Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line. The rays or waves may be diffracted, refracted, reflected, or absorbed by atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles.
Radio signals, like all electromagnetic radiation including light emissions, travel in straight lines. At low frequencies (below approximately 2 MHz or so) these signals travel as ground waves, which follow the Earth's curvature due to diffraction with the layers of atmosphere. This enables AM radio signals in low-noise environments to be received well after the transmitting antenna has dropped below the horizon. Additionally, frequencies between approximately 1 and 30 MHz, can be reflected by the F1/F2 Layer, thus giving radio transmissions in this range a potentially global reach (see shortwave radio), again along multiply deflected straight lines. The effects of multiple diffraction or reflection lead to macroscopically "quasi-curved paths".
However, at higher frequencies and in lower levels of the atmosphere, neither of these effects apply. Thus any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye may sense. Therefore, since the ability to visually see a transmitting antenna (disregarding the limitations of the eye's resolution) roughly corresponds to the ability to receive a radio signal from it, the propagation characteristic of high-frequency radio is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon".
In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM radio, at comparatively low frequencies of around 100 MHz, easily propagates through buildings and forests.
Line-of-sight propagation as a prerequisite for radio distance measurements
Travel time of radio waves between transmitters and receivers can be measured disregarding the type of propagation. But, generally, travel time only then represents the distance between transmitter and receiver, when line of sight propagation is the basis for the measurement. This applies as well to RADAR, to Real Time Locating and to LIDAR.
This rules: Travel time measurements for determining the distance between pairs of transmitters and receivers generally require line of sight propagation for proper results. Whereas the desire to have just any type of propagation to enable communication may suffice, this does never coincide with the requirement to have strictly line of sight at least temporarily as the means to obtain properly measured distances. However, the travel time measurement may be always biased by multi-path propagation including line of sight propagation as well as non line of sight propagation in any random share. A qualified system for measuring the distance between transmitters and receivers must take this phenomenon into account. Thus filtering signals traveling along various paths makes the approach either operationally sound or just tediously irritating.
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