GPS

GPS From zero !

Any system that can provide intelligent vehicle location and navigation information will let us avoid congested freeways and find more efficient routes to our destinations, saving millions of euros in gasoline and tons of air pollution. Travel aboard ships and aircrafts will be safer in all weather conditions. Businesses with large numbers of outside plants (e.g., railroads and utilities) will be able to manage their resources more efficiently, reducing consumer costs. A Global Positioning System (GPS) provides the answer to facilitate all these issues.

A GPS is a space-based radio positioning system that combines computer mapping techniques to provide 24-hour three-dimensional position, velocity, and time information to suitably equipped users anywhere on or near the surface of the earth (and sometimes off the earth).

GPS is amongst the major developments of the wireless telecommunication industry. It is an important tool for map-making and land surveying and has become a vital global utility, which is indispensable for modern navigation on land, sea, and air.

A GPS is basically divided into 3 major components as shown in Figure 1.1. These are the user segment, the control segment, and the space segment. A description of these components is given in many papers. I decided to adopt the one given in

Realy What is GPS?

Originally conceived as a navigation aid for the military, the Global Positioning System, or GPS, has since grown from relatively humble beginnings as different supporting technologies have been developed, some off which are within reach of consumer budgets.

All that GPS does is provide a set of coordinates which represent the location of the GPS unit with respect to it’s latitude, longitude and elevation on planet Earth. It also provides time, which is as accurate as that given by an atomic clock.

The actual application of the GPS technology is what leads to such things as navigation systems, GPS tracking devices, GPS surveying and GPS mapping.

GPS in itself does not provide any functionality beyond being able to receive satellite signals and calculate position information. But it does that very well!

How a GPS Device Works

The actual principle of GPS is very easy to appreciate, since it is exactly the same as traditional “triangulation” (although this is not quite correct, as GPS does not use angles). If one imagines an orienteer needing to locate themselves on a map, they first need to be able to find at least three points that they recognize in the real world, which allows them to pinpoint their location on the map.

They can then measure, using a compass, the azimuth that would be needed to take them from the point on the map totheir current position. A line is then drawn from each of the three points, and where the three lines meet is where they are on the map.

Translating this into the GPS world, we can replace the known points with satellites, and the azimuth with time taken for a signal to travel from each of the known points to the GPS receiver. This enables the system to work out roughly where it is located - it is where the circles representing the distance from the satellite, calculated on the basis of the travel time of the signal,intersect.

Of course, this requires that the GPS locator has the same coordinated time as the satellites, which have atomic clocks on board. To do this, it cross checks the intersection of the three circles with a fourth circle, which it acquires from another satellite.

If the four circles no longer intersect at the same point, then the GPS system knows that there is an error in it’s clock, and can adjust it by finding one common value (one second, half a second and so on) that can be applied to the three initial signals which would cause the circles to intersect in the same place.

Behind the scenes, there are also many complex calculations taking place which enable the system to compensate for atmospheric distortion of the signals, and so forth, but the principle remains the same.

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