Airborne C-band SAR measurements of wet snow-covered areas

Using polarimetric synthetic aperture radar (SAR) data at C-band, the optimum polarization and range incidence angles for the classification of land covers found in the James Bay area, P.Q., Canada-open areas, lakes ice, and forests, all covered with wet snow-have been examined. The separability between classes shows that they can be classified by a single polarization. For &thetas;<30°, the overall classification accuracies were 97.1, 98, and 90.8% from HH-, cross, and VV-polarizations alone, respectively. They were greater than 99% for all polarizations at &thetas;>30°. However, the polarimetric parameters were not suitable for classification     

Millimeter-wave backscatter measurements on snow-covered terrain

Backscatter measurements of snow-covered terrain have been made using a polarimetric frequency-agile experimental 94-GHz radar. The radar scanned snow surfaces at radar depression angles varying from 15 to 55°. Data collection using the polarimetric and frequency-agile radar along with concurrent detailed snow characterization measurements was performed. Typical results for the temporal and spatial variability of the snow reflectivity are presented and discussed     

Millimeter-wave measurements and analysis of snow-covered ground

Initial investigations and analyses have indicated that, although not predicted by present theory, the millimeter-wavelength (MMW) scattering properties of certain types of snow known as metamorphic or refrozen snow can create a problem for the detection of man-made targets. In 1984, researchers from the US Army, the Georgia Institute of Technology, and others carried out the SNOWMAN test program, which was a comprehensive data collection program using 35- and 94-GHz instrumentation radars to measure the backscatter cross section of snow-covered terrain as a function of wavelength, coherent bandwidth, polarization, (for both the incident and scattered waves), and the incidence angle. The radar measurements were supplemented by an extensive ground truth program that provided details of the meteorology and the physical characteristics of the snow cover. The authors describe the data collection procedures, present examples of the results, identify gaps in the data, and suggest additional analyses that could be performed to maximize the utility of the SNOWMAN data base     

Passive active L- and S-band (PALS) microwave sensor for oceansalinity and soil moisture measurements

A passive/active WS-band (PALS) microwave aircraft instrument to measure ocean salinity and soil moisture has been built and tested. Because the L-band brightness temperatures associated with salinity changes are expected to be small, it was necessary to build a very sensitive and stable system. This new instrument has dual-frequency, dual polarization radiometer and radar sensors. The antenna is a high beam efficiency conical horn. The PALS instrument was installed on the NCAR C-130 aircraft and soil moisture measurements were made in support of the Southern Great Plains 1999 experiment in Oklahoma from July 8-14, 1999. Data taken before and after a rainstorm showed significant changes in the brightness temperatures, polarization ratios and radar backscatter, as a function of soil moisture. Salinity measurement missions were flown on July 17-19, 1999, southeast of Norfolk, VA, over the Gulf Stream. The measurements indicated a clear and repeatable salinity signal during these three days, which was in good agreement with the Cape Hatteras ship salinity data. Data were also taken in the open ocean and a small decrease of 0.2 K was measured in the brightness temperature, which corresponded to the salinity increase of 0.4 psu measured by the M/V Oleander vessel     

Passive remote sensing at microwave wavelengths

Passive remote sensing at microwave frequencies has applications which range from meteorology to oceanography and geology. The meteorological applications are the most fully developed and include measurements of the temperature profile of the atmosphere and of the atmospheric distribution of H2O and O3. Such measurements can he made from space or from the ground by utilizing the microwave resonances of O2, H2O, and O3which occur near 1-cm wavelength. Although infrared observations permit similar meteorological measurements, such optical devices are much more sensitive to aerosols and clouds. The small but finite nonresonant attenuation of most moderate clouds at microwave frequencies also permits their liquid water content to be estimated. At wavelengths longer than 2 cm the microwave properties of the terrestrial surface dominate observations from space, and measurements as a function of polarization and viewing angle yield information about surface temperature and emissivity. Such measurements of the ocean should also permit the sea state to he inferred. The review has two major parts. The first part reviews the physics of the interactions, the mathematics of data interpretation, and the instrumentation currently available. The second part is applications-oriented and emphasizes the types, accuracy, and relevance of possible meteorological measurements.     

Automatic microwave network measurements

Network measurements using scattering parameters at microwave frequencies date back to before World-War II in different forms. With the advent of sweepers, the first phase of automation was achieved. Errors in reflection and transmission measurements were reduced by means of better hardware design until the next step of automation took place. Digital computers not only increased the speed of measurements, but were capable of correcting vectorial errors. The chronological development of S parameter-measurement techniques and error analysis is discussed.     

