Retrieval of land and sea brightness temperatures from mixedcoastal pixels in passive microwave data

A technique is presented to separate uncontaminated land and sea brightness temperatures from mixed coastal pixels in 37-GHz vertically polarized passive microwave data from the Special Sensor Microwave Imager (SSM/I) instrument. Combining a mathematical model of the instrument response over several neighboring footprints with a GIS representation of the coastline yields a relationship between land and sea brightness temperatures and radiation measurements made at the satellite. Inverting this relationship allows separate land and sea brightness temperature values to be derived for each mixed coastal pixel in the original image. The technique has been successfully applied to 37-GHz vertically polarized SSM/I imagery for test areas covering the Gulf of Aden and the British Isles. Errors in the retrieved brightness temperatures were estimated to be of the order of 1-2 K     

Modeling of wind direction signals in polarimetric sea surfacebrightness temperatures

There has been an increasing interest in the applications of polarimetric microwave radiometers for ocean wind remote sensing. Aircraft and spaceborne radiometers have found a few Kelvins wind direction signals in sea surface brightness temperatures, in addition to their sensitivities to wind speeds. However, it was not clear what physical scattering mechanisms produced the observed brightness dependence on wind direction. To this end, polarimetric microwave emissions from wind-generated sea surfaces are investigated with a polarimetric two-scale scattering model, which relates the directional wind-wave spectrum to passive microwave signatures of sea surfaces. Theoretical azimuthal modulations are found to agree well with experimental observations for all Stokes parameters from near nadir to 65° incidence angles. The upwind and downwind asymmetries of brightness temperatures were interpreted using the hydrodynamic modulation. The contributions of Bragg scattering by short waves, geometric optics scattering by long waves and sea foam are examined. The geometric optics scattering mechanism underestimates the directional signals in the first three Stokes parameters, and predicts no signals in the fourth Stokes parameter (V). In contrast, the Bragg scattering was found to dominate the wind direction signals from the two-scale model and correctly predicted the phase changes of the upwind and crosswind asymmetries in T_υ and U from middle to high incidence angles. The phase changes predicted by the Bragg scattering theory for radiometric emission from water ripples is corroborated by the numerical Monte Carlo simulation of rough surface scattering. This theoretical interpretation indicates the potential use of polarimetric brightness temperatures for retrieving the directional wave spectrum of short gravity and capillary waves     

Polarimetric radar remote sensing of ocean surface wind

Experimental data are presented to support the development of a new concept for ocean wind velocity measurement (speed and direction) with the polarimetric microwave radar technology. This new concept has strong potential for improving the wind direction accuracy and extending the useful swath width by up to 30% for follow-on NASA spaceborne scatterometer mission to SeaWinds series. The key issue is whether there is a relationship between the polarization state of ocean backscatter and surface wind velocity at NASA scatterometer frequencies (13 GHz). An airborne Ku-band polarimetric scatterometer (POLSCAT) was developed for proof-of-concept measurements. A set of aircraft flights indicated repeatable wind direction signals in the POLSCAT observations of sea surfaces at 9-11 m/s wind speed. The correlation coefficients between co- and cross-polarized radar response of ocean surfaces have a peak-to-peak amplitude of about 0.4 and are shown to have an odd-symmetry with respect to the wind direction, unlike the normalized radar cross sections     

对流层散射无源测向中方位向散射特性研究

研究了对流层超视距无源测向系统中方位向散射信号的特性,通过定义对流层散射方位向功率因子,分析不同参数对因子的影响,得出了接收信号功率中值与接收天线方位向波束宽度的关系.重点研究了系统对目标测向存在的测向误差和测向虚像模糊问题,发现存在与接收天线无关的最小测向误差和最小虚像模糊角,分析了造成这些问题的原因,并推导出了最小测向误差和最小虚像模糊角与目标天线和距离等的关系.     

High-resolution passive polarimetric microwave mapping of oceansurface wind vector fields

The retrieval of ocean surface wind fields in both one and two dimensions is demonstrated using passive polarimetric microwave imagery obtained from a conical-scanning airborne polarimeter. The retrieval method is based on an empirical geophysical model function (GMF) for ocean surface thermal emission and an adaptive maximum likelihood (ML) wind vector estimator. Data for the GMF were obtained using the polarimetric scanning radiometer/digital (PSR/D) on the NASA P-3 aircraft during the Labrador Sea Deep Convection Experiment in 1997. To develop the GMF, a number of buoy overflights and GPS dropsondes were used, out of which a GMF of 10.7, 18.7, and 37.0 GHz azimuthal harmonics for the first three Stokes parameters was constructed for the SSM/I incident angle of 53.1°. The data show repeatable azimuthal harmonic coefficient amplitudes of ~2-3 K peak-to-peak, with a 100% increase in harmonic amplitudes as the frequency is increased from 10.7 to 37 GHz. The GMF is consistent with and extends the results of two independent studies of SSM/I data and also provides a model for the third Stokes parameter over wind speeds up to 20 m/s. The aircraft data show that the polarimetric channels are much less susceptible to geophysical noise associated with maritime convection than the first two Stokes parameters. The polarimetric measurement technique used in the PSR/D also demonstrates the viability of digital correlation radiometry for aircraft or satellite measurements of the full Stokes vector. The ML retrieval algorithm incorporates the additional information on wind direction available from multiple looks and polarimetric channels in a straightforward manner and accommodates the reduced SNRs of the first two Stokes parameters in the presence of convection by weighting these channels by their inverse SNR     

