How Do GPS Tracking Watches Work?

All GPS tracking devices, including GPS tracking watches, work around a few basic geometric principles. These principles are summed up in trilateration. Trilateration uses the know location of two or more points of reference (the more the better) and the measured distance between the subject and the reference points to give an exact location for the subject. To aid our understanding of this geo-speak, take a look at the two dimensional example below:

You are standing at point B in this picture (found where all three circles intersect). Let us imagine that you are standing out in the middle of nowhere and you want to find out where ‘here’ is. On your wrist is your GPS tracking watch, ready for instances just like this one. P1, P2, and P3 represent satellites in the NAVSTAR GPS constellation run by the US Government. There are twenty-four of these satellites orbiting approximately 12,600 miles (20,200 km) above the earth surface that are constantly transmitting microwave signals. At the same exact time, these satellites transmit a signal with information concerning their current location and the time that they were transmitted. The colored circles represent the path of the signal from the satellite at a specific time, let us say 8:45:03 AM, August 5, 1991.

Your GPS tracking device is designed to pick up these signals and record the time at which they arrive. Because the signals from the satellites travel at approximately the speed of light, knowing the difference between when the signal was generated and when your watch picks it up will let you know the distance between you and the broadcasting satellite. This is a very simple physics equation (speed x time = distance). When you combine this with the known location of each of the satellites (this information is contained in the signal that your watch picks up) your GPS tracking device has all the information it needs to give you an exact location for where you are.

Your GPS tracking watch will then do something with this newfound information. If your watch is a data logger then it will simply log your location in its memory and possibly give you a display of your longitude, latitude, and altitude. This is a very simple GPS method and is often used by bikers, hikers , and joggers to record their journeys, track the distances traveled for training purposes, or to build maps or guides if they are exploring new territory. This type of device has many useful applications, but unless you have a map with you in the middle of nowhere (Point B) this type of information will do you no good in the type of situation you are in right now. Hopefully you don’t have a data logger.

Instead, you want a data pusher. What these GPS devices do is they transmit their position information at specified intervals to a central database. This database can often be accessed through the internet, providing valuable information to the search party looking for you. Common applications of data pushers include tracking delivery vehicles, locating stolen property (LoJack), or keeping tabs on an object’s or individual’s location. Data pushers are extremely powerful because of the information they provide - but with this power also comes significant risk of abuse.

I hope that was a fairly straight forward explanation of how GPS tracking watches work. It was fairly simplified, so if you are interested in finding out more about the complex mechanics I would recommend checking out the following articles:

• Global Positioning System from Wikipedia
• Satellite Navigation a paper discussing the history and workings of GPS technology