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  • Line-of-sight propagation


    Line of sight propagation to an antennaLine-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions traveling in a straight line. The rays or waves may be diffracted, refracted, reflected, or absorbed by the atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles. In contrast to line-of-sight propagation, at low frequency (below approximately 3 MHz) due to diffraction, radio waves can travel as ground waves, which follow the contour of the Earth. This enables AM radio stations to transmit beyond the horizon. Additionally, frequencies in the shortwave bands between approximately 1 and 30 MHz, can be reflected back to Earth by the ionosphere, called skywave or "skip" propagation, thus giving radio transmissions in this range a potentially global reach. However, at frequencies above 30 MHz (VHF and higher) and in lower levels of the atmosphere, neither of these effects are significant.

  • Sextant


    A sextant A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celestial navigation. The estimation of this angle, the altitude, is known as sighting or shooting the object, or taking a sight. The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart—for example, sighting the Sun at noon or Polaris at night (in the Northern Hemisphere) to estimate latitude. Sighting the height of a landmark can give a measure of distance off and, held horizontally, a sextant can measure angles between objects for a position on a chart. A sextant can also be used to measure the lunar distance between the moon and another celestial object (such as a star or planet) in order to determine Greenwich Mean Time and hence longitude. The principle of the instrument was first implemented around 1731 by John Hadley (1682–1744) and Thomas Godfrey (1704–1749), but it was also found later in the unpublished writings of Isaac Newton (1643–1727).

  • Soil horizon


    A cross section of a soil, revealing horizons A soil horizon is a layer parallel to the soil surface, whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, i.e. ‘coarser’ or ‘sandier’ than the horizons above and below. The identified horizons are indicated with symbols, which are mostly used in a hierarchical way. Master horizons (main horizons) are indicated by capital letters. Suffixes, in form of lowercase letters and figures, further differentiate the master horizons. There are many different systems of horizon symbols in the world. It should be emphasised that no one system is more correct – as artificial constructs, their utility lies in their ability to accurately describe local conditions in a consistent manner. Due to the different definitions of the horizon symbols, the systems cannot be mixed. Below, some of these systems will be described. In most soil classification systems, horizons are used to define soil types. Some systems use entire horizon sequences for definition, e.g. the German system. Other systems pick out certain horizons, the “diagnostic horizons”, for the definition, e.g. the World Reference Base for Soil Resources (WRB), the USDA soil taxonomy and the Australian Soil Classification. Diagnostic horizons are usually indicated with names, e.g. the “cambic horizon” or the “spodic horizon”. The WRB, e.g., lists 37 diagnostic horizons. Of course, besides these diagnostic horizons, some other soil characteristics may be needed to define a soil type. Some soils even don’t have a clear development of horizons. A soil horizon sensu stricto is a result of soil-forming processes (pedogenesis). Layers that do not have undergone such processes may be simply called “layers”. Some soil scientists use the word layer in a more general way, including the horizons sensu stricto.

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