# GPS, When The Satellite Tells Us Where We Are

Satellites orbit the Earth by emitting signals in the form of microwaves. Once on Earth, they are picked up by receivers which, by integrating the information derived from at least four satellites, determine their position, speed, and time on the Earth’s surface.

The now familiar car satellite navigator can determine its position without transmitting any signals by calculating the delay between the signals coming from different satellites. Let’s imagine we are inside a dark corridor with two people talking at its ends. As we move inside the dark corridor, we will hear the voice of the person we are approaching louder and the voice of the person we are moving away from, and in this way, we will be able to get an idea of ​​how far we have come.

The satellite system makes a similar calculation, but it needs to pick up the signal of at least four satellites to determine the three spatial coordinates (x, y, z) and the

By evaluating the transmission delay of the signals received from the various satellites, the navigator can determine its position and time: one satellite allows you to establish the time difference, and the other three the spatial coordinates. However, since both the satellite and the navigator change position continuously and do not have a constant distance between them, to calculate their spatial coordinates, the navigator must know the instant of departure of the different signals and, therefore, the fundamental condition for the functioning of the whole system is that the various satellites are perfectly synchronized.

For this reason, there are atomic clocks inside them. The computation system for integrating this information is called trilateration. Each satellite completes a full orbit around the Earth twice a day. The satellites move on six different orbital planes, spaced about 60 ° from each other, to cover the entire Earth’s surface, except the polar areas (in fact, they are inclined at 55 ° C concerning the equator).

The control segment comprises the tracking stations and the data center. The rescue stations refer to the space segment, i.e., the one made up of satellites: a system of control stations located in various positions on Earth (the main one is in Colorado) receives information from the satellites, calculates their position, and sends the data to the satellites themselves.

### Some applications of GPS

Apart from the ability to accurately determine the location, GPS has several specific applications:
– it provides a universal reference for the measurement of time, thanks to the presence on each satellite of the numerous atomic clocks, used for the synchronization of different systems and as a reference for commercial operations, for example, to synchronize economic transactions on the various computers of a bank;
– reduced the costs of some space operations, thanks, for example, to the replacement of expensive sensors with cheap GPS antennas and the use of this technology to monitor and regulate space traffic;
– it is fundamental in aircraft and naval systems;

– it is also widely used in environmental control and monitoring for the agricultural industry, thanks to the possibility of carrying out rapid and precise documentation and being able to orient oneself even in case of bad weather or in the dark; it makes it possible to monitor animals at risk of extinction and migrations; finally, it allowed the use of automatic systems for the distribution of substances dedicated to agriculture, limiting human exposure to toxic agents;

– it is also used as an anti-theft system, for example, to locate cars.

### Other satellite tracking systems

GPS is a system entirely controlled by the United States Department of Defense. In addition, there is another global positioning system, the Russian one, called GLONASS, managed by the Russian army. It is composed of 24 satellites on three orbital planes and has been active since 1995, despite its ups and downs.
Europe is working on the development of a new positioning system: it is called Galileo and should go into operation in 2013, with 30 satellites arranged in three orbits and one greater accuracy and coverage than the current one.

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