Troposcatter Research Articles

Criteria for the Ultimate Capability of the Optimized Tropospheric Scatter System

Behavior of the tropospheric scatter mechanism is now sufficiently well understood to permit a determination of optimum transmission criteria. The requirements for optimization are established from the channel signal-to-noise ratio equation. It is shown that the optimum is related not only to the basic RF transmission loss, but also to the effective noise figure, antenna gain and medium aperture loss. It is considered that if a minimum product of antenna height, path length, and frequency are maintained, there will be a cubic relation between path loss and frequency. When the frequency dependence of the other factors is also taken into account, it is found that the net result will depend either directly or inversely upon frequency, depending upon the choice of system parameters. On this basis, and by using a graphical solution, an optimum frequency is found, for which the system gain maximizes. It is seen that for a given antenna, the product of frequency and path length is substantially constant. From this, it is found that the minimum product of antenna height, path length and frequency is achieved, so validating the use of the initial cubic relation. It is shown that when the optimum frequency is used, the associated medium aperture loss is rather small, so that improved accuracy in the determination of this factor is unlikely to produce substantial improvement in the optimized system. Also, the plane wave antenna gain is not exceptionally high, so that structures which are more economic than paraboloidal types may be contemplated. Curves for path loss vs distance are derived using optimum frequencies; total loss curves for 99.9 per cent reliability of the optimized system are also shown. This data is used to establish a series of system capability curves which are directly related to transmitter power, noise bandwidth and the signal-to-noise ratio at receiver input. It is considered that with the optimized system, very low power may be used to carry high capacity circuits over quite long spans.

Forward scatter from rain

The forward scatter of radio waves from rain has been observed over a 90-mile, 2720-mc path. The observations support the assumption of omnidirectional scattering from rain. On this link rain scatter may exceed the normal tropospheric scatter signal by 15 db. This latter signal may itself be increased by the presence of thunderstorms in the vicinity. Signal level distribution during rain and no-rain conditions are presented. Among the effects of rain on a tropospheric scatter link are the increased fading rate and decreased bandwidth. The fading rate may increase by a factor of ten or more, and pulse-to-pulse fluctuations have been observed at a pulse repetition frequency of 600 pulses per second. Evidence for a decreased bandwidth is presented in the form of pulse photographs. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.5-\mu</tex> sec pulse is commonly broadened to 3 or <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4 \mu</tex> sec and on occasions to lengths in excess of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20 \mu</tex> sec.

Carrier-frequency dependence of the basic transmission loss in tropospheric forward scatter propagation

A further interpretation is given of certain Lincoln Laboratory data obtained in an experiment using scaled antennas as published by Bolgiano [1959]. This paper has three objectives: (1) to consider the significance of these data as regards the theory of radio propagation through a turbulent atmosphere; (2) to describe a suitable method for the measurement of the meteorological parameters entering the theory; (3) to apply a further statistical analysis to these data. On the basis of this analysis of the Lincoln Laboratory data, it is concluded (1) that the {2/Γ(μ)}(ρ/2)μKμ(ρ) correlation model provides a description of tropospheric turbulence adequate for the description of these data, with μ assigned the fixed value 1, and (2) that the variation of the carrier-frequency dependence of the basic transmission loss from hour to hour arises simply from a lack of correlation in the hourly median losses on widely separated carrier frequencies rather than from changes of μ in this correlation model.

Experimental swept-frequency tropospheric scatter link

An experimental swept-frequency tropospheric scatter link, with a path length of 194 miles, has been established in France between Cholet and Corbeville. The transmitter (designed by Compagnie Générale de Télégraphie Sans Fil) sweeps in a nearly linear manner from 3100 to 3600 mc and back to 3100 mc at about 10 cps. The average transmitted power is 250 watts. The receiver and data display (designed by Airborne Instruments Laboratory) sweep in synchronism with the transmitter. The synchronizing signal is derived from the carrier frequency of a broadcast station (Paris-Inter). The receiver is designed to track swept signal amplitudes down to about 4 db above noise level, and after severe fades, to reacquire signals larger than 6 db above noise level in about 1 to 2 milliseconds. Received signal amplitude is presented as a simultaneous function of frequency and time. Preliminary data obtained from the link indicate that privileged transmission frequencies exist at all times in this band at which the transmission level is about 10 db above median. The preliminary data gathered from the link are described, as well as general design features of the system.

Passive Satellite Communication

A number of communications capabilities are noted as desirable goals and the characteristics of passive satellite reflectors are cited as a means of achieving them. Goals noted are the attainment of transoceanic ranges, high communications traffic per satellite, low launching and operating costs, high reliability, simplicity of operation, and long useful life of satellites. The properties of various types of passive reflectors are discussed and the characteristics of the sphere are shown to be particularly advantageous. Passive satellite communications systems are compared with current tropospheric scatter systems and are shown to be one order of magnitude better, using power per channel per unit range as a criterion. Some attention is given to Doppler shift as it bears upon the problem of transmitter-receiver channel coordination, satellite acquisition, and tracking.

