Whenever we hear the term ?GPS?, what would instantly come to our minds is a digital device with maps and other features that tells us our exact location on this planet. But do you know that such a device is just one part of GPS? The device is called a GPS receiver, and while it is indeed an important GPS device, it is just a part of a very large and complex system that allows us to measure our exact location on earth.

The three-lettered term ?GPS? stands for Global Positioning System. It is a satellite based navigation system that has the ability to measure an object's precise location on earth. The system was developed by the United States? Department of Defense (DOD) and was officially called in the US military as the NAVSTAR GPR (Navigation Signal Timing and Ranging Global Positioning System). It was intended to be used for military operations only but has been opened for civilian use since the 1980s.

The Global Positioning System is a very huge and complex system that can be divided into three segments: space, control, and user. The ?space? segment of the GPS refers to its network of satellites that transmit the signals used for location identification. The ?control? segment, on the other hand, refers to the various stations on earth that maintain and control the system. The ?user? segment refers to the GPS receivers?the actual unit that we often mistake as the system.

SPACE

The ?space? segment of the GPS is a constellation of 27 satellites that orbits the earth at some 12,000 miles above its surface. 24 of these satellites are in operation while the other 3 are on ?standby? mode and will be used in case one fails. Each of the satellites orbits the earth twice in less than 24 hours, traveling at a speed of approximately 7,000 miles per hour.

Each of the satellite used in the GPS constellation weighs about 3,000 to 4,000 pounds. All of these satellites are solar-powered, although all have backup batteries onboard to be used in the absence of solar power (i.e., solar eclipse). Small rocket boosters are also used to keep the satellites flying on the correct path.

The signals being transmitted by the GPS satellites are low powered radio signals. These signals contain three different bits of information'a pseudorandom code, ephemeris data, and almanac data?that are deciphered by the GPS receiver to obtain a precise measure of its location. The pseudorandom code tells the receiver which satellite is transmitting the signal while the ephemeris data provides information about the satellite's position. The almanac data, on the other hand, contains information like the status of the satellite, current date and time.

CONTROL

The ?control? segment of the GPS refers to the various ground stations run by the US Department of Defense that controls various aspects of the system. In particular, these stations monitors the flight of the GPS satellites, synchronizes the satellite's onboard atomic clocks, and uploads the data to be transmitted by the satellites.

USER

The ?user? segment of the GPS refers to the GPS receivers, whether they are used for military or civilian purposes. The GPS receiver is basically an electronic device that picks up the signal transmitted by the satellites and uses the information on those signals to figure out its precise location. Now that sounds simple! In reality, however, this function is a lot more complex.

The GPS receiver actually needs two things for it to figure out its location. First, it needs information about the location of at least three GPS satellites. Then, it would need to measure the distance between it and each of the three GPS satellites. The first information can be deciphered from the signal transmitted by the satellites while the second can be obtained by measuring the speed at which the signal was received.

ALL IN ONE SYSTEM

Combining the space, control, and user segments, we have a Global Positioning System that can tell us about our precise location here on earth.

You see, the GPS is not just the GPS receiver alone. It is a very big system, and a very complex one at that. But because every other part of the system is well maintained by the Department of Defense, we will never have to worry about those parts. All we have to do is to secure a GPS receiver and in an instant, we'll receive information about our current location and other things that the receiver may want us to know.