How does GPS work?

Modern GPS systems have revolutionized navigation and location determination. They work on a very simple principal called trilateration.

Trilateration:

When we refer to GPS, we generally are referring to a GPS, which is the handheld unit we may be looking at. GPS is really a group of 27 Earth-orbiting satellites in geostationary orbit 12,000 miles above earth. Of the 27 satellites, 24 are in use with the other 3 being spares; just in case one fails. The United States military developed and implemented the GPS network to function as a military navigational system. They allowed others to use it but with a built in error known as Selective Availability or SA. President Clinton ordered that SA be discontinued so now everyone has improved accuracy.

Each of these 3,500 pound solar-powered satellites circles the globe making two complete trips every day. At any spot on Earth at least 4 satellites are above the horizon.

The function of the GPS receiver is to "lock" on to as many satellites as it can find and from the trilateration process it is able to calculate the distance from every satellite to Earth. That in itself is pretty amazing don't you think? Once it knows the distance from Earth to each satellite it then applies a very complex algorithm (formula) to calculate your location. This is all done by very precisely measuring the time it take for the satellite signal to reach your receiver.

Measuring Distance:

No to go into all of the technical details, which is beyond the scope of this paper suffice it to say that the satellite sends out a radio frequency signal using what is known as a pseudo-random code. Your GPS receiver also internally runs the very same pattern. Because there is a time lag between the internal code and that coming from the receiver, there is a time lag in the satellite signal when compared with the internal code.

The amount of delay time equals the time it takes the satellite radio signal to reach the GPS receiver. Your GPS receiver then does a simple calculation and multiplies the measured time delay time by the speed a radio wave travels, which is the speed of light. The speed of light is 186,282 MPS (yes miles per second). The receiver then knows exactly how far it is from the satellite. This sequence is done for each and every satellite the GPS can lock onto.

Of course both the GPS satellite and the receiver need to have an extremely accurate and stable clock that can be synchronized to a nanosecond. Since this would require an atomic clock (at a nominal $75,000 each), the GPS system utilizes a simple quartz clock. This is one place where multiple satellite signals come into play. Not only to they allow triangulation (referred to a trilateration in the GPS world) but they allow the GPS receiver to gauge its own accuracy

By measuring distance from four (or more) satellites four spheres are drawn from Earth to each satellite. Those spheres only intersect at one Earth point; your location. If there were only 3 satellites, there is more then one place the spheres intersect which is why the system needs that forth satellite so a bad calculation can not be made. Now this is complex math so it is best to just take this at face value. Because the GPS receiver makes each of the individual distance measurements, using its built-in quartz clock, each distance will have the same error making all individual measurements proportionally inaccurate. Timing errors have now been essentially eliminated. Pretty ingenious system!

Putting the GPS Data to Use:

Once the GPS receiver calculates your location it can then present it in latitude, longitude and altitude. Now we normally don't often talk about our location being at a specific latitude, longitude and altitude, but we do often do have attitude. Sorry, couldn't resist that one!

This is the unit I use in my GMC Sierra. This is typical of a multi-channel GPS receiver. The StreetPilot III has a total of 12 channels that is can use to determine position. We talked before about 4 satellites being required. That is true but another fact of GPS receivers is the more the better. Accuracy is improved with each satellite over 4. I have seen all 12 channels active while driving through the Central Valley of California.

This and other GPS receivers are able to tell you:

Distance traveled
Current speed (usable to over 900 MPH)
Average speed of travel
How long you've been traveling (if you tell it when to start keeping track)
Where you have been, and help you retrace your route to get back
Estimated time of arrival at your destination (if you tell it where you are going)
Keep track of waypoints (specific manually recorded locations)

For Amateur Radio use, GPS equipment can be used for Automatic Position Reporting System (APRS), antenna pointing, or even finding that mountain top contest location.

Some GPS receivers have a data port that can be used to upload receiver information to a computer. There, software can do all sorts of things for you. Anything from direction finding, street level navigation and a whole host of others uses.

There is also a Russian set of GPS satellites in space and a group of European countries are also working on a new system. More on these systems later.

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