Optimum design considerations for radio relays utilizing the tropospheric scatter mode of propagation

There are many areas of the world where the service which can be provided by the scatter link is needed, but cannot be justified economically. One approach toward reducing costs, suggested here, is to investigate the improvements achieved through optimizing system-design parameters.

Radio Transmission by Ionospheric and Tropospheric Scatter

Radio Transmission by Ionospheric and Tropospheric Scatter

Results from a Three-Hop Tropospheric Scatter Link in Norway with Parallel Operations on 900 mc and 2200 mc

A three-hop troposcatter system in Norway with simultaneous operation at 900 mc and 2200 mc is described briefly. From measurements on a 360-km hop in the period October 1957 to June 1958, it is concluded that 1) the monthly amplitude distributions are approximately Gaussian, and the 1-minute amplitude distributions are approximately of the Rayleigh type, 2) the signals are generally considerably stronger (differences up to the order of 10 db) in summer than in winter, 3) the monthly median strength of the 900-mc signals is generally 0-2 db stronger than that of the 2200-mc signals, 4) the foreground conditions may be critical, 5) the 1-minute fade duration distributions are approximately log-normal, 6) there are indications that the normalized 1-minute fade duration distributions are about equal for 900 mc and 2200 mc, 7) considerable reductions in telegraph error rate are effected by increasing orders of diversity reception, 8) the telegraph error rates are equal for 900-mc and 2200-mc signals of equal median strengths, 9) frequency-modulated telegraph multiplex equipment is slightly superior to two-tone telegraph multiplex equipment, when adjusted to equal loadings, 10) antenna radiation diagrams depend critically on local surroundings, such as woods.

Transmitter Power Control in Two-Way Communications System

A typical scatter communications loop consists of two sites, each having transmitters and receivers operating simultaneously over a two-way microwave loop. In tropospheric scatter systems, disturbances in the microwave paths produce dynamic fading of RF signal levels at the receivers. To achieve high system reliability, the current practice is to continuously transmit high power, adequate to maintain RF reception during the presence of severe fading. This wastes power and creates serious radio interference during favorable propagation periods. Feedback control is applied to high reliability systems to vary the transmitted power as a function of the fading and consequently 1) to reduce operating cost by conserving transmitted power, and 2) to reduce interference to other radio networks in the area. Other secondary advantages, including reduction of the dynamic range of received signal strength (which will ease receiver AGC and RF amplifier design), will be apparent to the reader. In order to maintain high system reliability, the automatic control at each site is simultaneous, continuous, and effective for nonreciprocal as well as for reciprocal fading. The system has been referred to as Controlled Carrier Communications.

Path-Loss Measuring Techniques and Equipment

The planning of tropospheric scatter communication systems requires precise and reliable knowledge of the path loss. This paper describes the techniques and equipment used to evaluate proposed installations. The equipment operates in the frequency range of 875 to 940 or 1650 to 1950 mc with output powers of 10 or 100 watts. A logarithic recording system is used which makes possible a dynamic signal recording range of 50 to 100 db. Automatic frequency control is employed to achieve a 300-cps 3-db bandwidth with moderate oscillator stabilities.

Tropospheric scatter propagation and atmospheric circulations

Tropospheric scatter propagation and atmospheric circulations

General theorems on the transmission coefficient from a transmitting to a receiving system

Some theorems on the transmission coefficient from a transmitting to a receiving system will be presented. They can be considered as very general theorems in physics, since they can be applied to fields other than electromagnetic theory, such as wave mechanics and fluid mechanics. In electromagnetics they have been applied to the theory of surface waves and to propagation problems, particularly tropospheric scatter propagation.

The filling in of an antenna null by off-path scattering on a tropospheric scatter circuit

The concept and characteristics of Antenna-to-Medium Coupling loss have already been discussed in the literature in some detail. These earlier analyses show that the coupling loss is not significant unless the beamwidth of the antenna becomes equal to or less than the angular width of the scatter volume as seen from the antenna. An inverse phenomenon occurs if one tries to design an antenna null to protect a given location beyond the horizon; namely, if the width of the null is narrower than the angular width of the scatter volume, the null can be filled in by off-path scattering and the desired protection might not be achieved. This paper outlines the analysis used in evaluating the "filling in" of the antenna null by off-path scattering and presents curves which should prove useful in obtaining quantitative estimates. An examination of the interrelation between the scattering process and the free-space antenna pattern reveals that the scattering process averages the free-space antenna pattern, the averaging interval being of the order of several degrees. Thus a qualitative conclusion can be drawn that if it is desired to protect an installation with a 40 db null, this null must be maintained over at least several degrees in order to be effective. A cosine squared pattern is not satisfactory.

