Global Positioning 

The global positioning system, GPS, is a fully functionally navigation system that uses satellites to determine location. The system includes a total of 27 satellites that constantly orbit the Earth. Of these 27 satellites, only 24 are in operation. The other three are contingencies in the event that one fails. The system was developed by the U.S. military and initially implemented as a military navigation tool. Not long after its implementation, the global positioning system was opened to the rest of the world.

Each of the satellites makes two complete rotations around the globe daily. At about 12,000 miles above the Earth, at least four of these satellites can be “seen” by a GPS receiver at any time of the day, anywhere on the Earth. By locating these four satellites, a GPS receiver can determine the distance to each of them. Then, based on the distance to these satellites, the receiver can figure out its own location.

The way the GPS receiver determines the distance to a satellite is a fairly complex process. The simplest explanation is that the receiver uses the amount of time it takes to receive a signal from a particular satellite to determine its distance. Since the satellite is at least 12,000 miles away from the receiver, there is naturally some delay in the time that signals are transmitted and received. With this delay in mind, the GPS receiver compares the time the signal was transmitted by a satellite to the time the signal is received. The delay in the two times is the signal’s travel time. This signal travel time is multiplied by the speed of light to yield the distance the signal traveled – the distance of the satellite.

The receiver and the satellite clocks must be perfectly synchronized for the measurement to be correct. The only way to ensure that all the clocks are synchronized is to use atomic clocks in all the satellites and the receivers. Considering the cost of a single atomic clock, global positioning receivers would be too costly for the average consumer to use. To solve this problem, each of the satellites is equipped with an atomic clock, even though they are expensive. The receivers, on the other hand, have ordinary clocks that are constantly being reset. To ensure that it has the accurate time, the GPS receiver uses the time of the signals coming from the satellites to judge whether its own time is accurate.

Solar energy powers the GPS satellites. In case there is a solar eclipse and the satellites are not able to receive any solar power, they have backup batteries that will keep them running. The satellites are also equipped with small rocket boosters that keep them flying.

The signals transmitted by the GPS satellites can easily pass through clouds, glass, and plastic. However, they cannot pass through many solid objects like mountains and buildings. Each signal includes vital information: a code that identifies the satellite, data about where each satellite should be located at a point in time during the day, the health of the satellite, and the current date and time. The current date and time is used for determining the satellites’ position.

While the global position system is a very sophisticated system, it is still prone to errors. As the signal passes through the atmosphere, it slows down. The GPS system attempts to calculate the amount of this delay and account for it. Signals can sometimes be reflected off objects; thereby increasing the travel time and causing accuracy errors. Since the receivers do not include atomic clocks, there is a slight chance that timing errors can occur.