Andrew J. Blanchard

Soil Moisture Information And Thermal Microwave Emission

This paper presents theoretical and experimental results that demonstrate the depth to which soil moisture can be directly measured using microwave radiometers. The experimental results also document the effect of uniform surface roughness on the response of thermal microwave emission to soil moisture. Experimental measurements were executed in July 1980 at the Texas A&M University Research Farm near College Station, TX. Thermal microwave emission measurements were made at 1.4, 4.9, and 10.7 GHz at both vertical and horizontal polarization at off nadir angles from 0 to 50°. It has been demonstrated that passive microwave measurements at frequencies down to 1.4 GHz can only measure soil moisture directly to very shallow soil depths, approximately 2 cm. This is due to the fact that the soil moisture dependence of the transmission coefficient across the air-soil interface predominates over the soil moisture dependence of the total energy originating within the soil volume. It also has been demonstrated that the combination of low incident angle and measurement frequency in the C-band range does not minimize the effect of surface roughness for passive microwave measurements. This result is significant in view of the fact that this combination of frequency and incident angle has been described as the optimum combination for minimizing the effect of surface roughness on the response of radar-backscatter measurements to soil moisture.

A Simple Relation between Active And Passive Microwave Remote Sensing Measurements of Earth Terrain

A simple approximate relation between backscattering coefficient and emissivity measurements is derived. The relation is applicable to active and passive microwave remote sensing of earth terrain where volume scattering plays a dominant role. From the relation, one can obtain a rough estimate of the backscattering coefficient from the emissivity and vice versa. Such estimation is useful in checking experimental measurements and also helps to ascertain the validity of theoretical models. It also safeguards against obtaining arbitrarily high values for both backscattering coefficients and emissivities.

Utilization of Active Microwave Roughness Measurements to Improve Passive Microwave Soil Moisture Estimates Over Bare Soils

Investigators have researched operational microwave techniques for the remote estimation of soil moisture for sometime now. Both active and passive microwave sensors respond to variations in soil moisture, but also respond to vegetation and roughness parameters. This has led to research in multisensor techniques which account for the interference. Previously, techniques have been developed which used visible and infrared bands (similar to Landsat) to compensate for the vegetation masking on the L-band passive radiometers response to soil moisture. In contrast, this study compensates for the surface roughness effect by using microwave scatterometer data on the same L-band radiometer. It was found that the L-band radiometers capability to estimate soil moisture over bare fields was significantly improved when surface roughness was accounted for with scatterometers.

Automated measurement of polarized bidirectional reflectance

Abstract An Automated Bidirectional Reflectance Acquisition Measurement System (ABRAMS) has been constructed to facilitate measurement of bidirectional reflectance from soil and vegetative samples in the laboratory. The system illuminates a sample with linearly-polarized laser light, λ = 632.8 nm, and measures the like- and cross-polarized scattered intensities over half a hemisphere. System design and polarized bidirectional reflectance measurements from a soil sample and SiO 2 spherical particles are discussed in this work. It is shown that polarization information in the plane of incidence is useful for identifying certain scattering mechanisms associated with soil reflectance. This is because the like-polarized intensity, I vv , is influenced by single-scattered light and the cross-polarized intensity, I HV , is strongly influenced by multiple-scattered light. For example, comparable levels of I VV and I HV indicated that the reflectance of soils, especially those made up of translucent particles, is dominated by significant multiple scattering because single scattering causes minimal depolarization in the plane of incidence. Furthermore, I HV is less dependent on the illumination angle (hence, more Lambertian) than I VV , indicating that the scattering mechanisms which caused them are not the same.

Antenna Effects in Depolarization Measurements

The depolarization of electromagnetic energy scattered from natural terrain has been of interest to experimenters and theoreticians for many years. However, the reported measurements have not agreed with theoreticaly predicted results. Boresight axial ratio has commonly been used as a figure of merit to describe the polarization isolation properties of the measurement system antennas. Since depolarization measurements from terrain involve extended targets which fill the full antenna beam, boresight axial ratio is often an inadequate measure of antenna polarization purity. This paper describes antenna isolation characteristics which impact the quality of the depolarized measurements. Representative calculations demonstrate the effects of nonideal antenna systems on the observed measurements. A criterion for antenna specification is given which will provide high-quality cross-polarized measurements.

