M. W. Whitt

Michigan microwave canopy scattering model

Abstract The Michigan Microwave Canopy Scattering model (MIMICS) is based on a first-order solution of the radiative-transfer equation for a tree canopy comprising a crown layer, a trunk layer and a rough-surface ground boundary. The crown layer is modelled in terms of distributions of dielectric cylinders (representing needles and/or branches) and discs (representing leaves), and the trunks are treated as dielectric cylinders of uniform diameter. This report describes MIMICS I, which pertains to tree canopies with horizontally continuous (closed) crowns. The model, which is intended for use in the 0·5-10GHz region at angles greater than 10° from normal incidence, is formulated in terms of a 4 × 4 Stokes-like transformation matrix from which the backscattering coefficient can be computed for any transmit/receive polarization configuration.

A general polarimetric radar calibration technique

A polarimetric radar calibration procedure is introduced and verified with experimental results. The procedure requires measurements of three known targets in order to determine the distortion matrices that characterize the effect of the measurement system on the transmitted and received waves. The scattering matrices for the known targets can be of any form, provided that a limited set of constraints is satisfied. A special case, wherein the transmit and receive distortion matrices are the transpose of each other, is considered. This case is useful for some single antenna systems and has the advantage that only two known targets are required. >

AVNA-based polarimetric scatterometers

The polarization synthesis technique for measuring the scattering properties of point and distributed targets is discussed, and it is pointed out that accurate measurements of both the magnitude and phase of the scattered signal are now possible by using the signal-processing and error-correction techniques of the automatic vector network analyzer (AVNA). The principles of operation of the AVNA are given, and an overview of AVNA-based scatterometer configurations in use today at centimeter and millimeter wavelengths is provided.<<ETX>>

A millimeterwave network analyzer based scatterometer

The Millimeterwave Polarimeter (MMP) is a network-analyzer-based scatterometer and reflectometer system that has been developed to characterize radar clutter at 35, 94, and 140 GHz. A Hewlett-Packard 8510A network analyzer is used in the MMP system as a signal conditioner and processor to facilitate real-time data reduction, to reduce the short time-delay leakage noise inherent in traditional FM/CW radar, and to further enhance the signal-to-noise ratio of the system through signal processing techniques. Operation of the system at millimeter wavelengths is achieved with upconversion and harmonic downconversion. The use of harmonic downconverters permits low-frequency signal connections between components of the system and allows easy reconfiguration in either scatterometer, bistatic, or reflection/transmission modes. >

Measuring the propagation properties of a forest canopy using a polarimetric scatterometer

A truck-mounted 1.6-GHz polarimetric scatterometer was used from a 19-m high platform to measure the backscattering from a dense canopy of pine trees at an incidence angle of 40 degrees . Two sets of measurements were made at each of many spatial locations, one set with and the other without a trihedral corner reflector present on the ground surface underneath the canopy. From the two sets of polarimetric measurements, it was possible to determine the mean values and the statistical distributions of the canopy attenuation factors for horizontal and vertical polarizations. The mean values of the one-way attenuation factors were found to be 9.31 dB for horizontal polarization and 9.16 dB for vertical polarization. The precision associated with the values of the canopy loss factor measured using the polarimetric technique is estimated to be on the order of +or-0.3 dB. >

Millimeter-wave polarimetric measurements of artificial and natural targets

The millimeter-wave polarimeter (MMP), a scatterometer system uses the HP 8510A vector network analyzer for coherent processing of the received signal, provides the polarization and phase measurement capability needed to measure the complete scattering matrix of a given target. A calibration and measurement technique that was used with the MMP at 35 GHz to measure the scattering matrix for both distributed and point targets is described. The measurement accuracy was analyzed by comparing theoretical and measured values for a set of conducting spheres and finite-length conducting cylinders. As an extension of the analysis to natural targets, the scattering matrix was measured for a series of twigs and various smooth and rough surfaces. >

Radar response of periodic vegetation canopies

Abstract Vector radiative transfer theory is used to model the scattered intensity from a layer of randomly oriented particles over a periodic rough surface. To account for the periodic nature of row-structured vegetation, the number density of particles within the layer is assumed to be varying periodically in the horizontal direction. Using Fourier series expansions and orthogonality properties, the radiative transfer equation is solved for the transformation matrix relating the incident and scattered intensities, from which the backscattering coefficient of the layer can be computed for any incidence direction and polarization configuration. The experimental component of this investigation consisted of radar observations at 1–5,4–75, and 9–5 GHz made by a truck-mounted system for a field of corn under three conditions: (a) full, which means that the corn plants were in their natural state, (b) defoliated, which was accomplished by stripping off the leaves and removing them, thereby leaving behind only ...

Observation And Modelling Of Look-direction Effects In Microwave Backscatter From Corn

A vector radiative transfer model for periodic row-structured vegetation will be presented. The model accounts for the periodic variation in the number density of leaves and stalks by expressing the incident and scattered intensities as Fourier series expansions, and then using orthogonality to remove the horizontal dependence from the radiative transfer equations. An iterative technique is then used to solve for the first order transformation matrix relating the incident and scattered intensities. The periodic structure of the ground height profile is also modelled using a two-scale rough surface backscattering model [F.T. Ulaby, et al., IEEE Trans. Geosci. and Remote Sensing, GE-20, 518-528, 19821 that has been extended to the polarimetric case. In order to verify the model, multi-polarization backscatter measurements were made for a corn canopy at different defoliation and developmental stages. A truck-based polarimetric scatterometer operating at L/C/X-band was used. Incidence angles of 10-60 degrees were considered, and the azimuth lookdirections were 0, 45, and 90 degrees with respect to the row direction. The backscatter was first measured on the fully foliated canopy, then the different vegetation types (cobs, leaves, and stalks) were removed sequentially, making additional backscatter measurements after removing each type. The model and measurements show a significant increase (as large as 8-10 dB) in the L-band co-polarized backscatter as the look-direction changes from 0 to 90 degrees. This effect decreases with frequency such that at X-band the difference between the 0 and 90 degree look-directions is less than 2-3 dB for lower canopy heights. When the corn is fully developed, the look-direction dependence is seen only at L-band. From the defoliation data, it is evident that the look-direction effects are a function of both direct backscatter from the ground and scattering from the vegetation, with the stalk-ground interaction becoming important at higher incidence angles. The data also indicates that second order scattering effects between stalks is important at L-band for the fully developed canopy.