|Global positioning System (GPS)
Global positioning System (GPS) technology has changed the way people can find their way around the Earth, GPS, which is a satellite-based navigation system is able to show the exact position on the Earth anytime, in any weather, anywhere.
GPS navigation is an all weather worldwide precise positioning system, which Was funded and controlled by the U. S. Department of Defence (DOD), in 1980s GPS has became a fully operating even for civil use. GPS has a very high positioning accuracy, and since no telecom operator owns it, GPS is publicly available to every one; moreover there is no subscription fees or set up charge to use GPS. 
GPS is using 24 satellites, which are constantly moving with speed of 7,000 miles an hour making two complete orbits in less than 24 hours, these satellites are continuously monitored by a ground stations located worldwide. The satellites transmit specially coded signals that can be processed in GPS receiver, enabling the receiver to compute position, velocity and time.
The accuracy of GPS receivers is about 15 meters on average, on the other hand the accuracy of three to five meters can be obtained by using the differential GPS (DGPS), which consist of network of towers that receive the GPS signals then correct them and transmit the correct signals by using beacon transmitters. [John]
GPS consist of 3 parts: the space segment, the user segment, and the control segment
The Space Segment of the system consists of the 24 GPS satellites circle the earth twice a day, the satellite orbits repeat almost the same ground track (the satellites repeat the same track and configuration over any point approximately each 24 hours), theses satellites transmit coded signals information from the space to the earth. 
The Control Segment consists of a system of tracking stations located worldwide, these monitor stations measure the signals transmitted from the satellites. 
The GPS User Segment consists of the GPS receivers and the users. The GPS receivers convert the signals transmitted from the satellites into position, velocity, and time. Four GPS satellites are required to compute the user position in three dimensions (X, Y, and Z position) and the Time. GPS receivers are quite useful that can be used for navigation, positioning, time dissemination, and many other researches. 
How GPS Works
The GPS system consists of satellites and receivers; each satellite transmits two low power radio signals, L1 with a frequency of 1575.42MHz for civilian GPS use, and L2. These signals can travel through clouds, glass, and plastic but can’t pass through solid objects such as buildings and mountains.
The satellites are synchronized to coded signal information to the earth, a GPS signal contains three different bits of information, the pseudorandom code, which is an ID code that identify which satellite is transmitting the information, the ephemeris data, which contains information about the satellite’s status and current date and time, and almanac data, which tells the receiver where each GPS satellite should be at anytime throughout the day.
The GPS receiver will intercept the satellite’s signals information, and then compute how far away is the satellite by measuring the time difference between the transition time and receiving time multiplied by the speed of light. Using distance measurement from a few more satellites enable the receiver to determine the users position.
To measure user’s 3D position( latitude, longitude, and altitude), the GPS receiver must measure the time it took for the signals from at least four separate satellites to get to the receiver. After calculating the user’s position, the GPS unit can calculate other information such as speed, track, trip distance, sunrise time, sunset time, and more. 
-Peter H. Dana, Peter H. Dana University of Texas at Austin, 1994
- “Welcome to GPS: Has anyone ever asked you what career you wanted to pursue? ” Aerospace Corporation, 1995.
- John Aa. Sørensen ,”Global and Mobile Application: Mobile Positioning Technique” ,IT University of Copenhagen, 2003.