Retrievals of low integrated water vapor using MIR and SSM/T-2 measurements

Satellite radiometric measurements at 150, 183.3/spl plusmn/3, and 183.3/spl plusmn/7 GHz have previously been used to retrieve integrated water vapor <1 g/cm/sup 2/ over Antarctica. The effects of the frequency dependence of surface emissivity and the variation of surface temperature on the retrieval, which have not been closely examined in the studies, are analyzed. Using four days of near-concurrent airborne and satellite radiometric measurements, it is shown that the previously derived retrieval algorithm could overestimate or underestimate integrated water vapor by up to 0.1 g/cm/sup 2/, depending on whether the surface emissivity increases or decreases with frequency. The average of the absolute value of the bias for each flight case studied is /spl les/0.04g/cm/sup 2/. Additionally, surface skin temperature is shown to vary substantially over a range from 240-270 K during these four days of measurements; the corresponding effect on the retrieval of integrated water vapor is comparable to that due to frequency dependence on surface emissivity. The quantitative correction needed for this effect is dependent upon the magnitude of integrated water vapor. At high values of integrated water vapor of 0.6-0.8 g/cm/sup 2/, the corrections are as large as 0.1 g/cm/sup 2/ for changes of surface temperature of /spl plusmn/10 K. A simple procedure is implemented to correct for this error, which significantly improves the retrieval. Correction for the frequency dependence of surface emissivity is nontrivial when using currently available satellite measurements; in order to properly correct this effect, an additional channel of measurements, e.g., at 220 GHz, is required.     

Impact of variable atmospheric water vapor content on AVHRR datacorrections over land

This paper explores the impact of the integrated water vapor content (IWV) in the atmospheric column on the corrections of optical satellite data over land. First, simulation runs were used to quantify the trends in red and near infrared parts of the electromagnetic spectrum. Second, advanced very high resolution radiometer (AVHRR) measurements obtained over Canada during the 1996 growing season, together with reanalyzed IWV content data, were employed to determine the actual impact of constant IWV values. Third, various options in characterizing IWV for atmospheric corrections of AVHRR composites were examined. It was found that (1) as expected, IWV affects near-infrared radiation substantially more than red, although the latter is also altered; (2) that additional, subtle interactions take place between IWV, radiance levels, and viewing geometry that influence the retrieved surface reflectance; (3) that spatial and temporal variation in IWV caused changes in the normalized difference vegetation index up to 7.5% in relative terms during the peak green period; and (4) that IWV varies so substantially that pixel and date-specific values need to be used for the atmospheric correction of AVHRR data. At present, subdaily gridded IWV data sets from atmospheric data reanalysis projects are the only candidate source for such purpose     

Normalized differential spectral attenuation (NDSA): a novel approach to estimate atmospheric water vapor along a LEO-LEO satellite link in the ku/K bands

We introduce here the normalized differential spectral attenuation (NDSA) approach, which is a novel differential measurement way for estimating the total content of water vapor integrated water vapor (IWV) along a tropospheric propagation path between two Low Earth Orbit (LEO) satellites. The NDSA approach requires a transmitter onboard the first LEO satellite and a receiver onboard the second one. It is based on the simultaneous measurement of the total attenuation at two relatively close frequencies in the Ku/K bands, and on the estimate of a "spectral sensitivity parameter" that can be directly converted into IWV. NDSA is potentially able to emphasize the water vapor contribution, to cancel out all spectrally flat unwanted contributions and to limit the impairments due to tropospheric scintillation. The objective of the paper is to analyze the level of correlation between the spectral sensitivity parameter and the IWV at a given altitude from ground of the LEO-LEO link (tangent altitude), in order to single out the best performing frequencies. Simulation results are based on microwave propagation models and on radiosonde data. The results shows the potential of the NDSA approach to provide direct estimates of IWV along LEO-LEO tropospheric propagation paths in the 15-25 GHz frequency range, under different atmospheric conditions.     

Modelling drain and gate dependence of HEMT 1-50 GHz, small-signalS-parameters, and d.c. drain current

We present refinements to a previously validated HEMT model that improves the model's accuracy as a function of drain bias for simulating d.c. drain current and 1-50 GHz, small-signal S-parameters. By comparing simulation data with experimental data for a 0.4-μm-gate pseudomorphic HEMT, we have been able to establish the accuracy of the refined model, which predicts the device's d.c. current and S-parameters as a function of the applied drain and gate biases to within an accuracy of ~5%. The core of the model and, in particular, its bias dependence, are directly dependent on the HEMT wafer structure and the physical gate length     

Retrieval of tropospheric water vapor scale height from horizontalturbulence structure

A scale height of the vertical water vapor distribution in the troposphere is shown to be related to the rate at which the total integrated water vapor (IWV) decorrelates with horizontal separation. This relationship is based on the departure from simple Kolmogorov behavior of the turbulence structure of the IWV as the horizontal separation becomes a significant fraction of the scale height of the three dimensional (3D) turbulence. The relationship is demonstrated by comparisons between direct measurements of the vertical water vapor distribution, by radiosondes, and coincident estimates of the horizontal turbulence structure, using the TOPEX Microwave Radiometer (TMR). This provides a new method by which to resolve some of the vertical structure of lower tropospheric water vapor from space. The turbulence structure estimator is applied to a larger body of TMR data to produce a sequence of images describing the dynamics of water vapor scale height across the tropical Pacific Ocean. The cyclical evolution of a basin scale east/west ridge of water vapor with high scale height near 5° north latitude is detected which is consistent with other observations of the Madden and Julian Oscillation. The general technique presented should be applicable to many other existing data sets which image the horizontal distribution of IWV, e.g., those of the Defense Meteorological Satellite Program's special sensor microwave/imagers     

Forward model studies of water vapor using scanning microwave radiometers, global positioning system, and radiosondes during the cloudiness intercomparison experiment

Brightness temperatures computed from five absorption models and radiosonde observations were analyzed by comparing them with measurements from three microwave radiometers at 23.8 and 31.4 GHz. Data were obtained during the Cloudiness Inter-Comparison Experiment at the U.S. Department of Energy's Atmospheric Radiation Measurement Program's (ARM) site in North-Central Oklahoma in 2003. The radiometers were calibrated using two procedures, the so-called instantaneous "tipcal" method and an automatic self-calibration algorithm. Measurements from the radiometers were in agreement, with less than a 0.4-K rms difference during clear skies, when the instantaneous method was applied. Brightness temperatures from the radiometer and the radiosonde showed a bias difference of less than 0.69 K when the most recent absorption models were considered. Precipitable water vapor (PWV) computed from the radiometers were also compared to the PWV derived from a Global Positioning System station that operates at the ARM site. The instruments agree to within 0.1 cm in PWV retrieval.     

Multiple resolution analysis of L-band brightness temperature forsoil moisture

Passive microwave Earth observing systems provide coarse resolution data. Heterogeneity in physical characteristics will typically be present within footprints, especially over land. How this affects the development and validation of methods of retrieving soil moisture has not been verified. In this study, aircraft-based 1.4 GHz microwave radiometer data were collected sit several altitudes over test sites where soil moisture was measured concurrently. The use of multiple flightlines at lower altitudes allowed the direct comparison of different spatial resolutions using independent samples over the same ground location. Results showed that the brightness temperature data from 1.4 GHz sensor in this study region provides the same mean values for an area regardless of the spatial resolution of the original data. The relationship between brightness temperature and soil moisture was similar at different resolutions. These results suggest that soil moisture retrieval methods developed using high resolution data can be extrapolated to satellite scales     

Improvements and complications involved with adding an 85-GHzchannel to cloud liquid water radiometers

