Measurements have been made at P-band (0.707 GHz) to construct a model for the dielectric constant of sea water and extend the model for the dielectric constant to high salinity [$S \gt 50$practical salinity scale (pss)]. The measurements are part of research to develop a model for the dielectric constant suitable for future wide-bandwidth (BW) remote sensing of salinity and for application to water bodies, such as the Great Salt Lake with salinity significantly above that found in the open ocean. Measurements have been made at temperatures from 2 °C to 30 °C and salinity from 0 to 138 pss. The data have been fit to a Debye model for the dielectric constant with a single relaxation mode as has been employed at L-band (1.413 GHz), where remote sensing of salinity is currently done. Comparison with contemporary models developed from data at L-band indicates that the L-band model and new P-band model do well at both frequencies for salinity less than 50 pss, but at higher values of salinity, the L-band models diverge from the data. The data have also been used to test at high salinity the mathematical relationship between salinity and conductivity, which is the basis for the pss.

above article [1], the corrections are needed for signs in (3b) and (5a) and for the location of the decimal point in the first term in (8b). Equations (3b) and (5a) with the correct signs are \begin{align*} & \varepsilon _{s}\left ({{S, T}}\right) =\varepsilon _{s}\left ({{0, T}}\right)\left [{{1-\text {SR}\left ({{S, T}}\right)}}\right]\tag {3b}\\[8pt] & \tau \left ({{0, ~T }}\right) = \left [{{17.9539-0.6955 ~T+ 0.0177 ~T ^{2}-0.0002 ~T ^{3}}}\right]10^{-12.} \tag {5a}\end{align*}

The paper reports the measurements of the dielectric constant of seawater at the P-band frequency on both typical ocean low salinity waters and the high salinity waters such as occur in the Great Salt Lake, Utah. The measurements are compared with the predictions from the GW 2022 model that is originally built based on the low salinity measurements at L-band and modified to apply to high salinity. For low salinity (below 40 psu), the GW 2022 model demonstrates excellent agreement with the P-band measurements within the temperature range 5°C to 35°C.. At higher salinity levels (50, 75, 100, and 150 psu), slight divergence is observed between the model predictions and the measurements, particularly as temperature increases. If the measurements are correct, this divergence indicates that the GW 2022 model will require modification for P-band especially for high salinity and high temperatures.

This article explores the potential of P-band and L-band radiometry for estimating soil moisture and temperature profiles. A total of 3977 hourly in situ soil data were collected at depths (d) of 5–60 cm in Beltsville, MD, USA. Using the data, a coherent model was used to generate synthetic brightness temperatures at an incidence angle of 40° for frequencies of 0.8, 0.9, 1.1, and 1.4 GHz. These synthetic brightness temperatures facilitated the estimation of soil moisture ($m_{v}$) and temperature (T) profiles, which were modeled as quadratic functions with three coefficients. The inversion problem was formulated as a least-squares problem and optimized by the adaptive simulated annealing (ASA) algorithm. Regression analysis highlighted the sensitivity of the soil moisture and temperature function coefficients on the brightness temperature at 1.4 GHz and the differential between 1.4 and 0.8 GHz ($R^{2}=0.62-0.97$), yielding the best-fit models to describe the relationships. The application of the ASA algorithm incorporating the best-fit models to constrain the search spaces showed the RMSE of soil moisture${m_{v}}\leq 0.04~\rm cm^{3}/cm^{3}$and soil temperature${T}\leq 1.35~^{\circ }\text {C}$for depths$d\leq 20~ \rm cm$, and${m_{v}}\leq 0.10~\rm cm^{3}/cm^{3}$and${T}\leq 1.60~^{\circ }\text {C}$for$d\leq 60 ~\rm cm$. A streamlined inversion method was also investigated which used the best-fit models as the retrieval formula and solely relied on the V-pol data. The streamlined inversion method achieved comparable retrieval accuracy but reducing the runtime from 770 to 0.54 s for one inversion. This approach simplifies the data collection process by eliminating the need for H-pol data, potentially broadening its flexibility and applicability.

A physics-based microwave emission model for layered vegetation (MEMLV) is developed to simulate the vegetation optical depth (VOD) and scattering albedo of coniferous forests from P- to Ka-band (0.4 GHz–37 GHz). This study aims to use physics-based forward modeling to guide and support multifrequency VOD retrieval. The MEMLV consists of three major components: 1) the single-layer discrete scatter model (SL-DSM) that calculates the VOD and single scattering albedo of a single layer; 2) a new Tree Structure Model (TSM) that represents forests by stratified multilayer media; and 3) the Two-Stream microwave emission model (2S-MEM) that combines SL-DSM and TSM to calculate the brightness temperature of forests and their corresponding effective VOD,$\tau _{\text {eff}}$, and effective scattering albedo,$\omega _{\text {eff}}$. Simulation of an exemplary coniferous forest shows that$\tau _{\text {eff}}$increases as frequency increases until reaching saturation at K-band. Meanwhile,$\omega _{\text {eff}}$increases rapidly at low frequencies until reaching a peak at S-band, then decreases until reaching X-band, and finally increases monotonically as frequency increases. Notably,$\omega _{\text {eff}}$is negligible at P-band for most cases. Sensitivity analyses demonstrate the saturation of$\tau _{\text {eff}}$when canopy height is substantial, and the decreasing trend of$\omega _{\text {eff}}$as canopy height or the areal fraction of canopy gap increases. The MEMLV has the potential to improve the parameterization of retrieval algorithms and to enhance the understanding of the retrieved VOD over a wide frequency range.

This paper is concerned with the accurate measurement of the dielectric constant of seawater at P band. The measurement of seawater salinity at L band (1.414 GHz) by satellite becomes less accurate as the seawater temperature decreases. At lower frequencies such as 707 MHz, the sensitivity to salinity of the radiometer measurements increases. Designers of P band radiometers for ocean sensing will need to know how the dielectric constant of seawater varies with salinity (S) and temperature (T). To determine this relationship, a P band cavity, based on an L band design, has been constructed. Initial measurements are made at S=35 psu and T= 10°, 20°, 30°C. The results are compared with the values obtained from L band models evaluated at P band (f=707MHz.). The model predictions are close to the measurement results.

Synthetic Aperture Radar acquisitions in nadir (sounding) and side-looking imaging modes are planned for the International-Mars Ice Mapper (I-MIM) orbiter mission to accurately detect ice buried just below the Martian surface. In an effort to gain a comprehensive understanding of the ice detection capability of the I-MIM L-band polarimetric SAR, we developed a Nadir Scattering Radar Model (NRSM), and evaluated it in comparison to a previously implemented Side-looking Radar Scattering Model (SRSM). Together, these Radar Scattering Models (RSMs) permit a thorough evaluation of key radar performance metrics for given sets of radar parameters and Martian surface and shallow subsurface scenarios, and offer new insights into potential depths of detection of buried water ice on Mars.

