Page 332 - Fundamentals of Radar Signal Processing

incidence. Other ranges may exhibit clutter seen through the antenna sidelobes,

called sidelobe clutter (SLC). SLC is weaker than MLC because it is observed
through the lower-gain sidelobes. It is also weaker than the AL due to longer
ranges and non-vertical incidence. SLC can potentially extend from Doppler
shifts approaching +2v/λ (directly ahead of the radar) to –2v/λ (directly behind
the radar), though the amplitude will drop rapidly due to increasing range at
those maximum Doppler shifts.

Figure 5.3b is a sketch of the notional MLC spectrum at the range bin
corresponding to the radar boresight, assuming the PRF is high enough to avoid
any aliasing. The MLC will be centered at F MLC = 2vcosψ MLC /λ, where ψ MLC is
the depression angle from the horizontal velocity vector to the radar boresight
vector. The width β MLC of the MLC will be approximately the sum of the
intrinsic clutter spectrum width β and the mainlobe widening due to platform
C
motion of β = 2vθsinψ MLC /λ, where θ is the antenna azimuth beamwidth in
D
radians. Figure 5.3c is a similar sketch for the altitude line, which would occur
at a closer range bin. Range bins nearer than that of the AL can contain only
noise.
Figure 5.4 is a more detailed simulated range-Doppler clutter spectrum for
a down-looking airborne radar viewing flat homogeneous ground clutter. The
aircraft is traveling level at an altitude of 10,000 feet (3048 m) and a velocity of

300 mph (134.1 m/s). The RF is 10 GHz. The radar antenna has a 2.5°
beamwidth, and its boresight is scanned 45° clockwise (right) of the velocity
vector in azimuth and 10° down in elevation. The boresight will then intersect
the ground at a range of 17.3 km. The component of velocity along the boresight
(i.e., vcosψ MLC ) is 93.4 m/s, giving a Doppler shift at the center of the mainlobe
clutter of 6.23 kHz. The maximum clutter Doppler shift that could be observed
is ±89.4 kHz, corresponding to relative velocities of ±134.1 m/s, the aircraft

velocity.