The improvement in cloud-liquid estimates by a microwave radiometer with the addition of measurements at 85 GHz is quantified. Atmospheric emission is simulated from radiosonde data at frequencies commonly used by ground-based water-vapor radiometers (22.235 and 31.65 GHz) and also at 85.5 GHz. Retrieval algorithms are developed from opacities based on full Mie extinction by cloud droplets and under an assumption that ice effects are not significant for downwelling emission. The algorithms use either three frequencies or only the lower two. The inclusion of 85-GHz information significantly improves liquid-water path estimates at all levels of integrated liquid water. The Rayleigh approximation is shown to be valid for most cloudy conditions. Uncertainty in the calculated opacities due to varying cloud droplet-size distributions and liquid-water content profiles is quantified. The accuracy of a retrieval algorithm trained by Rayleigh approximation opacities and including the additional uncertainty is shown to provide estimates with error levels similar to those from the algorithm trained with full Mie opacities     

The measurement of absolute absorption of millimeter radiation ingases: the absorption of Co and O2

An apparatus is described that will measure absolute absorption of millimeter radiation in gases. The method measures the change in the quality factor of a Fabry-Perot resonator with and without gas present. The magnitude of the change is interpreted in terms of the absorption of the lossy medium inside the resonator. Experiments have been performed on the 115 GHz CO line and the 119 GHz O₂ line at two different temperatures to determine the linewidth parameter and the peak absorption value. These numbers can be combined to give the integrated intensity which can be accurately calculated from results of spectroscopy measurements. The CO results are within 2% percent of theoretically predicted valves. Measurements on O₂ have shown that absorption can be measured as 0.5 dB/km with this technique. Results have been obtained for oxygen absolute absorption in the 60-80 GHz region     

The Microwave Reflective Properties of Water Surfaces-I: The Reflectivity of Smooth Water at 19.24 and 22.43 GHz as Measured with a Free-Wave Microwave Reflectometer

The reflectivity of smooth water has been measured with a free-wave reflectometer at frequencies of 19.24 and 22.43 GHz and for temperatures between 0/spl deg/ and 40/spl deg/C. Reproducibility of the data for water temperatures below 20/spl deg/C is better than /spl plusmn/ 0.15 percent and the absolute accuracy of the reflectometer is thought to be /spl plusmn/ 0.5 percent. The results deviate significantly from the early reflectivity measurements of Saxton and Lane but agree to within experimental error with values calculated from the absorption cell measurements of Lane and Saxton.     

Bias dependence of capacitances in modulation-doped FET's at 4 GHz

Scattering-parameter measurements were made on 1.0- µm gate normally-on and 1.6-µm gate normally-off modulation-doped field-effect transistors (MOD/FET's) as a function of bias at 4 GHz. A maximum oscillation frequency of 38 GHz, which is about 8 GHz larger than that for a comparable GaAs MESFET, due to larger transconductances for the MODFET's, was obtained. The input capacitance was found to vary more than expected with gate bias, while the feedback capacitance was nearly independent of bias in the saturation regime. Output capacitance increases as Vgsapproached the pinchoff voltage. These results should be of great importance in large-signal modeling of integrated circuits based on modulation-doped FET's.     

应用于60GHz无线收发机内的带宽大于3GHz的无电感CMOS可编译增益放大器

这篇文章呈现了一个应用于60GHz无线收发机内的带宽大于3GHz的无电感CMOS可编译增益放大器,使用了改进的带负电容抵消技术Cherry-hooper放大器作为增益单元,采用了新颖的电路技术来实现增益调节,该技术在宽带PGA的设计中具有普适性,并且可以大大简化宽带PGA的设计。PGA通过两级增益单元和一级输出BUFFER的级联获得了最大增益30dB和远宽于3GHz的带宽。该PGA集成进整个60GHz无线收发机里面并且用TSMC65nm的CMOS工艺获得实现。整个接收机前端的测试结果表明接收机前端获得了18dB的可变增益范围和>3GHz的带宽,这证明提出的PGA本身获得了18dB的可变增益范围并且带宽是远大于3GHz的。该PGA电源电压为1.2V,功耗为10.7mW,核心版图面积仅仅为0.025mm^2。     

Electromagnetic bias in sea surface range measurements atfrequencies of the TOPEX/Poseidon satellite

