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Friendly reflections: monitoring water level with GNSS / A. Egido in GPS world, vol 21 n° 9 (September 2010)

Public [article]  inGPS world > vol 21 n° 9 (September 2010) . - pp 50 - 55 Titre : Friendly reflections: monitoring water level with GNSS Type de document : Article/Communication Auteurs : A. Egido, Auteur ; M. Caparrini, Auteur Année de publication : 2010 Article en page(s) : pp 50 - 55 Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Applications de géodésie spatiale [Termes IGN] diffusion du rayonnement [Termes IGN] hydrographie de surface [Termes IGN] niveau de l'eau [Termes IGN] récepteur GNSS [Termes IGN] réflectométrie par GNSS Résumé : (Editeur) Why is the sky blue? This is an age-old question, interesting to anyone with a curiosity about his or her surroundings. But what has it got to do with global navigation satellite systems? Believe it or not, there is a connection. Some of you might remember the explanation of the sky’s color from your Physics 101 course but to bring everyone up to the same level, let’s review. Everything we see is the result of the interaction of light and matter. And by matter, we mean the atoms, molecules, and particles making up matter. Light causes matter to vibrate. And vibrating matter (due to its electrical charges) in turn emits light, which combines with the original light. But matter not only re-emits light in the forward direction, it re-emits light in all other directions. This is called scattering. Now, the light from the sun includes all colors and so if look directly at the sun when it is high in the sky (don’t try this at home), it looks white or slightly yellowish. We are seeing the light propagating directly toward our eyes. When we look at the sky away from the sun, we are seeing scattered light. And this scattered light is predominantly blue. Why? It turns out that scattering is proportional to the fourth power of frequency. Light that is of a higher frequency, say a factor of two, is sixteen times more intensely scattered. So, blue light, which has about twice the frequency of light from the red end of the visible spectrum, is scattered much more than red light. Violet light is scattered even more but our eyes are not as sensitive to violet light as they are to blue light. Hence the sky looks blue. So what has this got to do with GNSS? As we know, for the best positioning and navigation results, we need the satellite signals to travel along a direct path to the receiver’s antenna. There may be slight changes in the speed and direction of propagation of these direct-path signals caused by the interaction of the electromagnetic waves with the matter making up the ionosphere and the neutral atmosphere, but these are readily accounted for in the position fixes. However, once they reach the Earth’s surface, the signals can be reflected by buildings, vegetation, the ground, water surfaces, and so on. The signals are actually being scattered by the matter they encounter. A receiver can selectively acquire the scattered signals and the resulting measurements can be interpreted to reveal certain characteristics of the source of the scattering. In this month’s column, we learn about the design and application of a GNSS instrument that uses scattered signals for monitoring the level and roughness of inland and coastal water surfaces–yet one more use of GNSS signals for the betterment of planet Earth. Numéro de notice : A2010-375 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Article DOI : sans Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=30569 [article]

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