摘要:

Vertically polarised MF/HF groundwave radar has been used over the frequency range 2–30 MHz for sea-state sensing, surface-current measurement and ship tracking. The paper reviews the propagation factors and noise environment which control the performance of such systems. The various definitions of antenna gain, propagation loss and sea backscattering coefficient are discussed and related to physical concepts. The various first-order and higher-order mechanisms of scattering are reviewed and related to the relevant electromagnetic and hydrodynamic processes involved. These processes, linked with the velocities of the water waves responsible, result in a Doppler spectrum of the radar echoes which may be used to infer the wave height and wave directional spectra on the sea surface. Particular attention is directed towards remaining problems in understanding these processes. In evolving radars for ship tracking, detectability depends on the knowledge of the Doppler spectra of the sea clutter and, in particular, on limitations in directional determination of ship echoes with practical antenna systems located on available coastal sites. In conclusion, experimental measurements of sea echo, ship tracks and antenna-array characterisations are compared with theoretical considerations.

DOI:10.1049/ip-f-1.1983.0092

出版商:IEE    

年代:1983

数据来源: IET

摘要:

Two approaches for computing scattering from a random layer with an irregular interface are shown using the radiative-transfer principle. One approach is applied to a random layer to develop a scattering model for snow and sea ice, while the other is used to generate a scattering model for vegetation. It is noted that to model the scattering characteristics of a special medium, it is necessary to relate the electromagnetic parameters to the measurable parameters of the scatterers in the medium. Such relations are given and used to calculate theoretical estimates to compare with measurements acquired by microwave scatterometers.

DOI:10.1049/ip-f-1.1983.0093

出版商:IEE    

年代:1983

数据来源: IET

摘要:

First, a systematic theory is developed for a deterministic surface upon introduction of a surface Green function defined on a reference plane of the rough surface and determined by an equivalent surface impedance. The surface impedance is obtained by an exact transformation of a given boundary equation on the real boundary surface onto the reference plane and is given explicitly in an operator form, in terms of given surface displacement. An exact integral equation is also derived for the reflection coefficient and is applied to a rough surface of large scale, with the aid of an entirely new tangent-plane method. The scattering cross-section per unit area thus obtained is consistent with the power conservation, shadowing and multiple scattering, in contrast with those given in the literature. Governing equations of the statistical-surface Green functions of first and second orders are derived unperturbatively in form of the Dyson and Bethe-Salpeter equations, respectively, by following exactly the procedure established for a random medium. In particular the power conservation and associated optical relations are investigated in detail, in view of a complete lack of theories on this aspect. Results of specific problems are extensively summarised, including scattering cross-section per unit area and detailed optical relations inherent between various quantities involved, with several examples not only for a scalar wave but also for electromagnetic waves.

DOI:10.1049/ip-f-1.1983.0094

出版商:IEE    

年代:1983

数据来源: IET

摘要:

Contrived focusing is achieved by the disturbance of waves by man-made devices such as lenses or antenna dishes, and the theory of the process is well established. Natural focusing or caustics, on the other hand, are produced in waves subjected to natural, or nonspecial, disturbance such as propagation through an irregular medium or reflection from an irregular surface. The appropriate theory of such focusing had to await the mathematics of catastrophe theory developed only in the last decade, the different natural focusings being the different catstrophes. With it the caustic geometry of the focusings are described, as is the characteristic associated wave diffraction pattern. These can then be used, for example, to calculate intensity fluctuations in a randomly disturbed wave of short wavelength.

DOI:10.1049/ip-f-1.1983.0095

出版商:IEE    

年代:1983

数据来源: IET

摘要:

This paper reviews IREPS and some of its major products, and follows with a description of a recent effort to improve IREPS signal-prediction capabilities in tropospheric ducting environments. The IREPS products discussed are the altitude-versus-range coverage display, the path-loss-versus-range display, and a new historical propagation-conditions summary display derived from a computer-based refractivity climatology. In the GHz range elevated tropospheric layers are multimode systems, and practical beyond-the-horizon field calculations for system applications such as IREPS require simplified procedures. These simplified procedures could involve approximations directed towards speeding up waveguide calculations, or could take the form of replacing waveguide methods by ray methods or by a hybrid mixture of waveguide and ray concepts. Involved in each of these alternatives, and within the context of the modified refractivity formalism, is the need for calculating plane-wave reflection coefficients or their equivalents for layered media. The modelling effort discussed in this paper deals with the feasibility of using asymptotic forms of the plane-wave reflection coefficients for the purpose of waveguide analysis for a trilinear ducting environment.

