The GPS system (originally named Navstar) used in the United States is a satellite-based radionavigation system owned and operated by the
government and consists of a constellation of 31 satellites. A minimum of 24 satellites are required with a maximum of 32.
The satellites orbit the Earth in a MEO (medium earth orbit) at an altitude of approximately 12,550 miles. The satellites orbit the Earth
twice a day. The satellites are in six equally spaced orbital planes that surround the Earth. Each of these orbital planes contain four
slots occupied by baseline satellites. This ensures that users any where on the planet can view at least four satellites.
GPS receivers use signals from at least 4 satellites to calculate the position of the receiver. The receivers use two basic mathematical
concepts to calculate their position.
The two concepts are known as trilateration and the second is the relationship between distance traveled, speed of travel and the amount
of time spent travelling. This second concept may be expressed as the formula.
Trilateration finds the receiver’s position on the Earth by knowing the location of the GPS satellites and the distance of them from the
receiver. To measure the distance between the satellites and receiver the system uses the second concept relating distance, rate, and time.
The rate is how fast the radio signals travel. Radio signals travel at the speed of light, approximately 186,000 miles per second. We can
calculate time by how long it takes for the radio signals to travel from the satellites to the receiver. Since we know the rate and time,
we can calculate the distance. When we have the distance from at least 3 satellites, we can determine a three-dimensional position on the
surface.
There is an excellent video on YouTube from SparkFun that demonstrates how GPS works. You can find it below.
OTHER POSITIONING SYSTEMS
In addition to the US-owned and operated navigational system there are other systems run by other countries that provide position
information. They include: