Evaporation duct influences on beyond‐the‐horizon high altitude signals

Abstract

A case study of beyond‐the‐horizon propagation previously examined in terms of an earth‐detached duct produced by an elevated layer at 2000 ft is reexamined in terms of ducting produced by evaporation ducts (duct heights ≤100 ft). The case study applies to a frequency of 3087.7 MHz, a transmitter altitude of 68 ft and a receiver altitude of 3000 ft. It is concluded that relatively shallow surface ducts of the type examined can significantly affect signal level beyond the horizon at elevations which are an order of magnitude or more greater than the duct height and could conceivably account in large measure for the disparities previously reported.

Global sensitivity of parabolic equation radar wave propagation simulation to sea state and atmospheric refractivity structure

“…At altitudes well above the evaporation duct (>600 m), propagation is sensitive to duct height at close range for both 9 and 3 GHz but reduces with range only for 9 GHz. This frequency-dependent behavior is most likely due to higher duct leak rates at lower frequencies, consistent with the findings of Hitney et al [1978]. Figure 6a also clearly shows that sensitivity falls off more rapidly with range as frequency increases, particularly at low altitude.…”

“…This frequency‐dependent behavior is most likely due to higher duct leak rates at lower frequencies, consistent with the findings of Hitney et al . []. Figure a also clearly shows that sensitivity falls off more rapidly with range as frequency increases, particularly at low altitude.…”

Whispering gallery effects in the troposphere

“…These results also confirm a proposal by Wait [1968] that the usually neglected whispering gallery modes can substantially contribute to the total field in transhorizon propagation. In any case, the modeling presented here seems to be a much more likely explanation of the high signals than the evaporation duct model presented by Hitney et al [1978].…”

“…They also suggested that a strong low-level evaporation duct might account for the discrepancies between the modeled and measured loss values. Hitney et al [1978] reexamined this case study in terms of evaporation duct propagation. Since the evaporation duct was not expected to affect the relatively low microwave frequency of the experiment, accurate meteorological measurements near the sea surface were not made in the 1974 experiment.…”