Polarimetric signatures of terrain features and man-made objects have been measured using unique Direct Detection
Polarimetric Radiometers (DDPR). The DDPRs are lightweight inexpensive systems operating at 35 and 94 GHz. Each
system consists of a single antenna, amplifier, and a truncated cylindrical waveguide that directly measures Q, U, and V.
The highly portable DDPRs are ideal for obtaining the Stokes vectors needed to study the physical characteristics of
natural and man-made features. Field evaluations using the DDPR systems include measurements from an airborne
platform over different terrain features and water, and ground based measurements of the polarimetric signature of grass,
asphalt, buildings, and concealed munitions. The DDPR can function as a bistatic system by using an active source of
polarization. Using this configuration and a soil chamber, we have investigated the effect of soil type and soil moisture
on linear and circular polarization. This report will describe the DDPR and present the analysis of the airborne and
ground based measurements, including the effects of soil type and soil moisture on sources of linear and circular
Soil properties have a significant impact in the observed responses of various sensors for mine detection. Ground
penetrating radar (GPR) is an important sensor for mine detection. The performance GPR is largely governed by the soil
moisture content. Characterizing the spatial and temporal changes in the dielectric properties of soil surrounding the
landmines represents a major challenge for radar evaluation studies. Laboratory and field studies are currently in
progress to better document the effect of soil moisture variability on radar sensing of buried landmines. These studies are
conducted using commercially available GPRs operating at 400 MHz and 1.5 GHz. The study site is a government mine
test facility with various anti-tank (AT) and anti-personnel (AP) mines buried at different depths. The test lanes at this
facility are grass-covered and the sub-surface root system plays an important role in modulating the soil properties. Our
goal is to investigate the seasonal changes in soil processes at this site and to document how these processes impact the
radar signatures of landmines.
Soil properties make a significant impact in the observed responses of various sensors for subsurface target detection.
Ground penetrating radars (GPRs) have been extensively researched as a tool for subsurface target detection. A key
soil parameter of interest for evaluating GPR performance is the soil attenuation rate. The information about the soil
attenuation rate coupled with target properties (size, shape, material properties and depth of burial) can be used to
estimate the effectiveness of radar sensors in a particular soil environment. Radar attenuation in desert soil is of interest
in today's political and military climate. Laboratory measurements of desert soil attenuation were conducted using
samples collected from a desert in Southwestern United States and in Iraq. These measurements were made in a coaxial
waveguide over the frequency ranging from 250 MHz to 4 GHz. The soil grain size distribution, mineralogy, moisture
and salinity were also measured. This report describes the experimental procedure and presents the radar attenuation
rates observed in desert soils. The results show that the soluble salt content is an important parameter affecting the
attenuation behavior of desert soils.
Soil properties make a significant impact in the observed responses of various sensors for mine detection. Soil moisture affects the performance of electromagnetic sensors through its effects on soil thermal and dielectric properties. We have initiated laboratory, field and numerical studies to advance our fundamental understanding of the properties and governing processes of moisture distribution and flow around buried landmines. The laboratory component features magnetic resonance imaging (MRI) to map water distribution around a mine-like obstacle placed in a test soil sample. The field component investigates the moisture migration around landmines under realistic weather and soil conditions. We use anti-tank mines instrumented with moisture and temperature sensors to monitor the weather-driven processes. The numerical component investigates existing physics models underlying current simulations of moisture transport in soils. We use existing flow simulators to evaluate the completeness of process descriptions and to estimate the relative importance of individual processes on micro-scale moisture movement. These existing simulators include both continuum codes designed to work at scales much larger than the grain size and pore-scale models that discretize individual pores. We present the preliminary results of our investigations and discuss the potential impact of our findings on infrared and radar detection of buried landmines.
Creating a minefield requires disturbing the soil. This disturbance alters the soil properties and processes in a measurable way. The U.S. Army is investigating techniques to exploit the altered properties of disturbed soil to assist in the detection of buried landmines. The differential quartz reststrahlen signatures between disturbed and undisturbed soil at the long wave infrared (LWIR) region have shown promise in past field tests.(1,3)We have initiated ground-based measurements using a non-imaging spectral sensor to investigate the phenomenology of LWIR disturbed soil signature. Our primary goal is to develop rainfall-dependent models to predict the degradation of the differential reststrahlen signature for varying soil types. A bare soil test site with strong quartz reststrahlen signature was selected for our initial investigation. The disturbed and undisturbed soil spectral signatures at the LWIR regions were obtained after multiple rain events using a Design and Prototypes field portable Fourier transform infrared (FTIR) spectrometer. The intensity and total amount of rainfall were recorded using a high-resolution tipping-bucket rain gauge. In addition to these measurements, photomicrographs of the disturbed soil were obtained after rainfall events, and X-ray diffraction analyses were conducted to obtain detailed soil mineralogy of the test site. We present these results and discuss the changes in the spectral characteristics of disturbed soil as a function of rainfall amount and intensity.
A potential strategy for wide area airborne mine/minefield detection is to identify localized areas of soil that have been disturbed due to mine emplacement amidst the undisturbed soil. Disturbed and undisturbed soils are rough in varying degrees and this roughness affects the backscattering behavior at the microwave frequencies. We investigated the feasibility of using high-frequency radar (8-18 GHz) backscatter measurements to detect the residual surface disturbances caused by recent mine emplacement. Radar backscatter measurements from recently buried landmines were obtained at a government minefield data collection site. Case studies of radar backscatter from landmines buried in dirt and gravel for varying incident angles are presented. These results demonstrate that the surface roughness contrast between disturbed and undisturbed soils can be exploited to assist in mine detection operations. The maximum radar backscatter contrast between the disturbed and undisturbed soils was observed at normal incidence. The minimum contrast (radar backscatter crossover angle) occurred between 15 and 30 degree incident angles. These experimental results are shown to be consistent with rough surface scattering assumptions.
