Julius Goldhirsh
Johns Hopkins University
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IEEE Transactions on Antennas and Propagation | 1989
Julius Goldhirsh; Wolfhard J. Vogel
Field tests related to planned mobile satellite systems (MSS) were performed, and results that add to the existing database of propagation measurements at L-band (1.5 GHz) are described. They are considered particularly useful in that propagation effects were studied systematically with repeated and controlled runs pertaining to different path elevation angles, road types, and path geometries defining shadowing and line-of-sight modes. In addition, simultaneous L-band and UHF measurements were performed for the purpose of establishing scaling factors applicable to previous UHF (870 MHz) results. The control of the experimental parameters was made possible by using a helicopter as the source platform and a mobile van which housed the receiver. >
IEEE Transactions on Antennas and Propagation | 1987
Julius Goldhirsh; Wolfhard J. Vogel
Tree attenuation results at 870 MHz are described for experiments conducted in October 1985 and March 1986 in Central Maryland. These experiments employed a helicopter as a source platform and a van with receiver and data acquisition instrumentation. Tree attenuation results were obtained for the cases in which the van was stationary and in motion. The experiments were performed for the purpose of providing the designers of planned land mobile satellite systems with important elements in the determination of link parameter requirements; namely, the expected fading statistics due to roadside trees for both mobile and stationary vehicles. Single tree attenuation results gave worst case median fades as high as 15 dB although roadside tree values were noted to produce fades in excess of 20 dB for small percentages of time. The cumulative fade distributions and their relative contributions as a function of path elevation angle, right side versus left side driving, and different road types are derived from the field measurements. Upon comparing the attenuations from bare deciduous trees (March 1986) with those due to trees in full foliage (October 1985), the increase in dB attenuations were, in general, less than 25 percent for the dynamic cases, and less than 40 percent for the worst case static configuration. This result demonstrates thai the dominant fading is caused by the wooded tree branches as opposed to the leaves on these branches. The tail end of the observed fade distributions was observed to follow lognormal distributions with respect to dB attenuation.
IEEE Transactions on Antennas and Propagation | 1986
Wolfhard J. Vogel; Julius Goldhirsh
Tree attenuation results are described based on data acquired from an experiment employing UHF transmissions at 869 MHz between a remotely piloted aircraft and a stationary vehicle. The objective of the experiment was directed toward providing input to the land mobile satellite community where the extent of shadowing from roadside trees represents important information to be used for system design. Single trees were found to attenuate between 10 to 20 dB with an average median attenuation of 12 dB.
IEEE Transactions on Antennas and Propagation | 1982
Julius Goldhirsh
Meager information has hitherto been reported as to the influence of dust storms on radars operating in desert regions. This work represents an attempt to bring together the results of a body of diverse investigations and present a unified quantitative treatment of the attenuation and backscatter characteristics of radars operating in the range 1 to 10 GHz with particular emphasis at L - and S -band. The results demonstrate that for extreme mass loading values (40-60 gm/ m-3) the two-way attenuation may be as high as 44 dB over a one-way range of 20 km. Such a mass loading uniformly spread over this range is, however, considered very unlikely. Assuming typical S - and L -band radar parameters and the above extreme dust concentrations filling the pulse volumes at a range of 100 km, equivalent point target cross sections of approximately 2 and 1 m2result at the respective frequencies.
IEEE Transactions on Antennas and Propagation | 2001
Julius Goldhirsh
Fundamentals of attenuation and radar backscatter through duststorms are reviewed. A duststorm is modeled as circularly symmetric having a visibility with a minimum at its center (e.g., maximum mass loading) and which exponentially increases radially to a fixed maximum visibility threshold level (minimum mass loading). This model enables the convenient calculation of the two dimensional (2-D) structure of radar backscatter and path attenuation. As an example, the parameters of the exponential function describing the visibility distribution for a particular duststorm was derived using measurements made in the Sudan by other investigators operating a 10.5 GHz, 25 km link. A comparison of the calculated and measured attenuation time-series showed relatively close agreement. Both attenuation levels and backscatter levels due to even intense duststorms are expected to be relatively small for frequencies up to 10 GHz. For example, the peak attenuation for the duststorm that contained visibilities smaller than 2 m was less than 6.5 dB. Modeled backscatter due to this duststorm gave levels smaller than that obtained by an equivalent rainrate of 0.6 mm/h. Although the calculations were obtained for X-band, they may be extended to higher frequencies. Frequency scaling at 37 GHz, for example, showed a peak equivalent path attenuation level of at least 26.6 and 48 dB under varying assumptions.
IEEE Transactions on Antennas and Propagation | 1990
Wolfhard J. Vogel; Julius Goldhirsh
A mobile satellite system (MSS) propagation experiment at 1.5 GHz was performed near the east coast of the United States in central Maryland during December 1987 using the MARECS-B2 satellite as a transmitter platform. A receiving system in a vehicle measured signal fades caused by shadowing and multipath from roadside trees and utility poles. The propagation degradations were characterized for a system of three roads previously examined using a helicopter as the transmitter platform. The objectives of the MARECS-B2 MSS tests were to: (1) establish cumulative fade distributions for the particular satellite geometry for both rural and suburban roads; (2) validate the consistency of previous roadside tree measurements which employed a helicopter as the transmitter platform for the same system of roads; (3) obtain an additional set of fade levels at a lower angle hitherto not measured in central Maryland; and (4) combine the satellite-acquired data set with previous helicopter results and establish an analytic, empirically derived function describing the cumulative fade distributions for a family of path angles. An analysis of the satellite data has demonstrated the successful achievement of these objectives. >
IEEE Transactions on Antennas and Propagation | 1982
Julius Goldhirsh; E. J. Walsh
The incorporation in the 13.5 GHz Seasat-type radar altimeter of a mode to measure rain rate is investigated. Specifically, an algorithm is developed relating the echo power at the various range bins, to the rain rate taking into consideration Mie scattering and path attenuation. The dependence of the algorithm on rain drop size distribution and nonuniform rain structure are examined and associated uncertainties defined. A technique for obtaining drop size distribution through the measurements of power at the top of the raincell and power difference through the cell also is investigated together with an associated error analysis. A description of the minor hardware modifications to the basic Seasat design is given for implementing the rain measurements.
IEEE Transactions on Geoscience and Remote Sensing | 1982
Julius Goldhirsh; John R. Rowland
A large body of information from a number of sources is brought together and an error budget is deduced giving the projected overall height uncertainty correction for a suggested next generation high-precision radar altimeter. Uncertainties introduced by the wet and dry troposphere, clouds, and the ionosphere are reviewed. A suggested next-generation precision altimeter is assumed to be dual frequency (13.5 and 6 GHz) designed to correct out the ionospheric error. The altimeter-carrying satellite will include a nadir pointing near coincidentbeam dual-frequency microwave radiometer for mitigating the wet tropospheric uncertainty. Although there are a number of caveats, the combined uncertainty in the height correction due to the atmosphere for the suggested system should be nominally 3 cm rms compared to at least 6 cm associated with the Seasat-A mission. Improvements in height resolution of the kind referred to here are vital for future satellite missions designed to monitor ocean currents (e.g., TOPEX).
Proceedings of the IEEE | 1997
Julius Goldhirsh; Bert H. Musiani; Asoka W. Dissanayake; Kuan-Ting Lin
A three-site rain-fade space-diversity measurements experiment at 20 GHz has been in near continuous operation since September 1, 1994. Two receiver sites (at the Applied Phys. Laboratory ~APL\ of The Johns Hopkins University and COMSAT Laboratories) are located in Central Maryland, and the other is located in Virginia. The APL and COMSAT locations are separated by 33 km. The Virginia site is separated from APL by 44.5 km and from COMSAT by 30.5 km. Receivers at each of the sites measure the 20-GHz CW signal level from the radiating beacon onboard the geostationary Advanced Communications Technology Satellite (ACTS). Because of the lateral variability of rain, the likelihood is diminished that intense rain cells will simultaneously intersect all three Earth-satellite paths. There is also a reduced likelihood that intense rain will simultaneously intersect two Earth-satellite paths associated with different two-site scenarios. Hence, a substantially smaller rain-fade margin is required when the sites operate in a diversity mode (e.g., connected together such that the largest signal is used) vis-a-vis uncoupled single terminal operation. In this effort, we examine the efficacy of employing three-site and two-site space-diversity systems to reduce the required fade margin. Single- and joint-terminal rain-fade distributions are calculated for both the three-site and different combinations of two-site scenarios. Diversity gains, which give a measure of the reduced fade margin, are determined from these distributions. The distributions and diversity gains are also individually determined for two six-month periods of the year for which precipitation is predominantly convective or stratiform, respectively. Comparisons between measured diversity gains and those estimated using the model of the radiocommunications sector of the International Telecommunications Union are made.
IEEE Journal on Selected Areas in Communications | 1995
Wolfhard J. Vogel; Julius Goldhirsh
We characterize multipath fading at low elevation angles for unshadowed line-of-sight propagation over a land-mobile satellite link. Equipment aboard a van recorded a CW signal from INMARSATs geostationary satellite MARECS-B2 while driving in the states of Utah, Nevada, Washington, and Oregon. Elevation angles ranged from 7/spl deg/ to 14/spl deg/. The van carried a tracking helix antenna with beamwidths in the principal planes of approximately /spl plusmn/18/spl deg/. Multipath fading was observed in flat and hilly terrain and is compared with an example of fading due to tree shadowing. The measurements demonstrate that multipath effects at low elevation angles may result in fades exceeding 7 dB for approximately 1% of the driving distance for a worst case scenario corresponding to driving in rolling, hilly terrain where the satellite is in front or behind of the vehicle. Fading in such a case is typically dominated by a single multipath reflection from a terrain feature. On the other hand, tree shadowing at low elevation (7/spl deg/) may result in fades which exceed 25 dB at the 1% level. >