R.D. De Roo

A semi-empirical backscattering model at L-band and C-band for a soybean canopy with soil moisture inversion

Radar backscatter measurements of a pair of adjacent soybean fields at L-band and C-band are reported. These measurements, which are fully polarimetric, took place over the entire growing season of 1996. To reduce the data acquisition burden, these measurements were restricted to 45/spl deg/ in elevation and to 45/spl deg/ in azimuth with respect to the row direction. Using the first order radiative transfer solution as a form for the model of the data, four parameters were extracted from the data for each frequency/polarization channel to provide a least squares fit to the model. For inversion, particular channel combinations were regressed against the soil moisture and area density of vegetation water mass. Using L-band cross-polarization and VV-polarization, the vegetation water mass can be regressed with an R/sup 2/=0.867 and a root mean square error (RMSE) of 0.0678 kg/m/sup 2/. Similarly, while a number of channels, or combinations of channels, can be used to invert for soil moisture, the best combination observed, namely, L-band VV-polarization, C-band HV- and VV-polarizations, can achieve a regression coefficient of R/sup 2/=0.898 and volumetric soil moisture RMSE of 1.75%.

Sensitivity of the Kurtosis Statistic as a Detector of Pulsed Sinusoidal RFI

A new type of microwave radiometer detector that is capable of identifying low-level pulsed radio frequency interference (RFI) has been developed. The Agile Digital Detector can discriminate between RFI and natural thermal emission signals by directly measuring other moments of the signal than the variance that is traditionally measured. The kurtosis is the ratio of the fourth central moment of the predetected voltage to the square of the second central moment. It can be an excellent indicator of the presence of RFI. A number of issues that are related to the proper calculation of the kurtosis are addressed. The mean and standard deviation of the kurtosis, in both the absence and the presence of pulsed sinusoidal RFI, are derived. The kurtosis is much more sensitive to short-pulsed RFI-such as from radars-than to continuous-wave RFI. The minimum detectable power for pulsed sinusoidal RFI is found to be proportional to (M 3 N)-1/4, where N is the number of independent samples and M is the number of frequency subbands in the receiver.

Bistatic specular scattering from rough dielectric surfaces

An experimental investigation was conducted to determine the nature of bistatic scattering from rough dielectric surfaces at 10 GHz. This paper focusses specifically on the dependence of coherent and incoherent scattered fields on surface roughness for the specular direction. The measurements, which were conducted for a smooth surface with ks >

Vegetation canopy anisotropy at 1.4 GHz

We investigate anisotropy in 1.4-GHz brightness induced by a field corn vegetation canopy. We find that both polarizations of brightness are isotropic in azimuth during most of the growing season. When the canopy is senescent, the brightness is a strong function of row direction. On the other hand, the 1.4-GHz brightness is anisotropic in elevation: an isotropic zero-order radiative transfer model could not reproduce the observed change in brightness with incidence angle. Significant scatter darkening was found. The consequence of unanticipated scatter darkening would be a wet bias in soil moisture retrievals through a combination of underestimation of soil brightness (at H-pol) and underestimation of vegetation biomass (at V-pol). A new zero-order parameterization was formulated by allowing the volume scattering coefficient to be a function of incidence angle and polarization. The small magnitude of the scattering coefficients allows the zero-order model to retain its limited physical significance.

95-GHz scattering by terrain at near-grazing incidence

This study, consisting of three complimentary topics, examines the millimeter-wave backscattering behavior of terrain at incidence angles extending between 70 and 90/spl deg/, corresponding to grazing angles of 20/spl deg/ to 0/spl deg/. The first topic addresses the character of the statistical variability of the radar backscattering cross section per unit area /spl sigma//sub A/. Based on an evaluation of an extensive data set acquired at 95 GHz, it was determined that the Rayleigh fading model (which predicts that /spl sigma//sub A/ is exponentially distributed) provides an excellent fit to the measured data for various types of terrain covers, including bare surfaces, grasses, trees, dry snow, and wet snow. The second topic relates to the angular variability and dynamic range of the backscattering coefficient /spl sigma//sup 0/, particularly near grazing incidence. We provide a summary of data reported to date for each of several types of terrain covers. The last topic focuses on bare surfaces. A semi-empirical model for /spl sigma//sup 0/ is presented for vertical (VV), horizontal (HH), and cross (HV) polarizations. The model parameters include the incidence angle /spl theta/, the surface relative dielectric constant /spl epsiv/, and the surface roughness ks, where k=2/spl pi///spl lambda/ and s is the surface root mean square (RMS) height.

