Measuring soil moisture with imaging radars

An empirical algorithm for the retrieval of soil moisture content and surface root mean square (RMS) height from remotely sensed radar data was developed using scatterometer data. The algorithm is optimized for bare surfaces and requires two copolarized channels at a frequency between 1.5 and 11 GHz. It gives best results for kh⩽2.5, μ_υ⩽35%, and &thetas;⩾30°. Omitting the usually weaker hv-polarized returns makes the algorithm less sensitive to system cross-talk and system noise, simplifies the calibration process and adds robustness to the algorithm in the presence of vegetation. However, inversion results indicate that significant amounts of vegetation (NDVI>0.4) cause the algorithm to underestimate soil moisture and overestimate RMS height. A simple criteria based on the σ_hv⁰/σ_vv⁰ ratio is developed to select the areas where the inversion is not impaired by the vegetation. The inversion accuracy is assessed on the original scatterometer data sets but also on several SAR data sets by comparing the derived soil moisture values with in-situ measurements collected over a variety of scenes between 1991 and 1994. Both spaceborne (SIR-C) and airborne (AIRSAR) data are used in the test. Over this large sample of conditions, the RMS error in the soil moisture estimate is found to be less than 4.2% soil moisture     

Retrieving soil moisture from simulated brightness temperatures bya neural network

The authors present the retrievals of surface soil moisture (SM) from simulated brightness temperatures by a newly developed error propagation learning backpropagation (EPLBP) neural network. The frequencies of interest include 6.9 and 10.7 GHz of the advanced microwave scanning radiometer (AMSR) and 1.4 GHz (L-band) of the soil moisture and ocean salinity (SMOS) sensor. The land surface process/radiobrightness (LSP/R) model is used to provide time series of both SM and brightness temperatures at 6.9 and 10.7 GHz for AMSRs viewing angle of 55°, and at L-band for SMOS's multiple viewing angles of 0°, 10°, 20°, 30°, 40°, and 50° for prairie grassland with a column density of 3.7 km/m². These multiple frequencies and viewing angles allow the authors to design a variety of observation modes to examine their sensitivity to SM. For example, L-band brightness temperature at any single look angle is regarded as an L-band one-dimensional (1D) observation mode. Meanwhile, it can be combined with either the observation at the other angles to become an L-band two-dimensional (2D) or a multiple dimensional observation mode, or with the observation at 6.9 or 10.7 GHz to become a multiple frequency/dimensional observation mode. In this paper, it is shown that the sensitivity of radiobrightness at AMSR channels to SM is increased by incorporating L-band radiobrightness. In addition, the advantage of an L-band 2D or a multiple dimensional observation mode over an L-band 1D observation mode is demonstrated     

Ground-based passive microwave remote sensing observations of soilmoisture at S-band and L-band with insight into measurementaccuracy

A ground-based experiment in passive microwave remote sensing of soil moisture was conducted in Huntsville, AL, from July 1-14, 1996. The goal of the experiment was to evaluate the overall performance of an empirically-based retrieval algorithm at S-band and L-band under a different set of conditions and to characterize the site-specific accuracy inherent within the technique. With high temporal frequency observations at S-band and L-band, the authors were able to observe large scale moisture changes following irrigation and rainfall events, as well as diurnal behavior of surface moisture among three plots, one bare, one covered with short grass and another covered with alfalfa. The L-band emitting depth was determined to be on the order of 0-3 or 0-5 cm below 0.30 cm³/cm³ with an indication that it is less at higher moisture values. The S-band emitting depth was not readily distinguishable from L-band. The uncertainty in remotely sensed soil moisture observations due to surface heterogeneity and temporal variability in variables and parameters was characterized by imposing random errors on the most sensitive variables and parameters and computing the confidence limits on the observations. Discrepancies between remotely sensed and gravimetric soil moisture estimates appear to be larger than those expected from errors in variable and parameter estimation. This would suggest that a vegetation correction procedure based on more dynamic modeling may be required to improve the accuracy of remotely sensed soil moisture     