Accurate knowledge of the dielectric constant of sea water is important for remote sensing of surface parameters such as sea surface temperature (SST) and sea surface salinity (SSS). The advent of sensors in space, SMOS [1] , Aquarius [2] and SMAP [3] capable of measuring SSS motivated modern measurements [4] , [5] and modelling [5] , [6] of the dielectric constant at L-band (1.4 GHz). In the past, the range of salinity included in the data used to create these models has been restricted to values typically encountered in the open ocean (e.g., less than 40 psu). However, there are many smaller water bodies with much higher salinity. Notable examples are the Great Salt Lake in Utah with salinity on the order of 180 psu and Garabogazköl lagoon in Turkmenistan with even higher salinity. Unfortunately, existing models for the dielectric constant can’t necessarily just be extended to higher values of salinity. The problem is that the polynomials in salinity and temperature used to represent the unknown parameters in the models are not constrained outside the range of SSS and SST used to determine their coefficients. While the models for the dielectric constant may be very good within that range, outside that range they can lead to unrealistic behavior. Research is underway to develop a model that represents the dielectric constant well over the ocean and behaves well at high salinity. In preparation for possible wideband remote sensing of salinity [7] , [8] , [9] , the laboratory measurements made at 1.413 GHz [4] , [5] are being repeated at 0.707 GHz (P-band) and the plan is to include values of high salinity (50, 100, 150 psu).

The use of polarimetric SAR to detect buried water/ice at shallow depths under the Martian surface is quantitatively explored. An electromagnetic model consisting of a dielectric layer that represents the Martian regolith (upper surface layer), with an underlying half space of water/ice, is considered. The top surface of the regolith and the regolith/ice interface are assumed to be rough at radar wavelength scales. It is also assumed that throughout the regolith, small rock particles with arbitrary shapes and prescribed orientation statistics are randomly distributed. Backscattering with co-polarization and cross-polarization from vertical or horizontal polarized waves incidence at L-band frequencies are considered. The Advanced Integral Equation Method (AIEM) is applied to irregular surfaces and the Distorted Born Approximation (DBA) applied to particle scattering and regolith attenuation.

The accuracy of the Sea Surface Salinity (SSS) retrieved from L-Band radiometer measurements is strongly dependent on the reliability of the dielectric constant model. Two new parametrizations were recently developed based on one hand on the Soil Moisture and Ocean Salinity (SMOS) satellite multi-angular brightness temperature measurements by Boutin et al. (2021) (BV), and on the other hand on new George Washington University laboratory measurements by Zhou et al. (2021) (GW2020). These two approaches are fully independent. For most SSS and Sea Surface Temperature (SST) conditions commonly observed over the open ocean, the relative variations of brightness temperatures Tb simulated through the BV and GW2020 parametrizations agree particularly well, and better than with earlier parametrizations previously used in the SMOS, Soil Moisture Active Passive (SMAP) and Aquarius SSS retrievals. Nevertheless, uncertainty remains, especially below 10°C where a ~0.1K relative difference between the two models is observed. This motivates the development of a revised parameterization, BVZ, based on a methodology similar to that used to derive BV but using GW2020 instead of SMOS measurements. Compared to the GW2020 parameterization, BVZ is derived with a reduced number of degrees of freedom, it relies on TEOS10 PSS78 conductivity-salinity relationship and on previously derived static permittivity of fresh water. One month per season of SMOS data have been reprocessed in 2018 using BV, GW2020 and BVZ. We find the best overall agreement between SMOS SSS and Argo SSS with BVZ parametrization, with noticeable improvement in the 5°C-15°C SST range.

This paper describes a new system for measuring the dielectric constant of seawater near 700 MHz. The purpose of this research is to obtain data at P-band for the dielectric constant of seawater that can be combined with previous L-band data for developing a model function valid from 500 MHz to 2000 MHz. A transmission-type cylindrical cavity has been constructed with the$\text{TM}_{010}$mode resonance occurring near 700 MHz. The seawater is introduced via a capillary quartz tube. The dimensions of the cavity have been determined by electromagnetic modelling. The model takes the cavity wall loss into account and can provide accurate estimation of resonance condition for both the empty cavity and cavity with seawater. The errors between the theory and measurement are about 0.07% for the resonant frequency and 0.4% for the quality factor,$Q$. The cavity design, experimental setup and measurement schedule will be presented in the paper.

This paper demonstrates the potential use of P and L band passive measurements to determine root zone soil moisture (SM) and soil temperature (ST). SM and ST data have been taken as a function of depth during the NASA GSFC PLEX 19 experiment in the summer of 2019 at Beltsville, MD, USA. Using these data, a coherent model has been used to compute H and V brightness temperatures at frequencies of 0.8 and 1.4 GHz with an observation angle of 35 degrees. These synthetic brightness data are then used to estimate the SM and ST profiles which are represented by linear polynomials. The inversion problem is formulated as a least square problem that is solved by a global optimization method known as the Adaptive Simulated Annealing (ASA) method. Four inversion examples having different SM and ST profiles are presented. Selected results show that the standard deviation between the retrieved and measured data is less than 0.077$\text{cm}^{3}/\text{cm}^{3}$for SM, and 2.245 °C for ST.

We implemented a 3-D electromagnetic scattering model to gain a better understanding in the observations of L- and P-band radar returns from the multi-layered Martian upper surface layer. The model, based on the Integral Equation Model (IEM) developed by Adrian Fung, treats the Martian regolith (upper 10-meter layer of surface sediments) as an inhomogeneous medium with irregular boundaries, and employs surface and shallow subsurface parameters representative of a variety of Martian terrains. The model is well-suited to study the near-subsurface ice detection capability of the L-band (32 cm wavelength) Synthetic Aperture Radar (SAR) planned for the international Mars Ice Mapping (I-MIM) mission being considered for launching in the late 2020s.

This manuscript reports an improvement in the model for the dielectric constant of seawater used to fit laboratory measurements of the dielectric constant at L-band. The new model (dielectric constant as a function of salinity, temperature and frequency) is based on the response of a polar molecule proposed by Debye and fits the same measurement as reported in earlier work, but uses a functional form for conductivity, σ(S,T), that is given by the definition of salinity. The new version of this model fits the data well and has the advantages that the relaxation time constant is allowed to be a function of temperature and salinity and is well behaved when extrapolated to high salinities.

The model expressing the dielectric constant of sea water at microwave frequencies as a function of salinity and temperature is an important element in remote sensing of sea surface salinity. It is also important independently as a description of the physical properties of salt water. A major milestone was the development in the late 1970’s by Klein and Swift of a model based on laboratory measurements at L- and S-band and a functional form supported by theory for polar molecules and previous work on freshwater. Much of the subsequent work has focused on measurements at higher frequency and determining model parameters tuned to apply for applications such as remote sensing of sea surface temperature. Interest in the dielectric constant at 1.4 GHz (L-band) increased again with the development of SMOS and Aquarius to measure salinity from space. But there have been few new measurements at L-band and often confusion regarding the applicability of new models at 1.4 GHz. The objective of this manuscript is to compare available models in the context of how well they represent the dielectric constant of sea water at 1.4 GHz. Among the criteria applied will be the recent measurements at the George Washington University of the dielectric constant at 1.4 GHz.

A model function of seawater, which specifies the dielectric constant of seawater as a function of salinity, temperature, and frequency, is important for the retrieval of sea surface salinity using satellite data. In 2017, a model function has been developed based on measurement data at 1.4134 GHz using a third-order polynomial expression in salinity ($S$) and temperature ($T$). Although the model showed improvements in salinity retrieval, it had an inconsistent behavior between partitioned salinities. To improve the stability of the model, new dielectric measurements of seawater have been made recently over a broad range of salinities and temperatures to expand the data set used for developing the model function. The structure of the model function has been changed from a polynomial expansion in$S$and$T$to a physics-based model consisting of a Debye molecular resonance term plus a conductivity term. Each unknown parameter is expressed in$S$and$T$based on the expanded measurement data set. Physical arguments have been used to limit the number of unknown coefficients in these expressions to improve the stability of the model function. The new model function has been employed in the retrieval algorithm of the Aquarius satellite mission to obtain a global salinity map. The retrieved salinity using a different model function is compared within situdata collected by Argo floats to evaluate the impact and the performance of model functions. The results indicate that the new model function has significant improvements in salinity retrieval compared with other existing models.

New seawater dielectric measurements have recently been made over a broad range of salinities and temperatures at the George Washington University (GW). These measurements have been used to refine the existing dielectric model function of seawater for salinity retrieval. The mathematical structure of the new model function has been investigated to: (a) look for a physical basis for the mathematical form; and (b) to optimize the accuracy of data fitting. The Debye model has been chosen to represent the dielectric constant of seawater as a function of salinity (S), temperature (T) and frequency (f). The retrieved salinity using the Debye model has good agreement with the in-situ data collected by Argo floats. The global differences in retrieved salinity and in-situ data will be presented at the meeting.

Modeling L-band backscatter from a corn canopy continues to be a challenge due to the complex dynamics in both plant phenology and the underlying soil. An experiment has been conducted to better understand the relationship between L-band backscatter and canopy parameters such as soil moisture, vegetation water content, dew, and periodic rows. The experiment consists of field measurements that take into account plant phenology and are concurrent with L-band backscatter returns from a corn canopy over a full growing season. The field measurements of the corn plants' constituents highlight modeling complexities, such as an inhomogeneity in the dielectric constant of the stalk and cobs. A simple method to replace the stalk and cob with a homogeneous dielectric constant is validated. Using the field measurements in a scattering model developed at George Washington University (GW), both coherent and incoherent backscatter are computed. The results show coherent effects contributing to enhanced backscatter by up to 2.7 dB for both HH-pol and VV-pol. The coherent model and the detailed measurements, especially, the dielectric constant of the stalks, resulted in good agreement with the measurements. These measurements have an average root mean square difference (RMSD) with the results from the coherent model of around 1 dB for both HH-pol and VV-pol over the entire growing season. The incoherent mode does not perform as well.

The dielectric constant of seawater at L-band is determined by a resonant cavity technique. Based on the measurement data, an accurate dielectric model function has been developed and employed to retrieve the ocean surface salinity from the satellite data. The retrieved salinities indicate that the accuracy of the dielectric model needs to be improved to resolve the bias correlated with sea surface temperature. This paper reports the new measurements that have been recently made for the development of a more accurate model function. These new measurements are made using 20, 34 and 36 psu seawater samples from 10°-30° C with a 5° C interval. An improved model function is developed based on the new measurements and on the previous measurements. The salinity retrieval results using the new model function will be presented at the meeting.

SCoBi-Veg stands for Signals of opportunity Coherent Bistatic scattering model for Vegetated terrains. It simulates polarimetric reflectometry of vegetation canopy over a flat ground using a Monte Carlo scheme. The model is aimed at assessing the value of navigation and communication satellite Signals of Opportunity in a range of frequencies from P- to S-bands for remote sensing of a number of geophysical land parameters such as soil moisture and biomass. A fully polarimetric expression for bistatic scattering from a vegetation canopy is first formulated for a general case and is then specialized to the practical case of ground-based/low-altitude platforms with passive receivers overlooking vegetation using the signals transmitted from large distances. Using analytical wave theory in conjunction with distorted Born approximation, the transmit and receive antenna effects (i.e., polarization crosstalk/mismatch, orientation, and altitude) are explicitly accounted for. The forward model developed here enables the understanding of the effect of different geophysical parameters and system configurations on the coherent and incoherent components of the reflected signatures. It can thus help developing robust inverse algorithm for extraction of soil moisture and biomass. The model is applied to P-band signals of geostationary communication satellites to describe polarimetric reflections from tree canopies as observed from down-looking platforms at various altitudes. The relative contributions of diffuse and specular scattering on total reflected power and reflectivity are quantified for various observing scenarios.

In the past the effect of soil roughness was often considered secondary within the determination of soil moisture from remote sensing data. Several studies showed that accurate determination of soil roughness leads to an improved estimation of soil moisture. Two standard parameters in microwave sensing to describe the surface roughness are the standard deviation of the surface height variation s and the surface correlation length l with its corresponding autocorrelation function (ACF). Both parameters (s, l) affect the emissivity measured by radiometers as well as the backscattering observed by radars. In this study, we develop a physics-based approach to retrieve s and l by combining both microwave signals based on active-passive microwave covariation. To test the approach, containing a forward model and a retrieval algorithm, we used active/passive microwave data measured with the ComRAD truck-based SMAP simulator at L-band. Results and validations with corresponding field measurements on ground show that s and l can be estimated when using this approach. The physics-based retrieval algorithm works robustly for two investigated test fields having an RMS-Error of 0.68 cm and 0.69 cm between the microwave-based and field-measured s-values, and of 3.13 cm and 3.04 cm for l-values. Validation of the results reveals that the influence of the ACF, needed within the retrieval, is distinct.

Permanently frozen and seasonally frozen soils occur over a large portion of the Earth's land surface. Changes in the freeze/thaw state of the land surface reflects major changes in thermal and hydraulic properties as well as acting as a “switch” for many ecological processes. In short, soil freeze/thaw state is a fundamental land surface variable in the water and energy cycles, and it connects to the carbon cycle. Surface freeze/thaw state is observable by passive and active microwave sensors. For example, NASA's Soil Moisture Active Passive (SMAP) mission includes a freeze/thaw data product. Such satellite sensing offers routine all-season and all-weather global observations of soil freeze/thaw state with the application of suitable algorithms. We describe early finding from the SLAPex Freeze/Thaw campaign, believed to be the first airborne campaign of its type, focusing on soil freeze/thaw.

Electromagnetic backscattering from hemlock trees in Howland forest, Maine, is studied. The forest is assumed to be a sparsely distributed medium with discrete dielectric scatterers, namely; leaves, branches and trunks. The earth beneath is a moderately rough surface. Backscattering is computed as a sum of direct, direct-reflected and surface backscattering coefficients. The trunk-ground contribution can be represented as a component reflected from the average surface and a contribution for the surface fluctuations. Previously, Kirchhoff's Method (Lang 2004) had been used to represent the scattering from the rough surface. The Integral Equation Method (IEM) which incorporates the Kirchhoff's approach will be used here. Comparisons of the results for the Kirchhoff and IEM approaches will be made. Both approaches show that when the surface is very rough, the backscatter from the trunk rough surface fluctuation component is dominant. These model results will then be compared to backscatter measurements made by Air SAR overflights in 1990.

Recently, the dielectric constant of seawater at L-band was determined by employing a resonant cavity technique. A dielectric model function has been developed based on the measurement data and the model function has been used for retrieving the ocean salinity. The results indicate that additional accuracy is still needed to resolve the bias correlated with sea surface temperature. This paper reports the improvements that have been made recently for the development of a more accurate seawater dielectric model function. The additional measurements for the open ocean will be addressed in the paper.

A coherent bistatic vegetation scattering model, based on a Monte Carlo simulation, is being developed to simulate polarimetric bi-static reflectometry at VHF/UHF-bands (240-270 MHz). The model is aimed to assess the value of geostationary satellite signals of opportunity to enable estimation of the Earth's biomass and root-zone soil moisture. An expression for bistatic scattering from a vegetation canopy is derived for the practical case of a ground-based/low altitude platforms with passive receivers overlooking vegetation. Using analytical wave theory in conjunction with distorted Born approximation (DBA), the transmit and receive antennas effects (i.e., polarization, orientation, height, etc.) are explicitly accounted for. Both the coherency nature of the model (joint phase and amplitude information) and the explicit account of system parameters (antenna, altitude, polarization, etc) enable one to perform various beamforming techniques to evaluate realistic deployment configurations. In this paper, several test scenarios will be presented and the results will be evaluated for feasibility for future biomass and root-zone soil moisture application using geostationary communication satellite signals of opportunity at low frequencies.

Microwave back and forward scattering from a layer of vegetation continues to have important applications in remotely sensing soil moisture and vegetation biomass. Satellite radars detect backscattered returns while GNSS receivers can measure forward scatter. The scattering cross sections can be computed by the Distorted Born Approximation (DBA) and by 1st order transport theory. The first is a field based method while the second is based on conservation of energy. These two theories give the same results except for the presence of enhancement terms in the DBA theory. The DBA results will be derived by a simple voltage based pencil beam approach. The role of enhancement will be explored in both the backscatter and forward scatter cases. Results for realistic vegetation canopies such as agriculture crops at L-band frequencies will be used to demonstrate where enhancement effects are important; both like and cross polarization calculations. The effect of the antenna beamwidth will be presented.

Existing model functions for the dielectric constant of sea water produce viable retrievals of ocean salinity but with differences that are important for eventual optimization of the science product. Issues exist in cold water and the dependence on sea surface temperature. Additional accuracy is needed to resolve the behavior in cold water and a residual bias correlated with sea surface temperature. The model function developed from direct measurements of the dielectric constant highlights the issues and points to regimes where new measurements and accuracy are needed.

This paper describes a new model of the seawater dielectric constant as a function of salinity and temperature at L-band. The model function is developed by fitting the accurate measurement data made at 1.413 GHz to a third-order polynomial. The purpose of this study is to provide an accurate model for earth-observing satellites to retrieve seawater salinities from remote sensing data. In this paper, the development of the model function is introduced along with an analysis of the goodness of fit. The model function is then compared with the model functions of Klein-Swift and Meissner-Wentz. Finally, the comparison is made between the retrieved salinity from the satellite data with the in situ data measured by Argo floats.

This paper reports on a recent field campaign that aims to collect time-series multi-frequency microwave data over winter wheat during the entire growth cycle. The data are being collected to characterize vegetation dynamics and to quantify its effects on soil moisture retrievals. A C-band radar was recently incorporated within the existing L-band radar/radiometer system called ComRAD (SMAP's ground based simulator) and an additional VHF receiver is being constructed as well. With C-band's ability to sense vegetation details and VHF's root-zone soil moisture within ComRAD's footprint, we will have an opportunity to test our `discrete scatterer' vegetation models and parameters at various surface conditions. The purpose of this investigation is to determine optical depth and effective scattering albedo of vegetation of a given type (i.e. winter wheat) at various stages of growth that are needed to refine soil moisture retrieval algorithms for the SMAP mission.

This paper traces the development vegetation modeling during the 1980's. In this time period, discrete random media methods were applied to model vegetation. The distorted Born approximation and transport theory were successfully employed to understand the different contributions to the backscattering cross section from a layer of vegetation. Leaves were modeled by thin dielectric discs and stems, trunks and branches by dielectric cylinders. The discussion will emphasize active modeling but many of the methods apply to the passive case. In addition, most of the references refer to US authors. Contributions form researchers in other countries have not been considered at this time.

Vegetation water content (VWC) is an important component of microwave soil moisture retrieval algorithms. This paper aims to estimate VWC using L band active and passive radar/radiometer datasets obtained from a NASA ground-based Soil Moisture Active Passive (SMAP) simulator known as ComRAD (Combined Radar/Radiometer). Several approaches to derive vegetation information from radar and radiometer data such as HH, HV, VV, Microwave Polarization Difference Index (MPDI), HH/VV ratio, HV/(HH+VV), HV/(HH+HV+VV) and Radar Vegetation Index (RVI) are tested for VWC estimation through a generalized linear model (GLM). The overall analysis indicates that HV radar backscattering could be used for VWC content estimation with highest performance followed by HH, VV, MPDI, RVI, and other ratios.

NASA's Soil Moisture Active Passive (SMAP) mission is scheduled for launch in November 2014. In the prelaunch time frame, the SMAP team has focused on improving retrieval algorithms for the various SMAP baseline data products. The SMAP passive-only soil moisture product depends on accurate parameterization of the tau-omega model to achieve the required accuracy in soil moisture retrieval. During a field experiment (APEX12) conducted in the summer of 2012 under dry conditions in Maryland, the ComRAD truck-based SMAP simulator collected active/passive microwave time series data at the SMAP incident angle of 40° over corn and soybeans throughout the crop growth cycle. A similar experiment was conducted only over corn in 2002 under normal moist conditions. Data from these two experiments will be analyzed and compared to evaluate how changes in vegetation conditions throughout the growing season in both a drought and normal year can affect parameterizations in the tau-omega model for more accurate soil moisture retrieval.

Once launched in late 2014, NASA's Soil Moisture Active Passive (SMAP) mission will use a combination of a four-channel L-band radiometer and a three-channel L-band radar to provide high resolution global mapping of soil moisture and landscape freeze/thaw state every 2-3 days. These measurements are valuable to improved understanding of the Earth's water, energy, and carbon cycles, and to many applications of societal benefit. In order for soil moisture to be retrieved accurately from SMAP microwave data, prelaunch activities are concentrating on developing improved geophysical retrieval algorithms for each of the SMAP baseline products. The ComRAD truck-based SMAP simulator collected active/passive microwave time series data at the SMAP incident angle of 40° over corn and soybeans during 2012 for use in refining SMAP retrieval algorithms.

