Retrieval of crop biomass and soil moisture from measured 1.4 and 10.65 GHz brightness temperatures

Physically based land surface process/radiobrightness (LSP/R) models may characterize well the relationship between radiometric signatures and surface parameters. They can be used to develop and improve the means of sensing surface parameters by microwave radiometry. However, due to a lack in the skill to properly understand the behavior of the data, a statistical approach is often adopted. In this paper, we present the retrieval of wheat plant water content (PWC) and soil moisture content (SMC) profiles from the measured H-polarized and V-polarized brightness temperatures at 1.4 (L-band), and 10.65 (X-band) GHz by an error propagation learning back propagation (EPLBP) neural network. The PWC is defined as the total water content in the vegetation. The brightness temperatures were taken by the PORTOS radiometer over wheat fields through three month growth cycles in 1993 (PORTOS-93) and 1996 (PORTOS-96). Note that, through the neural network, there is no requirement of ancillary information on the complex surface parameters such as vegetation biomass, surface temperature, and surface roughness, etc. During both field campaigns, the L-band radiometer was used to measure brightness temperatures at incident angles from 0 to 50/spl deg/ at L-band and at an incident angle of 50/spl deg/ at X-band. The SMC profiles were measured to the depths of 10 cm in 1993 and 5 cm in 1996. The wheat was sampled approximately once a week in 1993 and 1996 to obtain its dry and wet biomass (i.e., PWC). The EPLBP neural network was trained with observations randomly chosen from the PORTOS-93 data, and evaluated by the remaining data from the same set. The trained neural network is further evaluated with the PORTOS-96 data.     

Reanalysis of L-band brightness predicted by the LSP/R model-forprairie grassland: incorporation of rough surface scattering

L-band brightness predicted by the land surface process/radiobrightness (LSP/R) model for prairie grassland appears to be somewhat lower than expected. A crucial reason for the underestimate of the L-band brightness is that the soil surface was treated as smooth. In this paper, surface scattering of the soil determined by the IEM model is incorporated into the LSP/R model to examine its impact on the predicted L-band brightness. Eight sets of surface parameters, two correlation lengths (L) of 3 and 6 cm×4 root mean squared (RMS) heights (σ) of 0.3, 0.6, 0.8, and 1.0 cm, are utilized to characterize the emission of the soil surface. It is found that H-polarized, L-band brightness is expectedly increased by different levels for all of the eight rough surface cases compared to the smooth surface case. The increase in the average of the H-polarized, L-band brightness is by as much as 13.2 K for the case with L=3 cm and σ=1.0 cm. In addition, L-band's sensitivity to soil moisture is found to be approximately equal with and without the scattering effects. An increase in H-polarized, L-band brightness by about 12 K at the end of a 14-day simulation by the LSP/R model is in response to a decrease in soil moisture by 7% for all of the nine cases of concern (eight rough plus one smooth soil surfaces)     

L-band land mobile satellite (LMS) amplitude and multipath phasemodeling in urban areas

A study is presented of the amplitude and multipath phase variations found in the land mobile satellite channel at L-band (1.820 GHz) in urban areas. The received time-series are split into three categories or “states” according to the degree of shadowing experienced: line-of-sight, light shadow, and deep shadow. Statistical models and their corresponding parameters (extracted from experimental data) are provided for a wide range of elevation angles (10°-70°) both for the amplitude and for the phase variations due to multipath     

InGaAs microwave switch transistors for phase shifter circuits

A new InGaAs insulated-gate FET (IGFET) with 1 μm gate length and three different gate widths has been designed, fabricated and characterized as switch devices for microwave control applications in phase shifter circuits. The devices employed a plasma deposited silicon dioxide gate insulator and had multiple air bridged source regions. The details of the DC current-voltage (I-V) characteristics and small signal S-parameter measurements up to 20 GHz are presented. The switch IGFET's had a drain saturation current density of 300 mA/mm gate width with breakdown voltages of higher than 35 V. An insertion loss of 1.0, 0.6, and 0.4 dB at 10 GHz and 1.4, 0.8, and 0.4 dB at 20 GHz have been measured for the 300, 600, and 1200 μm gate width IGFET's, respectively. Equivalent circuit models fitted to the measured S-parameters for IGFET's yielded on-state resistances from 10.7 to 3.3 Ω, off-state resistances from 734.4 to 186.8 R and off-state capacitances from 0.084 to 0.3 pF as the gate width is increased from 300 to 1200 μm The simulation results using IGFET models for the phase shifter circuits indicated a maximum phase error of 0.11°, 0.26°, and 0.479 with 0.74, 0.96, and 1.49 dB maximum insertion loss and greater than 33, 26, and 19 dB return loss for the 11.25°, 22.5°, and 45° phase bits, respectively, over the 9.5-10.5 GHz frequency band     

