GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone.

These days GPS is finding its way into cars, boats, planes, construction equipment, movie making gear, farm machinery, even laptop computers.

Soon GPS will become almost as basic as the telephone. Indeed, at Trimble, we think it just may become a universal utility.

Here's how GPS works in five logical steps:

1. The basis of GPS is "triangulation" from satellites.
2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals.
3. To measure travel time, GPS needs very accurate timing which it achieves with some tricks.
4. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.
5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere.

   We'll explain each of these points in the next five sections of the tutorial. We recommend you follow the tutorial in order. Remember, science is a step-by-step discipline!

   Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for locations here on earth.

   That's right, by very, very accurately measuring our distance from three satellites we can "triangulate" our position anywhere on earth.

   Forget for a moment how our receiver measures this distance. We'll get to that later. First consider how distance measurements from three satellites can pinpoint you in space.

   The Big Idea Geometrically:

   Suppose we measure our distance from a satellite and find it to be 11,000 miles.

   Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11,000 miles.

   We saw in the last section that a position is calculated from distance measurements to at least three satellites.

   But how can you measure the distance to something that's floating around in space? We do it by timing how long it takes for a signal sent from the satellite to arrive at our receiver.

   If measuring the travel time of a radio signal is the key to GPS, then our stop watches had better be darn good, because if their timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error!

   On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board.

   But what about our receivers here on the ground?

   Remember that both the satellite and the receiver need to be able to precisely synchronize their pseudo-random codes to make the system work. 

   If our receivers needed atomic clocks (which cost upwards of $50K to $100K) GPS would be a lame duck technology. Nobody could afford it.

   Luckily the designers of GPS came up with a brilliant little trick that lets us get by with much less accurate clocks in our receivers. This trick is one of the key elements of GPS and as an added side benefit it means that every GPS receiver is essentially an atomic-accuracy clock.

   The secret to perfect timing is to make an extra satellite measurement.

   That's right, if three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing.

   This idea is so fundamental to the working of GPS  If you have time, cruise through that.

    

    

    

    

    

    

    

    

    

    

    

    

   If measuring the travel time of a radio signal is the key to GPS, then our stop watches had better be darn good, because if their timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error!

   On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board.

   But what about our receivers here on the ground?

   Remember that both the satellite and the receiver need to be able to precisely synchronize their pseudo-random codes to make the system work.

   If our receivers needed atomic clocks (which cost upwards of $50K to $100K) GPS would be a lame duck technology. Nobody could afford it.

   Luckily the designers of GPS came up with a brilliant little trick that lets us get by with much less accurate clocks in our receivers. This trick is one of the key elements of GPS and as an added side benefit it means that every GPS receiver is essentially an atomic-accuracy clock.

   The secret to perfect timing is to make an extra satellite measurement.

   That's right, if three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing.

   This idea is so fundamental to the working of GPS that we have a separate illustrated section that shows how it works. If you have time, cruise through that.

    

    

    

    

    

    

    

    

    

    

    

    

    

   If measuring the travel time of a radio signal is the key to GPS, then our stop watches had better be darn good, because if their timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error!

   On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board.

   But what about our receivers here on the ground?

   Remember that both the satellite and the receiver need to be able to precisely synchronize their pseudo-random codes to make the system work.

   If our receivers needed atomic clocks (which cost upwards of $50K to $100K) GPS would be a lame duck technology. Nobody could afford it.

   Luckily the designers of GPS came up with a brilliant little trick that lets us get by with much less accurate clocks in our receivers. This trick is one of the key elements of GPS and as an added side benefit it means that every GPS receiver is essentially an atomic-accuracy clock.

   The secret to perfect timing is to make an extra satellite measurement.

   That's right, if three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing.

   This idea is so fundamental to the working of GPS that we have a separate illustrated section that shows how it works. If you have time, cruise through that.

   Up to now we've been treating the calculations that go into GPS very abstractly, as if the whole thing were happening in a vacuum. But in the real world there are lots of things that can happen to a GPS signal that will make its life less than mathematically perfect.

   To get the most out of the system, a good GPS receiver needs to take a wide variety of possible errors into account. Here's what they've got to deal with.

   If you want to know where DGPS might be headed, take a look at your hand, because soon DGPS may be able to resolve positions that are no farther apart than the width of your little finger.

   Imagine the possibilities. Automatic construction equipment could translate CAD drawings into finished roads without any manual measurements. Self-guided cars could take you across town while you quietly read in the back seat.

   To understand how this kind of GPS is being developed you need to understand a little about GPS signals. If two receivers are fairly close to each other, say within a few hundred kilometers, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and so will have virtually the same .