Seawater salinity measurements are currently being made at L-band (1.413 Ghz) by NASA's Aquarius instrument (on the Aquarius/SAC-D observatory). The goal of Aquarius mission is to measure the salinity of seawater to an accuracy on the order of 0.2 psu; this requires a model function of seawater permittivity with a high accuracy. Since 2011, the George Washington University (GW) has employed a cavity technique to determine the complex permittivity of seawater at 1.413 GHz. In this paper, a new seawater dielectric model function is introduced including the latest permittivity data for seawater with salinity 33 psu. Finally, the validation of the end-effect, measurement variance and data fitting will be discussed.

Tree scattering problems at L band contain tree components in close proximity of each other. In this case, the multiple scattering effects between tree components become more significant. Scattering properties of a simple tree with primary branches over the ground are studied in this paper. The Method of Moments is applied to find the scattering amplitude of the tree. Results for the bistatic scattering cross sections including the multiple scattering are compared with single scattering results. They suggest that coherent multiple scattering effects in trees can be significant in certain cases.

This study reports on the utilization of microwave modeling, together with ground truth, and L-band (1.4-GHz) brightness temperatures to investigate the passive microwave characteristics of a conifer forest floor. The microwave data were acquired over a natural Virginia Pine forest in Maryland by a ground-based microwave active/passive instrument system in 2008/2009. Ground measurements of the tree biophysical parameters and forest floor characteristics were obtained during the field campaign. The test site consisted of medium-sized evergreen conifers with an average height of 12 m and average diameters at breast height of 12.6 cm. The site is a typical pine forest site in that there is a surface layer of loose debris/needles and an organic transition layer above the mineral soil. In an effort to characterize and model the impact of the surface litter layer, an experiment was conducted on a day with wet soil conditions, which involved removal of the surface litter layer from one half of the test site while keeping the other half undisturbed. The observations showed detectable decrease in emissivity for both polarizations after the surface litter layer was removed. A first-order radiative transfer model of the forest stands including the multilayer nature of the forest floor in conjunction with the ground truth data are used to compute forest emission. The model calculations reproduced the major features of the experimental data over the entire duration, which included the effects of surface litter and ground moisture content on overall emission. Both theory and experimental results confirm that the litter layer increases the observed canopy brightness temperature and obscure the soil emission.

A Fresnel double scattering (FDS) method is presented in this paper to accurately and efficiently calculate the scattering cross section from two tree branches not necessarily in the far field of each other at L-band frequencies. The branches, modeled as dielectric cylinders, are assumed to have lengths comparable to one or more wavelengths. It is demonstrated that the FDS method provides a good approximation to the exact solutions of two branches modeled as one scatterer. The FDS method can be employed to study the double scattering effects between tree branches in microwave forest scattering models and accurately model a tree where single scatter and double scatter are the only important scattering mechanisms.

From a campaign conducted in 2002, hourly L-band H polarized TB(Brightness temperature) measurements are available for five episodes distributed over the corn growth cycle. In this study, fitting the τ-ω model to the TBmeasurements shows that the empirical parameter b, defining the optical depth or canopy opacity (r), and its dependence towards the incidence and azimuth angles both change during the growth cycle. The b found for the early growth stage is about three times larger than expected based on the literature, while near peak biomass and at senescence its value is about half. Moreover, the soil moisture dependence of the roughness and crop row orientation are found to be important uncertainties in the TBsimulations. The latter effect is particularly significant at senescence.

This paper investigates tree scattering effects at L-band by using a first-order radiative transfer (RT) model and truck-based measurements of brightness temperature over natural conifer stands to assess the performance of the τ - ω (tau-omega) model, a zero-order RT solution, over forest canopies. The tau-omega model accounts for vegetation effects in terms of "effective" vegetation parameters (single-scattering albedo and vegetation opacity) which represent the canopy as a whole. This approach inherently ignores multiple-scattering effects and it thus has a limited validity depending on the level of scattering within the canopy. The fact that the scattering from large forest components such as branches and trunks is significant at L-band requires that retrieved vegetation parameters be evaluated (compared) with their theoretical definitions to provide better understanding of these parameters in the soil moisture (SM) retrievals over moderately to densely vegetated landscapes. In this paper, the tau-omega model is fitted to a first-order RT model with an "effective" albedo assuming that "effective" vegetation optical depth is same as the "theoretical" opacity [1]. The "effective" albedo is found to be less than half of the "theoretical" one, which is generally around 0.5-0.6 for tree canopies at L-band. The "effective" albedo differs from the albedo of a single forest canopy element and becomes a global parameter which depends on all the processes taking place within the canopy including multiple-scattering and ground reflection.

A Fresnel Double Scattering (FDS) method is presented in this paper to accurately and efficiently calculate the backscattering cross sections from two tree branches in the Fresnel zone of each other. The branches have a comparable size to the wavelength for L band frequencies in length and in radius. It is demonstrated that the FDS results provide a good approximation to the exact solutions where two branches are modeled as one scatterer. The FDS method can be employed to study the double scattering effects between tree branches in microwave forest scattering models and accurately model a tree with single scatter and double scatter.

The 2009 IEEE Geoscience and Remote Sensing Society's (GRS-S) Awards were presented at IGARSS 2009. These awards included Fellow Recognitions, Major Awards, Publication Awards, and Certificates of Recognition. Those presented with awards are listed here.

In this study, a first-order radiative transfer (RT) model is developed to more accurately account for vegetation canopy scattering by modifying the basic τ-ω model (the zero-order RT solution). In order to optimally utilize microwave radiometric data in soil moisture (SM) retrievals over vegetated landscapes, a quantitative understanding of the relationship between scattering mechanisms within vegetation canopies and the microwave brightness temperature is desirable. The first-order RT model is used to investigate this relationship and to perform a physical analysis of the scattered and emitted radiation from vegetated terrain. This model is based on an iterative solution (successive orders of scattering) of the RT equations up to the first order. This formulation adds a new scattering term to the τ-ω model. The additional term represents emission by particles (vegetation components) in the vegetation layer and emission by the ground that is scattered once by particles in the layer. The model is tested against 1.4-GHz brightness temperature measurements acquired over deciduous trees by a truck-mounted microwave instrument system called ComRAD in 2007. The model predictions are in good agreement with the data, and they give quantitative understanding for the influence of first-order scattering within the canopy on the brightness temperature. The model results show that the scattering term is significant for trees and modifications are necessary to the τ-ω model when applied to dense vegetation. Numerical simulations also indicate that the scattering term has a negligible dependence on SM and is mainly a function of the incidence angle and polarization of the microwave observation.

The 19 papers in this special issue were originally presented at MicroRad 2010, held in Washington, DC from March 1 to 4, 2010.

