towards the plains, using two microwave signals separated by a difference in frequency corresponding to very wide communication bandwidths. The beyond line-of-sight measurements are being made between Boulder, Colo., and Altus, Okla., using large parabolic antennas ranging in size from 14 to 60 ft in diameter at the receiving and transmitting terminals. The information obtained will help to determine the ultimate volume of information that can be transmitted over long tropospheric paths and assist in the design of systems employing this type of propagation.

Mutual Interference Between Surface and Satellite Communication Systems. Artificial earth satellites have opened up new horizons in long distance communication possibilities. Optimum frequencies for satellite communication purposes lie generally in the 1,000 to 10,000 Mc/s portion of the radiofrequency spectrum. Since this portion of the spectrum is presently in use by many communication services, satellite communication

Receiving antennas used in studies of obstacle-gain propagation over Pikes Peak. Under certain conditions, signals propagated across mountain ridges are found to be far stronger than if the obstacle were not there (page 130).

systems will be expected to share frequencies with these existing services. Predictions have been made of the conditions under which these frequencies can be shared by conventional point-to-point microwave relays and satellite systems. A study sponsored by the Joint Technical Advisory Committee of the Institute of Radio Engineers predicts that frequency assignments may be shared if adequate geographical separation of terminals is provided. Theoretical studies and an experimental program using the NBS 60-foot parabolic antennas were conducted to determine the minimum separation distance and the minimum antenna elevation angles for the space communication system, such that the interfering signal power appearing at the receiver input terminals was below the interfering level. In order to make reliable estimates of these interfering conditions, these measurements will necessarily be conducted over an extended period.

The advent of space

Technical Factors Influencing Allocations. telecommunications, together with the increasing use being made of the crowded radio spectrum, requires the application of technically sound procedures in radio frequency allocations to assure maximum efficiency of usage and adequate provision for all radio services. The Central Radio Propagation Laboratory, in cooperation with the Interdepartment Radio Advisory Committee and the Federal Communications Commission, has undertaken a general study of propagation factors important in radio frequency allocation, giving consideration to all types of radio service presently in use or of future concern.

Under cer

Signal Characteristics of Mountain Obstacle Paths. tain conditions signals propagated across mountain ridges are found to be far stronger than if the mountain were not there. In order to take optimum advantage of this effect it is necessary to be able to estimate the variation in transmission loss over these paths and to understand the conditions under which space diversity may be used to overcome fading. A long-term series of measurements was completed in eastern Colorado using Pike's Peak as a knife-edge type obstacle. Sample recordings were made at two frequencies (100 and 750 Mc/s) over a period of nine months. Empirically derived functions based on line-of-sight fading phenomena specially adapted to this type of propagation give results which closely approximate the measured fading characteristics.

Refraction Effects in Microwave Tracking Systems. Modern precision missile radio guidance systems using microwaves have their ultimate accuracy limited by the refractive index irregularities in the troposphere. A program is being conducted to measure the effects of atmospheric inhomogeneities and turbulence on such systems. Using specially developed techniques on the unique terrain of the Boulder area, an experimental tracking system was constructed to simulate the basic functions of the Mistram system being built for the Air Force. This system is being used to record the variations in apparent positions which result from atmospheric variations. Simultaneous recordings are made of various atmospheric quantities such as

Airborne refractometer equipment used in studying the effects of atmospheric inhomogeneities and turbulence on missile radioguidance systems using microwaves (page 130).

Radio propagation paths under study by the Bureau to provide information on factors affecting the design and use of radio systems. Ionospheric, groundwave, and line-of-sight paths are investigated to define the limitations, disturbances, and capacity of the transmission medium as a channel (page 128).

refractive index at each of the antennas in the system, and at a number of height levels on a tower near the terminals of the system. In addition, microwave refractometer measurements are made by an aircraft flying approxi mately along the propagation paths. These data are being examined for correlation with the apparent position variations of a fixed target simulating a missile to investigate the feasibility of using them for correcting the radio system data. Preliminary work has shown that some of the long-term (several hours or more) atmospheric effects can be reduced significantly by proper atmospheric measurements. However, no methods have been found as yet to make significant or reliable correction for the short-term effects (hourly or less).

