Saturday, February 9, 2008
Global Positioning System
The Global Positioning System (abbreviated in GPS, in its turn abbreviation of NAVSTAR GPS, acronym of NAVigation System Time And Ranging Global Position System), is a positioning system on base to satellitare, to total cover and continues, managed from the department of the American defense.
History of the GPS
In the 1991 the USA opened to the world the service with name SPS (Standard Positioning System), with detailed lists differentiated that called soldier PPS (Precision Positioning System). In practical it came introduced the so-called Selective Availability (KNOWS) that it introduced intentional errors in marks them satellitari in order to reduce the accuracy of the survey, concurring precisions single in the order of 100-150 m.
The GPS has been created in substitution of the previous system, the Transit, when the USA, rinunciando to the Selective Availability, have rendered the first one taken care of how much the second, supporting it with a net of 24 satellites crafts them.
The degradation of marks them has been disabilitata from the month of May 2000, thanks to a decree of the President of the United States Bill Clinton, putting therefore to disposition the precision puts into effect them of approximately 10-20 m. In the models for civil use a device is present that inhibits the operation to heights and advanced speeds to sure values, in order to prevent of the assembly on unexpected missiles.
The UE has in plan the completion of an own net of satellites, Sistema of Galileo positioning, for civil scopes, between which the GPS. This plan has an obvious strategic valence in how much the net American is property of the single United states and is managed military authorities, than, in particular conditions, they could decide discretionarily and unilaterally to reduce the precision or block the access selective to the system; the sharing of the investment and the property from part of the utilizzatori states guarantees continuity, accessibility and interoperabilità of the service.
The GPS has been created in substitution of the previous system, the Transit, when the USA, rinunciando to the Selective Availability, have rendered the first one taken care of how much the second, supporting it with a net of 24 satellites crafts them.
The degradation of marks them has been disabilitata from the month of May 2000, thanks to a decree of the President of the United States Bill Clinton, putting therefore to disposition the precision puts into effect them of approximately 10-20 m. In the models for civil use a device is present that inhibits the operation to heights and advanced speeds to sure values, in order to prevent of the assembly on unexpected missiles.
The UE has in plan the completion of an own net of satellites, Sistema of Galileo positioning, for civil scopes, between which the GPS. This plan has an obvious strategic valence in how much the net American is property of the single United states and is managed military authorities, than, in particular conditions, they could decide discretionarily and unilaterally to reduce the precision or block the access selective to the system; the sharing of the investment and the property from part of the utilizzatori states guarantees continuity, accessibility and interoperabilità of the service.
Satellites
They are disposed on 6 orbital plans tilts to you of 55° regarding the equatorial plan (therefore do not cover the polar zones) to shape of ellipses to low eccentricity. Every slowly orbital it has 3 or 4 satellites, and the plans are disposed in such way that every utilizzatore on the earth can receive marks them at least 5 satellites. Their quota is of 20 200 km. Every satellite emits on derived the 1,5 frequencies of 1,2 and GHz from an only oscillator to high stability. The scope of the double frequency is that one to eliminate the error which had to the atmospheric refraction. On these carrying frequencies, modulated in phase, come emitted the messages of ephemeris, everyone of the durations of two minuteren; they begin and finish entire to the equal minuteren of the GMT. These messages of ephemeris contain mark them hour and the orbital parameters of the satellite.
In such a way receiver GPS, while he carries out conteggio the Doppler, receives the parameters of the orbit from which drift the position of the satellite: he comes therefore to arrange of all the elements necessary to define in the space the position surface.
In such a way receiver GPS, while he carries out conteggio the Doppler, receives the parameters of the orbit from which drift the position of the satellite: he comes therefore to arrange of all the elements necessary to define in the space the position surface.
