Automotive GPS Receivers

It seems that almost everywhere you travel these days, the auto, pickup, motorhome, or truck next to you on the freeway has a little box mounted on the windshield or dash. That box would, of course, be a GPS receiver, used for navigation along our highways, boat ways, airways, or just about anywhere a person can get lost.

Originally, GPS was a military system (actually, it still is) designed to help soldiers navigate and to accurately deliver ordinance. It was conceived as early as the 1940s, but not actually built until technology became available in the 1970s. While testing began in the early 70s, the complete system was not fully operational until 1993. Maintained by the US Air Force, the GPS (Global Positioning System) was made available free for civilian use by presidential directive (President Reagan) in 1983 after the Korean Air 007 incident. Since then, GPS has blossomed into a widely used and highly useful global navigation system.

So how does GPS work? It's both incredibly simple, and staggeringly complicated. The simple part is that each satellite, of which there are between 24 and 32 in operation at any given time, orbits the earth in a medium orbit at about 12,000 miles. This allows each satellite to make two complete revolutions (actually called sidereal revolutions) per day. At any point in time, there are usually six or more satellites “in view” from which to navigate. The Air Force tracks the precise orbit of each satellite and transmits orbital data up to each satellite, so it “knows” where it is at all times.

Each satellite, as it orbits the earth, transmits data about its orbit, and an incredibly accurate time hack. The receiver on the ground (or in the air) knows what time it is by a clever algorithm that allows it to be kept accurate based on existing signals. It knows what time a signal left a satellite, and knows what time it received it, and therefore knows how far it is from the satellite. When one satellite is received, the receiver can calculate the distance around a sphere where it is compared to the location of the satellite. When two signals are received, it can derive a circle made up of two intersecting spheres, but still can't locate the precise location. When three or more signals are received, the receiver can determine its location. The illustration at the right, with a little imagination, shows how only one point in space is common to all three spheres (which represent travel time of the signal from and to the satellite), and places a receiver with respect to the orbiting satellites with good accuracy.

GPS satellites transmit at a frequency that's similar to the universe's background noise. The reason for this is so that receivers can acquire a signal with relative ease, then differentiate the small differences between the background noise and the small differences added by the satellite. This means that satellite antennas can be very small and still be effective.

All of this technology translates down to a relatively small device being able to place its position on a coordinate grid. From there, it's simply (actually, nothing with this technology is simple) a matter of overlaying a map, some fancy software to make things human friendly, and we have a handy navigation gizmo for our cars so we don't have to look at a map.

Fine, but what will it do? This is one of those technologies that, until you get accustomed to one, you never miss it. Most of us have gone through most of our lives just fine using paper maps, and word of mouth instructions. We would probably have made it just fine without GPS had we not know it existed. With a good GPS, however, we know how far our destination is, how long it will take, where our next turn is, and which direction it will be. There are other bells and whistles too, and each brand has its own features. Most receivers have a built-in database that takes you to your favorite store or attraction, and nearly all will allow you to enter an address.

So which one is best? As a little magazine, you might imagine that the GPS manufacturers haven't exactly gone out of their way to send us evaluation units, so we're not going to offer an evaluation. Rather, here's what we can say about features based on a few years use of our own:

1. Look for a GPS unit that has the most complete database. None of them will be totally  complete, but they are still sometimes useful, and certainly some are more complete than others.
2. Look for a GPS unit that comes equipped with the largest number of maps and has the capability to add more, with updates periodically. We never need our GPS at home. We're familiar with that area. When we're in a strange town, or different state or country, however, a GPS receiver lets us drive almost as efficiently as a local. We're never straining to read the street signs and holding up traffic because we know the turns as they come up.
3. At least one manufacturer, Tom Tom, takes input from users to help update their database. Whether this is useful is yet to be seen, but it certainly has possibilities.
4. Our experience has shown that one of the most useful features are voice instructions. When driving in heavy traffic, there's little time to look at a GPS receiver, but voice commands can keep you informed about your route.
5. The next step when comparing features is to determine which “peripheral” features are important to you. Many have MP3 players, book readers, and at least one has an HTML file reader. Others transmit voice instructions via your FM radio and operate your bluetooth phone.
6. Finally, you should determine which receivers seem to be the easiest to operate to you. This can be done by either visiting a retail store and getting some hands on time with the unit (they usually have a simulator or training mode), or by downloading the instructions and see if it all makes sense.

Most receivers, from our experience, will not always select the most desirable route. This is evident when you're using it to navigate to a destination and you're familiar with the best routes. But remember, you probably won't be using it to get to work. You should already know how to do that. What they will do is get you to a place you've never been. It may not be the most efficient way, but it will still get you there.

Are GPS receivers the end all for navigation? Hardly. But experience has shown us that using one is a significant improvement. If you've ever thought that a GPS would be useful, you were right. If you don't have one and travel in new areas for any period of time, the investment will be well worth it.