How GPS Works



Your GPS receiver uses a GPS signal to figure out exactly where you are.

GPS stands for Global Positioning System and is dependent on a matrix of satellites orbiting in precise orbits around the Earth to locate your precise position on the Globe.

The US GPS satellite system uses 24 satellites circling high above our heads (20,300 kilometres / 12,600 miles) in six orbital paths so that at any one time, your GPS receiver is receiving signals from at least 4 satellites, the number of satellites you need to accurately pinpoint your position.

Each satellite transmits a unique low power radio signal (pseudo-random code). Using “Trilateration,” a process similar to triangulation, your GPS receiver can figure out your exact location.

How GPS Works

1- Your GPS receiver receives signal from satellite #1 and knowing that satellite #1 is, say 15km away (each GPS signal sent by a satellite contains information that allows your GPS receiver to identify its precise orbit), then it knows that you are somewhere on an imaginary sphere with a radius of 15km (if you imagine that satellite #1 is at the center of the sphere, then you are somewhere on the surface of the sphere).

2- Your GPS receiver receives signal from satellite #2 and knowing that satellite #2 is, say 10km away, then it knows that you are somewhere on a second imaginary sphere with a radius of 10km (if you imagine that satellite #2 is at the center of the second sphere, then you are somewhere on the surface of the second sphere).

Using the information from both satellites, and since the two satellites are not at the same spot in space, your GPS receiver now knows that you are located somewhere where the 2 spheres intersect and form a two-dimensional circle.

3- Your GPS receiver receives signal from satellite #3 and knowing that satellite #3 is, say 20km away, then it knows that you are somewhere on a third imaginary sphere with a radius of 20km (if you imagine that satellite #3 is at the center of the third sphere, then you are somewhere on the surface of the third sphere).

Using the information from the 3 satellites, and since the 3 satellites all occupy different spots in space, your GPS receiver now finds that the 3rd sphere intersects the previous two-dimensional circle at 2 opposite points.

4- So far, we have imagined the spheres hanging in space without regard to the Earth itself. When your GPS receiver factors in the Earth, it finds that only one of the 2 intersecting points will be on the Earth’s surface, your precise location on the planet!

5- But, we mentioned that we need at least 4 satellites and so far we have made use of only 3. Why do we need the 4th satellite?

Satellite #4 is necessary to synchronize your GPS receiver’s (imprecise) quartz clock with the satellite (precise) atomic clocks. This is important when you consider that, due to the great distances involved, even a slight 1/1000th of a second timing error results in a 300km / 200 miles error on Earth.

In the explanation above, we assumed that your GPS receiver also had a precise atomic clock and its calculations based on that precise clock would allow all spheres to intersect at the one point on Earth. But because your GPS receiver only has an imprecise quartz clock, the spheres do not actually quite intersect at that one point on Earth.

Instead, your GPS receiver has to figure out the one correction factor that would allow all signals to intersect at one point on Earth, using the signal from satellite #4 as a cross-check.

Line of Sight

GPS signals can pass through clouds, glass and plastic, but not through solid objects. This means that you need a “line of sight” — out in the open, away from buildings — to receive GPS signals.

Which is why you should always ensure you are in an open area (parked and stopped safely) before using your GPS receiver.

For example, if you are driving somewhere, wait until you are out of the underground parking garage before setting your destination and letting your GPS receiver calculate a route. Without first establishing your current location, it will not be able to do its job properly. If you try to set your GPS receiver while your car is travelling fast, the GPS receiver may not be able to get a fix on your location (since it is constantly changing).

Besides the US GPS satellite system, there are other US, Russian and European satellite systems.

Related links:
GPS Official Page
GPS Overview
How GPS Receivers Work

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