristics is beneficial to the development of a clutter
suppression algorithm.

FIRST-ORDER SEA CLUTTER FREQUENCY
The Doppler frequency of first-order sea clutter in bistatic radar
can be calculated by the equation [16]
fb = ±

g

πλ

cos ( β 2 )

(4)

where g is the gravity acceleration, λ is the radar operating wavelength, and β is the bistatic angle. The Bragg resonance condition
of HFSSWR is shown in Figure 5, where the blue arrows and the
red arrows are the incident wave and the reflection wave, respectively. Taking elevation angle into account, the Bragg resonance
condition of HFSSWR is
Lsea ( cos Δ i + cos Δ s ) cos ( β 2 ) = mλ

IEEE A&E SYSTEMS MAGAZINE

m = 1, 2,3,

(5)
MARCH 2018



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