Comparison of modeled and measured second azimuthal harmonics ofocean surface brightness temperatures

Second azimuthal harmonics of ocean surface brightness temperatures predicted by the second order small slope approximation (SSA) are compared to an empirical model based on WindRAD experiments performed by the Jet Propulsion Laboratory (JPL), Pasadena, CA. SSA predictions are illustrated for three differing models of the ocean surface directional spectrum, and results as a function of wind speed are shown to be in reasonable agreement with the WindRAD model at 19.35 and 37 GHz and at polar observation angles of 45°, 55°, and 65°. None of the three spectral models, however, completely matches all the trends of the empirical data. A slight modification to one of the spectra is demonstrated to yield an improved agreement     

Airborne imaging microwave radiometer. I. Radiometric analysis

The airborne imaging microwave radiometer (AIMR) was designed and built for regional scale sea ice mapping. It operates at 37 and 90 GHz (nominal), and collects radiance at two orthogonal polarizations from which one can compute horizontal and vertical polarizations. The sensitivity or precision (ΔT) of the radiometric data is on the order of 0.5-0.8 K for the 37 GHz channels and 0.8-1.5 for the 90 GHz channels. A detailed error analysis was conducted to assess the accuracy of the radiometric measurements. The error in the brightness temperatures of the original orthogonal polarizations channels was found to be on the order of 0.35-0.45 K for the 37 GHz channel and 0.55-0.65 K for the 90 GHz channel. The polarization conversion introduces additional errors and these are analyzed and computed for the LIMEX-89 data. The total error due to both calibration and polarization conversion for incidence angles greater than 20° is on the order of 0.65-0.70 K for 37 GHz and 0.75-0.85 K for 90 GHz. For incidence angles between 10° and 20° the error can be up to 1.5 K. As the incidence angle approaches zero the distinction between horizontal and vertical polarization breaks down and the error approaches infinity     

Multiwavelength DFB laser array with integrated spot sizeconverters

We describe the design, fabrication, and performance of a five-element quarterwave-shifted distributed feedback laser array with monolithically integrated spot size converters intended for use as a multiple-wavelength source in dense wavelength-division telecommunications systems. Facet power in excess of 10 mW with less than 150 mA bias and longitudinal side mode suppression greater than 40 dB were routinely achieved. Narrow far-field full-width at half-maximum angles of 6.9°×16.3° provided 3.5-dB coupling loss into single-mode fiber with 1.0-dB misalignment tolerances of ±2.0 μm. With ±10°C thermal tuning, 22 1555-nm channels spaced by 50 GHz were accessed with this device. Thorough field evaluation indicates that such a device is consistent with manufacturing requirements     

Multifrequency Microwave Radiometer Measurements of Soil Moisture

Ground-based microwave radiometer experiments were performed to investigate the effects of moisture, temperature, and roughness on microwave emission from bare soils. Measurements were made at frequencies of 0.6-0.9, 1.4, and 10.7 GHz using van-mounted radiometers to observe prepared soil sites in Kern County, CA. The sites were instrumented for monitoring soil characteristics and surface meteorological conditions. Brightness temperature variations of approximately 15 K at 1.4 GHz and 25 K at 10.7 GHz were observed as a result of diurnal changes in the soil temperature. Increasing the soil moisture content from 2 to 15 percent by volume resulted in brightness temperature decreases of approximately 70 K at 0.775 and 1.4 GHz, and 40 K at 10.7 GHz, depending, to a lesser extent, on polarization and viewing angle. The results show the significance of soil temperature in deriving soil moisture from microwave radiometer measurements. Comparisons of the microwave measurements with theoretical predictions using a smooth surface model show reasonable agreement and support previous results of this nature obtained with other soil types. Approximately equal sensitivity to soil moisture was observed at 0.775 and 1.4 GHz, although the sampling depth is greater at the lower frequency.     

