Estimation of tropospheric refractive bending from atmospheric emission measurements

Abstract

Refraction effects cause radio waves travelling through the atmosphere to be bent. Without knowledge of the atmospheric parameters along the ray path, the degree of bending cannot be calculated deterministically. Since atmospheric emission at microwave wavelengths is dependent on temperature, pressure, and relative humidity, radiometric data contain information about these parameters. This study will make use of a Monte Carlo simulation of the atmosphere to demonstrate that atmospheric emission measurements can be used with surface refractivity to increase greatly the accuracy with which refractive bending can be estimated.

Use of atmospheric emission to estimate refractive errors in a non‐horizontally stratified troposphere

“…The bending and range error were calculated using (6), (9), and (13). The emission data were calculated from a program similar to GABTAWF [Gaut and Reifenstein, 1971;Gallop and Telford, 1973] with a modification to account for the horizontally varying atmosphere.…”

“…Since the index of refraction can be specified as a function of the atmospheric parameters of temperature, pregsure, and water vapor density, corrections for refractive errors can be made deterministically where these parameters are known along the wave path [Bean and Dutton, 1966;Gallop and Telford, 1973]. However, such knowledge will be unavailable in the typical operational environment.…”

“…Herein it is shown that the assumption of a horizontally stratified troposphere at low elevation angles can lead to significant errors in the determination of bending and range error. It has been demonstrated in the context of estimating refractive bending that even where the atmosphere is horizontally stratified, the use of atmospheric emission measurements in addition to surface refractivity provides useful improvement in estimation performance over use of surface refractivity alone [Gallop and Telford, 1973; see also Decker et al, 1973;Guiraud et al, 1971;Schaper et al, 1970;Westwater, 1967]. The following results further establish that estimation with emission data proves even more useful Where the troposphere is horizontally variable, because of the degradation in performance of an estimator which employs surface refractivity alone.…”