ISSN:0920-5071    

DOI:10.1163/156939395X00488

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Polarimetric radars make use of the polarizations of the incident and backscattered waves to achieve the polarimetric target information. This information is described by the scattering matrix (SM) S(fk) of a target, where fkis the carrier frequency of a narrowband pulse radar. Depending on the number N of frequencies used, we distinguish between narrow- and broadband polarimetric radars. The purpose of this paper is to compare measured narrow- and broadband SM data and to show a new polarimetric broadband signal description. Furthermore, a comparison of narrow- and broadband signal processing techniques is presented using the polarimetric CFAR detector and a classification concept. The presented broadband measurements show that the polarimetric information changes significantly in the frequency range. This leads to improvements in the broadband polarimetric signal processing compared to the narrowband case.

ISSN:0920-5071    

DOI:10.1163/156939395X00497

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Polarimetric pulsed Doppler radar measurements of meteorological targets yield mean backscattering properties of hydrometeors in the considered range cells and provide information on propagation effects along the radar ray of propagation. In this paper, two aspects of choosing an optimum polarimetric measurement basis are examined. First, the polarization dependence of path integrated differential phase shift is studied starting from the 2 x 2 transmission matrix. It is shown that the linear HV-basis maximizes differential propagation phase. For the general case of an elliptical polarization basis, propagation effects can no longer be completely characterized by using the differential propagation phase shift (at frequencies lower that 6 GHz). Second, the choice of a particular measurement basis has a great impact on polarimetric weather radar data quality and, therefore, microphysical information content considering real world polarimetric weather radar limited channel isolation. It follows from the mean backscattering properties of hydrometeors in conjunction with polarimetric weather radar performance that the set of elliptical EFbases (|E-F| =E,F= ±/8) yields particularly suited orthogonal measurement bases for rain radar applications having considerable advantages over the conventionally used linear HV- or circular LR-basis. For ensembles of almost sphere-like hydrometeorological scatterers co- and crosspolar power levels will attain roughly the same amplitude if an elliptical EF-basis is used as measurement basis. Considering limited interchannel isolation, polarimetric weather radar measurements performed in an elliptical EF-basis promise minimum relative errors and improved data quality. However, a trade-off situation does not exist between the elliptical EF-basis and the linear HV-basis. Since the elliptical EF-basis provides optimum measurements with respect to system induced cross-talk distortion, the optimum measurement strategy for polarimetric weather radar applications is to perform the measurement in elliptical EF-basis and then transform the measured time series of 2 x 2 scattering matrices to the linear HV-basis to conveniently estimate the differential propagation phase and established radar observables usually defined in the HV-basis such as, in particular, linear depolarization ratio.

ISSN:0920-5071    

DOI:10.1163/156939395X00505

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The K-distribution model for clutter over distributed targets in Synthetic Aperture Radar (SAR) images is extended to include clutter along edges and thin linear targets. The physical mechanisms generating edge features are analyzed. Assuming a boxcar system response, the detected field from a resolution cell along an edge is shown to be expressible in terms of sums of random contributions from different scatterer populations, plus single phasors representing specular and secondary scattering contributions. The clutter statistics resulting from this expression are derived assuming that speckle is fully developed and that the number of scatterers from each population is controlled by a birth-death-immigration process. In the absence of specular and secondary scattering the detected field accords with the product model for radar imaging, i.e. the field is given by the product of a complex circularly symmetric zero-mean Gaussian speckle process and the square-root of the imaged surface cross-section (SCS). The imaged SCS is given by a weighted sum of gamma distributed random variables, each one of which represents the SCS due to a single distributed target. Under certain limiting conditions the imaged SCS is gamma distributed or constant. Some of the results are confirmed by statistical measurements taken along edges and thin linear targets.

ISSN:0920-5071    

DOI:10.1163/156939395X00514

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Two identical Ku band scatterometers were deployed at a number of locations in the region of the Filchner-Ronne ice shelf, Antarctica, during the austral summer '91/92. These were used to investigate the vertical incidence radar properties of the top 10m of firn, and these measurements were compared with those made by the ERS-1 radar altimeter over the region. The radar backscatter coefficient, on the ice shelf was found to remain constant over a period of 18 days, but the value differed significantly from that obtained on the nearby Coats Land plateau. Field measurements, coincident with ERS-1 overpasses, were made throughout a 4-week field season during the three day repeat cycle of the 'Ice Phase' of the satellite's operation. These show spatial agreement with the ERS-1 altimeter backscatter coefficient to within 1dB. Distinct sub-surface layering was detected at several locations, contributing up to 3dB to the total backscattered power with a variation over 30km of 1dB. The major surface factor contributing to the 7dB of spatial and temporal variation of backscatter coefficient, across the ice shelf, is the presence and formation of surface crusts.

