1. The position of the satellites is known by GPS receiver based on the ephemeris (5 orbital elements), parameters that are transmitted by the satellites themselves. The collection of ephemerides of the entire constellation is completed every 12 minutes and stored in the GPS receiver.
2. The GPS works by measuring its distance from the satellites, and uses that data to calculate its position. This distance is measured by calculating the time that signals take to reach the receiver. Known that time and based on the fact that the signal travels at the speed of light (except for some corrections that apply), we can calculate the distance between receiver and satellite.
3. Each satellite indicates that the receiver is at a point on the surface of the sphere centered at the own satellite radio and the total distance to the receiver.
4. Getting information from two satellites shows that the receiver is on the circle that results when the two spheres intersect.
5. If we acquire the same information from a third satellite we note that the new sphere intersects the circumference just above two points. One of them can be discarded because it offers an absurd position. In this way we would have the position in 3-D. However, since the clock incorporating with GPS receivers it is not synchronized with atomic clocks in GPS satellites; the two given points are not precise.
6. Taking information from a fourth satellite, we eliminate the inconvenience of a lack of synchronization between the clocks of the GPS receiver and satellite clocks. And this is when the GPS receiver can determine a precise 3-D position (latitude, longitude and altitude). As they are not synchronized (clocks between the receiver and satellites), the intersection of four spheres centered at these satellites is a small volume rather than a point. The correction is made to adjust the time of the receiver so that this volume becomes a point.
Reliability of received data
Because of the military nature of GPS system, it is also used by the Department of Defense. It is reserved the possibility of including some degree of random error, which could vary from 15 to 100 m. The so-called selective availability (S / A) was eliminated on May 2, 2000. Although it is not currently operated such induced error, the inherent accuracy of GPS depends on the number of visible satellites at the certain time and place.
When a large number of satellites are recruited (there could be seven, eight or nine satellites), and if they have a proper geometry (if they are scattered), all the details can be obtained in less than 2.5 meters in 95% of cases. If you activate the SBS system called DGPS (WAAS, EGNOS, MSAS), the accuracy improvement can be less than one meter in 97% of cases. But, there are some countries that do not use this advanced system. For example, these systems do not apply SBS in South America because this part of the world does not have this type of geostationary satellites. For More Information Please visit. http://www.gps-teknik.se/
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