The generation of surface targets with specified surface statistics

Abstract The analysis of scatter from rough surfaces has been of interest to researchers for many years. The comparison between theory and measurement has not always produced results that instill confidence in either the theory or the measurements. One would like to be able to construct the required surfaces to have control of the target as well as the measurements. There has been some work in the past to construct target surfaces, however, the statistics of the surface could only be determined after the fact. This paper presents some results of work to generate physical surfaces from known (i.e. desired) surface statistical properties. This study extends previous work on the generation of random surfaces for use in computer simulation approaches. The known statistical surface is extended using a bicubic spline technique and these results are interfaced to a numerically-controlled machine to generate the physical surface. A portion of a complete surface with Gaussian statistics was constructed and tested ...

The effect of measurement error and confusion from vegetation on passive microwave estimates of soil moisture

Abstract When using passive microwave sensors to estimate soil moisture it is important to understand the sensitivity of the estimate to (1) the effects of confusion due to surface roughness and vegetation and (2) the effect of measurement error due to noise. The sensitivity of error in soil moisture estimates to passive microwave measurement error as a function of vegetation is presented using a simple model and measurements. Roughness confusion is not considered. The direct problem is defined as investigating the sensor response as the dependent variable to the parameter of interest while the inverse problem uses the parameter of interest as the dependent variable. The inverse method must be used for operational remote sensing applications. It is shown that this subtle difference becomes very important when both confusion and measurement error are considered. A methodology is also presented whereby the sensitivity to measurement error attributed to vegetation can be estimated from the perpendicular vege...

Volumetric Effects in Cross-Polarized Airborne Radar Data

In recent years the detection of soil moisture using remote sensing technology has become of interest to investigators and a variety of state and federal government agencies. The parameter of interest is important in agricultural, meteorological, biological, and hydrological applications. The use of active microwave devices has shown to provide capability for remote measurement from space. Problems do exist however, in isolating moisture information from the effects of other parameters such as roughness and vegetation. Of special concern is the suppression of the roughness effects in the radar return. This paper presents an analysis of airborne cross-polarized radar measurements of agricultural scenes. The relative responses of the system to moisture and surface roughness are presented and compared to predicted responses using radar backscatter models. It is shown that the depolarized model predictions are sensitive to soil moisture, but are much less sensitive to surface roughness effects.

Enhanced binary tree genetic algorithm for automatic land cover classification

The development of automatic land cover classification maps using validated statewide datasets supported by state and federal agencies are becoming an important tool for monitoring change, planning, and land use impact assessment. Accurate and fast classification algorithms, adaptive to different data sources and scales, will facilitate cost effective routine updating of land cover maps using current satellite imagery to monitor and detect a broad array of land cover phenomena. A new binary decision tree classifier incorporating an evolutionary genetic learning algorithm is proposed for land cover classification. The new classifier referred to as the Enhanced Binary Tree Genetic Algorithm (BTGA+) has been applied to automatically classify tens of millions of pixels in two full scenes of Landsat TM data using multispectral multitemporal radiance features. The BTGA+ classifier can assign pixels into eight land cover categories for Landsat TM scenes in central Missouri with nearly 90% classification accuracy.

Bistatic Frequency Diverse Imaging Of Complex Scattering Targets

Tomographic imaging of a complex shape and a perfectly conducting object is discussed. The far field data was collected at a selected bistatic transmitheceive antenna configuration. The data was transformed to create tomographic image of the object. The Frequency Swept technique in the range of 4-12 GHz was used to achieve centimeter resolution in the reconstmeted image. The cross section of the scattering object for a number of transmithceive polarization is presented as well as the combination of like and cross polarized images.