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Making a difference with GPS: time differences for kinematic positioning with low-cost receivers / J. Traugott in GPS world, vol 19 n° 5 (May 2008)

Public [article]  inGPS world > vol 19 n° 5 (May 2008) . - pp 48 - 54 Titre : Making a difference with GPS: time differences for kinematic positioning with low-cost receivers Type de document : Article/Communication Auteurs : J. Traugott, Auteur ; Dennis Odijk, Auteur ; Oliver Montenbruck, Auteur ; et al., Auteur Année de publication : 2008 Article en page(s) : pp 48 - 54 Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Traitement du signal [Termes IGN] GPS en mode différentiel [Termes IGN] phase GPS [Termes IGN] positionnement cinématique [Termes IGN] récepteur GPS [Termes IGN] traitement de données GNSS Résumé : (Editeur) Most radio signals consist of a carrier wave that is modulated in some way. This includes the GPS satellite signals. The pseudorandom-noise ranging codes and the navigation message are modulated onto the L-band carriers using binary biphase modulation. A GPS receiver uses the ranging codes to determine its distance from multiple satellites and then, through the process of multilateration, its position. But what about the carrier phase? Is it just a means to convey the ranging codes and navigation message? Definitely not. A GPS receiver determines its velocity as well as its position and it does this not by differencing sequential code-based positions, which would not be very accurate, but rather by measuring the Doppler shift of the received carrier. But the carrier can be used in other ways too. In fact, it can be used for determining positions, just like the code, but with much higher precision. Over 20 years ago, surveyors and geodesists devised ways to make use of recorded measurements of the phase of the received carriers to determine accurate relative positions between a roving receiver and a base or reference receiver at a known location. The technique was enhanced over the years, evolving into an approach known as RTK or real-time kinematic positioning. As its name suggests, RTK is usually employed in real time using auxiliary radio communications (often cell-phone-based) between the base and rover receivers. However, RTK-style positioning can also be used to postprocess collected data, achieving the same high-accuracy standards. But one of the difficulties with the RTK approach is resolving the so-called carrier-phase ambiguities. One cycle of the carrier looks just like the next, so how can you determine the exact number of cycles in the carrier between the satellite's antenna and the receiver's antenna? Well, it can be done, but even with increasingly sophisticated techniques, there is a limit to how far away a rover can be from the base station. Isn't there a way to get rid of the integer ambiguity problem? There is. If you time-difference sequential carrier-phase measurements, the ambiguity actually disappears! As we'll see in this month's column, you can determine accurate relative positions using time-differenced carrier-phase measurements. But there are some caveats. Read on. Copyright Questex Media Group Inc Numéro de notice : A2008-164 Affiliation des auteurs : non IGN Thématique : IMAGERIE/POSITIONNEMENT Nature : Article DOI : sans Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=29159 [article]

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