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Anomalous Propagation and Interference Fields __ OT REPORT 76-107 ANOMALOUS PROPAGATION AND INTERFERENCE FIELDS HT. DOUGHERTY BA. HART or I ‘4TES O U.S. DEPARTMENT OF COMMERCE Elliot L. Richardson, Secretary Betsy Ancker-Johnson, Ph. 0. o..UTIO,v Assistant Secretary for Science and Technology OFFICE OF TELECOMMUNICATIONS John M Richardson. Director December 1976 UNITED STATES DEPARTMENT OF COMMERCE OFFICE OF TELECOMMUNICATIONS STATEMENT OF MISSION The mission of the Office of Telecommunications in the Department of Commerce is to assist the Department in fostering, serving, and promoting the nation’s economic development and technological advancement by improving man’s comprehension of telecommuni cation science and by assuring effective use and growth of the nation’s telecommunication resources. In carrying out this mission, the Office • Conducts research needed in the evaluation and development of policy as required by the Department of Commerce • Assists other government agencies in the use of telecommuni cations • Conducts research, engineering, and analysis in the general field of telecommunication science to meet government needs • Acquires, analyzes, synthesizes, and disseminates information for the efficient use of the nation’s telecommunication re sources. • Performs analysis, engineering, and related administrative functions responsive to the needs of the Director of the Office of Telecommunications Policy, Executive Office of the Presi dent, in the performance of his responsibilities for the manage ment of the radio spectrum • Conducts research needed in the evaluation and development of telecommunication policy as required by the Office of Tele communications Policy, pursuant to Executive Order 11556 IS)IM RI i-i TABLE OF CONTENTS Page LIST OF FIGURES iv ABSTRACT 1 1. INTRODUCTION 2 2. PROPAGATION VIA HIGH ELEVATED LAYERS 5 2.1 Enhanced Field Characteristics 9 2.2 Occurrence of Elevated Layers 11 2.3 Tropospheric Versus Sporadic—E 19 Enhanced Fields 3. PROPAGATION VIA STRONG SURFACE DUCTS 20 3.1 Sea—Surface Ducting 21 3.2 Superrefractive Advection Layers 23 3.3 Evaporation Layers 25 3.4 Subrefractive A9vection Layers 30 3.5 Overwater Prediction Charts 31 4. THE ANOMALOUS FIELDS 41 4.1 Duct Geometry 43 4.2 Approximations For Interference Estimates 49 4.3 Occurrence 52 5. CONCLUSION 55 6. REFERENCES 57 iii LIST OF FIGURES Figure Page 1. Geometry for the enhanced-field mode of 6 propagation via an elevated layer. 2. Transhorizon 185.75 MHz signals recorded at GBH, Colorado. 3. Percent occurrence contours for the Atlantic 12 trade wind inversion in February. 4. Contours of the mean refractivity gradient (in 13 N units/kin) for the Atlantic trade wind inver sion in February. 5. Contours of the mean layer thickness (in meters) 14 for the Atlantic trade wind inversion in February. 6. Contours of the mean layer base height (in 15 kilometers) for the Atlantic trade wind inversion in February. 7. Areas of the world’s oceans where conditions are 16 favorable for the formation of elevated layers. 8. Mean heights (in meters) and percent occurrences 18 (>40%) for elevated layers with gradients -l57. 9. Identification of the propagation mode for loca— 22 tions of a transmitter above the duct (top drawing) and within the duct (bottom drawing). 10. Surface currents of the oceans in February-March. 24 11. Surface temperatures of the oceans in February. 26 12. Surface temperatures of the oceans in August. 28 13. Areas of the world where evaporation exceeds 29 precipitation. 14. Regions for which charts are available primarily 32 of advective layers and primarily of evaporative layers. 15. Percent expectancy of extended coverage, S-band 34 radar, Southeast Asia, January. iv LIST OF FIGURES (Continued) Figure Page 16a. Percent of time trapping frequency 35 <300 Hz, August. 16b. Percent of time trapping frequency 35 <3000 Hz, November. 17a. Refractivity lapse rates (N/km) not exceeded 36 5 percent of time for 100 m layer, August. 17b. Refractivity lapse rates (N/km) not exceeded 36 2 percent of time for 100 m layer, August. 18a. Percent of time gradient s—lOO (N/km), August. 38 l8b. Percent of superrefractive layers thicker than 38 100 m, August. 19. February and August median values of 100 m 39 refractivity gradient. 20. Observed gradients at Bangui, Central Africian 40 Republic. 21. Diurnal variability in surface refractivity 42 gradients. 22. Elevated—layer and surface-layer parameters 44 for ducting conditions. 