>> No matter where you go, a compass and a good map can help you figure out where you are. But they're awfully hard to use while you drive. That's one of the reasons GPS has become so popular. I'm Tom Merritt from CNET.com. On this edition of Insider Secret, I'll show you how GPS works so you can pick the right one for you.
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A GPS device can show you where you are on the surface of the earth, within several meters. But how does it do it? Satellites. Right now, 30 satellites are in orbit around the earth, broadcasting a signal that GPS receivers pick up. A receiver like this one, knows where each satellite is supposed to be. The signal takes some time to get to the ground from space and the time it takes can help determine where you are. Here's an example. Let's pretend the people in this fine tavern represent the satellites. You're lost and you wander in to ask directions. The locals don't talk much but they'll each tell you one simple fact. Pretty unhelpful. Horrible road trip I know. You ask one person where you are and he says, you're 45 miles from Saint Louis. Well great, 45 miles in which direction? That could be anywhere in this circle. So you ask another person.
>> Well I can tell you're 263 miles from Chicago.
>> Well that narrows it down a little. You could be anywhere along this line here. So you tackle a trucker.
>> You're 194 miles away from Evansville.
>> Ah ha, now we're getting somewhere. With three references that include no direction, we can figure out you're in Greenville, Illinois. And that's how your GPS receiver works too. The satellites broadcast a time code which are GPS receiver uses to calculate how long the signal took to get there. With three satellites, it can get a fix on where. Throw in another satellite and it can calculate your altitude as well. In fact, the more satellite signals your receiver can pick up at once, the better the accuracy. This is usually expressed as the amount of channels on your receiver. Just remember, it costs more to have more channels. Now another way to get more accuracy is to find a receiver with differential GPS or DGPS. DGPS involves a system where base stations on the ground also transmit a signal to your receiver. A base station is in a fixed position, unlike the satellites. That means it always knows it's own position and that helps correct errors. Another thing to know before you go get a GPS is that the signal can be blocked or delayed by things like trees, buildings or even clouds. To get the best GPS reception, always get as high and as clear as you can. Not always possible. Now everything we talked about will get you a set of coordinates. Pretty useless without a map, so the smart people who make GPS receivers include maps within the devices. The most popular are detailed street level maps that help you navigate your way around town. Be sure to find out how your GPS device is going to update it's maps. Some require you to buy a new set of dvd's. The cooler ones and therefore the most expensive, update themselves over a wireless connection. In any case, the maps aren't going to be up to the minute so pay attention while you're driving. You don't wanna drive off into some newly formed canal or something. Well that's the basics on GPS. I hope I've helped you find yourself or at least helped you find a new GPS device. I'm Tom Merritt for CNET.com. Where am I?
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