The Global Positioning System (GPS) is a satellite navigation system placed into orbit by the U.S department of defense. It is made up of 27 satellites. 24 satellites are active while the three remaining ones are backups. It is a system that provides location, time and weather changes anywhere in the world, 24 hours a day. These satellites transmit signals to the ground. There are no subscription fees or service charges. It was first started by the U.S Government of defense for military purposes. In the 1980s, it was made available for the civilian use and could be accessed by anyone with a GPS receiver. It works in any weather condition, anywhere in the world (Parkinson 1996).
A lot of research has been done in the past, and people have gone to extreme measures so that they do not get lost or for tracking the weather patterns. The GPS system was established in 1973. This was established to beat the defaults of the previous navigation systems. It was created and established by the U.S. Department of Defense (DOD). A lot of research was done, and by 1994 the GPS was fully functional (Agnew, 2007).
GPS provides specially coded satellite signals that can be processed in a GPS receiver. This has enabled in the computing of position, time and velocity. With the advancing technology, it has made it clear that the GPS system has to be modernized too. In 2000, the congress and the office of the president authorized the modernization of the GPS system also referred to as GPS III. The GPS network we all use is called Navstar.
The design of the GPS is partially based on the navigation systems used in the World War II. In 1960, the first successful test of the GPS was done by the United States Navy. It operated on a constellation of five satellites and could provide a navigational fix approximately once per hour. Timation satellite was developed in 1970 that proved the ability to place accurate clocks in space (Weiffenbach, 1997). Omega Navigation Systems was also developed in the 1970s. This was based on the comparison of signal transmission from pairs of stations. This was the first worldwide radio navigation system.
Putting up the GPS system required billions of dollars. The cost in research, development, deployment and operation for a constellation satellites is enormous (Agnew, 2007). The nuclear threat against the United States justified the GPS project being funded. This was a well invested project as with time and technological advances, it has become a vital structure for both the military and civilian use (Parkinson 1996).
A GPS receiver calculates its position by precisely timing the signals by GPS satellites, which are located on the earth’s orbit. Each satellite transmits the time of the message and the position of the satellite when the message was transmitted. The speed of light is used to compute the distance of each message and the time it was received (Weiffenbach, 1997). It can ascertain the latitude and longitude of a GPS receiver on a device. Most GPS systems show information such as direction and speed which are calculated from the positions of the satellites.
Over the years, the GPS system has been modernized, with the advancing technology, to meet the overall and current needs. The latest GPS system consists of three major segments. These include space segment (SS), control segment (CS) and a user segment (US). The development, maintenance and operation of space and segments are done by the U.S. Air Force (Teven 2010).
The GPS satellites transmit signals from space, and every GPS receiver uses the three dimensional location (latitude, longitude and altitude) and also the current time. The control segment consists of a master control station, an alternate; host dedicated and shared ground antennas (Parkinson 1996). GPS operates well on a clear view of the sky where there are no obstructions from tall buildings or forested areas. Atomic clocks are located on each and every GPS satellite. They provide very accurate time reference (Teven 2010).
All GPS satellites synchronize operations so that the repeating signals are transmitted at the same time. Some satellites are farther away in the sky, so signals are received by the GPS receiver at different intervals. The ground stations are used to track each satellites orbit (Agnew, 2007). The GPS receiver determines the location of the satellite.
This is because the information is included in satellite transmission. By approximation of how far-off a satellite is, the receiver additionally knows it is situated someplace on the surface of an imaginary sphere centered at the satellite. This then determines the sizes of several spheres, one for each satellite. Receiver is always located where these spheres intersect (Teven 2010)
There are at least 24 operational GPS satellites, orbit with a period of 12hours (two orbits per day) at a height of about 11,500 miles travelling at 9,000mph (3.9km/s or 14000kph). The GPS signal consists of three bits of information- pseudo random code, almanac data and ephemeris data (Agnew, 2007). The pseudo random code identifies the satellite that is transmitting information. Almanac data describes the orbital courses of the satellites and determines which satellite it expects to see in the local sky. Ephemeris data informs the GPS receiver where every GPS satellite ought to be at all times during the day (Weiffenbach, 1997).
Some of the uses of GPS systems are surveying, telematics, GPS air tracking, car tracking, Robotics, Geotagging, GPS tours, recreation, clock synchronization, cellular telephony, navigation and many other systems. GPS have become a part of our lives, and everyday activities somehow co-relate to GPS in one way or another. Each satellite is built to last for at least 10 or more years, weighs around 2,000 pounds and is about 17 feet across with the solar panels extended (Agnew, 2007).
The GPS system was initially devised for the military, but nowadays the civilians also have access. They only have access to the Acquisition signal and not the precision signal that is only used by the military. Manufacturers/ devices provide a feature known as Quickfix. To be able to understand the concept of Quickfix, you need to calculate your GPS chipset needs to find at least 4 satellites with a strong signal of 28dBHz or more. It must maintain at least a minute so as to be able to download the data and calculate your position day (Weiffenbach, 1997).
If the signal is below 28dBHz, then the transmission starts from the beginning yet again to be able to calculate your position (Agnew, 2007). The GPS system has contributed largely to our day to day lives. The defense system can be able to detect the enemy’s position and prepare early in advance. The navigation system has also greatly been aided by the GPS system to be able to know the direction it is taking. Theft has also reduced due to the GPS car tracking. GPS is a part of our lives and easened communication all over the world (Agnew, 2007).

References
Agnew, D.C. (2007). Finding the repeat times of the GPS constellation” GPS solutions. London: Routledge
Parkinson Spilker (1996). The global positioning system. American Institute of Aeronautics and Astronautics
Teven Johnson (2010), where good ideas come from, the natural history of innovation, New York: Riverhead Books. Weiffenbach, George C. (1997). "Genesis of Satellite Navigation". John Hopkins APL Technical, PP.178-181