Spatial resolution enhancement of terrestrial features usingdeconvolved SSM/I microwave brightness temperatures

A method for enhancing the 19-, 22-, and 37-GHz measurements of the SSM/I (Special Sensor Microwave/Imager) to the spatial resolution and sampling density of the high-resolution 85-GHz channel is presented. An objective technique for specifying the tuning parameter, which balances the tradeoff between resolution and noise, is developed in terms of maximizing cross-channel correlations. Various validation procedures are performed to demonstrate the effectiveness of the method, which, it is hoped, will provide researchers with a valuable tool in multispectral applications of satellite radiometer data     

The influence of clouds on microwave brightness temperatures viewing downward over open seas

Microwave brightness temperatures for the case of downward viewing from above the earth's atmosphere over water for the 1- to 2-cm wavelength range are calculated for comparison with observations. A model of the troposphere which contains homogeneous layer clouds of varied thickness and liquid water content is used to compute estimates of the influence which clouds would have on real observations. It is assumed that only pure absorption is important for the cloud droplet-size distributions and droplet densities used. Results of the computations indicate that most water clouds will contribute a measurable amount to the microwave emission of the atmosphere and, in some cases, can be the principal source of received radiation. Comparisons of the computed cases with measurements obtained with a high flying aircraft are shown to be in reasonable agreement. These results are significant because they demonstrate that water clouds cannot be neglected in the application of passive microwave techniques to remote probing of the earth's atmosphere and because they indicate that quantitative measures of cloud liquid water contents and cloud thickness might be acquired through multi-frequency measurements.     

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     

Retrieving ocean surface wind speed from the TRMM Precipitation Radar measurements

Spaceborne scatterometery has been used for many years now to retrieve the ocean surface wind field from normalized radar cross-section measurements of the ocean surface. Though designed specifically for the measurement of precipitation profiles in the atmosphere, the Precipitation Radar (PR) of the Tropical Rainfall Measuring Mission (TRMM) also acquires surface backscattering measurements of the global oceans. As such, this instrument provides an interesting opportunity to explore the benefits and pitfalls of alternative radar configurations in the satellite remote sensing of ocean winds. In this paper, a technique was developed for retrieving ocean surface winds using surface backscattering measurements from the TRMM PR. The wind retrieval algorithm developed for TRMM PR makes use of a maximum-likelihood estimation technique to compensate for the low backscattering associated with the PR configuration. The high vertical resolution of the PR serves to filter-out rain-contaminated cells normally integrated into Ku-band scatterometer measurements. The algorithm was validated through comparisons of ocean surface wind speeds derived from PR with remotely measured winds from TMI and QuikSCAT, as well as in situ observations from oceanographic buoys, revealing good agreements in wind speed estimations.     

Sensitivity improvement in digital microwave signal analysis

Sensitivity in digital microwave signal analysis strongly depends on the signal/noise ratio. The letter describes a method which improves the signal/noise ratio and therefore the sensitivity of monofrequent amplitude and phase measurements by means of sampling below the Nyquist frequency.     

微波无源器件的快速有限元全波分析

从电磁场有限元法出发,采用有限元撕裂对接法将大型复杂模型转化为若干个小型简单模型来进行分析,引入高阶传输条件和并行计算策略进一步提升了有限元撕裂对接法的性能。对基片集成波导和微带滤波器两种典型微波无源器件的S参数进行了仿真分析,数值结果表明了方法的正确性和有效性。     

Surface wind speed retrieval using passive microwave polarimetry:the dependence on atmospheric stability

The results of the Joint US/Russia Internal Wave Remote Sensing Experiment are presented. An airborne Ka-band radiometer-polarimeter was used for surface wind vector retrieval based upon the polarizational anisotropy effect. The anisotropy versus wind speed curve was noted to be approximately twice as steep under unstable atmospheric stratification than under stable or neutral stratification. The dependence of anisotropy on both friction velocity and stability parameter z/L was examined. The correlation between anisotropy and stability parameter is significant, especially for slightly stable stratification. The paper concludes that atmospheric stratification should be taken into consideration to improve the wind speed retrieval algorithm     

A systematic comparison of QuikSCAT and SAR ocean surface wind speeds

We performed a systematic comparison of wind speed measurements from the SeaWinds QuikSCAT scatterometer and wind speeds computed from RADARSAT-1 synthetic aperture radar (SAR) normalized radar cross section measurements. These comparisons were made over in the Gulf of Alaska and extended over a two-year period, 2000 and 2001. The SAR wind speed estimates require a wind direction to initialize the retrieval. Here, we first used wind directions from the Navy Operational Global Atmospheric Prediction System (NOGAPS) model. For these retrievals, the standard deviation between the resulting SAR and QuikSCAT wind speed measurements was 1.78 m/s. When we used the QuikSCAT-measured wind directions to initialize the inversion, comparisons improve to a standard deviation of 1.36 m/s. We used these SAR-scatterometer comparisons to generate a new C-band horizontal polarization model function. With this new model function, the wind speed inversion improves to a standard deviation of 1.24 m/s with no mean bias. These results strongly suggest that SAR and QuikSCAT measurements can be combined to make better high-resolution wind measurements than either instrument could alone in coastal areas.     

