Phase II Final Report - NASA's Institute for Advanced Concepts

Planetary Exploration Using Biomimetics
An Entomopter for Flight on Mars
5.4 Scatterometers and Spectrometers
The communications/control subsystem could also potentially be used as a scatterometer.
Radars used to measure ground return are called scatterometers, because the ground return is
almost invariably due to scattering. Scatterometers capable of measuring a response over a wide
range of frequencies are called spectrometers. Scatterometers are used to measure the surface
reflectivity as a function of frequency, polarization, and illumination direction (angle of incidence)
of the sensing signal. The depolarization and spectral reflectivity of returned radar echoes
from a surface provide information about the roughness structure, geometric structure,
morphology, and dielectric constant of the surface and immediate subsurface [72]. The communications/control
subsystem and flight configurations could be reconfigured to exploit different
polarizations, angles of illumination and center frequencies. Single-frequency, single-polarization
radar measurements are useful, but use of multiple polarizations (particularly cross-polarizations)
and multiple frequencies significantly increase their value.
Radar ground return is described by σ 0 , the differential scattering cross-section, or scattering
coefficient (scattering cross-section per unit length), rather than by the total scattering cross-section
σ used for discrete targets [242]. Since σ varies with illuminated area and this is determined
by geometric radar parameters (pulse width, beam width, etc.), σ 0 was introduced to obtain a
coefficient independent of these parameters. Use of σ 0 implies that return from the ground is
contributed by a large number of scattering elements whose phases are independent. This is primarily
due to differences in distance that are comparable to many wavelengths.
Different wavelengths are sensitive to different elements on the surface. If the geometry of two
radar targets is the same, the returns would be stronger from the target with higher complex permittivity,
because larger currents (displacement or conduction) would be induced in it. Effective
permittivity for ground targets is very strongly influenced by moisture content, because the relative
permittivity of liquid water ranges from 60 at X-band to about 80 at S-band and longer
wavelengths. Most dry solids have permittivities less than 8 [242]. Attenuation is also strongly
influenced by moisture, because wet materials usually have higher conductivity than the same
materials dry. Thus, permittivity increases with moisture, and radar return increases with permittivity.
Qualitative information on surface roughness also can be determined. Relatively smooth surfaces
tend to reflect radio waves in accordance with Fresnel-reflection direction (angle of reflection
= angle of incidence), and so they give strong backscatter only when the look angle is nearly
normal to surface. Rough surfaces tend to reradiate almost uniformly in all directions, so they
give relatively strong radar returns in any direction. Scattering falls off more rapidly with angle
for smooth surfaces than rough.
Variations in amplitude of the returned signal can be converted directly into a scattering coefficient
[262]. Scattering coefficient also depends on polarization of transmitted and returned signals,
thus significant information about the target is contained in the ratio of the received likepolarized
and cross-polarized signals. The variation of scattering coefficient with angle of incidence
is different for different classes of targets. Thus, this information can be used for target
identification. Scattering versus angle of incidence can be measured with multiple flight passes
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Phase II Final Report