23 Ray-theory representations of elevated-duct 48 propagation. V ANOMALOUS PROPAGATION AND INTERFERENCE FIELDS + * H. T. Dougherty and B. A. Hart This report presents some estimates of the enhanced fields deduced from the presently avail able theory and observational data. The fields of interest are the enhanced fields that are encountered in the presence of the stratified atmosphere, the ducting conditions associated with supercritical refractivity gradients both near the surface and aloft. As a basis for estimating the likelihood of occurrence of these supportive conditions, the available information is surveyed. It is suggested that the access to computers and the availability of historical radiosonde data be exploited and the pertinent characteristics of elevated layers and their associated ducts be mapped on a worldwide basis to meet the interference prediction require ments of regulatory agencies. 1. INTRODUCTION As the operation of telecommunication systems has been implemented for higher and higher transmission frequencies, the earth’s atmosphere has played an increasingly significant role. For example, the atmospheric bending of a radio wave’s path of propagation (refraction) is determined by the atmo— sphere’s refractivity gradient. This is often expressed in terms of an equivalent earth’s radius factor, k = r/r0, r0 6370 km, that varies from approximately 1.0 at about 0.1 MHz to approximately 4/3 for 100 MHz and higher fre quencies [Rotheram, 1970]. At 30 MHz and higher frequencies, +The author is with the Institute for Telecommunication Sciences, Office of Telecommunications, U.S. Department of Commerce, Boulder, Colorado 80302. * The author is with the Policy esearch Division, Office of Telecommunications, U.S. Department of Commerce, Boulder, Colorado 80302. this median refraction is better described in terms of the refractivity structure, the change of refractivity with height above the earth’s surface. Then, the seasonal and geographical variation of this refractivity structure may he described in terms of a standard median refractivity model [Bean et al.,, 1966; Bean and T)utton, 1968; CCIR, 1974a]. For such standard conditions, the applicable mode of radio wave propagation is identifiable from the path parameters, and its characteristics are predictable by classical methods that approximate the atmosphere as a homogeneous medium. For some portion of the time at most locations, the refractivity structure near the surface and at elevated levels will depart markedly from the model structure. Nevertheless, for 30 Hz and lower frequencies, even this stratified atmosphere is no problem because the vertical dimensions of the associated refractivity structures are very small in terms of wavelength; the atmosphere averages to an exponential structure whose effect is readily approximated by modification of the standard prediction methods [Rotheram, 1970; (erks, 1971]. However, as the frequency of interest is extended to VHF and higher, the effects of these non—standard conditions become increasingly significant. The resulting modes of propagation are anomalous. Anomalous (or non—standard) refers to those conditions for which received radio fields are markedly stronger or weaker than would be expected for a standard atmosphere. There may he either a serious degradation or a marked improvement in the signal’s level, fading rate, or fade duration. For most paths, either effect is possible and is determined by both the path geometry and the spatial distribution of the marked departures of the refractivity structure from the model. The anomalous degradation of the received signal (fading) has been the subject of many theoretical and experimental investigations of line—of-sight propagation paths. These are 2 well—documented [Beckman and Spizzichino, 1963; Dougherty, 1964], and the results have been suveyed and categorized [Dougherty, 1968; White, 1968]. This signal degradation (fading) due to anomalous propagation is serious because it directly impairs system performance; it is an intrasystem problem for which remedies are applied (either in ariticipa tion or after—the—fact) so as to assure that the required system reliability is achieved. On the other hand, the anomalous improvement of the received signal (an enhanced field) is primarily of interest because the more volatile problem of intersystem interference can result, particularly for beyond-line-of-sight paths. These paths, generally referred to as transhorizon paths, normally involve propagation via the troposcatter or the diffraction modes. Since they pose the threat of interference to or from other systems, the occurrence of the enhanced fields take on increased importance. It is these anomalous fields (these uncommon, enhanced fields) which largely determine coordination distances required by the (national and inter national) regulatory agencies (CCIR, l974b). Somewhat optimistic estimates of unusually strong fields and the likli— hood of their occurrence are required in order to avoid their serious impact as intersystem interference. Anticipation of this latter problem is one of continuing urgency
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