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DGPS

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Differential Global Positioning System (DGPS)

GPS / DGPS Made Easy

So how does it work?

GPS receivers use the principle of "ranging". The receiver measures the distance from a location on earth to the positions of several satellites to determine the latitude and longitude of the position on earth.

Getting a position from GPS

Only 1 satellite

Figure 2 - One satellite

Suppose you turn on your GPS receiver and it picks up the signal of 1 satellite. The receiver tells you the location of that satellite and the distance between you and it.

But this single measurement is not a big help. At best, you can conclude that you are within a rather large circle whose circumference is determined by your distance from the satellite.

You are still lost!

2 satellites

Figure 3 - Two satellites

Now suppose that your GPS receiver picks up signals from another satellite. This helps to narrow down your location.

Now, you know that you are somewhere within the intersection of two large circles.

You still don't know exactly where you are, but things are starting to look better!

3 satellites

Figure 4 - Three satellites

Let's go one better. If your receiver picks up signals from a third satellite, then you know that your position will be within the intersection of three circles.

You have a position but no check on its quality!

4 satellites

Figure 5 - Four satellites

With four satellites you get a precise point for your position and you also know your elevation.

There, you have a position that is accurate to 30 metres!