ISSN:0920-5071    

DOI:10.1163/156939395X00523

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The European Space Agency's campaign, RENE'91, for the geophysical calibration and validation of its ERS-1 satellite took place in the Haltenbanken area of the Norwegian Sea in the Autumn of 1991. A primary objective was to collocate surface wind measurements over the sea with normalized radar cross section (0) measurements from the ERS-1 Scatterometer, in order to establish a radar sea-echo model from space derived data. The scatterometer's three-beam0products from 77 orbits were merged with 10 metre height, neutral stability, wind vector estimates from a field analysis program which combined aircraft, buoy, ship, and meteorological background field measurements. To provide enough data to study0against the main physical variables, raw0values were classified (binned) into ranges of incidence angle (), wind speed (v), and wind direction with respect to the scatterometer beam (), and a bin average0calculated for used in subsequent analysis. A by-product was lower noise and data volume. The Haltenbanken model was assumed to be linear and of the form:0= b0.(1 + b1. cos + b2. cos2), where the factor b0corresponds to the mean of0over , the factor in brackets contains the wind direction dependence, and all the b's are functions of v and . This is identical to the ESA pre-launch model cmod2. Studies of the two factors using binned collocation data confirmed the form of the direction dependence, but log (b0) was found to fit a √v dependence significantly more closely than the log (v) used in cmod2, so this √v relationship was preferred in the final model called cmod3_h1.

ISSN:0920-5071    

DOI:10.1163/156939395X00532

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Emissivity of agricultural crops is theoretically estimated through a model based on the Radiative Transfer Theory, and then compared with experimental data collected at X and Ka bands. The test is in general positive. The model and the experimental data agree in showing the ability of a dual-frequency radiometer system to discriminate different field types. The importance of a realistic choice of the dimensions, orientation and relative position of the scattering elements within a vegetation layer is pointed out.

ISSN:0920-5071    

DOI:10.1163/156939395X00541

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The conventional matched filter forms the basis for SAR image generation. Although the impulse response function obtained by the matched filter approach has the best achievable signal-to-noise ratio, high sidelobes, paired echoes and azimuth ambiguities will appear in the impulse response function (IRF) for severe phase and amplitude errors. This paper considers two types of error: 1) SAR transceiver chain errors and instabilities which influence the range IRF and 2) azimuth signal aliasing which leads to azimuth ambiguities. In order to suppress these errors and to improve the SAR image quality, an ideal filter concept is used which provides, in addition to the matched filtering, the deconvolution of the phase and amplitude errors. Several simulation results are presented concerning the improvement of the range and azimuth IRFs. In the case of range compression, the sidelobes can efficiently be suppressed if the replica of the transmitted signal is known with sufficient accuracy. For azimuth processing, the Doppler parameters' accuracy and the azimuth fading limits the efficiency of ambiguity suppression. The results of the image quality analysis show that the sidelobes of the IRF obtained by matched filtering are attenuated by more than 10 dB when using the ideal filter concept.

ISSN:0920-5071    

DOI:10.1163/156939395X00550

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We made backscatter measurements at C band (5.3 GHz) over sea ice in the Weddell Sea. These measurements were performed from the German research vessel F.S. Polarstem during September and October, 1989. We acquired data on First-Year (FY) and Second-Year (SY) snow-covered sea ice at stations where the ship stopped and detailed snow and ice characterizations were performed. Data were collected at incidence angles from 30° to 75° with all four linear polarizations. The results showed that the presence of slush at the snow-ice interface effectively masked the distinction between FY and SY ice in the Weddell Sea. Perhaps more importantly, we found that it is possible to distinguish slush-covered and slush-free FY ice floes, depending on incidence angle. We found that scattering from roughened, slush-free ice was stronger than slush-covered ice at large incidence angles. Essentially the presence of slush at the snow-ice interface created a new dielectric interface that was smoother than the original rougher ice surface below. At small incidence angles, quasi-specular reflection from slush-covered ice dominated the radar signal. The results further indicated that the volume scattering from the snow and from the ice was small up to 50° angle of incidence. We compared the results with data collected over sea ice from the Arctic. Backscatter for FY ice in the Arctic and the Weddell Sea were similar in magnitude. Returns from the Arctic SY ice were about 3-7 dB higher than those from the Weddell Sea SY ice. We believe that this difference is due to the presence of a slush layer over the SY ice surface studied in the Weddell Sea. The wet slush layer reduced the penetration depth into the ice, significantly reducing the volume scatter contribution from the ice.

ISSN:0920-5071    

DOI:10.1163/156939395X00569

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

For the utilization of higher frequencies (above 10 GHz) for satellite communications, propagation effects due to the atmosphere are a critical system factor. The main effects are the attenuation and depolarization of the signals by rain and other hydrometeors, but also the attenuation by water vapor and oxygen has to be considered. For the investigation of these effects, remote sensing techniques, in particular radiometry and radar polarimetry, are well applicable and useful tools. This contribution presents monthly single and joint parameter statistics of 20 GHz beacon attenuation, sky noise temperature, radiometer derived attenuation and rain rate. Radar predicted attenuation is compared with the measured beacon attenuation for a selected event. The polarimetric behavior of the sky noise temperature and the calibration of the radiometer are discussed.

ISSN:0920-5071    

DOI:10.1163/156939395X00578

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor