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GPS Basics
by: Zvi Braverman

This article is the first of series of three articles in which I'll explain some basic principles and concepts of GPS. I in this article I'll explain how exactly GPS works so it won't be a mystery anymore. I'll also mention some recent enhancements to the GPS system. In the second article I'll explain about some of the GPS core technologies, so when you hear the magic words "Sirf III" you'll know what people are talking about. And finally, in the third paragraph I'll make a short review of the leading GPS manufacturers on the market today.

First of all GPS stands for Global Positioning System and usually refers to the GPS receiver. The receiver basically receives the signals incoming from constellation of 27 satellites which are orbiting the Earth (actually there are only 24 satellites that are operational and 3 additional for backup in case one of the 24 fails) and from those signals can calculate it's absolute position on Earth. This satellite system was developed by the U.S. military and after a while U.S. government opened it up for public. Those satellites are circling the Earth making two complete rotations every day. Their orbits are planned the way that at any time and anywhere on Earth there are at least four satellites "visible" in the sky. A GPS receiver locates four or more satellites from the signals those satellites send constantly towards the Earth, calculates the distance to each of the satellites and from this information calculates its' absolute location on Earth. This process of pinpointing the location of the GPS receiver is based on mathematical principle called trilateration.

Let me explain a little about this principle regarding the GPS. It is a little tricky so if you don't want to bother yourself with details - just skip this part. Like I said, GPS receiver locates the satellites and calculates the distance to them. Lets say for example receiver knows that Satellite_1 is located 20 miles from it. In the whole space Satellite_1 can be anywhere on a huge Sphere with radius of 20 miles around the receiver. If receiver also knows that Satellite_2 is located 30 miles from it, it can overlap the sphere of satellite_1 with the sphere of satellite_2. The spheres intersect in a perfect circle (think in 3 dimensions). The distance to the third satellites gives us a third sphere which intersects with the circle in two points. One of those points is located in space and the other located on Earth. This second point is actually the location of the GPS receiver. Using the Earth as fourth sphere, receiver is able to choose that second point and this way it knows its exact location on earth.

Basically GPS receiver needs to locate only three satellites but it always tries to locate as many satellites as possible because it improves the accuracy of the result.

The signals that satellites are constantly transmitting are high-frequency, low power radio waves which contain information about the satellite and its' location. GPS receiver can calculate the distance to the satellite by counting the time it takes to the signal to get from the satellite to the receiver.

The whole process described above works well but it has inaccuracies due to several reasons like signal interference,weather and more. Differential GPS (DGPS) helps correct those inaccuracies. This enhancement is base on stations located on Earth. The hardware at those stations knows exactly the location of the station and constantly sends information to GPS receivers (but only to those in which this technology is implemented) how to correct their results so the will be more accurate.

Once the GPS receiver made the calculation, it can tell the latitude, the longitude and the altitude of its' current position. This doesn't tell much to the average user. So in order to make the use of the GPS receiver more user-friendly many receivers send this data to a program which displays a map and can show the position on it.

Zvi Braverman is a student of electronic engineering at the Tel-Aviv university.




 



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