Range measurements made by satellite radar altimeters experience an electromagnetic (EM) bias toward the troughs of ocean waves. Measurements taken with the NASA altimeter on the TOPEX/Poseidon satellite in a series of aircraft flights during the Surface Wave Dynamics Experiment (SWADE) indicate that EM bias is slightly higher at 5.3 GHz than at 13.6 GHz, and that the magnitudes of both biases increase with increasing wind speed, as does their difference. Tower, airborne, and satellite measurements show a consistency in the characteristics of the wind speed dependence but suggest that bias decreases with increasing altitude. The airborne measurements appear to be the most reasonable basis for correcting the NASA altimeter range data from the TOPEX/POSEIDON satellite. A preliminary analysis of data acquired at 20.3 m/s in the Southern Ocean Waves Experiment (SOWEX) has given confidence that the quadratic models for the prelaunch EM bias corrections are more appropriate for wind speed dependence than linear models     

Improved Retrieval of Total Water Vapor Over Polar Regions From AMSU-B Microwave Radiometer Data

The polar regions are among those where the least information is available about the current and predicted states of surface and atmosphere. We present advances in a method to retrieve the total water vapor (TWV) of the polar atmosphere from data from spaceborne microwave radiometers such as the Advanced Microwave Sounding Unit B (AMSU-B) on the polar-orbiting satellites of the National Oceanic and Atmospheric Administration (NOAA), NOAA-15, -16, and -17. The starting point of the retrieval is a recently proposed algorithm that uses the three AMSU-B channels centered around the 183-GHz water vapor line and the window channel at 150 GHz, and that can retrieve the TWV with little dependence on the surface emissivity. This works up to TWV values of about 7 $hbox{kg/m}^{2}$. We extend the retrievable range toward higher TWV values by including the window channel at 89 GHz. However, now, the algorithm needs information on the surface emissivity, which we have extracted from emissivity measurements over sea ice and open water during the Surface Emissivities in Polar Regions-Polar Experiment campaign. The resulting algorithm can retrieve TWV up to about 15 $hbox{kg/m}^{2}$, with reduced accuracy as compared to the original algorithm. It now allows the monitoring of the TWV over the central Arctic sea ice and over Antarctica, and the surrounding sea ice during most of the year with a spatial resolution of about 50 km. Such TWV fields can show details which might be missed out by standard weather model analysis data.      

Normalized Differential Spectral Attenuation (NDSA) Measurements Between Two LEO Satellites: Performance Analysis in the Ku/K-Bands

Normalized differential spectral attenuation (NDSA) is a novel differential measurement method to estimate the total content of water vapor [integrated water vapor (IWV)] along a tropospheric propagation path between two low Earth orbit (LEO) satellites. A transmitter onboard the first LEO satellite and a receiver onboard the second one are required. The NDSA approach is based on the simultaneous estimates of the total attenuation at two relatively close frequencies in the Ku/K-bands and of a “spectral sensitivity parameter” that can be directly converted into IWV. The spectral sensitivity has the potential to determine the water vapor contribution, to cancel out all spectrally flat unwanted contributions, and to limit the impairments due to tropospheric scintillation. In this paper, we focus on the measurement accuracy of the spectral sensitivity parameter. Specifically, we examine this accuracy at three different frequencies and for two models of atmospheric structure. We first provide an approximate expression of the accuracy and then validate this expression through Monte Carlo simulations based on microwave propagation models.      

The development of an 850 GHz waveguide receiver using tuned sis junctions on 1µm Si3N4 membranes

We report preliminary development work on a 850 GHz SIS heterodyne receiver employing a tuned niobium tunnel junction on a 1 µm Si₃N₄ supporting membrane. Since the mixer is meant to be operated well above the superconducting gap frequency of niobium (2δ/h ? 690 GHz) special care has been taken to minimize transmission line loss. We have therefore used junctions with an integrated radial stub RF matching network to tune out the large shunt susceptance of the junction and minimize the niobium film absorption loss. Scale model measurements of the waveguide embedding impedance have been made to aid in the design of the choke structure and RF matching network. Detailed Fourier Transform Spectrometer measurements of tuned junctions on both SiO₂ and silicon nitride membranes show response up to 1100 GHz and indicate that the absorption loss in the niobium film is in the order of 4–7 dB at 850 GHz, in fairly good agreement with the theoretical loss calculated from the Mattis-Bardeen theory. The junctions have a center frequency of 800 GHz which presents a 6% downshift from the designed value.     

