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     

Stable broadband variable noise source for microwave radiometry

A stable broadband microwave reference noise source has been designed using, as radiating elements, carbon-filament indicator lamps mounted in a waveguide. When used as a reference in an accurate radiometer, in place of a gas tube and attenuator, this source has a number of advantages. It is electronically controllable, insensitive to ambient temperature changes and the output may be set to match exactly any hot-load absolute standard of noise.     

HUT fully polarimetric calibration standard for microwave radiometry

This paper describes the Helsinki University of Technology's Fully Polarimetric Calibration Standard (FPCS). The developed standard generates a complete Stokes reference vector and it is applied for the end-to-end absolute calibration of a fully polarimetric microwave radiometer at 36.5 GHz. The FPCS is based on the function principle of a Gasiewski-Kunkee linearly polarized (tripolarimetric) standard, with an additional phase retardation plate to generate the fourth Stokes parameter. Design considerations and operational aspects of the standard are discussed in this paper. An advanced calibration procedure, which takes advantage of both the tripolarimetric and fully polarimetric calibration scenes to suppress calibration uncertainties, is introduced. The feasibility of the standard has been verified and the generated brightness temperatures in a sample calibration are presented. An extensive set of tests has been performed to evaluate the characteristics and performance of the calibration standard. Furthermore, the use of the advanced calibration procedure to measure the characteristics of the phase retardation plate has been successfully demonstrated. The achievable calibration accuracy is analyzed and discussed relative to requirements for maritime wind vector measurements; the results indicate that the pixel-to-pixel retrieval of the wind speed is possible with high accuracy and the retrieval of the wind direction with at least moderate accuracy. In addition to calibration of a fully polarimetric radiometer, other potential applications, e.g., linearity measurements, are discussed.     

Improved recognition of thermal structures by microwave radiometry

A radiometric system for medical applications, including a set of six probes, facilitates the positioning of the probes and improves the precision in their location. An example of an experiment and of signal processing is shown which improves the recognition of the thermal structure.     

Microwave radiometry for continuous non-contact temperature measurements during microwave heating.

Temperature measurement during microwave heating in industrial and commercial processes can improve quality, throughput, and energy conservation. Conventional ways of measuring temperature inside a microwave oven cavity are costly, inconvenient, or unsuitable for high-volume industrial applications. In this paper, we describe the theory of microwave radiometry as applied to the measurement of temperature during microwave heating. By extending the theory of radiative transfer to the case of thermal microwave radiation inside a cavity, we show that the same characteristics which make a microwave cavity suitable for heating materials also assist in obtaining meaningful temperature data with microwave radiometry. We present experimental data from the heating of liquid and solid materials which confirm the essential features of the theory, and show agreement between this method and more conventional methods of +/-4 degrees C.     

Multiangle method for temperature measurement of biological tissues by microwave radiometry

A new approach for deriving the temperature distribution in biological tissues of microwave radiometry is proposed. It consists in the measurement of the thermal radiation of the body, at a given frequency, as a function of the observation angle, for two mutually orthogonal polarizations. Theoretically, this method yields results comparable to those obtained with the multispectral method. In order to derive the relations between the body temperature and the emitted thermal signal, the biological body is modeled by a set of parallel planar layers, each characterized by constant permittivity and temperature. It is demonstrated that for all practical purposes the radiation pattern of the antenna may be approximated by that of an unbounded plane wave.<>     

Interferometric synthetic aperture microwave radiometry for theremote sensing of the Earth

Interferometric aperture synthesis is presented as an alternative to real aperture measurements of the Earth's brightness temperature from low Earth orbit. The signal-to-noise performance of a single interferometric measurement is considered, and the noise characteristics of the brightness temperature image produced from the interferometer measurements are discussed. The sampling requirements of the measurements and the resulting effects of the noise in the measurements on the image are described. The specific case of the electronically steered thinned array radiometer (ESTAR) currently under construction is examined. The ESTAR prototype is described in detail sufficient to permit a performance evaluation of its spatial and temperature resolution. Critical aspects of an extension of the ESTAR sensor to a larger spaceborne system are considered. Of particular important are the number and placement of antenna elements in the imaging array     

An example of thermometry in volume by microwave radiometry

A method of determination of the size D depth z and temperature T0+ΔT of a cylindrical thermal structure embedded in an homogeneous lossy material, in the present case, water, is presented. A microwave radiometric image at 3 GHz points out the location of the thermal structure; its threshold provides the diameter D of the structure. The depth z derives from the ratio of the maximal radiometric intensities at 1.5 and 3 GHz. The combination of D, z and of the radiometric intensities gives ΔT     

Satellite microwave radiometry of forest and surface types inFinland

10.7-, 18-, and 37-GHz data from the Nimbus-7 Scanning Multichannel Microwave Radiometer (Nimbus-7 SMMR) were used to investigate the microwave response to different surface and forest types. SMMR data for the fall seasons of 1978 through 1981 were compared against a digital surface type map that shows seven different surface types for southern Finland and six for northern Finland. For each land-cover category, the brightness temperature behavior as a function of frequency and polarization was determined. The precision of the results is evaluated, and they are compared to other published results. The discrimination of land-cover categories using the brightness temperature is discussed     

Correlations between slant wet delays measured by microwave radiometry

We present an analysis of the correlation between the atmospheric slant wet delays in different directions using data from a microwave radiometer. The correlations between wet delays observed in different directions using different temporal constraints are compared to a model derived from theories of turbulence. The agreement between the model and the radiometer data was good, and the average squared difference between zenith mapped slant wet delays could be predicted with an accuracy of 0.01-0.04 cm/sup 2/. Our analysis shows a large short-term variability which variance has a seasonal dependence of about /spl plusmn/26%, largely depending on the refractivity structure constant C/sub n/. We also demonstrate how the model can be used to characterize the stability of a microwave radiometer.     