The ability to detect buried land mines under a wide variety of environmental conditions is an important Army requirement. Both for interpreting signatures of mines and to ensure appropriate modeling of mine and background signatures, it is important to understand the phenomena that result in different signature patterns. The dynamic signatures can change quickly in time due to changing meteorological conditions and their impact on the mine, the soil, and on the mine-soil interaction. In field tests, infrared measurements of surface and near surface mines have shown anomalous concentric thermal signatures around the mine. The cause of these irregularities is not known. We conduct numerical multidimensional finite element calculations to investigate interactions between the meteorological conditions, the mine, and the nearby soil to elucidate the cause for these signatures. Both in-situ temperature measurements and model results show that thermal interactions between the mine and the soil are responsible for the signatures. The warm area around the mine in the nearby soil is predominant primarily at night. The warm ring effect is most likely to exist in dry soil and for mines whose heat capacity exceeds that of the soil, resulting in thermal dominance of the mine in the coupled mine-soil thermal regime. Wet soils are less likely to display the thermal contrast of the warm ring. Improved understanding of physical interactions between the mine and the background may facilitate improved discrimination between signatures of mines and of false alarms.
NASA's Earth Science Enterprise has identified the need for improved measurement of snow properties and frozen soils via a space-flight mission within the next decade. Microwave sensors appear ideal to measure these properties. Measurements of the Earth's surface in the microwave spectral regions can be largely insensitive to weather conditions and solar illumination, which is especially important during cold seasons. Both active and passive microwave sensors have demonstrated sensitivity to snow properties and the freeze/thaw status of soils. Microwave signal response is influenced by snow depth, density, wetness, crystal size and shape, ice crusts and layer structure, surface roughness, vegetation characteristics, soil moisture, and soil freeze/thaw status. These characteristics make microwave remote sensing attractive for providing spatially distributed information to improve and update land surface models for cold regions, either through assimilation of state-variable information estimated from microwave remote sensing observations using inversion algorithms, or through direct assimilation of microwave remote sensing data themselves. At the same time, the sensitivity of microwave signal response to several snow, soil, and vegetation characteristics also complicates the interpretation and analysis of these data. To better understand microwave remote sensing for measurement of snow and frozen soil properties, NASA is conducting the Cold Land Processes Field Experiment (CLPX). The CLPX is a large field experiment being conducted primarily over a two-year period (2002 and 2003) in Colorado, U.S.A. The purpose of the CLPX is to develop the quantitative understanding, models, and measurements necessary to extend our local-scale understanding of water fluxes, storage, and transformations to regional and global scales. Of particular importance is the development of a strong synergism between process-oriented understanding, land surface models and microwave remote sensing. Objectives of the CLPX include evaluation and improvement of algorithms for retrieving snow and frozen soil information from active and passive microwave sensors, evaluating the effects of sensor spatial resolution on retrieval skill, coupling forward microwave radiative transfer schemes to distributed snow/soil models to improve assimilation of microwave remote sensing data, and to develop sensor specifications for a new space-flight mission to measure cold land processes. This paper discusses the data sets collected during the CLPX-2002 to support these objectives.
An ultra-wideband frequency modulated continuous wave (FMCW) radar was used to detect plastic anti-personnel (AP) mines. The study was conducted sign AP mines that were flush buried in a test bed prepared form crushed gravel. The ultra- wideband radar resolved the signals reflected from the top and bottom surfaces of the AP mines. It was not possible to detect these mines using the surface reflection as a detection threshold because of the high ground clutter. However, the ability to detect these mines improved greatly when the sub-surface reflections were used as a detection threshold. The study demonstrated that an ultra-wideband FMCW radar can be used to reject the ground clutter and to detect the AP mines buried at shallow depth.
We are investigating the environmental effects on radar detection of simulant mines (SIMs). SIMs are standard test targets developed by the US Army Project Manager-Mines, Countermine and Demolitions, and VSE Corporation for testing and evaluation of mine detection equipment. These test targets are filed with RTV silicone rubber, which has similar dielectric properties as TNT and Composition B. Therefore, they interact with radar sensors in a way representative of live mines. We are using broadband frequency modulated continuous wave (FMCW) and impulse radars to obtain signatures of SIMs buried under controlled laboratory conditions and at a test site instrumented with sensors to monitor the state of the ground. We find that anti-tank SIMs buried in frozen soil, in our case a common, silty sand are easy to detect. The dominant resonances included within SIMs by a broadbeam, 1.5 GHz impulse radar are of-nadir responses that appear unique and not predictable by simple ray theories of diffraction. A narrow beam, 2-6 GHz bandwidth FMCW radar induced reflections from the top and bottom of SIMs that were clearly resolved due to the broad bandwidth of the FMCW radar.
Several studies are under way at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) to define environmental effects on detection and classification of buried mines and unexploded ordnance (UXO). Ground that is very wet, frozen, or snow covered can pose severe constraints on demining operations. The qualitative and quantitative nature of chemical signatures of buried land mines is being documented. Research to date indicates that although 2,4,6- trinitrotoluene constitutes over 99% of military-grade TNT, it is a minor component of the vapor signature at ground level. CRREL operates a year-round test site to determine the effect of weather on radar and IR systems used to detect buried mines. The New England site experiences many of the weather conditions likely to interfere with mine detection around the world. Short-pulse ground penetrating radar (GPR) was used to profile both ordnance and non-ordnance targets at the 40-acre UXO site at Jefferson Proving Ground. Analysis of the data indicates that future systems will have to operate at faster data acquisition rates. Radar modeling is being used to simulate the effects of the environment and identify new techniques for finding and classifying buried ferrous objects.