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

Abstract

Tropospheric refraction introduces errors into radar and radio communication systems by causing radio waves to travel along a curved path and at a speed which changes with position. Common error correction techniques, such as making estimates of refractive effects from surface refractivity, rely implicitly on the assumption that the troposphere is horizontally stratified. This study demonstrates that such an assumption may be unwarranted especially at elevation angles below 10°, and that such reliance can lead to substantial errors. The end product of this research is a system which employs atmospheric emission measurements in addition to surface refractivity to estimate refractive bending and range error. This approach relies on the similar dependence of refraction and emission on atmospheric parameters to account for nonhorizontal stratification. Performance results establish that such a system need employ no more than one radiometer channel to provide potential error reduction of more than 40% over conventional approaches, along with a potentially automated, real‐time operational capability.

Analysis of Tropospheric Effects at Low Elevation Angles

“…The departure of the measurements reported by Anway from the surface correction model can be attributed to the effects of large scale horizontal variations in refractive index, the problem described by Gallop and Telford (1975). The increase in variance about the correction model observed in this study was comparable to the variance about the mean value of elevation error observed within an hour and previously reported by Crane (1976b), The source of this increased variability relative to the results reported by…”

“…The surface correction scheme did not work as well as expected based on the simulation analysis. Sine the earlier analysis of Haystack observations (Crane, 1976b) A number of simulation studies were made in the late 1950's to evaluate the seriousness of refraction induced errors and to provide possible procedures for refraction correction Thayer, 1954, Bauer et al, 1958;Bean et al, 1960 Gallop and Telford (1975) simulated the effects of horizontal inhomogeneity and reported that correction based on only surface refractivity measurements would not be as successful as expected for a horizontally homogeneous atmosphere. They suggested that the effects of horizontal variation would be small at a So elevation angle and negligible at 10, not sufficient to explain the Anway results.…”

Computer modeling of multipath propagation: Review of ray‐tracing techniques

“…The concept of an effective earth radius was introduced by Schelleng et al, [1933] and is also found in the works of Elmore [1975], Inoue [1975], Comit• Consultatif International des Radiocommunications (CCIR) [1978a], and Sasaki and Akiyarna [1979]. The use of Snell's law for rays refracted in a spherical atmosphere is applied by Schulkin [1952], Bean and Thayer [1959], Bean [1964], Armand and Kolosov [1965], Thayer [1967], Blake [1968], Bean and Dutton [1968], Plotnikov [1975], Gallop and Telford [1975], and Vickers and Lopez 1-1975].…”