Improvement of Moisture Estimation Accuracy of Vegetation-Covered Soil by Combined Active/Passive Microwave Remote Sensing

The measured effects of vegetation canopies on radar and radiometric sensitivity to soil moisture are compared to first-order emission and scattering models. The models are found to predict the measured emission and backscattering with reasonable accuracy for various crop canopies at frequencies between 1.4 and 5.0 GHz, especially at angles of incidence less than 30°. The vegetation loss factor L (?) increases with frequency and is found to be dependent upon canopy type and water content. In addition, the effective radiometric power absorption coefficient of a mature corn canopy is roughly 1.75 times that calculated for the radar at the same frequency. Comparison of an L-band radiometer with a C-band radar shows the two systems to be complementary in terms of accurate soil moisture sensing over the extreme range of naturally occurring soil-moisture conditions. The combination of both an L-band radiometer and a C-band radar is expected to yield soil-moisture estimates that are accurate to better than +/-30 percent of true soil moisture, even for a soil under a lossy crop canopy such as mature corn. This is true even without any other ancillary information.     

Use of radar and optical remotely sensed data for soil moisture retrieval over vegetated areas

This work assesses the possibility of obtaining soil moisture maps of vegetated fields using information derived from radar and optical images. The sensor and field data were acquired during the SMEX'02 experiment. The retrieval was obtained by using a Bayesian approach, where the key point is the evaluation of probability density functions (pdfs) based on the knowledge of soil parameter measurements and of the corresponding remotely sensing data. The purpose is to determine a useful parameterization of vegetation backscattering effects through suitable pdfs to be later used in the inversion algorithm. The correlation coefficients between measured and extracted soil moisture values are R=0.68 for C-band and R=0.60 for L-band. The pdf parameters have been found to be correlated to the vegetation water content estimated from a Landsat image with correlation coefficients of R=0.65 and 0.91 for C- and L-bands, respectively. In consideration of these correlations, a second run of the Bayesian procedure has been performed where the pdf parameters are variable with vegetation water content. This second procedure allows the improvement of inversion results for the L-band. The results derived from the Bayesian approach have also been compared with a classical inversion method that is based on a linear relationship between soil moisture and the backscattering coefficients for horizontal and vertical polarizations.     

An observation system simulation experiment for the impact of landsurface heterogeneity on AMSR-E soil moisture retrieval

Using a high-resolution hydrologic model, a land surface microwave emission model (LSMEM), and an explicit simulation of the orbital and scanning characteristics for the advanced microwave sensing radiometer (AMSR-E), an observing system simulation experiment (OSSE) is carried out to assess the impact of land surface heterogeneity on large-scale retrieval and validation of soil moisture products over the U.S. Southern Great Plains using the 6.925 GHz channel on the AMSR-E sensor. Land surface heterogeneity impacts soil moisture products through the presence of nonlinearities in processes represented by the LSMEM, as well as the fundamental inconsistency in spatial scale between gridded soil moisture imagery derived from in situ point-scale sampling, numerical modeling, and microwave remote sensing sources. Results within the 575000 km² Red-Arkansas River basin show that, for surfaces with vegetation water contents below 0.75 kg/m², these two scale effects induce root mean squared errors (RMSEs) of 1.7% volumetric (0.017 cm_water³/cm_soil³ ) into daily 60 km AMSR-E soil moisture products and RMS differences of 3.0% (0.030 cm_water/3cm_soil³ ) into 60 km comparisons of AMSR-E soil moisture products and in situ field-scale measurements of soil moisture sampled on a fixed 25-km grid     

Global Simulation of Brightness Temperatures at 6.6 and 10.7 GHz Over Land Based on SMMR Data Set Analysis

