Discrimination between road and soil surfaces using CO2 laser reflectances

ABSTRACTIt is well known that CO2 laser reflectances of natural surfaces containing certain minerals, notably quartz, feldspar and kaolinite, exhibit differential reflectance features in the 9-11 jim mid-infrared spectral range. The useof off—normal reflectance ratios using measurements at four CO2 laser wavelengths to differentiate between varioustypes of soil surfaces has been established. Off-normal reflectance ratios are observed to be relatively independentof incidence angle compared to ratios computed at normal incidence, which makes them suitable for field remotesensing applications. Road surface materials, such as concrete and asphalt, contain large quantities of quartz, and assuch exhibit reflectance characteristics similar to soils. Our measurements indicate that it is, nevertheless, possibleto discriminate between road and soil surfaces using off-normal reflectance ratios. Discrimination from soil surfacesis better for concrete compared to asphalt. 1. INTRODUCTION One of the major goals of remote sensing is the ability to discriminate between broad classes of targets. In order todo this, the reflectance features of various target types need to be characterized in the appropriate wavelength range,either through appropriate models or actual measurements. Algorithms or techniques can then be developed fortarget discrimination or classification based on the knowledge of unique reflectance features. In battlefield scenarios,there exists a need to be able to differentiate between soils and road surfaces, in addition to conventional targetdetection functions. Many laser radars are currently in operation, or under development, for various tactical militaryapplications. As is well known, in comparison to conventional microwave radar systems, laser radar systems canemploy much smaller aperture diameters, possess almost four orders of magnitude less beam divergence, and almosteight orders of magnitude less spot size.1 CO2 lasers operating in the 9-11 pm wavelength range have been usedin a variety of remote sensing applications due to a unique combination of desirable features such as operation inthe 8-12 pm atmospheric transmission "window," moderate power outputs and efficiencies, emergence of rugged andcompact systems, and wavelength tunability.2 In addition, recent measurements have indicated the possibility ofutilizing CO2 laser reflectances to determine the mineralogical composition of the earth's surface.3'4Our prior investigation into the CO2 laser reflectance of eighteen benchmark soil samples indicated that thesoils could be classified based on their off-normal differential reflectance characteristics.5 Specifically, we have shownthat the ratios of reflectance measured at different wavelengths at off-normal incidence angles (> 20") are relativelyinsensitive to variations in incidence angle, and can thus be used for remotely identifying the soils investigated. Inthis paper, we extend the above results to assist discrimination between soils and road surface materials, such asconcrete and asphalt. In the following section, a brief description of the experimental set—up is provided. Next, themeasured reflectance characteristics of concrete and asphalt samples in the 9-11 jim range are discussed. We thencompare the off-normal reflectance ratios of concrete and asphalt to those of various soil types, and discuss how thesecan be used for discrimination purposes. Conclusions are presented in the final section.370