Passive localization from Doppler-shifted frequency measurements

The position and speed of a moving source that radiates a pure tone can be determined by measurements of Doppler-shifted frequencies at five or more different locations. The equations relating to the measured frequencies to the localization parameters are nonlinear, and there is no closed-form solution. Solving nonlinear equations by a grid search, whereby all possible combination of source parameters are entered into the equation to see if they constitute a solution, would require a five-dimensional search. A more efficient search scheme that uses intermediate variables which are products of some of the unknowns is introduced. The search dimension is two. The resultant intermediate equations become linear and are easier to solve: the major calculation at each grid point requires only an inversion of a 3×3 matrix. The Cramer-Rao bound for localization is given, together with simulation results that verify the method     

Passive microwave observation of diurnal surface soil moisture

Microwave radiometers operating at low frequencies are sensitive to surface soil moisture changes. Few studies have been conducted that have involved multifrequency observations at frequencies low enough to measure a significant soil depth and not be attenuated by the vegetation cover. Another unexplored aspect of microwave observations at low frequencies has been the impact of diurnal variations of the soil moisture and temperature on brightness temperature. In this investigation, observations were made using a dual frequency radiometer (1.4 and 2.65 GHz) over bare soil and corn for extended periods in 1994. Comparisons of emissivity and volumetric soil moisture at four depths for bare soils showed that there was a clear correspondence between the 1 cm soil moisture and the 2.65-GHz emissivity and between the 3-5 cm soil moisture and the 1.4-GHz emissivity, which confirms previous studies. Observations during drying and rainfall demonstrate that new and unique information for hydrologic and energy balance studies can be extracted from these data     

Accuracy of microwave cavity perturbation measurements

Techniques based on the perturbation of cavity resonators are commonly used to measure the permittivity and permeability of samples of dielectric and ferrite materials at microwave frequencies. They are also used to measure the local electric- and magnetic-field strengths in microwave structures including the shunt impedances of cavity resonators and the coupling impedances of slow-wave structures. This paper reexamines the assumptions made in the theory of these techniques and provides estimates of the errors of measurement arising from them     

Noise measurements of microwave local oscillators

A method for measuring the spectral distribution of amplitude fluctuations and frequency fluctuations in microwave oscillators is described. The method provides for evaluation of (1) amplitude fluctuations in terms of signal to noise ratio; (2) frequency fluctuations in terms of two parameters: (a) rms deviation; (b) rms rate of change of deviation. In addition, the method permits estimation of the correlation between these two noise components. Some typical results of measurements are described, and certain implications of the effects of system parameters on interpretation of these results are considered. For a specific system, two measurements are shown to be sufficient to determine the effect of frequency noise on system performance.     

Passive microwave relative humidity retrievals using feedforwardneural networks

A technique for retrieving atmospheric humidity profiles using passive microwave spectral observations from satellite and multilayer feedforward neural networks (MFNN) is introduced. Relative humidity retrievals on a global scale from simulated radiances at fifteen frequencies between 23.8 and 183.3 GHz yielded rms errors in relative humidity of 6-14% over ocean and 6-15% over land at pressure levels ranging from 131 mbar to 1013 mbar. Comparison with a combined statistical and physical iterative retrieval scheme shows that superior retrievals can be obtained at a lower computational cost using MFNN     

Passive microwave remote sensing of snow constrained byhydrological simulations

This paper describes a snow parameter retrieval algorithm from passive microwave remote sensing measurements. The three components of the retrieval algorithm include a dense media radiative transfer (DMRT) model, which is based on the quasicrystalline approximation (QCA) with the sticky particle assumption, a physically-based snow hydrology model (SHM) that incorporates meteorological and topographical data, and a neural network (NN) for computational efficient inversions. The DMRT model relates physical snow parameters to brightness temperatures. The SHM simulates the mass and heat balance and provides initial guesses for the neural network. The NN is used to speed up the inversion of parameters. The retrieval algorithm can provide speedy parameter retrievals for desired temporal and spatial resolutions, Four channels of brightness temperature measurements: 19V, 19H, 37V, and 37H are used. The algorithm was applied to stations in the northern hemisphere. Two sets of results are shown. For these cases, the authors use ground-truth precipitation data, and estimates of snow water equivalent (SWE) from SHM give good results. For the second set, a weather forecast model is used to provide precipitation inputs for SHM. Additional constraints in grain size and density are used. They show that inversion results compare favorably with ground truth observations     

Passive electrical measurements from three Oklahoma tornados

Passive electrical measurement of three tornados in the Oklahoma area have been made and are being reported upon. The measurements made include point discharge current, 18.5-kc/s sferics, along with 10-, 50-, 100-, and 175-kc/s, and 30-Mc/s sferics. The preceding measurements of tornados are compared with a typical local thunderstorm. In this comparison, it is shown that the electrical discharges within the tornados are much more intense than in local thunderstorms, particularly in frequency of occurrence. Data also show that the normal frequency spectrum of sferics radiation is different from the frequency spectrum of the tornados measured. Furthermore, the tornado sferics appear to have a definite aperiodic structure that may be present to a limited extent in the intense portion of a thunderstorm. Similarities between the measurements made on the Oklahoma tornados and Jones's work are pointed out. Finally, it is suggested that a coupling between the electrical and mechanical energies of the tornado may be responsible for the difference noted between tornado sferics and thunderstorm sferics.     