Polarimetric calibration of SIR-C using point and distributed targets

In preparation for the Shuttle Imaging Radar-C/XSAR (SIR-C/XSAR) flights, the University of Michigan has been involved in the development of calibration procedures and precision calibration devices to quantify the complex radar images with an accuracy of 0.5 dB in magnitude and 5 degrees in phase. In this paper, the preliminary results of the SIR-C calibration and a summary of the University of Michigans activity in the Raco calibration super-site is presented. In this calibration campaign an array of point calibration targets including trihedral corner reflectors and polarimetric active radar calibrators (PARCs) in addition to a uniform distributed target were used for characterizing the radiometric calibration constant and the distortion parameters of the C-band SAR. Two different calibration methods, one based on the application of point targets and the other based on the application of the distributed target, are used to calibrate the SIR-C data and the results are compared with calibrated images provided by JPL. The distributed target used in this experiment was a field of grass, sometimes covered with snow, whose differential Mueller matrix was measured immediately after the SIR-C overpass using The University of Michigan polarimetric scatterometer systems. The scatterometers were calibrated against a precision metallic sphere and measured 100 independent spatial samples for characterizing the differential Mueller matrix of the distributed target to achieve the desired calibration accuracy. The L-band SAR has not yet been adequately calibrated for inclusion here. >

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.

Brightness Temperatures of Snow Melting/Refreezing Cycles: Observations and Modeling Using a Multilayer Dense Medium Theory-Based Model

The ability of electromagnetic models to accurately predict microwave emission of a snowpack is complicated by the need to account for, among other things, nonindependent scattering by closely packed snow grains, stratigraphic variations, and the occurrence of wet snow. A multilayer dense medium model can account for the first two effects. While microwave remote sensing is well known to be capable of binary wet/dry discrimination, the ability to model brightness as a function of wetness opens up the possibility of ultimately retrieving a percentage wetness value during such hydrologically significant melting conditions. In this paper, the first application of a multilayer dense medium radiative transfer theory (DMRT) model is proposed to simulate emission from both wet and dry snow during melting and refreezing cycles. Wet snow is modeled as a mixture of ice particles surrounded by a thin film of water embedded in an air background. Melting/refreezing cycles are studied by means of brightness temperatures at 6.7, 19, and 37 GHz recorded by the University of Michigan Truck-Mounted Radiometer System at the Local Scale Observation Site during the Cold Land Processes Experiment-1 in March 2003. Input parameters to the DMRT model are obtained from snow pit measurements carried out in conjunction with the microwave observations. The comparisons between simulated and measured brightness temperatures show that the electromagnetic model is able to reproduce the brightness temperatures with an average percentage error of 3% (~8 K) and a maximum relative percentage error of around 8% (~20 K)

Detection of RFI by its Amplitude Probability Distribution

A new type of microwave radiometer detector has been developed that is capable of identifying low level Radio Frequency Interference (RFI) and of reducing or eliminating its effect on the measured brightness temperature. The Agile Digital Detector (ADD) can discriminate between RFI and natural thermal emission signals by directly measuring higher order moments of the signal than the variance that is traditionally measured. After detection, the ADD then uses spectral and temporal filtering methods to selectively remove the RFI. ADD performance has been experimentally verified and its performance characterized while connected to an airborne C-Band radiometer (the NOAA/ETL PSR) installed on a NASA WB-57 flying over major urban centers. Index Terms—microwave radiometer, radio frequency interference

MMW radar scattering statistics of terrain at near grazing incidence

The statistical behavior of clutter observed near grazing incidence and at 95 GHz is investigated for the specific cases of bare ground, snow cover, and for a heterogeneous scene. The bare ground constitutes a homogeneous target under homogeneous conditions and the magnitude of the amplitude is Rayleigh distributed. While the snow cover is a homogeneous target, the conditions under which it was observed are heterogeneous, and the Bayes rule is employed to describe its clutter distribution. The Bayes rule integrates variations due to signal fading with the underlying variations in the backscattering coefficient associated with the heterogeneity. The heterogeneous scene is also successfully described with the Bayes rule.