Operations of GPS

Posted on 22. Jul, 2011 by admin in Uncategorized

The position of the satellites can be determined in advance by the receiver with the information called almanac (a set of values ​​with 5 orbital elements ), parameters that are transmitted by the satellites themselves. The collection of the calendars of the entire constellation is completed every 12-20 minutes and stored in the GPS receiver.
The information is useful to the GPS receiver to determine its position is called ephemeris. In this case each satellite broadcasts its own ephemeris, which includes the health of the satellite (whether or not to be considered for taking the position), its position in space, its atomic time doppler information, etc. ..

The GPS receiver uses the information sent by the satellites (time at which broadcast signals, locating them) and tries to synchronize its internal clock with the atomic clock that have satellites. Synchronization is a process of trial and error a portable receiver that occurs once every second. Once synchronized the clock, you can determine its distance to the satellites, and uses that information to calculate your position on earth.

Each satellite indicates that the receiver is at a point on the surface of the sphere, centered on the satellite radio and the total distance to the receiver.

Getting information from two satellites indicates that the receiver is on the circle that results when the two spheres intersect.

If we acquire the same information from a third satellite we noticed that the new sphere intersects the circumference just above two points. One of them can be ruled out because it offers an absurd position. In this way we would have 3D position. However, since the clock incorporating GPS receivers are not synchronized with atomic clocks in GPS satellites, the two fixed points are not accurate.

Taking information from a fourth satellite, we eliminate the inconvenience of the lack of synchronization between the clocks of the GPS receiver and satellite clocks.And at this point that the GPS receiver can determine a precise 3D position ( latitude , longitude and altitude ). Not being synchronized clocks between the receiver and the satellites, the intersection of four spheres centered on these satellites is a small amount rather than a point. The correction is to adjust the receiver time so that this volume becomes a point.