In the framework of the Soil Moisture and Ocean Salinity mission, a two-year (1987–1988) global simulation of brightness temperatures (TB) at L-band was performed using a simple model [L-band microwave emission of the biosphere, (L-MEB)] based on radiative transfer equations. However, the lack of alternative L-band spaceborne measurements corresponding to real-world data prevented from assessing the realism of the simulated global-scale TB fields. In this study, using a similar modeling approach, TB simulations were performed at C-band and X-band. These simulations required the development of C-MEB and X-MEB models, corresponding to the equivalent of L-MEB at C-band and X-band, respectively. These simulations were compared with Scanning Multichannel Microwave Radiometer (SMMR) measurements during the period January to August 1987 (corresponding to the end of life of the SMMR mission). A sensitivity study was also carried out to assess, at a global scale, the relative contributions of the main MEB parameters (particularly the roughness and vegetation model parameters). Regional differences between simulated and measured TBs were analyzed, discriminating possible issues either linked to the radiative transfer model (C-MEB and X-MEB) or due to land surface simulations. A global agreement between observations and simulations was discussed and allowed to evaluate regions where soil moisture retrievals would give best results. This comparison step made at C-band and X-band allowed to better assess how realistic and/or accurate the L-band simulations could be.     

GaAs MESFET device dependences on ion-implant tilt and rotationangles

Lightly Cr-doped liquid-encapsulated Czochralski (LEC) GaAs wafers were implanted with 5×10¹² 100-keV Si²⁹ ions/cm² at tilt angles between 0 and 13° and at rotation angles between 0 and 45°C. Capacitance-voltage measurements were then made to determine electron profiles. It was found that cross-wafer device uniformity can be improved using implant tilt angles greater than 9°. For microwave MESFET devices, the maximum transconductances at low I_DS are achieved using tilt angles greater than 6° and rotation angles greater than 30°     

Improved inversion channel mobility for 4H-SiC MOSFETs followinghigh temperature anneals in nitric oxide

Results presented in this letter demonstrate that the effective channel mobility of lateral, inversion-mode 4H-SiC MOSFETs is increased significantly after passivation of SiC/SiO₂ interface states near the conduction band edge by high temperature anneals in nitric oxide. Hi-lo capacitance-voltage (C-V) and ac conductance measurements indicate that, at 0.1 eV below the conduction band edge, the interface trap density decreases from approximately 2×10¹³ to 2×10¹² eV^-1 cm^-2 following anneals in nitric oxide at 1175°C for 2 h. The effective channel mobility for MOSFETs fabricated with either wet or dry oxides increases by an order of magnitude to approximately 30-35 cm²/V-s following the passivation anneals     

High-resolution change estimation of soil moisture using L-band radiometer and Radar observations made during the SMEX02 experiments

The soil moisture experiments held during June-July 2002 (SMEX02) at Iowa demonstrated the potential of the L-band radiometer (PALS) in estimation of near surface soil moisture under dense vegetation canopy conditions. The L-band radar was also shown to be sensitive to near surface soil moisture. However, the spatial resolution of a typical satellite L-band radiometer is of the order of tens of kilometers, which is not sufficient to serve the full range of science needs for land surface hydrology and weather modeling applications. Disaggregation schemes for deriving subpixel estimates of soil moisture from radiometer data using higher resolution radar observations may provide the means for making available global soil moisture observations at a much finer scale. This paper presents a simple approach for estimation of change in soil moisture at a higher (radar) spatial resolution by combining L-band copolarized radar backscattering coefficients and L-band radiometric brightness temperatures. Sensitivity of AIRSAR L-band copolarized channels has been demonstrated by comparison with in situ soil moisture measurements as well as PALS brightness temperatures. The change estimation algorithm has been applied to coincident PALS and AIRSAR datasets acquired during the SMEX02 campaign. Using AIRSAR data aggregated to a 100-m resolution, PALS radiometer estimates of soil moisture change at a 400-m resolution have been disaggregated to 100-m resolution. The effect of surface roughness variability on the change estimation algorithm has been explained using integral equation model (IEM) simulations. A simulation experiment using synthetic data has been performed to analyze the performance of the algorithm over a region undergoing gradual wetting and dry down.     

