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

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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 264 Phase II Final Report
Chapter 5.0 Potential Payload Functions Using a Communication/ Control Subsystem along different flight paths at different altitudes using one or more Entomopters. Some radar scatterometers are specifically designed to measure scattering coefficient as a function of angle of incidence, because they have a beam pointed ahead of and behind the aircraft, such that the scattering coefficient can be measured at a variety of angles as the aircraft flies over the target. These can only illuminate along a narrow track, so numerous flights would be needed to get area coverage. Alternatively, the elements of the sunflower antenna could be switched on and off to scan a larger area compared to a single element. 265
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Planetary Exploration Using Biomime
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Table of Contents Table of Contents
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List of Tables List of Tables Table
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List of Figures List of Figures Fig
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List of Figures Figure 3-54: Pressu
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List of Figures Figure 3-138: Stere
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List of Figures Figure 4-24: Averag
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List of Contributors List of Contri
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Executive Summary Executive Summary
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Chapter 1.0 Introduction Chapter 1.
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Chapter 1.0 Introduction Figure 1-2
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Chapter 1.0 Introduction 1.1 Histor
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Chapter 1.0 Introduction 1.2 Origin
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Chapter 1.0 Introduction 1.3 Missio
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1.3.3.1 Surface Imaging The Entomop
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Chapter 2.0 Entomopter Configuratio
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Chapter 3.0 Vehicle Design 3.1 Wing
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3. Density: 1.40E-2 kg/m 3 4. Press
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Chapter 3.0 Vehicle Design 3.4 Reci
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Chapter 3.0 Vehicle Design 3.5 Fuel
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Chapter 3.0 Vehicle Design 3.6 Powe
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Page 291 and 292: Chapter 7.0 Conclusion Chapter 7.0
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Page 307 and 308: Appendix B: Sizing Results Appendix
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Phase II Final Report - NASA's Institute for Advanced Concepts

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