# GPS operation and use: system and tools

## Gps stands for Global Positioning System (global localization system)

## Brief notes on the use of the Gps system.

The following notes are addressed to those who have never had the opportunity or the time to deepen the theory and the use of this technological system, to those who have experience in using GPS “road” but not “outdoor” and also to those who want a deepening not too technical, but that allows an organic vision of the system and its use.

The considerations are mainly addressed to hikers and cyclists, with the aim of enabling those who want to use a GSP receiver to make profitable and conscious use of it.

## The GPS system

## What a GPS receiver does

A GPS receiver is able to calculate and show on the display and/or record the latitude and longitude of the position where you are (to make the point) with an accuracy in non-professional civilian use of the order of 5-10 m. So exactly what a captain did during the navigation using the stars (then a clear sky), a good clock and a sextant, obtaining at the end, after several calculations and use of tables, the position with an error of the order of km. All functions of reception, signal processing, calculation of position are generally carried out in a small micro integrated circuit, which can then be mounted on various receiving equipment.

What is the use of Gps in outdoor use.

The Gps is a very useful system in the use of hiking, both because it allows to follow a pre-recorded route, including the path (track), points of interest (Poi, waypoints) and other information, even outside of the ‘roads’ coded, and to display and / or record a path that we ourselves are traveling. Particularly interesting is the use for the preparation of an excursion in a place that is completely unknown to us, especially if complex and/or articulated in more than one day. All this today is made easier by the use of the internet, which allows us to find tracks that can guide us to places of new frequentation (a track does not only mean positions, but also quotas, slopes and times).

## Therefore a Gps system

– before the excursion it is necessary to prepare and analyze, with the help of a computer and possibly some electronic maps, a route to be loaded in the GPS receiver (often the route is already available on the net and it is enough to download it)

– during the excursion, in addition to the natural vocation of allowing us to follow the predetermined track by displaying our position (also useful for communicating coordinates to any rescuers), provides important information about where we are (using a map if available with the receiver), time and distance traveled, height above sea level, (comparable if available with the barometric trend, speed and average).

– after the excursion we can check exactly where we passed through and get a lot of information about our excursion, such as the time taken, the average and maximum speed of travel, the graph of the gradient, the altitude reached, the duration of stops, etc.. The route can be viewed on applications available online, such as Google Earth, which provides us with detailed 3D satellite maps. We can then store our track, as well as share it with others, after having eventually cleaned and optimized it.

## How the GPS System Works

The GPS system operates on the basis of a very simple and well-established concept: it finds the position by means of a geometric trilateration.

Given three fixed points C1,C2,C3 and knowing the distance of a point from each of these fixed points (respectively R1,R2,R3), it is immediate to find the position of the point in question. Drawing the 3 circumferences of center C1,C2,C3 the intersection of these is precisely the point P sought, having distance R1,R2 and R3 from the centers C1,C2,C3 by construction.

Let us now think of having 3 positions in space S1,S2,S3 above the earth’s surface (corresponding to the centers C1,C2,C3 in the plane) and be able to measure the distance between these points (satellites) and the place where we are (R1,R2,R3). We can construct 3 spheres with center S1 S2 S3 and radius equal to the measured distances. The three spheres meet in 2 points but only one of them will be on the Earth’s surface. Adding a further sphere (i.e. satellite) we can correct the poor precision of the quartz clock of the receiver (0.00001 s at the speed of light means 3 km on the radius!). In this mode, in extreme synthesis, the GPS system operates. Our receiver, today at the cost of a few tens of euros, must find at least 4 satellites (which move and change position), evaluate their distance (based on the time it takes for the signal to reach the receiver moving at the speed of light) and then calculate with a geometric procedure the required point of intersection.

In reality, even if the procedure is exactly the one described, the thing is much more complicated (think for example that since the speed of light is about 300000 km/s, to calculate the time an accurate quartz clock at 0,000001 s would lead to an error of 3km, as with the old sextant. Or, since satellites orbit at 20000 km, a precision of 10 m (that of our devices) corresponds to a calculation with a precision of a few parts in a million (like measuring the height of the Tower of Pisa with the precision of a hair). It is obvious that since the reception of satellite signals is a key point, the receiver must be used in an open space, if possible free of obstacles. Even a dense curtain of clouds or foliage in a forest reduces the satellite signal.