Intercomparison of microwave radiative transfer models forprecipitating clouds

An intercomparison of microwave multiple scattering radiative transfer codes used in generating databases for satellite rainfall retrieval algorithms has been carried out to ensure that differences obtained from retrieval techniques do not originate from the underlying radiative transfer code employed for the forward modeling. A set of profiles containing liquid water and ice contents of cloud and rain water as well as snow, graupel and pristine ice were distributed to the participants together with a black box routine providing Mie single scattering, atmospheric background absorption and surface emissivity. Simulations were to be carried out for nadir and off-nadir (53.1°) observation angles at frequencies between 10 and 85 GHz. Among the radiative transfer models were two-stream, multiple stream and Monte Carlo models. The results showed that there were two major sources of differences between the codes. 1) If surface reflection/emission was considered isotropic, simulated brightness temperatures were significantly higher than for specular reflection and this effect was most pronounced at nadir observation and over ocean-type surfaces. 2) Flux-type models including delta-scaling could partially compensate for the errors introduced by the two-stream approximation. Largest discrepancies occurred at high frequencies where atmospheric scattering is most pronounced and at nadir observation. If the same surface boundary conditions, the same multiple-stream resolution and the same scaling procedures are used, the models were very close to each other with discrepancies below 1 K     

Optimum design of waveguide E-plane stub-loaded phaseshifters

Broadband low-insertion-loss E-plane stub-loaded rectangular waveguide phase shifters are designed with the method of field expansion into normalized eigenmodes, which includes higher-order mode interaction between the step discontinuities. Computer-optimized three-stub prototypes of 90° differential phase shift with reference to an empty waveguide of appropriate length, designed for R140-band (12.4-18 GHz) and R320-band (26.5-40 GHz) waveguides, achieve typically ±0.5° phase shift deviation within about 20% bandwidth. For two-stub designs, the corresponding values are about +2.5°/-1° and 17%. Both designs achieve minimum return loss of 30 dB. The theory is verified by measurements of a compact R120-band (10-15 GHz) waveguide phase shifter design example milled from a solid block, showing measured insertion loss of about 0.1 dB and about +2.5°/-0.5° phase error between 10.7 and 12.7 GHz     

Radiative properties of deformed hydrometeors for commonly usedpassive microwave frequencies

Scattering parameters for ensembles of aspherical ice and liquid hydrometeors were calculated at an incidence angle (ψ) of 50° for frequencies of 10.7, 18.0, 37.0, 50.3, and 85.6 GHz. Hydrometeors are assumed to be oblate ellipsoids with semiminor axes aligned with ψ=0°. The angle ψ 50° is determined by the viewing angle of satellite-borne radiometers. The backscattering extinction coefficients at ψ=0° were also calculated at 18, 37, and 85.6 GHz to provide information for future nadir scanning satellite-borne radars. The hydrometeors are assumed to be characterized by Marshall-Palmer drop-size distributions. The extended boundary condition method is used to calculate the extinction and backscattering coefficients as well as the albedo for single scattering and the asymmetry factor of the phase function. Results are fitted to simple functions of the rainfall rate. Some comparisons illustrate the differences and similarities of these results with those obtained from equal volume spheres     

Variational data assimilation of microwave radiobrightnessobservations for land surface hydrology applications

Our ability to accurately describe large-scale variations in soil moisture is severely restricted by process uncertainty and the limited availability of appropriate soil moisture data. Remotely sensed microwave radiobrightness observations can cover large scales but have limited resolution and are only indirectly related to the hydrologic variables of interest. The authors describe a four-dimensional (4D) variational assimilation algorithm that makes best use of available information while accounting for both measurement and model uncertainty. The representer method used is more efficient than a Kalman filter because it avoids explicit propagation of state error covariances. In a synthetic example, which is based on a field experiment, the authors demonstrate estimation performance by examining data residuals. Such tests provide a convenient way to check the statistical assumptions of the approach and to assess its operational feasibility. Internally computed covariances show that the estimation error decreases with increasing soil moisture. An adjoint analysis reveals that trends in model errors in the soil moisture equation can be estimated from daily L-band brightness measurements, whereas model errors in the soil and canopy temperature equations cannot be adequately retrieved from daily data alone. Nonetheless, state estimates obtained from the assimilation algorithm improve significantly on prior model predictions derived without assimilation of radiobrightness data     

