What is GPS?

In this article, you will find out how the GPS works.

The Global Positioning System (GPS) was a satellite-based positioning system created by the Americans during the Cold War. Operational since 1995, its precision for civilians was deliberately degraded around a hundred meters.

It was not until May 2000 that President Clinton decided to give free access to the maximum accuracy of 5 to 10 meters, and thus paved the way for the commercialization of many types of GPS receivers for a wide variety of uses, including our GPS trackers.

What’S a GPS for?

Most often used as a guide, GPS allows you to locate yourself in space to find your way from point A to point B.

It is also used in agriculture to optimize the number of passages during the application of fertilizer for example, which avoids having to iron twice in the same place (saving of products, time and fuel). GPS operation, in this case, requires ground stations to improve accuracy.

In the maritime or aviation field, it also allows, with other systems, to guide the aircraft. Today, the scope of the geolocation satellite system has exploded.

How does GPS work?

The GPS constellation consists of at least 24 satellites, located at an altitude of 20184 km, and circling the Earth in 12 hours. Five ground stations manage the perfect alignment of the satellites and define possible errors.

To define a position in space, three coordinates (x, y, and z). GPS data also includes a fourth variable: time. It takes four satellites with atomic clocks to get a position, as well as a GPS receiver to decode and calculate the signals received. The precision of the best atomic clocks is measured in billionths of a second, that is, it would take 160 million years to lose a second of precision.

The operating principle of the American positioning system is based on triangulation.

First of all, we need to know that each satellite emits an electromagnetic wave of known velocity. This wave, which carries a “pseudo-random” code, is emitted at a specific time. The receiver then calculates the transmission time, that is, the time required for its signal (which carries the same pseudo-random code) to be in phase with the signal emitted by the satellite. By multiplying this time by the speed, it thus obtains the distance which separates it from the satellite.

At the end of this calculation, the receiver has first information: it is on a circle centered on the satellite. By repeating this procedure with a second satellite, it can again be placed on a second circle centered on the second satellite. By repeating the operation a third time and looking for the zone of intersection between these three circles, we obtain the position on the Earth. The fourth satellite determines the time difference between the GPS receiver and the exact time provided by the satellites, to refine the position. The greater the number of satellites captured, the greater the accuracy.

The extraordinary precision of atomic clocks is essential because an error of one-thousandth of a second in the transmission time calculation would result in a positioning error of 300 km!

In practice, the calculation of the distance requires precise knowledge of the position of the satellites. For this purpose, each satellite regularly sends its ideal position to a control station capable of determining the positioning error. This correction is then sent back to the satellite, which can transmit the information to the receiver, who can take it into account in its calculations.

Limits of use

The signal transmitted by the satellites is quite weak and does not cross buildings or metal.

Also, many factors can degrade the signal and thus the accuracy: wet weather, the presence of buildings or mountains, a forest, solar flares.

GPS positioning is, therefore, not infallible, but it is an excellent tool that provides us with many services, and many of us would find it difficult to do without.