Cable shielding measurements at microwave frequencies

A matched triaxial device is developed and constructed to measure the transfer impedance of braided coaxial cables at microwave frequencies. Full-design principles of the device have been developed and given in detail in the paper. The device is particularly suited for shielding measurements at frequencies up to 3.5 GHz, for which no comparable technique exists. The performance of the device is fully evaluated both theoretically and by measurements. The accuracy of the device is studied by comparing the results against those obtained from the low-frequency International Electrotechnique Committee triple-coaxial apparatus, and using well-known theoretical models.     

Multipath angle-of-arrival measurements on microwave line-of-sightlinks

The results obtained from the experimental measurement of angles-of-arrival (AOA) and amplitudes of individual multipath rays on three separate microwave links are presented. An extensive data base has been established from the operation of a wide-aperture vertical array of receiving antennas. From these data, statistical distributions of the AOAs and amplitudes of the incoming rays are presented which appear to be consistent with the predictions of ray-tracing     

移动通信微波部件无源互调干扰综述

在地面通信系统中,基站的微波部件长期暴露在外,受温度、湿度、污染等因素影响,原本洁净的部件表面受到氧化、沾污等,导致接触不良,产生无源互调(PIM),干扰通信系统,导致地面移动通信中PIM的干扰不可避免。卫星通信系统中,由于通信距离远、平台限制等,必须采用收发共用模式,随着接收机灵敏度提升,对PIM指标要求更高;因系统高可靠性要求,PIM成为系统成败的关键问题之一。本文介绍了地面移动通信和星载微波部件PIM干扰的特点和研究进展,以期对PIM研究提供参考。     

Passive microwave remote sensing of forests: a model investigation

A model, based on the radiative transfer theory and the matrix doubling algorithm, is described and used to compute the emissivity e of forests. According to model simulations, the L-band emissivity trend versus forest biomass is more gradual than that of the backscatter coefficient. This gradual behavior is observed, in absence of leaves, also at C- and X-bands, while leaves anticipate saturation and make e higher in coniferous forests and lower in deciduous forests. Model results are successfully validated by some available experimental data. Operational aspects, concerning the potential of airborne and spaceborne radiometers in identifying forest type and estimating biomass, are discussed     

单星测频无源定位法

提出了一种利用卫星在不同位置上测量信号的多普勒频率,实现单颗卫星对地面或空中静止的非协同性无线电辐射源定位的方法。在介绍了定位原理和定位计算方法后,分析了定位误差,并给出了计算机仿真结果。     

Radiometric measurements of the microwave emissivity of foam

Radiometric measurements of the microwave emissivity of foam were conducted during May 2000 at the Naval Research Laboratory's Chesapeake Bay Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal and vertical polarization measurements were performed at 36.5 GHz; horizontal, vertical, +45/spl deg/, -45/spl deg/, left-circular, and right-circular polarization measurements were obtained at 10.8 GHz. These measurements were carried out over a range of incidence angles from 30/spl deg/ to 60/spl deg/. Surface foam was generated by blowing compressed air through a matrix of gas-permeable tubing supported by an aluminum frame and floats. Video micrographs of the foam were used to measure bubble size distribution and foam layer thickness. A video camera was boresighted with the radiometers to determine the beam-fill fraction of the foam generator. Results show emissivities that were greater than 0.9 and approximately constant in value over the range of incidence angles for vertically polarized radiation at both 10.8 and 36.5 GHz, while emissivities of horizontally polarized radiation showed a gradual decrease in value as incidence angle increased. Emissivities at +45/spl deg/, -45/spl deg/, left-circular, and right-circular polarizations were all very nearly equal to each other and were in turn approximately equal to the average values of the horizontal and vertical emissivities in each case.     

Uncertainty charts for RF and microwave measurements

The scope of the calibration services for electrical quantities in the range of frequency from 0 to 100 GHz that are available from the National Bureau of Standards is discussed briefly in a historical context. Some plans for improved services that will be available in the near future are noted. Charts showing the variation of uncertainty with magnitude over the full range of the respective calibration services are presented.