Improved Spatial Mapping of Rainfall Events with Spaceborne SAR Imagery

The Seasat satellite acquired the first spaceborne synthetic-aperture radar (SAR) images of the earth's surface, in 1978, at a frequency of 1.275 GHz (L-band) in a like-polarization mode at incidence angles of 23 ± 30. Although this may not be the optimum system configuration for radar remote sensing of soil moisture, interpretation of two Seasat images of Iowa demonstrates the sensitivity of microwave backscatter to soil moisture content. In both scenes, increased image brightness, which represents more radar backscatter, can be related to previous rainfall activity in the two areas. Comparison of these images with ground-based rainfall observations illustrates the increased spatial coverage of the rainfall event that can be obtained from the satellite SAR data. These data can then be color-enhanced by a digital computer to produce aesthetically pleasing output products for the user community. When the methodology for extracting accurate information about soil moisture status from radar data is developed, it will prove useful in a wide variety of agronomic and hydrological investigations.     

A retrodirective antenna array using a spatially fed localoscillator

This work investigates a spatially fed local oscillator (LO) for a retrodirective antenna array. Compared to conventional planar transmission-line feed networks, the spatial feed offers a more flexible array layout and easier implementation, especially in large two-dimensional (2-D) arrays. This method of delivery involves radiating the LO through free space to the back-side of the array where it is received by slot-coupled patch antennas and delivered to the mixing elements. A four-element linear retrodirective array utilizing this approach is demonstrated at an LO frequency of 10.7 GHz. Retrodirectivity is observed at scan angles from -40° to +40°     

Calibration of the L-MEB Model Over a Coniferous and a Deciduous Forest

In this paper, the L-band Microwave Emission of the Biosphere (L-MEB) model used in the Soil Moisture and Ocean Salinity (SMOS) Level 2 Soil Moisture algorithm is calibrated using L-band (1.4 GHz) microwave measurements over a coniferous (pine) and a deciduous (mixed/beech) forest. This resulted in working values of the main canopy parameters optical depth (tau), single scattering albedo (omega), and structural parameters tt(H) and tt(V), besides the soil roughness parameters H _R and N _R. Using these calibrated values in the forward model resulted in a root mean-square error in brightness temperatures from 2.8 to 3.8 K, depending on data set and polarization. Furthermore, the relationship between canopy optical depth and leaf area index is investigated for the deciduous site. Finally, a sensitivity study is conducted for the focus parameters, temperature, soil moisture, and precipitation. The results found in this paper will be integrated in the operational SMOS Level 2 Soil Moisture algorithm and used in future inversions of the L-MEB model, for soil moisture retrievals over heterogeneous, partly forested areas.     

Deep-space calibration of the WindSat radiometer

The WindSat microwave polarimetric radiometer consists of 22 channels of polarized brightness temperatures operating at five frequencies: 6.8, 10.7, 18.7, 23.8, and 37.0 GHz. The 10.7-, 18.7-, and 37.0-GHz channels are fully polarimetric (vertical/horizontal, /spl plusmn/45/spl deg/ and left-hand and right-hand circularly polarized) to measure the four Stokes radiometric parameters. The principal objective of this Naval Research Laboratory experiment, which flys on the USAF Coriolis satellite, is to provide the proof of concept of the first passive measurement of ocean surface wind vector from space. This paper presents details of the on-orbit absolute radiometric calibration procedure, which was performed during of a series of satellite pitch maneuvers. During these special tests, the satellite pitch was slowly ramped to +45/spl deg/ (and -45/spl deg/), which caused the WindSat conical spinning antenna to view deep space during the forward (or aft portion) of the azimuth scan. When viewing the homogeneous and isotropic brightness of space (2.73 K) through both the main reflector and the cold-load calibration reflector, it is possible to determine the absolute calibration of the individual channels and the relative calibration bias between polarimetric channels. Results demonstrate consistent and stable channel calibrations (with very small brightness biases) that exceed the mission radiometric calibration requirements.     