Ku- and C-band SAR for discriminating agricultural crop and soilconditions

A method is proposed to estimate both green leaf area index (GLAI) and soil moisture (h_v), based on radar measurements at the Ku-band (14.85 GHz) and C-band (5.35 GHz) frequencies. The Ku-band backscatter at large incidence angles was found to be independent of soil moisture conditions and could be used alone to estimate GLAI. Then, the Ku-band estimate of GLAI could be used with a measurement of C-band backscatter in a canopy radiative transfer model to isolate the value of h_v. This concept was demonstrated with a set of Kuand C-band synthetic aperture radar (SAR) backscatter data acquired over agricultural fields in Arizona. The demonstration showed promise for operational application of the method, though several limitations were identified. Since both Ku- and C-band σ_° are sensitive to soil roughness, this approach must be applied only to fields of similar soil roughness or row direction. This limitation may be less serious for farm management applications since crop type and cultivation practices are generally well known and can be taken into consideration. Another limitation of the use of Ku- and C-band σ° is the apparent saturation of the Ku-band signal with increasing GLAI. Operational implementation of this approach will require dual-frequency sensors aboard an aircraft or orbiting satellite     

Optical channel waveguides in AlGaAs multiple-quantum-wellstructures formed by focused ion-beam-induced compositional mixing

Optical channel waveguiding in a AlGaAs multiple-quantum-well structure was demonstrated in a channel formed by compositional mixing induced by focused ion beam (FIB) implantation. Selective mixing was achieved by FIB implanting Si⁺⁺ with a dose of 5×10¹⁴ cm^-2 followed by rapid thermal annealing at 950°C for 10 s. Raman microprobe spectra were used to characterize the lateral variation of compositional mixing. Channel waveguide loss of 17.2 dB/cm was measured, compared to 10-12 dB/cm measured for planar waveguiding. Mode field pattern measurements indicate that a change in effective index of 2.7×10^-4 was induced, corresponding to an approximate mixing depth of 270 nm     

Seasat over-land scatterometer data. I. Global overview of theKu-band backscatterer coefficients

Statistics on the backscatter coefficient σ⁰ from the Ku-band Seasat-A Satellite Scatterometer (SASS) collected over the world's land surfaces are presented. This spaceborne scatterometer provided data on σ⁰ between latitude 80° S and 80° N at incidence angles up to 70°. The global statistics of vertical (V) and horizontal (H) polarization backscatter coefficients for 10° bands in latitude are presented for incidence angles between 20° and 70° and compared with the Skylab and ground spectrometer results. Global images of the time-averaged V polarization σ⁰ at a 45° incidence angle and its dependence on the incidence angle are presented and compared to a generalized map of the terrain type. Global images of the differences between the V an H polarization backscatter coefficients are presented and discussed. The most inhomogeneous region, which contains the deserts of North Africa and the Arabian Peninsula, is studied in greater detail and compared with the terrain type     

Microwave backscattering and emission model for grass canopies

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     

High-frequency measurements of AlGaN/GaN HEMTs at high temperatures

High-frequency measurements of the 1.3-μm-long gate AlGaN-GaN HEMTs have been performed at temperatures ranging from 23 to 187°C. The cutoff frequency f_T decreased with increasing temperature. It was 13.7 and 8.7 GHz at 23 and 187°C, respectively. The effective electron velocities υ_eff in the channel evaluated from the total delay time versus I_D-inverse relation were 1.2 and 0.8×10⁷ cm/s at 23 and 187°C, respectively     

Thin-film transistors in polycrystalline silicon by blanket andlocal source/drain hydrogen plasma-seeded crystallization