RFI detection and mitigation for microwave radiometry with an agile digital detector

A new type of microwave radiometer detector has been developed that is capable of identifying high and low levels of radio-frequency interference (RFI) and of reducing or eliminating its effect on the measured brightness temperatures. High-level, localized RFI can be easily identified by its unnatural appearance in brightness temperature imagery. Low-level or persistent RFI can be much more difficult to identify and filter out. The agile digital detector (ADD) can discriminate between RFI and natural thermal emission signals by directly measuring higher order moments of the signal than the variance that is traditionally measured. After detection, the ADD then uses spectral filtering methods to selectively remove the RFI. ADD performance is experimentally verified in controlled laboratory tests and in the field near a commercial air traffic control radar. High-level RFI is easily identified and removed. Very low level RFI contamination, with power levels as low as the radiometric measurement uncertainty of the radiometer, is also shown to be reliably detected and removed.     

On the possibility of detecting avalanche victims by microwave radiometry

An experimental investigation has been made of the possibility of using a microwave radiometer to find humans covered by snow. When a radiometer operating at 1.4 GHz was moved along a track it registered a highly fluctuating radiation temperature of the snow covered ground. The increase in temperature when the radiometer passed over a human was so small that detection in general was impossible.     

Adaptive Doppler-Kalman filter for radar systems

Time-dependent Fourier transform plays an important role in radar signal analysis. The Doppler frequency carries information about the relative velocity of a moving target regarding the radar antenna. However, because the target movement may be very complex, with temporary high-intensity manoeuvres, the Doppler frequency must be estimated by using a window function. In order to minimise the estimation error, the window function width must be dependent on the target manoeuvre as well as on the signal-to-noise ratio. Here, a system consisting of a Doppler filter and a second-order Kalman filter, connected in an adaptive structure, has been proposed as an efficient solution to this problem. The proposed approach enables the window width adaptation, based on the estimates of target acceleration and signal-to-noise ratio, generated by an adaptive Kalman filter. Simulation results demonstrate the advantage of this structure compared to the fixed-width, window-based algorithms, giving acceptable results even for very bad signal-to-noise conditions.     

Modeling and measurement of rainfall by ground-based multispectral microwave radiometry

The potential of ground-based multispectral microwave radiometers in retrieving rainfall parameters is investigated by coupling physically oriented models and retrieval methods with a large set of experimental data. Measured data come from rain events that occurred in the USA at Boulder, Colorado, and at the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) site in Lamont, OK. Rain cloud models are specified to characterize both nonraining clouds, stratiform and convective rainfall. Brightness temperature numerical simulations are performed for a set of frequencies from 20 to 60 GHz at zenith angle, representing the channels currently deployed on a commercially available ground-based radiometric system. Results are illustrated in terms of comparisons between measurements and model data in order to show that the observed radiometric signatures can be attributed to rainfall scattering and absorption. A new statistical inversion algorithm, trained by synthetic data and based on principal component analysis is also developed to classify the meteorological background, to identify the rain regime, and to retrieve rain rate from passive radiometric observations. Rain rate estimate comparisons with simultaneous rain gauge data and rain effect mitigation methods are also discussed.     

自适应LUM滤波器

低-高-中滤波器(Lower-upper-middlefilter,简称LUM滤波器)是中值滤波基础上发展起来的一种非线性滤波器.提出了一种改进的LUM滤波器的算法.算法对LUM滤波器进行了两方面的改进:一方面针对未被噪声污染的非边缘区域,不滤波;另一方面自动查找LUM滤波器锐化窗口的位置.与原有的LUM滤波器相比,它能缩短程序运行的时间,较好地降噪,保护图像的有用信息,更好地增强图像的边缘.     

Adaptive histogram-based filter for image restoration

A novel filter for image restoration is proposed in this paper. The filter estimates histogram of original image via input image. It gets a membership function through the histogram, and the membership function contains a lot of information of original image. Then a weighted fuzzy mean filter is established based on this membership function; meanwhile, the filter adaptively adopts different filter scale according to the character divergence of image region and intensity of impulsive noise. Experimental result shows that new filter gives superior performance to conventional filters and currently used fuzzy filter.     

Adaptive LSP filter

Line spectral pair (LSP) coefficients have been proposed as alternative representations of LPC parameters for low-bit-rate speech coding. The letter presents an LMS-type adaptive digital filter for directly calculating even-order LSP coefficients on a sequential basis. The technique is demonstrated for fourth-order adaption and may be generalised to higher orders.