DOI:10.1049/ip-f-1.1983.0096

出版商:IEE    

年代:1983

数据来源: IET

摘要:

Substantially more frequent occurrences of surface-based super-refractive and ducting layers have been noted in daytime than in night-time refractive climatological data derived from standard United States radiosondes. Evidence is presented which indicates that the diurnal bias in these data is largely fictitious. Although the problem is fundamentally a consequence of vulnerability of the carbon humidity sensor to temperature effects, several factors contribute to this bias. These factors are discussed, and steps suggested which could improve the quality of future measurements for refractive applications.

DOI:10.1049/ip-f-1.1983.0097

出版商:IEE    

年代:1983

数据来源: IET

摘要:

Air-to-ground propagation paths are used by many microwave equipments such as sensors, counter-measures and communications. Prediction methods for the coverage of these equipments have been studied elsewhere using powerful numerical modelling techniques. A fundamental consideration is the availability of sufficient and accurate atmospheric data. A review of the possible measurement techniques is given and their suitability for operational use is considered. It is concluded that, in the general case, a high spatial sampling rate is likely to be necessary for any successful exploitation of the prediction methods. The most likely area for application of such methods are open sea-regions where airborne assets with environmental sensors can be freely manoeuvred.

DOI:10.1049/ip-f-1.1983.0098

出版商:IEE    

年代:1983

数据来源: IET

摘要:

Above about 30 MHz the only tropospheric-propagation mechanism giving a significant field strength far beyond the radio-horizon is the scattering of radiowaves by tropospheric inhomogeneities. This mechanism has been extensively studied and has given rise to discussions between theoreticians, especially during the years 1950 to 1965. From that time, studies have been continued at a lower rate, and this propagation mechanism has been used for the planning of transhorizon radio links. Moreover, troposcatter propagation has to be taken into account for the prediction of long-distance interference. This paper summarises the main characteristics of the propagation by tropospheric inhomogeneities, including long-term and short-term variations of field strength, antenna gain degradation and limitations in bandwidth capabilities.

DOI:10.1049/ip-f-1.1983.0099

出版商:IEE    

年代:1983

数据来源: IET

摘要:

The purpose of the paper is to present a broad overview of propagation effects for frequencies predominantly within the 30 MHz to optical range, but excluding detailed discussion of depolarisation mechanisms which have been reviewed elsewhere. Emphasis is placed on atmospheric effects of direct relevance to systems design and operation, particularly for satellite communication systems. The topics considered include atmospheric attenuation, noise, scintillation, Faraday rotation and their frequency dependence. Various interference mechanisms are described and a summary of prediction methods relating to single-site slant path attenuation, frequency scaling and diversity operation is provided. A knowledge of the propagation characteristics at frequencies higher than those studied in depth hitherto is of crucial significance to the development of systems which embrace a wide variety of applications, especially in the field of remote sensing.

DOI:10.1049/ip-f-1.1983.0100

出版商:IEE    

年代:1983

数据来源: IET

摘要:

Experimental results obtained on two line-of-sight links at 36 GHz and 55 GHz operating on a 4 km path across central London are summarised. Measurements confirm the predicted low-frequency enhancement of the spectral density of the log amplitude scintillations in the region of the 60 GHz oxygen absorption peak and the results indicate that large horizontal-scale sizes have a strong influence on the amplitude scintillations. Results on long-term attenuation variations due to the oxygen absorption peak, attenuation due to rainfall and long-term statistics are also presented.

DOI:10.1049/ip-f-1.1983.0101

出版商:IEE    

年代:1983

数据来源: IET