Laboratory Measurements of Radar Transmission through Dust with Implications for Radar Imaging

Introduction: As NASA's exploration plan for Mars advances with additional missions, the type and abundance of data increase. High-resolution images are revealing numerous wind-related features and suggest widespread distribution of aeolian deposits. These deposits reflect the current and past wind regimes on Mars, but they also obscure some of the bedrock geology. A complementary experiment to those already proposed for Mars would be a Synthetic Aperture Radar (SAR) mission [1]. Because such a mission is constrained by antenna size and power, it is important to understand radar penetration through aeolian materials such as sand and dust so that the most effective radar frequencies would be used. Background: There have been many studies of radar penetration of sand in arid areas such as the Sahara Desert [2,3] and the U.S. Southwest [4]. The earliest works showed that L-band (λ=24 cm) radar was able to penetrate 1-2 meters of extremely dry sand [e.g. 3] to return images of the subsurface obscured by the sand. These radar images of the subsurface revealed ancient river channels that existed prior to the Sahara Desert [2] and have greatly added to the understanding of the climatic history of that region. A previous study of the ability of radar to penetrate sand involved laboratory measurements of the attenuation of a radar signal as it passed through quartz sand [5]. That experiment was conducted over a frequency range of 0.5-12.6 GHz and for moisture contents of 0.3-10.7 % [5]. Results showed that the lowest frequency (longest wavelength) signals penetrated through dry sand with an attenuation of less than 5 dB/m. Those frequencies enable deeper subsurface imaging but require a larger antenna. The experimental results for higher frequencies showed higher attenuations and thus less ability to penetrate. However, depending on the sensitivity of a Mars SAR, the higher frequencies could be used to penetrate several meters of sand using a smaller antenna. The results of [5] provide information on the ability of radar to penetrate sand over a large frequency range; however, a Mars radar imaging system could also penetrate dust (i.e., possibly 20 μm). Although some dust deposits on Earth have been covered in radar images, most radar penetration studies of geologic materials have focused on sand. Because a large part of Mars could be mantled by dust, it is important to understand the ability of radar to penetrate dust. Experimental method: The Electromagnetic Anechoic Chamber at Arizona State University was used to measure the radar signal transmitted through a sample container holding the dust (Fig. 1). The base of the container was made of plywood and the sides were made of Plexiglas. The radar emitter was supported above the sample container and the receiver was under the container. The transmitted signal was measured over four frequency ranges: 0.5-2, 2-6, 6-8, and 8-12 GHz. 801 measurements were made over each frequency range. Transmission was measured with the empty sample container in place and at dust thickness levels of 10, 20, 30, and 40 cm.