The Global Positioning System (GPS) was originally developed by the US Military who launched satellites (the first satellite was launched in 1978) into orbit. The system consists of a network of between 24 and 32 satellites orbiting the earth at about 7000 mph, in a precise manner, two times every day.

In the early days only the military could track the signals but in the 1980's it was opened up to the general public.

Before we get into the specifics it is important to realize that the use of the satellite network is free, there is no subscription required.

The other benefit is the system is essentially unaffected by weather and it is accurate anywhere in the world. Some might recall the Loran system which was popular with boaters before GPS became popular.

Loran was capable of providing a very accurate position but only in limited areas where the transmission of signals was sufficient and provided the weather was not adverse.

GPS transmission satellites orbit the earth in a very precise and constant manner. As they orbit 12,000 miles above, they transmit radio signals 24 hours a day, every day. The receivers we use simply receive the transmitted signals and compare the time the signal was transmitted against the time it was received. Knowing the signals travel at a fixed rate of speed makes it easy to calculate how far the satellite and the receiver are from each other.

If this process were done with only one satellite the result would not be useful for us since it would only indicate a possible arc of a position. To provide accuracy the receiver has to receive signals from a minimum of 3 satellites.

In actual use, the receivers typically track the signals from many more satellites.
In order to provide data like speed, direction, distance traveled, etc. the receiver repeats the position calculation on a continual basis and thus updates the position and other data. The signal transmitted includes the satellites ID, the satellites position at any given time and the date and time.

Accuracy of the receivers has improved with technology advances to an average of about 15 meters. In real practice it is common to see accuracy within a meter or two, at least at times.

When GPS was opened up for public use the signal was intentionally degraded by the Government which was referred to as Selective Availability (SA). SA has been discontinued so civilians now enjoy full accuracy of the system.

Technology referred to as Wide Area Augmentation System (WAAS) available with some receivers improves accuracy to about 5 meters. Receivers equipped with Differential technology apply corrections to the data based on other known references.

The end result is accuracy and functionality of GPS receivers to the point we all can rely upon. As a result we now see and enjoy GPS watches, auto and boat GPS navigation systems, GPS cell phones.