Intro to practical applications for global positioning systems
Years ago, global positioning system (GPS) was an exotic service involving expensive equipment and used by the government and professionals in such areas as surveying, earth resources, and commercial transportation. And then, as I've mentioned before, along came the magic of VLSI, and the rest is, as they say, history. Inexpensive GPS chips (See example) have made it possible to put GPS into many consumer-grade devices, such as devices for tracking lost people, lost pets, a lost (or stolen) car, and providing one implementation of the E-911 service that allows mobile phone users to be located (much of the time, anyway) when they dial 911. GPS is practically a fixture in consumer electronics today, and used in many types of businesses on a truly global basis. Future location-based services will also be able to use GPS. Note that GPS isn't the only way to track people and things, but it's one of the most common, accurate and effective.
What is GPS? In a nutshell, it's a network of 24 "Navstar" satellites, orbited by the U.S. Department of Defense, and originally designed for, well, defense applications. GPS allows a user or device to locate itself on (or above) the surface of the earth with potentially very great accuracy (we'll return to the "potentially" in a moment). Each satellite is essentially an orbiting atomic clock (there are actually four atomic clocks on each satellite, for redundancy), and it broadcasts the value of this clock using a CDMA technique not unlike that used in CDMA cellular phones, but much more reliable. It also transmits several related pieces of data needed for accuracy on a continuous basis. On earth, a GPS receiver can detect this signal and, in fact, can often see up to 12 satellites at once, although only four are required for proper three-dimensional functionality with excellent accuracy. But the more satellites observed, the greater the accuracy in determining position. The calculation itself is quite simple - given the location of each satellite, all we have to do is measure the time delay between when the satellite sends a signal and when it is received on earth. Given that the speed of light is a constant, and the position of the satellites relative to one another is known, a triangulation calculation can then be performed, and voila, we know where we are.
Of course, that's the optimal scenario. In reality, there are many possible errors that can occur in the signal between its transmission and its reception - it's a radio signal, after all, and, just as a cell phone will drop a call when the signal is too weak or distorted, GPS can be subject to similar challenges. For example, the speed of light is constant in a vacuum, but it will vary slightly as it goes through the air, and especially through clouds. There are a number of ways that GPS can correct for these errors. These techniques include observing as many signals as possible as often as possible, and clever processing algorithms. What's more, the civilian and military services provided by GPS vary greatly in accuracy. The civilian system was even "dithered" to produce inaccurate results for many years - a process called "selective availability", or "SA". SA is now turned off, and accuracy to a few meters is possible even with relatively inexpensive products.
There are some excellent tutorials located at Trimble and Garmin. The leading publication on GPS is GPSWorld. The official GPS sites are (among others) http://tycho.usno.navy.mil/gps.html and http://www.af.mil/factsheets/factsheet.asp?fsID=119, and you'll find lots of other information on GPS on the Web.
There's an excellent chance that, if you don't already, you'll own a handheld GPS device in the future; such capability might even be built into your cell phone or car (as is already the case with GM's OnStar service). Keep in mind, though, that GPS requires a clear view of the sky. The signals from the satellites are so weak that they can't penetrate solid objects like buildings. But, no matter - GPS is now so accurate and so inexpensive that its value and utility will continue to spread to a vast array of applications around the globe.
Copyright 2003 by Farpoint Group - All rights reserved.