ISSN:0143-1161    

DOI:10.1080/014311697217422

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Interpretation of Synthetic Aperture Radar (SAR) images of sea ice is complex because of the natural variability of sea ice and sensor-induced effects, such as speckle. Most of the research on SAR image interpretation has focused on the winter months and algorithms were developed to classify sea ice successfully under cold conditions. However, interpretation of SAR images during the seasonal transitions has proved difficult due to rapidly changing weather conditions. In this paper we address the application of SAR during the transition from summer to the fall freeze-up. This period is important because it signals the start of significant new ice growth, which affects the air-ocean heat exchange and injects brine into the upper layers of the ocean. We have interpreted SAR images of the sea ice in terms of the basic ice characteristics by using shipborne radar measurements of sea ice during the freeze-up and models derived from these measurements. We have shown that the model-based approach is effective in interpreting SAR images during this seasonal transition. This work also provides the physical mechanisms responsible for the large increase in backscatter observed at the end of the summer melt season.

ISSN:0143-1161    

DOI:10.1080/014311697217431

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Surface-based microwave studies made in the Austrian Alps during the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/XSAR) experiment in April 1994 are described, and are compared with the SAR data. The SAR test region included glaciers and unglaciated surfaces which were snow covered. Clear differences between these two surface types were observed with the SIR-C/X-SAR. In addition, the angular dependencies of the backscattering were useful in order to distinguish between the accumulation and the ablation areas of the glaciers, and the snow-covered areas showed characteristic differences depending on the presence or absence of layers containing wet snow. The surface-based studies were carried out at an alpine meadow in the valley Kaunertal at an altitude of 2160 m. The measurements included snow-physical observations supported by microwave dielectric probes, microwave radiometers at 21 and 35GHz and polarimetric scatterometers at 5.3 and 35 GHz. The dielectric measurements were used to retrieve the liquid-water profile of the snowpack. Three main layers were observed: a wet bottom layer as a remnant of a warm period in March, a thick crust (i.e., the refrozen part of the old pack), and a fresh and mostly dry snow layer on top. Except for the top layer, the temporal changes were small during the observation period from 8 to 12 April 1994. The radiometric measurements made at 21 and 35GHz showed pronounced temporal variations of the brightness temperature in the afternoons due to melt-and-refreeze cycles in the fresh snow. The liquid water in the top layer was monitored by a special radiometric method using a metal plate at the bottom of this layer. The radiometer and scatterometer measurements revealed the refrozen crust as the dominant scatterer, especially at 35 GHz. However, the wet bottom layer inhibited the detection of signals scattered at the snow-ground interface and therefore reduced the backscatter at 5.3 GHz. The comparison of the C-band SAR data with those of the C-band scatterometer shows higher returns in the SAR data for the considered test site. Explanations for the differences are discussed. Nevertheless, also the SAR backscatter data are clearly lower in the zones where wet snow was present than in areas either covered by dry snow or which were snow-free.

ISSN:0143-1161    

DOI:10.1080/014311697217440

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Numerical simulation of coupled vector radiative transfer equations for a multi-layer model of dense scatterers is developed. The effect from ice layer formed during melting is studied to take account of anomalous spectrum of brightness temperature at the SSM/I channels. This approach is applied to analysis of SSM/I data observed over the southern and central Greenland snow, which can show distinctly different spectral characteristics, T B19 > T B85 or T B19 < T B85. Ice layers produced in melting and ice grain sizes can significantly affect functions of thermal emission at SSM/I channels.

ISSN:0143-1161    

DOI:10.1080/014311697217459

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A temporal and spatial Empirical Orthogonal Function (EOF) analysis has been performed on monthly maximum sea surface temperature (SST) satellite data over eight years (July 1981-August 1989) to examine the SST patterns of variability off north-west Africa. The first temporal EOF mode (75 per cent) represents the cooling and warming cycle over the region. The temporal amplitudes show an interannual variability with 1982 summer temperatures abnormally much colder, immediately preceding the largest El Nino phenomenon of the century. The second temporal mode (9 per cent) distinguishes a large-scale behaviour of the northern and southern areas of the region, the two areas being out of phase presumably related to the variability of the wind field. The first spatial EOF mode (66 per cent), which resembles the mean of all images, is stronger in fall, with a smaller second maximum at the beginning of spring. The second spatial mode (10 per cent) shows the coastal upwelling region where a seaward extension of upwelled water around 300km offshore is observed off Cape Blanc. All the modes show complex eastern boundary dynamics with different regimes north and south of Cape Blanc. The Cape Blanc vicinity must have some interesting dynamics with a sharp boundary evident in all the modes. Only these two first temporal and spatial EOF modes are significantly above the noise level. The EOF method is efficient in compressing monthly SST satellite imagery off north-west Africa, with a great percentage of the total variance being accounted by only two modes.