This paper discusses the results from a series of field experiments using ground-based L-band microwave active/passive sensors. Three independent approaches are applied to the microwave data to determine vegetation opacity of coniferous trees. First, a zero-order radiative transfer model is fitted to multi-angular microwave emissivity data in a least-square sense to provide “effective” vegetation optical depth. Second, a ratio between radar backscatter measurements with a corner reflector under trees and in an open area is calculated to obtain “measured” tree propagation characteristics. Finally, the “theoretical” propagation constant is determined by forward scattering theorem using detailed measurements of size/angle distributions and dielectric constants of the tree constituents (trunk, branches, and needles). The results indicate that “effective” values underestimate attenuation values compared to both “theoretical” and “measured” values.

Vegetation scattering problems in L band contain plant or tree components in the near field of each other. In this case, the interleaf or inter-branch interaction effect becomes more significant. In this paper, scattering properties from a cluster of plant or tree components are analyzed to study the interaction effect in the cluster. A numerical approach, the Discrete Dipole Approximation, is applied to find the scattering amplitude of the cluster. Numerical results of bistatic cross sections including the mutual interaction effect are compared with those results not taking the interaction into account. The interaction effect in a cluster of plant or tree components is presented in the examples given in this paper.

This research presents a methodology to correct backscatter (σo) observations for vegetation effects. The proposed methodology is based on the concept that the ratio between the surface scattering over the total amount of scattering (CTsurface /σsooil) is affected only by the vegetation and can be described as a function of the vegetation water content. The data set used in this study was collected at USDA's Optimizing Production Inputs for Economic and Environmental Enhancement (OPE3) experimental site in Beltsville, Maryland (USA) over a corn growth cycle in 2002 and includes C-band (4.75 GHz) HH- and VV-polarized observations acquired at incidence angles of 15, 35 and 55 degrees. During this period the corn crops reached peak biomass of 6.6 kg m−2 and a soil moisture range varying from 0.02 to 0.26 cm3cm−3. The results show that through application of the proposed vegetation correction methodology the soil moisture retrieval accuracy can be improved from 0.033 to 0.032 cm3cm−3, 0.049 to 0.033 cm3cm−3, and 0.079 to 0.047 cm3cm−3 at incidence angles of 15, 35 and 55 degrees, respectively.

A first order scattering model is developed and tested at 1.4 GHz by using microwave brightness temperature data acquired over deciduous tree canopies in Maryland during 2007. Microwave measurements at several incident angles and supporting ground truth data (including size/angle distributions of tree constituents) have been collected over stands of deciduous Paulownia trees under full canopy and leaf-drop conditions. Detailed ground truth data obtained during this experiment have been used to compute the additional radiation due to scattering and emission by the vegetation components. The preliminary model predictions are in good agreement with the data and they give quantitative understanding for the influence of the first order scattering within the canopy on the radiometer brightness temperature. The model results using tree ground truth show that the scattering term is significant for trees and that the tau-omega model needs modification to account for additional scattering contribution. Numerical simulations also indicate that the single scattered radiation increases the canopy brightness temperature considerably. These simulations show that the scattering term has a negligible dependence on soil moisture and is only function of angle and polarization.

During 2007-2009 field experiments have been conducted using the ComRAD microwave truck instrument system with a goal of optimizing microwave soil moisture retrieval algorithms for small to medium deciduous and coniferous trees. A joint effort of NASA / GSFC and George Washington University, ComRAD consists of a quad-polarized 1.25 GHz radar and a dual-polarized 1.4 GHz radiometer sharing the same antenna. In the current study, ComRAD microwave data and ground truth measurements of soil moisture, temperature, soil texture, and vegetation water content and geometry statistics have been used to assess whether the zero-order tau-omega model can be employed successfully to retrieve soil moisture under tree canopies using effective values for tau (the vegetation opacity) and omega (the single scattering albedo). In addition, the tau-omega model has been modified to include a first-order scattering term, which will be discussed in a companion paper.

This paper presents a discrete interferometric model of random media. The forest is modeled by characterizing tree trunks, branches and leaves with randomly oriented, lossy dielectric particles whose area and orientation in a layer of random medium are prescribed. The model result is compared to the theoretical result in the literature and good agreement is found.

The 2008 IEEE Geoscience and Remote Sensing Society's (GRS-S's) Awards were presented at the International Geoscience and Remote Sensing Symposium (IGARSS) 2008. These included IEEE Fellow Recognition; GRS-S Distinguished Achievement Award; GRS-S Outstanding Service Award; GRS-S Education Award; IEEE Electromagnetics Award; several Publication Awards and two Certificates of Recognition.

In the radiometric sensing of soil moisture through a forest canopy, knowledge of canopy attenuation is required. Active sensors have the potential of providing this information since the backscatter signals are more sensitive to forest structure. In this paper, a new radar technique is presented for estimating canopy attenuation. The technique employs details found in a transient solution where the canopy (volume-scattering) and the tree-ground (double-interaction) effects appear at different times in the return signal. The influence that these effects have on the expected time-domain response of a forest stand is characterized through numerical simulations. A coherent forest scattering model, based on a Monte Carlo simulation, is developed to calculate the transient response from distributed scatterers over a rough surface. The forest transient-response model for linear copolarized cases is validated with the microwave deciduous tree data acquired by the Combined Radar/Radiometer (ComRAD) system. The attenuation algorithm is applicable when the forest height is sufficient to separate the components of the radar backscatter transient response. The frequency correlation functions of double-interaction and volume-scattering returns are normalized after being separated in the time domain. This ratio simply provides a physically based system of equations with reduced parameterizations for the forest canopy. Finally, the technique is used with ComRAD L-band stepped-frequency data to evaluate its performance under various physical conditions.

This research presents a methodology to correct backscatter (sigmadeg) observations for vegetation effects. The proposed methodology is based on the concept that the ratio between the surface scattering over the total amount of scattering (sigmadegsurface/sigmadegsoil) is affected only by the vegetation and can be described as a function of the vegetation water content. The data set used in this study was collected at USDA's Optimizing Production Inputs for Economic and Environmental Enhancement (OPE3) experimental site in Beltsville, Maryland (USA) over a corn growth cycle in 2002 and includes C-band (4.75 GHz) HH- and VV-polarized observations acquired at incidence angles of 15, 35 and 55 degrees. During this period the corn crops reached peak biomass of 6.6 kg m-2and a soil moisture range varying from 0.02 to 0.26 cm3cm-3. The results show that through application of the proposed vegetation correction methodology the soil moisture retrieval accuracy can be improved from 0.033 to 0.032 cm3cm-3, 0.049 to 0.033 cm3cm-3, and 0.079 to 0.047 cm3cm-3at incidence angles of 15, 35 and 55 degrees, respectively.

Truck-based measurements of brightness temperature at L- band over small deciduous stands located in Maryland were made in 2006 and 2007. Ground truth data related to forest stands and the ground were also collected. The deciduous trees were modeled by the Distorted Born Approximation (DBA) in conjunction with Peak's principle. The ground was modeled as a half space with surface roughness. The model has been used to investigate the sensitivity of L-band radiometers to soil moisture under forest stands. It was observed that it is possible to see through the forest to sense the underlying soil moisture and to see the seasonal changes.

Measurements of the complex dielectric constant of seawater at 30.00 psu, 35.00 psu and 38.27 psu over the temperature range from 5degC to 35degC at 1.413 GHz are given and compared with the Klein-Swift results. A resonant cavity technique is used. The calibration constant used in the cavity perturbation formulas is determined experimentally using methanol and ethanediol (ethylene gycol) as reference liquids. Analysis of the data shows that the measurements are accurate to better than 1.0% in almost all cases studied.

During 2007 a field experiment was conducted with a goal of optimizing microwave soil moisture retrieval algorithms for small to medium deciduous trees. After initial field checkout in Fall 2006, the ComRAD microwave truck instrument system was deployed to a test site with several stands of deciduous paulownia trees. A joint effort of NASA/GSFC and George Washington University, ComRAD consists of a quad-polarized 1.25 GHz radar and a dual-polarized 1.4 GHz radiometer sharing the same antenna. ComRAD can function as a ground-based instrument simulator for L band space missions such as SMOS, SMAP, and Aquarius. In the current study, ComRAD acquired data from April to November 2007 to monitor the seasonal difference in microwave response to soil moisture under deciduous trees. To conclude the three-year planned field measurement effort, ComRAD will deploy to a natural coniferous pine tree site in 2008.

The common two parameter `tau-omega' model for emission by vegetation assumes a uniform attenuating canopy with some darkening due to scattering. As a result, the structural features of the canopy are dismissed, rendering the model insensitive to the azimuthal dependence of brightness temperature. The objective of this study is to demonstrate the effects of canopy structure on the L-band emission by employing a full-wave approach in conjunction with Monte-carlo (MC) averaging. The simulations are performed on corn canopies consisting of only stalks.

This paper reports on the retrieval of soil moisture from dual-polarized L-band (1.6 GHz) radar observations acquired at view angles of 15$^{\circ}$, 35$^{\circ}$, and 55$^{\circ}$, which were collected during a field campaign covering a corn growth cycle in 2002. The applied soil moisture retrieval algorithm includes a surface roughness and vegetation correction and could potentially be implemented as an operational global soil moisture retrieval algorithm. The surface roughness parameterization is obtained through inversion of the Integral Equation Method (IEM) from dual-polarized (HH and VV) radar observations acquired under nearly bare soil conditions. The vegetation correction is based on the relationship found between the ratio of modeled bare soil scattering contribution and observed backscatter coefficient$(\sigma^{\rm soil}/\sigma^{\rm obs})$and vegetation water content$(W)$. Validation of the retrieval algorithm against ground measurements shows that the top 5-cm soil moisture can be estimated with an accuracy between 0.033 and 0.064$\hbox{cm}^{3}\cdot\hbox{cm}^{-3}$, depending on the view angle and polarization.

A new temperature controlled microwave cavity system to measure the complex dielectric constant of seawater at 1.413 GHz is discussed. The system is being developed to measure seawater for temperatures from 0degC to 30degC and salinities from 10 to 40 psu. The paper discusses the construction of the measurement system and initial stability tests.

The NASA/GSFC and George Washington University network analyzer-based multifrequency truck- mounted radar system has recently been upgraded with the addition of a dual-polarized 1.4 GHz total power radiometer. The system, now called ComRAD for Combined Radar/Radiometer, can function as a ground-based instrument simulator for L band space missions such as Hydros, Aquarius, and SMOS. In late summer 2006 ComRAD was deployed to the field to begin a series of coordinated active/passive L band measurements over small stands of deciduous and coniferous trees in order to improve our understanding of the microwave properties of trees and their effect on soil moisture retrieval algorithms. This paper describes the preliminary measurements obtained at the start of a three-year planned field measurement effort.

Retrieval of soil moisture under corn canopies has been investigated by many researchers and is still a problem that is not fully assessed. Corn is an organized canopy. This feature produces effects not observed from natural canopies. One such effect is the dependence of sensor response on the azimuthal direction. The aim of this study is to explore these distinguishing effects by Monte-Carlo simulations.

Relations between various definitions of albedo for a layer of random discrete scatterers are established. Albedo and attenuation values are obtained for soybeans and corn using a discrete model. Attenuation values are also obtained using an empirical approach. Brightness temperatures, computed by transport theory with these albedo and attenuation values, are compared with the measured brightness temperature values for soybeans and corn

This paper studies the feasibility of making backscatter measurements from rainfall with the NASA/Microwave Link system at Wallops Island, VA. The research entails the implementation of an FMCW radar at the Link frequencies to enable simultaneous forward and backscatter measurements from rain. The backscatter measurements will be used in conjunction with the forward measurements and the measurements from a ground-based network of disdrometers and rain gauges located under the propagation path to develop new microwave retrieval techniques, and to test established single-frequency and dual-frequency radar retrieval-algorithms relevant to the ongoing TRMM and up coming GPM missions

Accurate relationships between salinity and dielectric constant (which determines emissivity) are needed for sensor systems such as SMOS and Aquarius that will monitor salinity from space in the future. This paper describes a resonant cavity technique for the measurement of the dielectric constant of seawater as a function of its salinity. The purpose of the new measurements is to establish the dependence of the dielectric constant of seawater on salinity in contemporary units (e.g. psu) and to take advantage of modern instrumentation to increase the accuracy of these measurements.

Soil moisture is a critical state variable in land surface hydrology. Large-scale soil moisture mapping based on microwave remote sensing would be valuable in many different practical and theoretical applications, and a real potential exists for new space missions in the near future which well utilize simultaneous active/passive microwave measurements for global soil moisture retrieval. This paper discusses the experiment for the retrieval of soil moisture using radar and radiometric measurements. It was shown that combinations of simultaneous radar and radiometer data can enhance soil moisture retrievals, especially in the presence of dynamic vegetation.

Attenuation measurements at 25 and 38 GHz performed with the NASA/TRMM Microwave Link provide information about the drop size distribution (DSD) along the propagation path. Additional path-average measurements along the Link path, such as a third attenuation measurement or the rain rate from well-calibrated raingauges, can provide further DSD information. This paper explores an inversion procedure for determining simultaneously three parameters of a gamma DSD by using three measurements. Also, some preliminary results obtained using Link data are presented.

Radiometers require periodic calibration to correct for instabilities in the receiver response. Various calibration techniques exist that minimize the effect of instabilities in the receivers. The optimal technique depends upon many parameters. Some parameters are constrained by the particular application and others can be chosen in the system design. For example, the measurement uncertainty may be reduced to the limits of the resolution of the measurement (sensitivity) if periodic absolute calibration can be performed with sufficient frequency. However if the period between calibrations is long, a reference-differencing technique, i.e. Dicke-type design, can yield better performance. The measurement uncertainty not only depends upon the detection scheme but also on the number of pixels between calibrations, the integration time per pixel, integration time per calibration reference measurement, calibration reference temperature, and the brightness temperature of what is being measured. The best scheme for reducing the measurement uncertainty also depends, in large part, on the stability of the receiver electronics. A framework for evaluating calibration schemes for a wide range of system architectures is presented. Two methods for treating receiver non-stationarity are compared with radiometer measurements.

