### Origins

The global positioning system is designed by the Department of Defense and the Department of Transportation of the United States of America. On April 27, 1995 the system, containing 24 operational satellites, was formally declared as able to meet the requirement of Full Operation Capability. Since then, the system has been taken into full use.

### How it works

The GPS satellite system is based on the concept of trilateration. The heart of a GPS receiver has the ability to find the satellite's distance from four (or more) GPS satellites. Once it determines its distance from the four satellites, the receiver can calculate its exact location and altitude on Earth. If the receiver can only find three satellites, then it can use an imaginary sphere to represent the Earth and can give you location information but no altitude information.

For a GPS receiver to find your location, it has to determine two things:

• The location of at least three satellites above you
• The distance between you and each of those satellites

GPS satellites send out radio signals that your GPS receiver can detect. Then the receiver measures the amount of time it takes for the signal to travel from the satellite to the receiver. The receiver will receive signals at least from four satellites. Since radio signals travel at the speed of light, we can figure out how far they've traveled by figuring out how long it took for them to arrive.

The receiver looks at all the signals that it receives and calculates exact time and the exact location simultaneously using a normal quartz clock. When measuring the distance to four located satellites, four spheres that are drawn should intersect at one point. Since the receiver makes all of its time and distance measurements, using the clock it is equipped with, the distances could be proportionally incorrect. However, the receiver can easily calculate exactly what distance adjustment will cause the four spheres to intersect at one point. This allows it to adjust its clock to adjust its measure of distance. For this reason, a GPS receiver actually measures extremely accurate time.

Information that GPS receivers can provide

1)            Coordinates information on electronic maps which is stored in memory

2)            Distance traveled

3)            Current speed and average speed

### Remarks

One problem with this method is the measure of speed. The Earth is not a vacuum, and its atmosphere slows down the transmission of the signal. A GPS receiver estimates the actual speed of the signal, using complex mathematical models of a wide range of atmospheric conditions. The satellites can also transmit additional information to the receiver.

The satellites send radio signals to GPS receivers so that the receivers can find out how far away each satellite is. Because the satellites are orbiting at a distance of 12,660 miles (20,370 km) overhead, the signals are fairly weak by the time they reach your receiver. This implies that receivers have to be in a fairly open area in order to work.