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See also Non-line-of-sight propagation for more on impairments in propagation. It is important to take into account the curvature of the Earth for calculation of line-of-sight paths from maps, when a direct visual fix cannot be made. Designs for microwave formerly used 4 ⁄ 3 Earth radius to compute clearances along the path.
To set a sight line 13 feet (4.0 m) above the water, and thereby reduce the effects of atmospheric refraction. To add a pole in the middle of the length of canal that could be used to see the "bump" caused by the curvature of the Earth between the two end points.
s is along the surface of the Earth, d is the straight line distance, and ~d is the approximate straight line distance assuming h << the radius of the Earth, 6371 km. In the SVG image, hover over a graph to highlight it. If the observer is close to the surface of the Earth, then it is valid to disregard h in the term (2R + h), and the formula ...
Without taking into account the refraction through the atmosphere, the radar horizon would be the geometrical distance from the radar to the horizon only taking into account the height of the radar above sea-level, and the radius of the earth (approximately 6.4·10 3 km):
The frequency of radio waves used by most radars, in the form of microwaves, travel in straight lines.This generally limits the detection range of radar systems to objects on their horizon (generally referred to as "line of sight" since the aircraft must be at least theoretically visible to a person at the location and elevation of the radar transmitter) due to the curvature of the Earth.
Diagram showing displacement of the Sun's image at sunrise and sunset Comparison of inferior and superior mirages due to differing air refractive indices, n. Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. [1]
where R is the radius of the earth. The line of sight is horizontal at the instrument, but is not a straight line because of atmospheric refraction. The change of air density with elevation causes the line of sight to bend toward the earth. The combined correction for refraction and curvature is approximately: [3]
Ground wave is important for radio signals below 30 MHz, but is generally insignificant at higher frequencies where line-of-sight propagation dominates. AM and longwave broadcasting, navigation systems such as LORAN , low-frequency time signals , non-directional beacons , and short-range HF communications all make use of it.