Stations of tracciamento and center of calculation
The tracciamento of the satellites comprises all those operations apt to determine the parameters of the orbit. To that they exactly supply 4 master stations said of tracciamento (main tracking stations) and a calculation center (computing center), all situate to you in USA territory, and in particular to Wahiova (Hawaii), Point Mogu (California), Prospect Harbur (Maine) and Rosemount (Minnesota). Point Mogu is also center of the calculation center. Every time that every satellite in its orbital motion flies over the territory American the tracciamento stations of it records the Doppler data that they come starts you on the calculation center and here values you for the determination of the orbital parameters. In order to resolve this problem it has been necessary to come in possession of a faithful mathematical model of the land gravitational field. The construction of this model has been one of the problems more arduous solution in the development of the Transit plan from which it is derived puts into effect it them Navstar. It turns out you of this surveying on the land gravitational field, that they are of immense capacity from the geodetic point of view, can be reassumed in an image of the globe in which they come brought back the lines of equal scostamento of Geoide (LMM) fromthe ellissoide of reference APL.
Stations of soccorrimento
The orbital parameters of every satellite, as soon as it determines to you near the calculation center, are re-united in a message that comes forwarded to the satellite interested by means of one of the soccorrimento stations. The satellite records the parameters receipts in its memory and them reirradia to the customers.
Receiver GPS
The operation principle is based on a method of spherical positioning, that the time employed consists in measuring from marks them radio to cover the distance satellite-receiver. Knowing the time employed from it marks them in order to reach the receiver and the exact position at least 3 satellites for having a position 2D (bidimensional), and 4 for having a position 3D (three-dimensional), is possible to determine the position in the space of the same receiver. Such procedure, called trilaterazione, uses alone distance information and is similar to the triangolation, from which however the angles differ for the fact to make less than regarding information.
The precision can ulteriorly be increased thanks to the use of systems like WAAS (American) orthe EGNOS (European), perfectly compatible between of they. They consist one or two in geostazionari satellites that they send of marks them of correction. The modality Differential-GPS (DGPS) uses a connection radio in order to receive given to DGPS from one earth station and to obtain an error on the position of a pair of meters. Modality DGPS-IP takes advantage of, rather than radio waves, the Internet net for the shipment of correction information.
Receivers GPS ("external"), interfacciabili by means of door USB or logons without threads like the Bluetooth exist in commerce, that they concur to realize navigators GPS on several devices you: portable palmari, PC, computers, and, if it equips you of sufficient memory, also cellular telephones. For navigation they exist software I affixed to you, owners or open source that they use one cartography, anch' it publishes or owner.
The precision can ulteriorly be increased thanks to the use of systems like WAAS (American) orthe EGNOS (European), perfectly compatible between of they. They consist one or two in geostazionari satellites that they send of marks them of correction. The modality Differential-GPS (DGPS) uses a connection radio in order to receive given to DGPS from one earth station and to obtain an error on the position of a pair of meters. Modality DGPS-IP takes advantage of, rather than radio waves, the Internet net for the shipment of correction information.
Receivers GPS ("external"), interfacciabili by means of door USB or logons without threads like the Bluetooth exist in commerce, that they concur to realize navigators GPS on several devices you: portable palmari, PC, computers, and, if it equips you of sufficient memory, also cellular telephones. For navigation they exist software I affixed to you, owners or open source that they use one cartography, anch' it publishes or owner.
GPS and Theory of the Relativity
The satellitari clocks are affections from the consequences of the Theory of the Relativity. In fact, because of the effects arrange you of the relative speed, that the time slows down on the satellite of approximately 7 microsecond ones to the day, and of the smaller curving of the spaziotempo to level of the orbit of the satellite, than it accelerates it of 45 microsecond ones, the time on the satellite slides to a leggermente faster rhythm that to earth, causing an advance payment of approximately 38 microsecond ones to the day, and rendering necessary one automatic correction from part of the edge electronics. This observation supplies an ulterior test of the exactitude of the theory in an application of the real world. The found relativistico effect is in fact exactly correspondent to that calculable one theoretically, at least in the supplied limits of accuracy from the instruments of measure currently available.
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