Effects Of Ground Reflection Research Articles

Nomogram for estimating noise propagation in urban areas

A nomogram, generated numerically, is presented for estimating sound levels generated by point noise sources in urban street complexes. The nomogram accounts for street width, distance of the receiver from the source, reflection coefficient of the building walls, and includes cases where the receiver is across one or more street intersections or is around a street corner from the source. The effect of ground reflection is included. Subject Classification: 50.80; 20.20; 28.40.

Analysis of ground reflection of jet noise obtained with various microphone arrays over an asphalt surface

Ground reflection effects on the propagation of jet noise over an asphalt surface are discussed for data obtained using a 13-in.-diameter nozzle mounted 1234 ft above grade. One-third-octave sound-pressure levels were measured for microphone heights from 0.02 to 1234 ft and at distances from 25 to 75 ft from the nozzle. Limited ground-effect data are also presented for a 2-in.-diameter jet. Analysis of ground reflection effects is made using the concept of a reflected ray transfer function in a model. The range of values of the single complex number used to define the reflected ray transfer function was larger than that expected if the range was determined only by the asphalt surface. This may indicate that the atmosphere is affecting the propagation of the reflected signal The model used as a basis for the Computer program minimized the ground reflection effects. Also discussed is the selective placement of the ground-effect reinforcements and cancellations in the planning of an experiment in order to minimize their effects.

Theory of Scalloping Amplitude Errors in Standard VOR Bearing Indications

A rigorous theory is developed for the course scalloping amplitudes observed in the bearing indications of an airborne standard VOR receiver in the presence of multipath signals. The VOR transmitting antenna pattern characteristics, the scattering properties of the multipath source, and the effects of ground reflection are all built into the theory so that their combined effects on the scalloping may be quantitatively investigated. The analysis assumes that the VOR receiver characteristics are ideal.

Propagation of severe-storm infrasound to ionospheric heights: a narrow-band atmospheric filter

We set out to model theoretically the spectral features of infrasound observed in the ionosphere and believed to be radiated by severe thunderstorms. We explain the dominant 2–5-min wave period as the effect of atmospheric filtering; shorter periods are excessively attenuated by absorption in transit to the ionosphere, and longer periods are attenuated in portions of the atmosphere where the waves are evanescent because their frequencies are below the acoustic cutoff. An observed spectral “fine structure” within the 2–5-min band is explained in terms of resonant interactions between the waves and the atmospheric temperature structure. Accurate quantitative modeling of all these details of the storm-to-ionosphere transmission coefficient requires numerical integration of the acoustic-gravity wave equation, including the effects of ground reflection and partial reflections in atmosphere.

Acoustic antennas for atmospheric echo sounding

Utilization of acoustic waves for atmospheric studies through echo sounding requires an efficient antenna with high on-axis gain and suppressed sidelobes. The antenna pattern for a piston source, as it influences the evaluation of the acoustic radar equation, is calculated and the effective gain factor obtained. Existing microwave antennas and optical searchlights have been adapted for acoustic use and their performance measured. A conical horn-reflector antenna made of fiberglass gives excellent performance in the frequency region 1–5 kHz after being coated with a viscous dampening tar to shorten vibration time. This damping allows reception of weak echoes with minimum delay following the transmission of a high-power acoustic pulse from the antenna. Septum dampers have been used successfully on parabolic-dish antennas. Antenna beam patterns have been measured on an outdoor range, with some measurements being made across a canyon to avoid multipath ground-reflection effects. Mainlobe to 90° sidelobe ratios are 40 to 50 dB. The measured antenna pattern for the horn-reflector antenna agrees closely with predictions for a perfect piston source. The parabolic-dish antennas produce a more complex diffraction pattern. Absorbing cylindrical antenna enclosures have been found effective in further reducing sidelobe reception by 10 to 20 dB, in agreement with predictions. Attention to antenna design allows acoustic echo-sounder operation in a noisy urban environment, and keeps noise pollution from the sounder pulses below the annoyance threshold.

Concept of phase centre of an array applied to elevation-angle measurements

A linear receiving array can be regarded as possessing a phase centre the position of which varies with the direction of arrival of the signal. This concept has proved useful in a study of the effects of imperfect ground reflection on elevation-angle measurements made with the wavefront-analysis technique. A distinction is drawn between ground-interference minima and free-space nulls.

Ground reflection effects upon radiated and received signals as viewed via image theory

The ground surface affects radiated and received signals of antennas near the surface of ground. By considering spatially extended current distributions, we obtain both positive and negative image results for both horizontal and vertical antennas. We present numerical results for HF with ground surfaces of soil, seawater, and a perfect electrical conductor. To illustrate the angle and frequency-to-height dependence of the constructive and destructive interference, we plot contours (of equal amplitude) of the four different image factors in terms of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\theta</tex> and height/wavelength ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h/\lambda</tex> ). These results provide insight into design considerations involving antennas in the near vicinity of ground.

A survey of low velocity and coaxial jet noise with application to prediction

A review of recent published data on low velocity jet noise is given together with previously unpublished results taken from the Rolls-Royce Noise Research Programme on model rigs and full-scale engines. Noise correlations are given which show that at low jet velocities, the low frequency exhaust noise which is commonly referred to as jet noise, emitted from the fan stream of a turbofan engine is considerably lower in level than that from the (hot) centre stream. From this result, a new prediction procedure for coaxial jet noise of turbofan engines is then developed. Comparisons are given which show that this method gives good correlation with measured results from a number of full-scale turbofan engines. The importance of accurate estimation of the “ground reflection effect” is clearly demonstrated. A critical review of published jet noise data from model coaxial jets is given and the need for further extensive testing emphasized.

