The Global Positioning System was devised in the 1970s by the U.S. Department of Defence to improve the accuracy of navigation by land, sea and air. It is based on signals emitted by a group of 24 satellites orbiting 20,200 kilometres above Earth. The signals are received simultaneously by a receiver, and this data determines the receiver's position any place on the planet, day or night, under any weather conditions. The operation may seem complex but it is not: each satellite gives its location (position and altitude) and the exact time of the start of transmission. The GPS receiver captures the signal and measures the interval between transmission and reception to determine the distance between the receiver and the satellite. By taking and processing signals from three satellites, the receiver can determine is position on the earth's surface. Later, with a signal from a fourth satellite, it establishes its altitude and the timing signal from the satellite's atomic clock.

 

Although initially the GPS was designed for military use, many civilian applications soon emerged. To make this possible each satellite emits two kinds of signal: an encrypted military one and another domestic one. The three basic uses of this system and a GPS receiver are to calculate one's position and then find it on a map, guide a user to designated spot, and finally, memorise that position in the receiver's memory so as to be able to go back to it. 

 

Multiple civilian applications
Thanks to this simple way of working, GPS soon began to have many civilian applications. The first and best-known so far have been navigation systems for cars. All a user has to do is punch the data corresponding to his or her destination into the receiver (it looks like a cassette player) and the receiver, either vocally or on a screen, will tell the driver which route to follow. It warns the driver in advance about turns to take and changes in direction. 

 

Other winners with this technology are fans of adventure sports. For mountain-climbers, cyclists, boaters, hunters, fishermen, drivers of 4x4 vehicles in rough terrain or simply people who like to take long hikes in the country, having a GPS receiver gives them greater security than a compass does. The GPS becomes a veritable road map, with the ability to show routes, time consumed and the way back in case you get lost, even in complete darkness. All you need is a GPS receiver. Shops specialising in adventure sports and computers sell models at prices ranging from 300 to 650 euros, depending on the features or kind of screen (size, colour or black and white, etc.) Their size is in some cases very similar to that of a mobile telephone, although in general they are about the size of a hand-held electronic agenda or PDA. 

 

New applications
Other equally interesting applications have arisen of late. In the business world, GPS is being used to manage and monitor fleets of cars, ships, trains or planes with the goal of improving transportation of merchandise and speed up services. One novel case is that of some winemakers. They incorporated a GPS receiver next to a scale that weighs the grapes produced on each plot of land. This data is used to devise a map that shows exactly how many grapes were produced in each piece of land, and locate plants that need fertiliser such as nitrogen or potassium. Also, to avoid car theft, several security companies have started selling tiny GPS systems that are hidden in the car so the owner can know where the vehicle is if it is stolen. 

 

Other examples are the receivers that some taxi drivers in large cities like Madrid have installed in their cars to be able to alert authorities in case of theft or kidnapping, or the receivers being used by Alzheimer patients in Barcelona so their families can know where they are at all times. The patients are given a small receiver the size of a pack of cigarettes that transmits its position periodically to a monitoring centre which determines the location of each patient. Another application of GPS which has stirred controversy is an anti-kidnapping chip which a couple in Britain implanted last year in their daughter's arm to be able to find her if she went missing. This device is based on GPS technology as well.
It is a 1.5 cm microchip implanted under the skin, and allows you to locate the carrier quickly through mobile phone lines or GPS. It only activates if it receives a search signal. The idea is to be located at all times.