基于FFS架构实现宽窄视角切换

基于公共场合隐私保护的需求,宽窄视角可切换的液晶显示技术有着迫切的市场需求。现有宽窄视角切换技术不但增加了显示装置的厚度与成本,而且降低了原本宽视角模式下的显示效果。本文研究了一种基于负性液晶FFS模式的宽窄视角切换技术,它通过在彩色滤光片一侧形成高预倾角配向(40°～60°),在同侧第三电极施加弱电场时,液晶分子离轴方向具有较大相位延迟,形成大视角观看时的暗态漏光与对比度下降,实现窄视角显示模式。而同侧第三电极施加强垂直电场时,大的预倾角液晶分子因负介电各项异性使得倾角降低,可减少暗态漏光和提升对比度,实现宽视角显示模式。验证结果表明:宽视角模式时,其上下左右视角均可达到85°以上;而窄视角模式时,左右视角仅为40°,且具有较好的对称性。本技术结构简单,驱动灵活,在防窥显示领域具有很好的应用前景。     

An Ocean Surface Wind Vector Model Function for a Spaceborne Microwave Radiometer

Surface wind vector measurements over the oceans are vital for scientists and forecasters to understand the Earth's global weather and climate. In the last two decades, operational measurements of global ocean wind speeds were obtained from passive microwave radiometers (Special Sensor Microwave/ Imagers); and over this period, full ocean surface wind vector data were obtained from several National Aeronautics and Space Administration and European Space Agency scatterometry missions. However, since SeaSat-A in 1978, there have not been other combined active and passive wind measurements on the same satellite until the launch of Japan Aerospace Exploration Agency's Advanced Earth Observing Satellite-II in 2002. This mission provided a unique data set of coincident measurements between the SeaWinds scatterometer and the Advanced Microwave Scanning Radiometer (AMSR). The AMSR instrument measured linearly polarized brightness temperatures (TB) over the ocean. Although these measurements contained wind direction information, the overlying atmospheric influence obscured this signal and made wind direction retrievals not feasible. However, for radiometer channels between 10 and 37 GHz, a certain linear combination of vertical and horizontal brightness temperatures causes the atmospheric dependence to cancel and surface parameters such as wind speed and direction and sea surface temperature to dominate the resulting signal. In this paper, an empirical relationship between AMSR T_B's (specifically A ^. T_BV - T_BH) and surface wind vectors (inferred from SeaWinds' retrievals) is established for three microwave frequencies: 10, 18, and 37 GHz. This newly developed wind vector model function for microwave radiometers can serve as a basis for wind vector retrievals either separately or in combination with active scatterometer measurements.     

Global Millimeter-Wave Precipitation Retrievals Trained With a Cloud-Resolving Numerical Weather Prediction Model, Part I: Retrieval Design

This paper develops a global precipitation rate retrieval algorithm for the advanced microwave sounding unit (AMSU), which observes 23-191 GHz. The algorithm was trained using a numerical weather prediction (NWP) model (MM5) for 106 globally distributed storms that predicted brightness temperatures consistent with those observed simultaneously by AMSU. Neural networks were trained to retrieve hydrometeor water-paths, peak vertical wind, and 15-min average surface precipitation rates for rain and snow at 15-km resolution at all viewing angles. Different estimators were trained for land and sea, where surfaces classed as snow or ice were generally excluded from this paper. Surface-sensitive channels were incorporated by using linear combinations [principal components (PCs)] of their brightness temperatures that were observed to be relatively insensitive to the surface, as determined by visual examination of global images of each brightness temperature spectrum PC. This paper also demonstrates that multiple scattering in high microwave albedo clouds may help explain the observed consistency for a global set of 122 storms between AMSU-observed 50-191-GHz brightness temperature distributions and corresponding distributions predicted using a cloud-resolving mesoscale NWP model (MM5) and a two-stream radiative transfer model that models icy hydrometeors as spheres with frequency-dependent densities. The AMSU/MM5 retrieval algorithm developed in Part I of this paper is evaluated in Part II on a separate paper.     

Radiobrightness decision criteria for freeze/thaw boundaries

A freeze indicator (FI), based on a low 37-GHz radiobrightness and a low 10.7, 18, and 37-GHz radiobrightness spectral gradient, has been used to classify frozen surfaces in the northern Great Plains. By modeling the radiometer beampatterns as Gaussian, freeze/thaw boundaries can be located at the (fine) resolution of the 37-GHz channel. The performance of the freeze indicator, and subsequent boundary location estimate, depends on the accuracy of the boundary decision criteria. It is shown that decision criteria based on clustering and unsupervised classification yield good performance. A simple algorithm for registering coarse-resolution FI boundaries to equivalent boundaries in fine-resolution 37-GHz radiobrightness images is also presented