Transoceanic Communication by Means of Satellites

The existence of artificial earth satellites and of very low-noise maser amplifiers makes microwave links using spherical satellites as passive reflectors seem an interesting alternative to cable or tropospheric scatter for broad-band transatlantic communication. A satellite in a polar orbit at a height of 3000 miles would be mutually visible from Newfoundland and the Hebrides for 22.0 per cent of the time and would be over 7.25° above the horizon at each point for 17.7 per cent of the time. Out of 24 such satellites, some would be mutually visible over 7.25° above the horizon 99 per cent of the time. With 100-foot diameter spheres, 150-foot diameter antennas, and a noise temperature of 20°K, 85 kw at 2000 mc or 9.5 kw at 6000 mc, could provide a 5-mc base band with a 40-db signal-to-noise ratio. The same system of satellites could be used to provide further communication at other frequencies or over other paths

The Effects of Low-Noise Techniques on Tropospheric Scatter Communications

This paper defines the expected benefits to be derived from the use of low-noise receiver techniques when applied to tropospheric scatter systems in the 1000 to 2000-mc range. Calculations were made of the expected antenna noise temperatures for "smooth" and "rough" ground locations. These data, plus the known and anticipated performance of various low-noise techniques, permitted the calculation of increased sensitivity. The sensitivity improvement is then expressed in terms of increased range, decreased transmitter power, and decreased antenna size.

A Study of the Technical and Economic Feasibility for Tropospheric Scatter Circuits in Primary Toll Networks of Underdeveloped Countries

The inherent capabilities of the tropospheric scatter circuit have emphasized the need for careful economic and technical evaluation. It is suggested that this should be carried out in two phases. The first is a broad study which determines the essential system parameters for which detailed analysis is necessary. The second phase is concerned with this detail. Outlines are given relevant to the principal factors associated with the first of these phases. These cover such areas as trunk circuit growth, required channel capacity at the end of the amortization period, revenue potential, optimum routing, choice of transmission method, and the optimum number of tandem tropospheric scatter links. The performance sacrifice for a minimal cost "first in" system and the manner in which design accuracy may be traded against probable propagation reliability are two of the problems discussed. Evaluation of the scatter link may be based on a performance index and data relevant to this are given. It is concluded that adequate analysis of the modern system capability requires a sophisticated engineering approach for which the present paper is a guide.

Theoretical research on tropospheric scatter propagation in the United States, 1954-1957

This paper outlines recent progress in the theory of tropospheric scatter propagation in the United States. In the past three years, the emphasis of theoretical research has shifted from the analysis of the average signal level to the analysis of the signal statistics and to the underlying hydrodynamics of atmospheric turbulence. As might be expected in such a new and complex field, there is far from unanimity of opinion as to the "best" model to explain the myriad experimental results.

Tropospheric Scatter Path Loss Tests--Florida Bahamas

Telephone service between the United States and the Bahama Islands is now provided by means of high-frequency radio systems working in the 2-7 mc range. To handle properly the large volume of traffic during the peak winter season requires that more than the present nine circuits be provided. The lack of available frequencies in the 2-7 mc range, together with the comparatively poorer grade of facility obtained from these radio systems, makes it desirable to investigate other methods of providing telephone facilities. The fact that the present Miami-Havana tropospheric scatter system is performing so well, and that the distance from Miami to Nassau is the same as that to the Cuban terminal, made this type of system very attractive. At the time the Cuban system was being developed, klystron tubes capable of high outputs above 1000 mc were not available. For this reason, the Cuban system was designed to operate in the 700-900 mc range. Since that time tubes capable of high output levels at frequencies above 2000 mc have been developed, and it was considered desirable to operate at these higher frequencies rather than in the lower part of the UHF spectrum. Path loss tests were made over a nineweek period on the Florida-Nassau path using a frequency of 1970 mc. These tests are discussed and some comparisons are drawn between them and the path loss tests made to Cuba at 800 mc two years earlier. The Nassau tests indicated that a satisfactory 60-channel radio system could be provided through use of 10-kw transmitters and 30-foot parabolic antennas.

A Formalized Procedure for the Prediction and Analysis of Multichannel Tropospheric Scatter Circuits

In order to evaluate the potentialities of the scatter link and to minimize the financial risk involved in its implementation, examination of many combinations of path, sites, and system configurations is desirable. To assist this process, and to minimize the financial risks involved in uncertainties, a formal analytical procedure for the study of these circuits is desirable. The essential structure for such a procedure is outlined. A step-by-step process is involved, commencing with determination of an objective weighting factor for site evaluation. Data are presented for resolution of further steps dealing with median path loss, design accuracy, traffic loading, instantaneous fading, SNR, error rates, bandwidth and power. A performance index for path evaluation is used so that an optimum path-site combination may be selected. An index for comparison of various paths carrying different traffic loads with different reliabilities is suggested. Equations for the use of this data in specific design situations are also given.

Angular Diversity Reception at 2290 MC Over a 188-Mile Path

Experiments were performed over the 188-mile Round Hill-Crawfords Hill path at 2290 mc to determine the feasibility of using angular reception in a tropospheric scatter system. Using a 28-foot reflector, two beams were produced with two separate feed systems. The correlation of the signals received on one of the two beams with that received on the other was determined for various spacing of the beams, as well as for the azimuthal position of the antenna. These experiments show that angular techniques can be effective depending on the proper choice of frequency, antenna size, and beam separation for paths in the neighborhood of 200 miles in length. A substantial diversity gain can be achieved even though partial correlation exists. These results also appear to be in good agreement with theoretical predictions for equal means and for the short periods of time applicable to obtaining reliable voice and high speed teletype communications.