In an effort to study the drop size distribution (DSD) a state-of-the-art instrument arrangement was deployed on Wallops Island, VA. The instrumentation consisted of a 2.3-km multi-frequency microwave link, three impact disdrometers, and a network of optical and tipping bucket raingauges. A dual-frequency inversion technique was implemented with the link measurements of attenuations at 25 GHz and 38 GHz to estimate the path-average DSD. Concurrently, an X-band, dual-polarization radar, located in the vicinity, collected polarization and reflectivity measurements over the link path. The evaluation of the estimates and measurements generated some preliminary results.

Microwave attenuation measurements at 25 and 38 GHz made on a 2.3-km microwave link are employed to estimate drop size distributions (DSD), rainfall rate, and rainfall accumulation. A theoretical model for the propagation of microwaves in a link system sets forth the basis for the development of a dual-wavelength analytical technique to invert two parameters of a path-average gamma DSD. The DSDs obtained from the technique are evaluated in conjunction with point measurements performed with a 2-D video disdrometer. Additionally, the DSDs yield path-average rainfall rates and rainfall accumulation which are compared with path-average measurements from a network of optical and tipping bucket rain gauges located beneath the link path, and with estimates based on empirical power law relations.

Complex permittivity spatial distributions of two conifers, a 5.0-m high Caucasian fir (Abies normanniana) and a 1.9-m high spruce (Picea omorika), have been measured in the frequency range 1-10 GHz. Unlike earlier studies, particular attention was paid to the anisotropy of the dielectric properties. The measurements were performed in the frame of polarimetric scattering, and imaging experiments conducted on both trees in the European Microwave Signature Laboratory (EMSL), Space Applications Institute, Joint Research Centre, Ispra, Italy. They will be used, together with carefully established architectural models of the trees, to validate forest remote-sensing algorithms. The measurement method was based on an open-ended coaxial probe reflection technique with a rational function approximation model for the probe tip aperture admittance. With this model, no calibration on reference liquids is required and sufficiently accurate results for the dielectric constant and loss factor can be obtained. Results are presented for branches, parts of the trunks, and needles from different tree heights. Values obtained with the probe oriented along different stem directions of the trunk confirm the anisotropic nature of wood. The longitudinal complex permittivity is roughly 1.5-three times higher than the transverse component. Inside the trunk, early and late wood layers and small heterogeneities give rise to fluctuations; the phloem layer and the new needles have the highest complex permittivity. Representative average values for trunks, branches, and needles for modeling purposes are also given.

As part of the Multisensor Aircraft Campaign, MACHYDRO, two microwave sensors, NASA's Airborne Synthetic Aperture Radar (AIRSAR) and Pushbroom Microwave Radiometer (PBMR) collected data over the same corn fields during the summer of 1990. During these flights, measurements were made on the ground of soil moisture and plant parameters. In this paper the measured canopy and soil parameters are used in a discrete scatter model to predict the response of both sensors (radar and radiometer). A distorted Born approximation is used to compute the scattering coefficient for the corn canopy. The backscatter coefficient gives the radar response and the radiometer response is obtained by integrating the bistatic coefficient over all scattering angles above ground. The objective of this analysis is to test the model and, in particular, to determine how well a single set of plant parameters and single model can yield agreement with both the radar and radiometer measurements. The model values are in reasonably good agreement with the measurements at horizontal polarization and reflect observed changes in soil moisture.>

MACHYDR0'90 was an experiment conducted in Pennsylvania in 1990 to study the synergistic use of remote sensors in multitemporal hydrologic studies. As part of this mission the pushbroom microwave radiometer was flown and used to produce brightness temperature maps. Verification studies and vegetation algorithms for mixed land cover areas are described.>

Microwave radar and radiometer measurements of grasslands indicate a substantial reduction in sensor sensitivity to soil moisture in the presence of a thatch layer. When this layer is wet it masks changes in the underlying soil, making the canopy appear warm in the case of passive sensors (radiometer) and decreasing backscatter in the active case (scatterometer). A model for a grass canopy with thatch is presented in order to explain this behavior and for comparison with observations. The canopy model consists of three layers: grass, thatch, and the underlying soil. The grass blades are modeled by elongated elliptical discs and the thatch is modeled as a collection of disk shaped water droplets (i.e., the dry matter is neglected). The ground is homogeneous and flat. The distorted Born approximation is used to compute the radar cross section of this three layer canopy and the emissivity is computed from the radar cross section using the Peake formulation for the passive problem. Results are computed at L-band (1.4 GHz) and C-band (4.75 GHz) using canopy parameters (i.e., plant geometry, soil moisture, plant moisture, etc.) representative of Konza Prairie grasslands. The results are compared to C-band scatterometer measurements and L-band radiometer measurements at these grasslands.>

A microwave scattering model was developed for layered vegetation based on an iterative solution of the radiative transfer equation up to the second order to account for multiple scattering within the canopy and between the ground and the canopy. The model is designed to operate over a wide frequency range for both deciduous and coniferous forest and to account for the branch size distribution, leaf orientation distribution, and branch orientation distribution for each size. The canopy is modeled as a two-layered medium above a rough interface. The upper layer is the crown, containing leaves, stems, and branches. The lower layer is the trunk region, modeled as randomly positioned cylinders with a preferred orientation distribution above an irregular soil surface. Comparisons of results obtained using this model with measurements from deciduous and coniferous forests show good agreement at several frequencies for both like and cross polarizations.>

A solution is presented in the time domain for the scattering of a short pulse incident on a layer of discrete random media. The layer consists of identical, but randomly oriented particles situated above a homogeneous (i.e. deterministic) half-space. A solution is found for the backscattered pulse when the incident pulse is obliquely incident on the layer. The solution is obtained in the case of low particle density using the distorted Born approximation. The short pulse solution is an approximation to the impulse response (Green's function) for the layer and as such can be used to compute the response to pulses of arbitrary shape.>

The authors report on the use of microwave modeling, ground truth, and synthetic aperture radar (SAR) data to investigate the characteristics of forest stands. A mixed coniferous forest stand has been modeled at SAR frequencies (P-, L-, and C-bands). The extensive measurements of ground truth and canopy geometry parameters were performed in a 200 m-square hemlock-dominated plot inside a forest. Hemlock trees in the forest are modeled by characterizing tree trunks, branches, and needles (leaves) with randomly oriented, lossy dielectric cylinders whose area and orientation distributions are prescribed. The distorted Born approximation is used to compute the backscatter at P-, L-, and C-SAR frequencies.>

A new average equation for the adaptive weights which takes into account the effects of statistical weight fluctuations has been developed. Past researchers have derived results by using an approximate average equation for the weights obtained under the assumption that the weights vary much more slowly than the input process. The weights are then treated as constants for averaging. The new method employs a two-variable perturbation analysis which uses as a small parameter the ratio of the loop bandwidth to the input process bandwidth. The analysis yields a significant correction term for the directivity pattern in interference nulls. This correction term is the result of stochastic weight fluctuations which are usually ignored in most analyses. Comparison of these results to those found by past researchers is given and the results of a computer simulation substantiate the analysis.