Monopole radiation in the presence of an absorbing plane

Two theoretical solutions for the field of a point sound source above a plane boundary between air and an absorbing material are discussed. The validity of approximations based on the plane-wave reflection coefficient and on the modified image method are also examined. Results of measurements made with a source in and above slabs of absorbent material are shown to be in agreement with theoretical results. The performance of free-field rooms having plane absorbent treatment and ground-reflection effects in the measurement of aircraft noise spectra can thus be predicted.

Monopole radiation in the presence of an absorbing plane

Two theoretical solutions for the field of a point sound source above a plane boundary between air and an absorbing material are discussed. The validity of approximations based on the plane-wave reflection coefficient and on the modified image method are also examined. Results of measurements made with a source in and above slabs of absorbent material are shown to be in agreement with theoretical results. The performance of free-field rooms having plane absorbent treatment and ground-reflection effects in the measurement of aircraft noise spectra can thus be predicted.

Multiple ground reflection effects on fading behavior of VHF/UHF satellite transmissions

A computerized method to predict fading occurrence in VHF/UHF satellite transmissions, originating from multiple ground reflections is described. Based on ray optics the effective antenna gain is computed as a function of the elevation angle and ground parameters of unidealized irregular terrain.

The Effect of Ground Reflections and Scattering on an Interferometer Direction Finder

The azimuth and elevation angles of an airborne beacon transmitter can be determined using a direction finder comprised of two orthogonal interferometers located near the earth's surface. In this paper we consider the reflection of the incident electromagnetic field by the earth's surface and the resultant effect on the direction-finding system. The analysis yields an upper bound or limit to the interferometer phase error and the corresponding error in bearing angle that can result because of specular reflection and diffuse scattering.

The Effects of Scattering and Ground Reflection on the Solar Energy Absorbed by Ozone in a Rayleigh Atmosphere

Abstract The effects of Rayleigh scattering and Lambert ground reflection on the solar energy absorbed by ozone in the earth's molecular atmosphere are evaluated on the basis of a complete solution of the transfer equation. The results show that these processes increase the following over that for a purely-absorbing atmosphere: 1) the total solar energy absorbed by ozone in a unit atmospheric column, 2) the energy absorbed per unit mass of air at all levels (except below 100 mb at low sun), and 3) the heating rates due to the absorption of solar energy by ozone. In each case, the magnitude of the increase can vary considerably depending on the solar zenith angle and the reflectivity of the underlying surface.

An analysis of the effects of ground reflection in line-of-sight phase systems

The presence of a reflected component in a line-of-sight radio system influences the phase variability resulting from atmospheric changes along and near the path. An analysis is made of a simple case showing the general nature of this effect and the parameters upon which it depends. A method is presented for calculating the phase behavior of the resultant signal, in terms of that expected from a single direct component, and the relevant system parameters.

The Effect of Lambert-Type Ground Reflection on Umkehr Measurements

Abstract The problem of the effect of ground reflection on the scattered sky radiation in the presence of Ozone absorption has been solved considering only primary scattering. The numerical results for the case of the zenith direction show a considerable increase in the intensity of the primary scattered radiation due to ground reflection. The effect of ground reflection on the ratio of the intensities of the zenith sky at two wavelengths in the ultraviolet was also studied for its possible implication in “umkehr” work.

Multipath phase errors in CW-FM tracking systems

High-precision, cw, missile-tracking systems are subject to multipath errors due to ground reflections and reflections from neighboring objects. To reduce the effects of ground reflection, advantage may be taken of the characteristic of frequency modulation to discriminate against a weaker interfering signal. This paper discusses the magnitude of the phase error reduction to be obtained by use of frequency modulation. It is demonstrated that the phase error can be reduced to any desired value if sufficient band width is utilized in the frequency modulation signal and significant improvement over an amplitude modulation system can be obtained with a wide-band FM system.

Global Radiation Resulting from Multiple Scattering in a Rayleigh Atmosphere

Using Chandrasekhar’s solution for the radiative transfer problem in a plane-parallel model of the earth’s atmosphere, under the assumption of perfect, conservative scattering according to the Rayleigh law, expressions are derived for the relative global and sky radiation received on a horizontal surface, as a function of normal optical thickness and inclination of incoming parallel radiation. The corrections representing the effect of ground reflection are also expressed in terms of certain functions already defined by Chandrasekhar. These expressions, which include the effect of all orders of scattering, are then used to compute the absolute global and sky radiation as a function of wavelength and solar zenith distance, based on the solar extraterrestrial energy curve as given recently by Nicolet. The results show that, under these assumptions, the global radiation received at the surface of the earth should remain essentially constant in spectral distribution for a wide range of solar altitudes, and that the pure sky radiation should have two maxima, centered at 3,300 Å and 4,100 Å respectively, for solar zenith distances ranging between o° and 45°.&#x0D; The theoretical work is compared with Bernhardt’s results obtained by means of certain simplifying assumptions about the successive orders of scattering.&#x0D; The absolute global and sky radiation is integrated over the range 0.29 μ to 4.00 μ, and it is shown that in general the computation is in good agreement with existing measurements under clear sky conditions.