Time-Frequency Analysis of Micro-Doppler Phenomenon - Time-Frequency Transforms for Radar Imaging and Signal Analysis - page 181

the same data but in a different short-time interval and at the range cell 20.
Superposition of the time-frequency signatures over several range cells, which
correspond to different walking steps, gives a full time-frequency signature
of the walking man as shown in Figure 8.4(f ). We can see that the body's
Doppler shift is almost constant but the arm's micro-Doppler shift becomes
time-varying and has a sinusoidal-like curve. Again, from the additional time
information, the swinging rate of the arm can be estimated and is about
1.2 cycle/sec in this example.

8.2 Rotation-Induced Micro-Doppler

Modulation induced by rotation structure (such as rotor blades of a helicopter,
propellers of an aircraft, or rotating antennas on a ship or an aircraft) can
be regarded as a unique signature of the target. The micro-Doppler signature
becomes an important feature for identifying the target of interest. Here,
we examine micro-Doppler induced by rotor blades of a helicopter, show
the time-domain and the frequency-domain signatures of the micro-Doppler,
and introduce the time-frequency signature of the micro-Doppler induced
by rotor blades.

8.2.1 Rotor Blade Motion
In a helicopter, the main rotor blades, the tail rotor blades, and the hub
have unique signatures suitable for target identification. Generally, radar
returns from a helicopter are back-scattered from the fuselage, the rotor
blades, the tail blades, the hub, and other structures on the helicopter. The
motion of the rotor blades depends on the interdependent coupling between
the aerodynamics and the rotor dynamics [5]. Each blade is a rotating aerofoil
having bending, flexing, and twisting. The radar cross section of a segment
in the blade depends upon its distance from the center of rotation, its angular
position, and the aspect angle of the rotor [6, 7].
Because we are especially interested in electromagnetic back-scattering
from the main rotating blades of a helicopter, for simplicity, the rotor blade
is modeled as a rigid, homogeneous, and linear rod rotating about a fixed
axis with a constant rotation rate. No flapping, lagging, and feathering are
considered for the calculation of electromagnetic back-scattering.

8.2.2 Radar Returns from Rotor Blades
As illustrated in Figure 8.5, the radar is located at the origin of the radar
coordinates ( X , Y , Z ); reference coordinates ( x , y , z ) are translated from