ISSN:0143-1161    

DOI:10.1080/014311697217468

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Whenever remote sensing data are used in conjunction with in situ measurements for modelling a physical phenomenon, the major problem is in the selection of digital data which are representative of the ground points under consideration. This problem can be solved in most cases, including the present study on water quality modelling, by placing dependence not on a single pixel number of the estimated point in question. The study reports the analyses of different size pixel arrays for date of 11 April 1993 in the form of CCT obtained from NRSA, Hyderabad. In order to select the best pixel array configuration to represent the sample station point on the multi-band image, nested arrays of pixels for 4 bands with 7 different sizes are sampled at 52 different sampling stations in the Gautami-Godavari river estuary where water quality variables had been measured. These seven array sizes, namely three symmetric arrays of sizes: 5 by 5, 3 by 3 and 1 by 1 and 4 non-symmetrical arrays of 2 by 2 with the centre pixel located at different corners of each 2 by 2 arrays are analysed using Analysis of Variance \[ANOVA] and t -test. The data bank of 52 by 7 by 4 average pixel values was available for ANOVA and t -test. The results of ANOVA highlight the importance of considering all four IRS bands and each band contains different information regarding physical conditions of the study points, sensor characteristics and water and atmospheric responses. The pixel data are also significantly different at 99 per cent confidence level among sample locations. This indicated that all the four bands have to be considered in developing water quality models. The results of t -test proved that the 5 by 5 and 3 by 3 array sizes are the best among the 7 configurations, but no significant difference is indicated between the two array types. The 3 by 3 window is selected for water quality modelling because it is smaller and involves less computational work.

ISSN:0143-1161    

DOI:10.1080/014311697217477

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Salinization-alkalinization is a time- and space-dynamic soil degradation process in semiarid regions. This study implements a synergistic approach to map salt- and sodium affected surfaces, combining digital image classification with field observation of soil degradation features and laboratory determinations. Salinity-alkalinity classes were established using the electrical conductivity (EC) and pH values. A neighbourhood operator, with spatial and spectral user-defined constraints determined the spectral objects constituting the training set. Six combined Landsat TM bands (1,2,4,5,6,7) provided the highest separability between salt- and sodium-affected soil classes. Although the overall accuracy was slightly low (64 per cent), accuracies of 100 per cent were obtained for some classes. Main causes of spectral confusions, masking different salinity-alkalinity degrees were the type and abundance of salt-tolerant vegetation cover, the topsoil textures, and the mixture of topsoil properties under field conditions.

ISSN:0143-1161    

DOI:10.1080/014311697217486

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This work is aimed at deriving canopy component (soil and foliage) temperatures from remote sensing measurements. A simulation study above sparse, partial and dense vegetation canopies has been performed to improve the knowledge of the behaviour of the composite radiative temperature and emissivity. Canopy structural parameters have been introduced in the analytical parameterization of the directional canopy emissivity and directional canopy radiance:namely, the leaf area index (LAI), directional gap fraction and angular cavity effect coefficient. The parameterization has been physically defined allowing its extension to a wide range of Leaf Inclination Distribution Functions (LIDF). When single values are used as leaves and soil temperatures, they prove to be retrieved with insignificant errors from two directional measurements of the canopy radiance (namely at 0 and 55 from nadir), provided that the canopy structure parameters are known. A sensitivity study to the different parameters shows the great importance of the accuracy on LAI estimation (an accuracy of 10 per cent is required to retrieve the leaves temperature with an accuracy better than 0.5 degK, the same requirement being 5 per cent for the retrieval of soil temperature). The radiometric noise is important too, but its effects may be limited by using very different angles for the measurements: for 0 and 55, the effect of a Gaussian noise (NEDeltaT 0.05deg K) is lower than 0.5degK on the retrieved soil and foliage temperatures). Uncertainties on the leaf and soil emissivities (Delta epsilon 0.01) cause little errors in the retrieval (lower than 0.5degK). If the inclination dependence of the leaves temperature is considered, a 1 degK error is observed in the retrieved soil and foliage temperatures. This error is due to the fact that the effective foliage temperature varies with the view angle (a few 10 -1 deg K at 55 ), which implies errors in the inversion scheme. This effect may be corrected for by using an angular corrective term delta depending only on the off-nadir angle used.

ISSN:0143-1161    

DOI:10.1080/014311697217495

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This paper contains the results of measured dielectric constants of leaves of two tropical crops, rubber ( Heavea brasiliensis ) and oil palm ( Elaeis guineensis ) at microwave frequency region. Using a microcomputer-based automated system consisting of a Wiltron 360B vector network analyser,measurements are conducted based on the waveguide thin sheet technique. Leaf samples are inserted within waveguide sections, and the dielectric constants are calculated from the complex reflection coefficients. The measurements cover a frequency range of 4-18GHz. In general, the dielectric constant of a leaf sample is a function of water content and frequency. Theoretical calculations from three dielectric mixing models, i.e., the Debye-Cole Dual-Dispersion dielectric model by Ulaby and Rayes, Matzler's Model and Carlson's Model are compared with the measurements. From the comparisons, the dual-dispersion model is found to show more promise in prediction of the dielectric constants, if proper physical parameters of the model, such as bound and free-water volume fractions, are used.

ISSN:0143-1161    

DOI:10.1080/014311697217503

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Forest fires in large sparsely populated areas in the boreal forest zone are difficult to detect by ground based means. Satellites can be a viable source of information to augment air-borne reconnaissance. The Advanced Very High Resolution Radiometer (AVHRR) sensor aboard the National Oceanic and Atmospheric Administration (NOAA) satellites has been used to detect and map fires in the past mainly in the tropics and mainly for environmental monitoring purposes. This article describes real-time forest fire detection where the aim is to inform local fire authorities on the fire. The fire detection is based on the 3.7 mu m channel of the NOAA AVHRR sensor. In the fire detection algorithm, imaging geometry is taken into account in addition to the data from the near-infrared and thermal infrared channels. In an experiment in summer 1995, 16 fires were detected in Finland. One was a forest fire, 11 were prescribed burnings and 4 false alarms. Three of the false alarms were due to steel factories. We conclude that satellite-based fire detection for fire control is feasible in the boreal forest zone if the continuous supply of frequent middle-infrared data can be guaranteed in the future.

ISSN:0143-1161    

DOI:10.1080/014311697217512

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor