The Global Positioning System (GPS) is a navigation system owned by the United States government that's made up of three main segments²:
- The Space Segment: This is a navigation system of at least 31 satellites, 24 (or more) of which are usually in flight and operational. These satellites fly within Earth's orbit at an altitude of 12,550 miles.
- The Control Segment: This is an international network of control stations that track, monitor, and maintain the satellites in orbit.
- The User Segment: This is for civilians and our GPS devices, also known as GPS receivers since they receive signals from the satellites in orbit to pinpoint our locations.
A device with built-in GPS – whether it’s a dedicated in-car GPS navigation unit or a smartphone – only acts as a GPS receiver. A device with GPS isn’t actually “contacting” satellites to determine its location. Instead, it’s just listening for the radio signals that are being broadcast from these satellites all the time.
Once the receiver calculates its distance from four or more GPS satellites, it can figure out where you are. They can usually determine where you are within a few yards of your actual location. More high-tech receivers, though, can figure out where you are within a few inches!
As for the code used in GPS, there are two main types: the coarse/acquisition (C/A) code and the precision (P) code. The C/A code is freely available to the public, while the P-code is usually reserved for military applications. The P code is called the Precise code. It is a particular series of ones and zeroes generated at a rate of 10.23 million bits per second. Each GPS satellite is assigned a part of the P-code all its own and then repeats its portion every 7 days. This assignment of one particular week of the 37-week-long P-code to each satellite helps a GPS receiver distinguish one satellite’s transmission from another.
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