Statistical iterative scheme for estimating atmospheric relativehumidity profiles

Estimation of atmospheric relative humidity profiles using passive remote sensing techniques is difficult when the temperature profile is not well known, and such retrievals approach singularity when the atmosphere is nearly isothermal. A retrieval method that is more robust near isothermal regions and temperature inversions is described. Its robust character results from an iterative combination of statistical methods based on a priori data, which stabilize the effects of any singularities, and physical methods that reflect the nonlinear character of the equation of radiative transfer and the dependence of measurements on uncertain surface reflectivities and temperature profiles. This method can be used to interpret data from meteorological satellites. It was tested extensively using simulated clear-sky microwave observations from space at 89 GHz, 166 GHz, and three frequencies near the 183-GHz water vapor resonance and the 60-GHz oxygen band, which is sensitive to the atmospheric temperature profile. Humidity profiles from the tropical, midlatitude, and arctic regions were retrieved. Relative humidity profiles retrieved using the statistical iterative method typically had errors between 5 and 10% in the 300-1000 mbar pressure region. These errors were somewhat less in the tropics and greater in the polar regions, and represented significantly better performances than a linear statistical retrieval method     

The Millimeter Wave Atmospheric Sounder (MAS): a shuttle-basedremote sensing experiment

The Millimeter Wave Atmospheric Sounder (MAS) will be launched in the spring of 1992 as part of the ATLAS 1 (Atmospheric Laboratory for Application and Science) mission. Using passive limb-scanning millimeter-wave radiometry, it will sense the thermal emission produced by ozone at 184 GHz, water vapor at 183 GHz, chlorine monoxide at 204 GHz, and oxygen (for retrieval of temperature and pressure) at 60 GHz. From these observations, concentration profiles of these gases throughout the middle atmosphere will be made. The fundamentals of the measurements, the design of the radiometers, and the approaches used for the data analysis are described     

Ground-based multifrequency microwave radiometry for rainfallremote sensing

Inversion algorithms for ground-based microwave radiometric retrieval of surface rain-rate, integrated cloud parameters, and slant-path attenuation are proposed and tested. The estimation methods are trained by numerical simulations of a radiative transfer model applied to microphysically-consistent precipitating cloud structures, representative of stratiform and convective rainy clouds. The discrete-ordinate method is used to solve the radiative transfer equation for plane-parallel seven-layer structures, including liquid, melted, and ice spherical hydrometeors. Besides ordinary multiple regression, a variance-constrained regression algorithm is developed and applied to synthetic data in order to evaluate its robustness to noise and its potentiality. Selection of optimal frequency sets and polynomial retrieval algorithms for rainfall parameters is carried out and discussed. Ground-based radiometric measurements at 13.0, 23.8, and 31.7 GHz are used for experimentally testing the retrieval algorithms. Comparison with rain-gauge data and rain path-attenuation measurements, derived from the three ITALSAT satellite beacons at 18.7, 39.6, and 49.5 GHz acquired at Pomezia (Rome, Italy), are performed for two selected cases of moderate and intense rainfall during 1998     

A Variable Conversion Gain Star Mixer for Ka-Band Applications

A variable conversion gain star mixer for Ka-band applications has been presented. This monolithic microwave integrated circuit was implemented on AlGaAs/InGaAs/GaAs pseudomorphic high-electron-mobility transistor process with a chip size of 1.7times1.7 mm². The mixer is modified from conventional star mixer to apply dc bias. The conversion gain of the mixer, controlled by the voltage of the diodes, could be applied to meet gain compensation requirements in communication systems. From the measured results, the circuit can provide 11.9 dB conversion gain and 9.3 dB gain adjustment by controlling voltage from 0 to 0.7 V at 30 GHz.