What GPS is
As it often happens with high-tech projects, initiators of development and implementation of GPS (Global Positioning System) were the military. Project of satellite network for determining the coordinates in real time-mode and in any spot on the Earth was named NAVSTAR (Navigation signal timing and ranging), whereas the abbreviation GPS appeared later when the system was used not only for the purposes of defence, but also for civilian purposes (Howell). Today, GPS “provides accurate, continuous, world-wide, three-dimensional position and velocity information to users with the appropriate receiving equipment” (Kaplan, 3).
Short history of GPS
First steps of deploying navigation network were undertaken in the mid-seventies, the commercial operation of the system in the way we know today began in 1995. Currently, there are 31 satellites that are evenly distributed at the orbits of approximately 20,200 km (it needs to be mentioned that, for full functionality, it is sufficient to have 24 satellites) (“Space Segment”).
The year of birth of the technology, without which it is hard to imagine the modern world, is considered to be 1978 – when the first satellite was put into orbit. This event marked the beginning of what is now called GPS, though the system of global positioning started fully operative only in December 1993 (“NAVSTAR GPS - Summary”).
Truly key moment in the history of the GPS appeared to be President Bill Clinton’s decision to abolish on May 1, 2000 the regime of so-called Selective Availability – an error, artificially introduced into the satellite signals to make them distort the data transmitted to the civilian GPS-receivers. From May 1 year 2000, any regular freely purchased GPS terminal received the precision of navigation data “improved from 330 feet to 66 feet” (Deffree).
Principle of GPS operation
In the basis of operation of the GPS is the idea of determining the coordinates of the location of objects on the ground, based on the calculation of distances to a group of satellites in the outer space. The satellites, it shall be noticed, act as the precisely coordinated starting points.
The primary idea of determining the coordinates of the GPS-receiver is calculation of the distance from it to several satellites. Global Position System as a service is composed of three constituents: satellites orbiting the Earth, control stations on the Earth, and the GPS receivers owned by users. GPS satellites transmit signals from the outer space that are perceived and identified by the GPS receivers. Every GPS receiver then estimates three-dimensional location (latitude, longitude, and altitude) and the current time. “For the determination of its position on earth, the GPS receiver compares the time when the signal was sent by the satellite with the time the signal was received” (“Position Determination with GPS”).
Purposes of GPS
A user with a GPS receiver can get an accurate location, and can easily determine where to go further in accordance with a preliminarily chosen route. GPS has become a pillar of transportation systems worldwide by providing navigation for aviation, ground transportation, and maritime traffic. GPS is a must nowadays during disasters and calamities; it helps emergency services to determine the location and timing capabilities for the rescue missions. Everyday activities such as banking, mobile network services, control of electric power transmission and many others became much more convenient and hard to imagine without GPS that provides the accurate timing and precisely locates objects when it is needed for concrete services. Farmers, inspectors, geologists and a countless number of other experts do their job more efficiently, safely, economically with the help of GPS signals.
Errors of location positioning
It is worth noting that a certain miscalculation often happens (because of inaccurate synchronization of the clocks of a receiver and a satellite, due to dependence of the speed of light on the state of the atmosphere, etc.). Therefore, to determine the three-dimensional coordinates of the receiver not three, but at least four satellites are involved.
Upon receiving the signal from four (or more) satellites, the receiver searches the corresponding point of intersection of the spheres. If there is no such a point, the processor of the GPS receiver starts adjusting its clock until it gets all the spheres intersected at one point.
A mode of the so-called differential correction (DGPS – Differential GPS) allows eliminating possibility of inaccuracy in measuring the coordinates. “The GDGPS System provides decimeter (10 cm) positioning accuracy and sub-nanosecond time transfer accuracy anywhere in the world, on the ground, in the air, and in space, independent of local infrastructure” (“The Global Differential GPS System”). Differential mode is represented with the use of two receivers – the first one, “base”, is motionlessly located in a certain point, and the second is mobile. Data received by the base receiver are used for correcting information gathered by a mobile receiver. Usually a base receiver is a high-end device belonging to a company specializing in providing services of navigation.
It should be noted that the main clients of differential correction are geodesic and topographic services – for private user DGPS is out of interest because of the high cost and because of cumbersome equipment. And it is unlikely to happen when it is necessary to know the precise geographical coordinates with an accuracy of 10 cm.
Every year, the GPS technology is gaining larger recognition among people of different occupations, ranging from professional travellers and sailors at sea to people leading an active life. And there is an explanation:
At first, it is its price. Today, GPS system is available practically for everyone even for persons having low incomes.
Secondly, operation of GPS- navigation is free.
At third, it is the massive production of programs and devices related to GPS for different categories of users. These are smartphones, tablet computers with GPS modules, car GPS navigation, etc. Anyone who needs to know his location or get directly to the intended destination point, who is interested in his speed and is eager to know the arrival time to the concrete place can easily find it out by taking advantage of GPS.
Defdree, Suzanne. Accurate GPS access available to civilians in US, May 2, 2000. EDN Network. 02 May. 2014. Web. 16 Feb. 2014. http://www.edn.com/electronics-blogs/edn-moments/4413261/Accurate-GPS-access-available-to-civilians-in-US--May-2--2000
Howell, Elizabeth. Navstar: GPS Satellite Network. Space.com. 14 Feb. 2014. Web. 16 Feb. 2014. http://www.space.com/19794-navstar.html
Kaplan, D. Elliott, and Hegarty, J. Christopher. Understanding GPS: Principles and Applications. Norwood, MA: Artech House, 2006. Print.
“NAVSTAR GPS - Summary”. Space and Tech. n.d. Web. 16 Feb. 2014. http://www.spaceandtech.com/spacedata/constellations/navstar-gps_consum.shtml
“Position Determination with GPS”. Kowoma.de. 19 Apr. 2009. Web. 17 Feb. 2014. http://www.kowoma.de/en/gps/positioning.htm
“Space Segment” GPS.gov. 2 Jan. 2014. Web. 16 Feb. 2014. http://www.gps.gov/systems/gps/space
“The Global Differential GPS System”. Jet Propulsion Laboratory. n.d. Web. 16 Feb. 2014. http://www.gdgps.net