ISSN:0143-1161    

DOI:10.1080/01431168508948260

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

A multifrequency continuous wave radar has been developed. By means of this, the directional spectra of ocean irregularities (waves and turbulence) can be measured by illuminating the sea surface simultaneously over a frequency band of some 30 MHz in the 6 GHz region, using 15 frequency spacings. By measuring amplitude and phase, the power spectrum (Doppler spectrum) associated with each of the 15 discrete sea surface irregularity scales is obtained, as is, independently, information about velocity. From the Doppler spectra associated with each of the 15 irregularity scales investigated (6-150 m) it is possible to distinguish between the contribution from the coherent gravity waves and the incoherent random irregularities. Experimental results from a set of different geophysical situations are given with reference to a general theoretical presentation.

ISSN:0143-1161    

DOI:10.1080/01431168508948261

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Digital synthetic-aperture radar (SAR) imaging techniques have previously only been reported in the literature in a fragmentory manner. This article presents a comprehensive review of the theory of digital SAR imaging from Earth-orbiting satellites. The digital SAR imaging process is explained, including a discussion of various aspects which are specific to satelliteborne SAR. A number of relevant digital-processing techniques are reviewed and it is shown how these techniques may be applied to the processing of digital SAR data. The range migration problem is discussed and various techniques for overcoming it are presented. The paper should be useful not only to the designer of SAR processors, but also to the user of digital SAR data and images.

ISSN:0143-1161    

DOI:10.1080/01431168508948262

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The effects of sea-surface velocities in the imaging of waves by synthetic aperture radar (SAR) are considered using the ‘facet’ concept of the backscattering process. It is shown that if the sea wave spectrum is divided at the nominal limit of resolution of the SAR the effect of the long and short wavelength parts can be considered separately, the former being treated by numerical simulation and the latter by statistical methods. It is found that the motions due to: the short wavelengths produce an azimuthal smearing which can be represented by a Gaussian low-pass filter acting on the azimuthal component of wavenumber in the image. The cut-off wavelength is typically some hundreds of metres in moderate winds. Images obtained with the SEASAT SAR frequently show such an effect.

ISSN:0143-1161    

DOI:10.1080/01431168508948263

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The case is considered of a CW microwave radar looking at the sea surface at moderate angles of incidence. This is treated using the ‘facet’ concept, in which the sea surface is considered as being covered by spatially uncorrelated facets, each a small number of Bragg wavelengths across and being transported by the particle velocities in the longer sea waves. The facets are considered to have an unknown decorrelation time. In two cases simplifications allow approximate numerical values for the coherent decorrelation time of the radar return to be calculated. These are compared with measured bandwidths and decorrelation times reported in the literature. These agree adequately with the theory and indicate that the facet decorrelation time is long. However, some properly thought-out measurements of coherent decorrelation time are badly needed.

ISSN:0143-1161    

DOI:10.1080/01431168508948264

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Frequency agility is applied to dual-frequency scattering from the ocean using an L-band microwave system in order to reduce the clutter background which has previously limited signal detectability in such scatterometers. Doppler spectra of the return show improvements of up to 13 dB in signal-to-clutter ratio using this technique compared with the standard non-frequency-agile technique. This improvement greatly enhances the ability of a dual-frequency scatterometer to measure ocean surface currents. Composite surface scattering theory is applied to explain the signal improvement. The clutter background is shown to be proportional to the ocean surface wave spectrum evaluated at a wavenumber corresponding to the wavenumber separation between the carrier frequencies of two transmitted pairs of frequencies. Thus, if the frequency separation between these carriers is equal to the frequency separation between the lines of each pair, the entire signal received by the system is proportional to the ocean surface wave spectrum evaluated at the common wavenumber difference. This three-frequency technique completely removes previous limitations on the measurement of ocean wave spectra by dual-frequency scatterometers by converting the clutter background into part of the desired signal. Signal detectability is then limited only by thermal noise.

ISSN:0143-1161    

DOI:10.1080/01431168508948265

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

SAR images of the European SAR 580 campaign taken over a mountainous region from three parallel flight lines are available for stereo measurements. The positions of the three flight lines with respect to a set of ground control points have been calculated by the parallaxes on each image. Using the three available stereo bases, the horizontal and vertical components of the distances between a number of identifiable ground points, along a line approximately perpendicular to the flight direction, have been calculated. The limitations of the accuracy of this procedure are mainly due to the ill-determined stereo base and the confusing appearance of ground features on the different stereo mates. In the best cases, the accuracy of the distance determination is about three times the range resolution of the radar, i.e. ∼ 7·5  m.

ISSN:0143-1161    

DOI:10.1080/01431168508948266

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

A comparison of the soil moisture response of three 1–4  GHz radiometers from truck and aircraft platforms at a variety of lest sites indicates that microwave remote sensing of bare soil produces repeatable and quantifiable results regardless of geographic location and sensor system used. The combined microwave sensitivity of these data sets appears to be of the order of − 3–4deg K per 1 per cent change in volumetric soil moisture for L-band wavelengths ( − 2–7 deg K per 1 per cent soil moisture if watersheds characterized by vegetation and surface roughness are included). With the addition of appropriate algorithms to handle the effect of roughness and vegetation, these results demonstrate the potential of microwave remote sensing for estimating soil moisture over large areas.

ISSN:0143-1161    

DOI:10.1080/01431168508948267

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Extensive studies conducted by several researchers using truck-mounted active microwave sensors have shown the sensitivity of these instruments to soil-moisture variations for agricultural land covers. The logical extension of these results is the evaluation of similar systems at lower resolutions typical of operational systems and for other types of land cover. Data collected during a series of aircraft flights in 1978 and 1980 over four rangeland watersheds located near Chickasha, Oklahoma, were analysed in this study. These data included scatterometer measurements made at 1·6 and 4·75  GHz using a NASA aircraft and ground observations of soil moisture for a wide range of moisture conditions. Data were compared with previous truck and aircraft results. Results indicate that if the sensor system is calibrated it is capable of providing estimates of surface soil moisture for the rangeland conditions tested.

ISSN:0143-1161    

DOI:10.1080/01431168508948268

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Results of radiometric measurements over bare soil obtained with a horizontally polarized microwave radiometer at 19·1  GHz frequency are presented. Radiometer measurements were made with incidence angles varying from 10 to 50°. Ground-truth acquisition of soil moisture in the 0–1  cm layer and of soil temperature near the surface was made concurrently with radiometer measurements. The measured brightness temperatures over a bare field are higher than those calculated from an emmissivity model.

ISSN:0143-1161    

DOI:10.1080/01431168508948269

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor