Evaluation of atmospheric refraction effects on UHFVHF radio propagation has long been accomplished with the convenient four-thirds earth concept of Schelling, Burrows, and Ferrell. This method has proven particularly useful in evaluating performance of point-to-point radio communications systems. However, relatively new long-range applications have demanded a model of atmospheric radio refractive index more representative of observed refractive index profiles than the simple linear decay inherent in the four-thirds earth approach. This paper introduces two models of atmospheric radio refractive index which can be used to predict refraction effects from the value of the refractive index at the transmitting point. Both models offer considerable improvement over the four-thirds earth model, particularly for applications at long distances and high elevations in the atmosphere. Further, both models may be adjusted to represent mean conditions at different times of year and in different geographical locations. A new method of predicting radio-ray refraction at very low initial elevation angles is introduced which utilizes both the initial value and the initial height-gradient of the refractive index over roughly the first 100 meters above the earth's surface. This method, which is dependent only upon the first two radiosonde reporting levels or simple tower measurements of the common meteorological elements, results in a considerable improvement of the values of ray-refraction predicted by the model.

Models of the Atmospheric Radio Refractive Index

The paper describes a simple and accurate method for computing ionospheric and tropospheric bending. The only assumptions made are that the refractive gradient is radial and that the refractive index profile can be approximated by a finite number of linear segments whose thickness is small compared with the earth's radius. These assumptions are readily justifiable in all practical cases. Since there are no limitations on the angle of elevation and the shape of the refractive index profile, the method has a wide application and it is extended to cover other refractive effects such as retardation, Doppler error and Faraday Rotation.

Simple Methods for Computing Tropospheric and Ionospheric Refractive Effects on Radio Waves

The radio refractive index gradient over the British Isles

The problem of guidance to near planets with low specific impulse engines is examined. Trajec­ tories to which the vehicle must adhere in order to minimize fuel consumption are treated in order to evolve which measurable parameters are suited for guidance. Criteria are established for the judging of suitability of these measurement parameters. Various possible measurement tech­ niques are catalogued into active electromagnetic techniques, passive electromagnetic techniques and inertial techniques. The merits of the individual methods and the classes are studied.

Guidance Techniques for Interplanetary Travel

Limits of applicability of ray tracing in computing tropospheric refraction at White Sands Missile Range have been further explored. 286 comparisons were made, all for a path from radar to fixed beacon of about 45 miles and an elevation angle of 17.99 milliradians. A horizontally stratified atmosphere was assumed. Refractive index profiles were prepared from a variety of weather data, and classified A, B, C, or R, in descending order of reliability prior to ray-tracing calculations. Angle observations were made with an FPS-16 C-band radar having a quoted instrumental accuracy of 0.14 milliradian rms. Angle readings varied from 18.36 to 20.54 milliradians, with mean of 19.01 milliradians and standard deviation of 0.41 milliradians. The rms deviation of computed from experimental angles ranges from 0.28 to 0.41 milliradian for different classes of data. The ratio of this deviation to the deviation from over-all mean varies from 0.68 for Class A to 1.00 for Class R. Thus the improvement over a "standard atmosphere" varies from 32 per cent to 0, and correlates directly with quality of weather information. For this experiment it is concluded that most of the rms elevation angle error is contributed by atmospheric conditions. Although ray tracing methods provide a significant correction when sufficiently good weather information is available, there still remains a large uncertainty not accounted for by equipment.

Comparison of experimetal with computed tropospheric refraction

Refractive-index variations in the atmosphere cause errors in radar measurements. This paper presents the results of a theoretical study of the elevation-angle error due to the refraction of electromagnetic waves in the troposphere. The study is based upon ray theory, using standard meteorological data reported by the United States Weather Bureau for the surface and the standard pressure levels. Computed errors are tabulated for thirty-four monthly mean refractive-index profiles selected as being representative of various type air masses for different seasons and latitudes. In order to get an indication of the spread in propagation errors to be expected during a particular season for fixed locations, computations have been carded out based on the 0300Z soundings for the odd days of January (1950-1954) and July (1950-1953) for nine U. S. locations. The diurnal influence is investigated by analyzing the variations in surface refractive index for January and July, 1953, and also for nine U. S. locations.

A study of radar elevation-angle errors due to atmospheric refraction

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A study of radar elevation-angle errors due to atmospheric refraction