A system for the measurement of the variations in electrical lengths of radio propagation paths is described The observed path-length instabilities are considered to be caused bythe same atmospheric turbulence responsible for the existence of very high frequency and ultra high frequency fields far beyond the radio horizon Results obtained on 1728 mc and 1,046 mc along 3?-, 10-, and 60-mile paths are reported It is pointed out that measurements of this type provide a powerful tool for the study of the size and intensity of the refractivity variations of the atmosphere giving rise to the observed phenomenon

Measurements of the Phase of Radio Waves Received over Transmission Paths with Electrical Lengths Varying as a Result of Atmospheric Turbulence

This paper presents a summary of airborne refractive index measurements which have been made at many locations in the United States both over land and off the east and west coasts, by several organizations. Included are the results of studies of the mean vertical structure of the index of refraction of the atmosphere as a function of such factors as location, season, horizontal distance, and time. Included also are the results of measurements of the fine detail variations of refractive index about its mean value at several locations for altitudes up to 20,000 feet mean sea level.

Survey of Airborne Microwave Refractometer Measurements

During the past five years, it has been definitely established that useful radio signals at all frequencies can be received consistently at distances far beyond the horizon. These facts have forced a considerable modification of the theories, concepts and charts found in most textbooks and handbooks. Even more important, these facts have opened up many interesting radio possibilities, particularly in the field of fixed point-to-point communication. This relatively long distance transmission has been called by various names, such as, beyond-the-horizon or extended range transmission, but perhaps it is most generally known by the term, "scatter". It is important to note that the word "scatter" is used in a specialized sense which is actually closer to the layman's concept of reflection than it is to the opposite concept of scattering in all directions. More recently, the term, "forward scatter" has been used in an attempt to minimize this language difficulty. There are two different types of beyond horizon or scatter transmission. One type is the ionospheric scatter which is useful for telegraph signals at frequencies below about 50 mc for distances up to a thousand miles or more. The second type is tropospheric transmission which is useful over a very wide band of frequencies but is limited for wide band transmission to distances of a few hundred miles. A review of the principal characteristics of tropospheric transmission shows that the differences between line-of-sight and beyond-the-horizon transmission are not as great as has been commonly believed. Until a few years ago, the general expectations were that, first, the beyond horizon signal would not be reliable, second, it could not be used effectively because antennas would not realize their free space gain, and, third, the useful bandwidth would be very small. All of these doubts had a logical basis and contained an element of truth. However, these fears have been exaggerated. It is now clear that reliable point-to-point radio circuits can be engineered for path lengths of 150 miles or more to provide good quality multichannel voice circuits or possibly even television for radio relay purposes. Such circuits do require high power and large antennas but they are technically feasible. It is expected that the use of beyond horizon circuits will supplement but not supplant line of sight systems. The principal applications will probably be for point to point circuits over difficult terrain.

Characteristics of beyond-the-horizon radio transmission

An X-band vertical array system is developed for measuring and characterizing electromagnetic (EM) propagation in the marine atmospheric boundary layer. In particular, the evaporation duct that commonly forms over water is investigated as part of the Coupled Air-Sea Processes and Electromagnetic ducting Research (CASPER) at-sea experimental campaign conducted off the coast of Duck, NC, USA, during October–November of 2015. Monte Carlo simulations are first used to develop an optimal array of four vertically spaced receiving antennas. In the experiment, the antennas are mounted on the stern A-frame of a research vessel and measure EM signals transmitted from beacons mounted on another research vessel and on the pier at the Army Field Research Facility. While the propagation loss versus range provides a data set similar to previous work, the vertical array can provide sampling of modes in the leaky waveguide formed in the duct. Combining both the range and height sampling results in a robust inversion method for evaporation duct estimation. In this paper, the efficacy of the four-element array is demonstrated by estimation of the evaporation duct height through comparison with a library of precomputed propagation curves generated using the parabolic wave equation. Low model error gives confidence in the estimates, which are consistent with the concurrent environmental measurements performed by the CASPER team. It is found that the evaporation duct can vary significantly with range and time over the duration of a data collection run.

Range and Height Measurement of X-Band EM Propagation in the Marine Atmospheric Boundary Layer

The effect of variations in index of refraction on line-of-sight propagation of electromagnetic waves in the troposphere is investigated using, in this paper, a ray theory approach. Mean-square variations in phase delay and phase correlation between two paths are calculated. It is shown that these quantities are relatively insensitive to the form of space correlation function assumed for the index of refraction. The mean-square phase is proportional to ?N2l0 where l0 is scale length and ?N2 the mean-square variation in index of refraction, whereas in beyond line-of-sight propagation the received power is proportional to ?N2/l0. Variations in angle of arrival are also calculated and it is shown that here the assumed space correlation of index of refraction is critical in determining the angle of arrival characteristics. It is shown, however, that a certain portion of the angle-of-arrival spectrum is insensitive to the choice.

Line-of-Sight Propagation Phenomena-I. Ray Treatment

A micro\yave refracto meter for aircraft use is described, and some of the sources of error in the measuremen t of variations of a t mospheric refractive index with t his instrumen t a re discussed. Observat ions were made up to 10,000 feet on 2 days near vVashington, D . C. Two refractive-index soundings taken 1Yz h ours apart showed changes that, in the coarser aspects, resembled changes shown by radiosonde data. The flu ctuation intensity changed erratically with ti me and place, but was usually greatest where the vertical gradient was changing. A rough analysis of the data showed that the larger fluc t ua tions occurred over distances of several hundred meters, whereas over distances less than 5 mete rs t he flu ctuations were negligible (the r esponse of the instrument wou ld have a llowed t he detection of ftuctuations occu rr ing within 0.5 meter). Large increases in index were observed on entering cumulus clouds and intense flu ctuations we I'e noted within the clouds.

/pdf/measurement-of-variations-in-atmospheric-refractive-index-4xcno994p5.pdf

Measurement of variations in atmospheric refractive index with an airborne microwave refractometer

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Measurement of variations in atmospheric refractive index with an airborne microwave refractometer