« We will continue to provide the service of GPS standard positioning for peaceful purposes of civil, commercial and scientific use on a global scale constantly and without imposing direct charges to users. »

Bill Clinton, on March 28, 1996.

What is GPS ?

GPS is a satellite-based navigation system designed to instantly provide information about position, speed and time pratically anywhere on Earth, any time and by any weather conditions.

The designation Navstar GPS means: navigation system by timing and ranking Global Positioning system.

The GPS allows instant positioning with a position which ranges from a hundred meters to a few meters (95% of the cases). Some equipment allow, after delayed corrections of measurements, to attain precisions of the order of centimetre. Of course It is clear that the best performance requires substantial infrastructure and generates higher costs.

A bit of history !

The stages of the development of the GPS:
- 1973, program start under the aegis of the Department of defense (DoD) USA.
- 1974-85, testing of the first receivers, launches the first satellites of the "block I".
- 1983, crash with the Boeing - 747 of Korean Airlines (KAL 007) shot down by a Soviet Mig. The navigation system opens to civilian applications.
- 1986, further development of the system with the launches of satellites of the "block II".
- 1990, on March 25, put according to the SA (Selective Availability).
- 1991, the SA is temporarily disabled during the first Gulf war resulting in a fantastic promotion of GPS.
- 1995, 17 July, the USA announced that GPS system is fully operational.
- 1997, launches of satellites of the "block IIR.
- 2000, may 1, the SA is disabled.

How do GPS work ?

The GPS system has three main components and is based on a network of satellites that emit radio signals :

i) The space component made up (nominally) of 24 satellites in orbit around the Earth (above 20000 km). 21 of these are navigation satellites and 3 are backup satellites. orbits are inclined at 55 ° compared to the plan of Ecuador and their period is approximately 12 hours. This configuration allows a tuner to the surface or above the Earth to receive signals from five to eight satellites, 24 hours per day. The satellites continuously transmit their position and time data, which are received and processed by GPS receivers to determine the three-dimensional position of the user (latitude, longitude, altitude), velocity and time.

ii) The control component consisting of a main control station located in Colorado Springs, five tracking stations and three Earth-based antennas distributed around the land. The tracking stations monitor all GPS satellites in view and gather the data contained in their messages. These remote stations are able to pursue and monitor the position of each of the GPS satellites.The tracking stations transmit the data obtained from satellites to the main control station that then calculates the very precise orbits of the satellites. These data are then formatted as updated navigational messages for each satellite.Updated data are then sent on an uplink to each satellite from Earth antennas. These antennas are used also to transmit and receive signals satellite monitoring and control.

iii) The user component that comprises the receivers.

Satellites emit so-called "carriers" radio waves on two different frequencies L1 at 1.6 GHz and L2 at 1.2 GHz, with wavelengths of 19 and 24 cm, respectively. Two codes are derived from these signals, they are called pseudo-random codes. This is the code p (precise or protected code) on frequencies L1 and L2, usually reserved for the US Army (wavelength of 30 m), It was replaced by a secret code named Y (in 1994) and, the code C/A (coarse/acquisition code) on frequency L1 (wavelength 300 m). The frequency of carrier waves and the sequence of codes are governed by atomic clocks on board satellites. Each of the satellites emits its own pseudo random codes C/A and p. The GPS receiver has in memory the complete list of codes to identify the satellites in which it receives signals. The maximum accuracy that can be obtained by the C/a code (only accessible) is the order of the metre. Therefore, alongside these codes, GPS also use the phase of these radio waves to improve accuracy there are 2 modes of use of the GPS system :
- Absolute positioning or pseudo distance or autonomous mode: it is calculated by the user's receiver which measures the distance between it and the different satellites using the C/A or P codes. This positioning is not very precise, it varies from a few meters to 100 meters. Its main use is marine or terrestrial navigation. We're using the so-called first class GPs.
- Relative or differential positioning: the distance is calculated either from the codes or the phase simultaneously in two stations (receivers). A receiver is fixed, called the station, and the other is mobile.

The result is the vector between these two GPS. This mode allows to correct the sources of error in large part. The accuracy is so improved, reaching up to the subcentimeter. This positioning may be:
- At the delayed time (with a post-processing)
- Real-time: observations of station are sent by radio to the mobile receiver equipment to allow instant correction and accuracy on the ground.

It is used for the precise location and surveys

The signal propagates at the speed of light, is the equation: Distance = travel time x speed of light.

The GPS system subtlety is based on a system of perfectly synchronized atomic clocks and accurate are equipped with the satellite. To know the time taken for the signal to reach the receiver, shall determine when the signal has started to issue. To do this, each satellite generates its own code that must be recognised by the receiver.

It then calculates what is called the phasing-out, i.e. the difference between the signal emitted by the satellite and the same signal reproduced by the receiver. Thus the results of the calculations depend on the accuracy of the clock, since the code must be generated simultaneously by the receiver and the satellite. To compensate for clock offsets of GPS receivers, it is necessary to use the measurements of a fourth satellite to adjust the error of its clock.

How the error correction in relative mode is performed

The main sources of system-related errors are:

- Atmospheric layers (troposphere and ionosphere): they can cause errors on localization results; The radio signal can be delayed or accelerated when it passes through its layers.

- System errors such as satellite trajectory problems (Ephemeris errors) or uncorrected errors of the satellite clock.

- The multi-path , it is encountered in unclear contexts type forest or urban environment. As the trees or buildings are obstacles on the trajectory of the signal, the latter is absorbed, attenuated, reflected or refracted.

- The dilution of precision or DOP: it's the geometrical configuration or geometry formed by the satellites at the time of the transmission. The quality indicator is generally (for devices trimble) PDOP (Position dilution of position) for the position in 3 D. There are others: the HDOP, for the horizontal position, VDOP, for vertical position, TDOP, for time.