Thin film n-channel transistors have been fabricated in polycrystalline silicon films crystallized using hydrogen plasma seeding, by using several processing techniques with 600 to 625°C or 1000°C as the maximum process temperature. The TFTs from hydrogen plasma-treated films with a maximum process temperature of 600°C, have a linear field-effect mobility of ~35 cm²/Vs and an ON/OFF current ratio of ~10⁶, and TFTs with a maximum process temperature of 1000°C, have a linear field-effect mobility of ~100 cm²/Vs and an ON/OFF current ratio of ~10⁷. A hydrogen plasma has also then been applied selectively a in the source and drain regions to seed large crystal grains in the channel. Transistors made with this method with maximum temperature of 600°C showed a nearly twofold improvement in mobility (72 versus 37 cm² /Vs) over the unseeded devices at short channel lengths. The dominant factor in determining the field-effect mobility in all cases was the grain size of the polycrystalline silicon, and not the gate oxide growth/deposition conditions. Significant increases in mobility are observed when the grain size is in order of the channel length. However the gate oxide plays an important role in determining the subthreshold slope and the leakage current     

Monte Carlo simulations for tilted-channel electron multipliers

Microchannel electron multipliers with tilted structures are simulated using the Monte Carlo method. Gains of secondary electrons are calculated for different structures of the electron multiplier. For a short tilted cylindrical channel of the electron multiplier, a maximum gain is achieved greater than 10⁴ at a tilt angle near 25°. The maximum gain is about 10³ times larger than that of the nontilted channel. An explanation for the improvement of gain in tilted channel is suggested     

Abrupt and asymptotic transience in DWDM systems using backward-pumped fiber Raman amplifier

Abrupt power transience in wide (C+L)-band dense wavelength division multiplexing systems caused by stimulated Raman scattering during channel add/drop is investigated both theoretically and experimentally. It is found that adding L- or C-band channels leads to abrupt decrease or increase in surviving C- or L-band channels and is constructively or destructively added to the asymptotic transience in a system using backward-pumped fiber Raman amplifiers. Dropping of L- or C-band channels also leads to reversed consequences. The impact of channel add/drop on the steady-state Raman gain in such systems is also investigated.     

A multifrequency algorithm for the retrieval of soil moisture on alarge scale using microwave data from SMMR and SSM/I satellites

The sensitivity of microwave emission at different frequencies to soil moisture in bare and vegetated soils has been investigated using experimental data. Since the best frequency for the measurement of soil moisture (L-band) is absent in current satellite sensors, it is necessary to seek alternative solutions. An algorithm is proposed for the retrieval of soil moisture based on the sensitivity to moisture of both the brightness temperature and the polarization index at C-band, one that is able to correct for the effect of vegetation by means of the polarization index at X-band. The algorithm has been tested by using experimental data collected with airborne microwave radiometers on agricultural areas and validated by using the data sets of special sensor microwave/imager (SMM/I) and scanning multichannel microwave radiometer (SMMR). These research activities are planned in view of coming new satellites: AQUA (NASA) and ADEOS-II (NASDA), which will be launched by the end of 2001. These will have new generation microwave radiometers (AMSR-E and AMSR) onboard, which show much better characteristics with respect to the previous sensors, in particular an enhanced spatial resolution     

Effects of trap-state density reduction by plasma hydrogenation inlow-temperature polysilicon TFT

The reduction of trap-state densities by plasma hydrogenation in n-channel polysilicon thin-film transistors (poly-TFTs) fabricated using a maximum temperature of 600°C has been studied. Hydrogenated devices have a mobility of ~40 cm²/V×5, a threshold voltage of ~2 V, an inverse subthreshold of ~ 0.55 V/decade, and a maximum on/off current ratio of 5×10⁸. The effective channel length decreases by ~0.85 μm after a short hydrogenation which may be attributed to the activation of donors at trap states near the source/drain junctions. Trap-state densities decrease from 1.6×10¹² to 3.5×10¹¹ cm^-2 after hydrogenation, concomitant with the reduction of threshold voltage. Using the gate lengths at which the trap-state densities deviate from the long-channel values as markets for the leading edge of passivation, the apparent hydrogen diffusivity is found to be 1.2×10^-11 cm²/s at 350°C in the TFT structure