Sensitivity, spatial heterogeneity, and scaling of C-band microwave brightness temperatures for land hydrology studies

Soil moisture is one of the most important hydrological variables that characterizes the land surface water and energy balance. Measurements from space suffer from the problem of subpixel heterogeneity, i.e., soil moisture has spatial variability at all scales; therefore, it is important to realize the exact physical implication of the single value of the satellite measurements. In this paper, we study the sensitivity of C-band passive microwave brightness temperatures to various land surface variables. The issue of heterogeneity and its role in interpretation of single spatially averaged value of satellite brightness temperature is investigated. Finally, we use the brightness temperatures from the Scanning Multichannel Microwave Radiometer to characterize spatial variability and to understand the variation of this variability with scale.     

Land surface temperature derived from the SSM/I passive microwavebrightness temperatures

Passive microwave brightness temperatures from the Defense Meteorological Space Program Special Sensor Microwave/Imager (SSM/I) were used to determine surface temperature over land areas in the central plains of the United States. A regression analysis comparing all of the SSM/I channels and minimum screen air temperatures (representing the surface temperature) showed good correlations, with root-mean-square errors of 2-3 degC. Pixels containing large amounts of water, snow, and falling rain, as classified with SSM/I brightness temperatures, were excluded from the analysis. The use of independent ground truth data such as soil moisture or land surface type was not required to obtain the correlations between brightness temperatures and surface temperatures     

Microwave radiometric determination of wind speed at the surface of the ocean during BESEX

Microwave radiometric measurements were made at wavelengths ranging from 0.8 to 2.8 cm at altitudes from 0.16 to 11 km under well documented meteorological conditions over the Bering Sea. It is shown that determinations of wind speed at the ocean surface and liquid water content of the clouds may be made from such data. Determinations were made from two simultaneous but independent sets of radiometric measurements. The wind speeds and liquid water contents made from these two sets showed remarkable agreement. Independent estimates of these parameters made from in situ measurements showed reasonable agreement as well.     

Ocean surface wind speed and direction retrievals from the SSM/I

A semiempirical model is developed that retrieves ocean surface wind direction information in addition to improved wind speeds from Special Sensor Microwave/Imager (SSM/I) measurements. Radiative transfer and neural network techniques were combined in the authors' approach. The model was trained and tested using clear sky cases, but atmospheric transmittance is retrieved so that retrieval in other than clear sky conditions is possible. With two SSM/I instruments currently providing operational ocean surface wind speed retrievals, the addition of wind direction information and improved wind speed retrievals will enhance the impact of this data in weather prediction models and marine weather forecasting     

The magnetic field dependency of brightness temperatures atfrequencies near the O2 microwave absorption lines

The dependence of microwave brightness temperatures on the Earth's magnetic field at frequencies within a few megahertz of the O₂ absorption line centers is examined in detail. The azimuthal angular dependence is shown to factor from the radiative transfer equation while the dependence on the angle between the propagation direction and the Earth's field direction is described by a linear or quadratic polynomial in the square of the cosine of this angle in some typical cases of potential interest for a mesospheric sounder. Dependences of the brightness temperature matrix on the magnitude of the Earth's field are derived and shown to be different according to whether linear or circular polarization is used     

Directional analysis of SAR images aiming at wind direction

Currently, the retrieval of wind fields from synthetic aperture radar (SAR) images suffers from inadequate knowledge of the wind direction. State-of-the-art spectral analysis works fine on open seas, but is limited in spatial resolution. The method described here is based on the local gradients computed with standard image processing algorithms. It handles image features not caused by wind and can be applied to irregularly shaped regions. The new method has already been applied to many images from the European Remote sensing Satellite SARs and RADARSAT-1 ScanSAR, usually supplying reasonable wind fields. The spatial sampling most frequently used was 20 /spl times/ 20 and 10/spl times/10 km/sup 2/. In some cases, samplings down to 1/spl times/1 km/sup 2/ were tested. This paper describes the local gradients method including the filtering of nonwind generated image features and gives some application examples.     

STUDY ON PASSIVE MICROWAVE REMOTE SENSING OF OIL POLLUTION ON SEA SURFACE

By using the model of air-oil slick-sea water system, the influence of the oil slick thickness, operating frequency of the radiometer and incidence angle on the effective emissivity are calculated and analysed. The optimum scheme for passive microwave remote sensing of oil pollution on sea surface is also proposed and used successfully for airborne microwave remote sensing experiments.