How GPS works
The Global Positioning System
consists of a set of 24 operational satellites and land-based control stations. The system was designed for and is operated by the U. S. military, owned by the US Department of Defense (DoD). But it is available on a worldwide basis to all civil GPS users with no direct charge.
GPS determines distance between a GPS satellite and a GPS receiver by measuring the amount of time
it takes a radio signal to travel from the satellite to the receiver. Radio waves travel at the speed of light, which is about 300,000 kilometer per second. So, if the amount of time it takes for the signal to travel from the satellite to the receiver is known, the distance from the satellite to the receiver (distance = speed x time) can be determined. If the exact time when the signal was transmitted and the exact time when it was received are known, the signal's travel time can be determined.
In order to do this, the satellites and the receivers use very accurate clocks which are synchronized so that they generate the same code at exactly the same time. The code received
from the satellite can be compared with the code generated by the receiver. By comparing the codes, the time difference between when the satellite generated the code and when the receiver
generated the code can be determined. This interval is the travel time of the code. Multiplying this travel time, in seconds, by 300,000 kilometer per second gives the distance from the receiver
position to the satellite in miles.
Each satellite transmits a coded signal on two carrier frequencies that says, amongst other things,
where it is in the sky. In its simplest form a GPS receiver receives the signal from the GPS satellite and uses the code to determine its distance from the satellite. If the distance to four satellites
can be measured at once then the receiver can calculate its own position in real time.
A position calculated in this way is accurate to better than 15 metres. Do you know the term DOP?
GPS System Segments
The GPS consists of three major segments: