Fluctuation statistics of millimeter-wave scattering fromdistributed targets

The applicability of the Rayleigh fading model for characterizing radar scattering from terrain is examined at 35 GHz for both backscattering and bistatic scattering. The model is found to be in excellent agreement with experimental observations for single-frequency observations of uniform targets such as asphalt and snow-covered ground. The use of frequency averaging to reduce signal fading variations was examined experimentally by sweeping the radar signal from 34-36 GHz in 401 steps. The results show that the formulation based on the Rayleigh model relating the reduction in signal fluctuation to the bandwidth used provides a reasonable estimate for the improvement provided by frequency averaging     

Variability in the measurement of radar backscatter

A variety of systems and platforms have been used over the past three decades to acquire radar backscatter data of terrain. The variability in the reported data was evaluated for agricultural crops under "similar" phenological conditions and for approximately the same sensor parameters (frequency, polarization, and angle of incidence). The evaluation reveals wide variations in the magnitude of the scattering coefficient among different measurement programs. While it is difficult to determine the exact causes of these variations it is quite evident that extreme care must be employed in 1) monitoring the measurement system transfer function, 2) calibrating the system on an absolute basis, and 3) acquiring and reporting detailed target parameter information.     

Extinction Behavior of Dry Snow in the 18-to 90-GHz Range

The extinction properties of several dry snow types were examined in the 18-to 90-GHz range. The snow types ranged from newly fallen snow to refrozen snow, and the density and mean grain size varied from 0.17 to 0.39 g/cm3 and from 0.2 to 1.6 mm, respectively. From measurements of the transmission loss as a function of sample thickness at a temperature of -15°C, the extinction coefficient and the surface scattering loss (due to surface roughness at the front and back surfaces of the snow slab) were determined for each snow type. The experimental values were compared against theoretical results computed according to the strong fluctuation theory. In general, good agreement with the experimental data was obtained at 18, 35, and 60 GHz when the grain size used in the theoretical calculations was chosen to be slightly smaller than the observed snow-particle size. However, the extinction coefficient of large-grained refrozen snow as predicted by the strong fluctuation theory is much larger at 90 GHz than the values determined experimentally. The attenuation in snow was observed to increase only slightly with increasing temperature in the -35 to -1°C range.     

Active Microwave Soil Moisture Research

This paper summarizes the progress achieved in the active microwave remote sensing of soil moisture during the four years of the AgRISTARS program. Within that time period, from about 1980 to 1984, significant progress was made toward understanding 1) the fundamental dielectric properties of moist soils, 2) the influence of surface boundary conditions, and 3) the effects of intervening vegetation canopies. In addition, several simulation and image-analysis studies have identified potentially powerful approaches to implementing empirical results over large areas on a repetitive basis. This paper briefly describes the results of laboratory, truck-based, airborne, and orbital experimentation and observations.     

Undergraduate electromagnetics laboratory: An invaluable part ofthe learning process

A newly developed pair of undergraduate courses in applied electromagnetics (EM) has succeeded in transforming student attitudes towards EM from dread and avoidance to acceptance and appreciation. One concrete measure of this change is in the enrolment figures in follow-up courses in microwave circuits and microwave propagation-within one year, the enrolments have doubled in one of them and just about tripled in the other one. The new EM courses incorporate an integrative approach towards teaching EM phenomena; this includes a new text, a large number of class demonstrations, and a laboratory component consisting of nine laboratory exercises and a final team project. This paper focuses on design and content of the instructional laboratory and learning contribution it provides     

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     

External Calibration of SIR-B Imagery with Area-Extended and Point Targets

Data takes on two ascending orbits of the Shuttle Imaging Radar-B (SIR-B) over an agricultural test site in west-central Illinois were used to establish end-to-end transfer functions for conversion of the digital numbers on the 8-bit image to values of the radar backscattering coefficient ?0 (m2/m2) in dB. The transfer function for each data take was defined by the SIR-B response to an array of six calibrated point targets of known radar cross section (transponders) and to a large number of area-extended targets also with known radar cross section as measured by externally calibrated, truck-mounted scatterometers. The radar cross section of each transponder at the SIR-B center frequency was measured on an antenna range as a function of the local angle of incidence. Two truck-mounted scatterometers observed 20-80 agricultural fields daily at 1.6 GHz with HH-polarization and at azimuth viewing angles and incidence angles equivalent to those of the SIR-B. The form of the transfer function is completely defined by the SIR-B receiver and the incoherent averaging procedure incorporated into production of the standard SIR-B image product. Assuming that the processing properly accounts for the antenna gain, all transfer function coefficients are known except for the thermal noise power and a system " constant" that has been shown to vary as a function of uncommanded changes in the effective SIR-B transmit power.     