Radio Meteorological Sensors. The results of an investigation concerned with the problem of time-lag constants in the humidity and temperature sensors of standard radiosonde instruments currently in use by civilian and military weather organizations shows clearly that corrections for the time lag in the sensors of both parameters are necessary for correct interpretation of the observed readings. Radiosonde observations have been universally used for determining the radio refractive index properties of the atmosphere with altitude; such observations are required not only for predicting the normal refraction of radio waves around the curved surface of the spherical earth, but are also used for predicting the strength of signals resulting from atmospheric turbulence and tropospheric waveguide propagation commonly known as ducting. It is especially important to take into account the time lag of these radio meteorological sensors in studying the climatological occurrence of radio ducts. By ignoring sensor time lag one tends to underestimate ducting incidence; by correcting only for humidity sensor lag, ducted incidence is overestimated.

Atmospheric Refractivity Models. The variation of the refractivity of the atmosphere with height above the surface of the earth is important in the prediction of tropospheric radio field strengths. A study of refrac tivity with height made by radiosonde observations throughout the world indicates that a satisfactory model of the atmosphere can be represented by the sum of two exponential quantities, one dealing with the dry properties of the atmosphere, and the other dealing with the humidity properties of the atmosphere. The dry and wet exponential terms are sensitive indicators of climatic differences, and in the course of the study maps of each were prepared for the United States for both summer and winter. The bi-exponential model yields more accurate estimates of refractivity structure in the troposphere than the earlier single exponential model, and consequently gives more reliable estimates of refraction for initial elevation angles in excess of 3. Only negligible improvement for the near zero angles of departure commonly used in tropospheric propagation are obtained.

Automatic Amplitude Distribution Analyzer. An analysis system for determining the principal statistical parameters of time varying radio propagation data was completed. These parameters are cumulative amplitude distribution, fade rate versus data level, fade or enhancement duration

distribution at specific data levels, and the distribution of percent of time preset fade or enhancement durations are exceeded. The system is designed to analyze data recorded on magnetic tape, making it possible to analyze most field data at a rate 100 times the speed at which it was recorded. The input data is in the form of variable voltage, representing the fading characteristics of the strength of the measured radio signal. The results are automatically recorded on a digital printer at the end of each analysis period. Engineering Standards for Tropospheric Communication. A revised and updated 360 page handbook of engineering standards for tropospheric telecommunications was prepared, partly in response to demand in excess of the 1960 edition and partly at the request of the Air Force for a shorter version of the material. Methods for calculating system performance and equipment requirements for line-of-sight, knife-edge diffraction, smooth and rough earth diffraction, and forward scatter are given with several new concepts and a general updating over the original handbook. Prediction of Radio Noise from Thunderstorm Counts. Since atmospheric radio noise originates in thunderstorms, an attempt has been made to predict the radio noise, at any location, from world-wide thunderstorm

counts.

From available records of thunderstorms, the probability of a thunderstorm occurring during any hour in each of the four seasons has been computed for any geographic location. Using the computed number of thunderstorms over the globe for any time, and season and propagation characteristics for the various frequencies and paths, the noise power received at any location may be calculated for the same time and season, providing the average power from an average thunderstorm is known. By assuming a value for this average power, comparisons of measured power and calculated power at each of the seventeen stations in the CRPL radio-noiserecording network can be made. The average power from a thunderstorm found from this comparison can then be used to calculate the noise power at any other location.

To date, due to the volume of computation necessary, only one check (at 50 kc/s) has been computed for one continental location. In this one check, the computed diurnal and seasonal variation of noise are in good agreement with the recorded noise.

RADIO SYSTEMS

The aim of the radio systems program is to provide technical information relating to propagation factors affecting the design and use of radio systems. The emphasis of this work is on long-range radio transmission problemsand methods of measurement-for radio communication, navigation, timing, detection, and positioning systems. Radio wave propagation studies are carried out for ionospheric, ground wave and line-of-sight paths to define the limitations, disturbances, and capacity of the transmission medium as a channel. The information obtained is directed toward guidance of engineering practices, allocation and use of radio frequencies, and evaluation