Dielectric properties of snow in the 3 to 37 GHz range

Microwave dielectric measurements of dry and wet snow were made at nine frequencies betweeo 3 and 18 GHz, and at 37 GHz, using two free-space transmission systems. The measurements were conducted during the winters of 1982 and 1983. The following parametric ranges were covered: 1) liquid water content, 0 to 12.3 percent by volume; 2) snow density, 0.09 to 0.42 g cm^-3; 3) temperature, 0 to-5 degC and-15degC (scattering-loss measurements); and 4) crystal size, 0.5 to 1.5 mm. The experimental data indicate that the dielectric behavior of wet snow closely follows the dispersion behavior of water. For dry snow, volume scattering is the dominant loss mechanism at 37 GHz. The applicability of several empirical and theoretical mixing models was evaluated using the experimental data. Both the Debye-like semi-empirical model and the theoretical Polder-Van Santen mixing model were found to describe adequately the dielectric behavior of wet snow. However, the Polder-Van Santen model provided a good fit to the measured values of the real and imaginary parts of wet snow only when the shapes of the water inclusions in snow were assumed to be both nonsymmetrical and dependent upon snow water content. The shape variation predicted by the model is consistent with the variation suggested by the physical mechanisms governing the distribution of liquid water in wet snow.     

An ultrafast wide-band millimeter-wave (MMW) polarimetric radar for remote sensing applications

With the advent of high-frequency radio frequency (RF) circuits and components technology, millimeter-wave (MMW) radars are being proposed for a large number of military and civilian applications. Accurate and high-resolution characterization of the polarimetric radar backscatter responses of both clutter and man-made targets at MMW frequencies is essential for the development of radar systems and optimal detection and tracking algorithms. Toward this end, a new design is developed for ultrafast, wide-band, polarimetric, instrumentation radars that operate at 35 and 95 GHz. With this new design, the complete scattering matrix of a target (magnitude and phase) can be measured over a bandwidth of 500 MHz in less than 2 /spl mu/s. In this paper, the design concepts and procedures for the construction and calibration of these radars are described. In addition, the signal processing algorithm and data-acquisition procedure used with the new radars are presented. To demonstrate the accuracy and applicability of the new radars, backscatter measurements of certain points and distributed targets are compared with their analytical radar cross section (RCS) and previously measured /spl sigma//spl deg/ values, respectively, and good agreements are shown. These systems, which can be mounted on a precision gimbal assembly that facilitates their application as high-resolution imaging radar systems, are used to determine the MMW two-way propagation loss of a corn field for different plant moisture conditions.     

Performance characterization of polarimetric active radarcalibrators and a new single antenna design

Several topics associated with the use of a polarimetric active radar calibrator (PARC), which is a high radar-cross-section transponder with a known scattering matrix, are addressed. The first involves experimental measurements of the magnitudes and phases of the scattering-matrix elements of a pair of PARCs that operate at 1.25 GHz and 5.3 GHz. The measurements were conducted over a wide range of incidence angles (relative to the boresight direction) in the azimuth, elevation, 45°, and 135° planes. The 5.3-GHz PARC, which consisted of two antennas placed several wavelengths apart, exhibited symmetrical patterns with no ripples and excellent isolation between orthogonal polarization channels. The 1.25-GHz PARC, whose antennas were in very close proximity to one another, exhibited unsymmetrical patterns as well as ripples in the phase patterns, thereby introducing errors in the elements of the scattering matrix. To avoid this problem, a single-antenna PARC is designed,, using an orthomode transducer. The single-target calibration technique is extended so that it applies to the use of a PARC as well as reciprocal passive calibration devices such as spheres and trihedral corner reflectors