1 00:00:00,000 --> 00:00:04,000 Be sure to look for the answers to the following questions. 2 00:00:04,000 --> 00:00:08,000 How do GPS receivers detect their distance from satellites? 3 00:00:08,000 --> 00:00:11,000 What is the electromagnetic spectrum? 4 00:00:11,000 --> 00:00:13,000 What is static electricity? 5 00:00:13,000 --> 00:00:17,000 When you see this icon, the answer is near. 6 00:00:17,000 --> 00:00:21,000 Dr. D's friend, Mr. Gnau, works at NASA Langley Research Center. 7 00:00:21,000 --> 00:00:26,000 He develops new instruments for global positioning systems and has agreed to meet with us. 8 00:00:26,000 --> 00:00:30,000 If anyone can help us learn more about GPS, he can. 9 00:00:30,000 --> 00:00:37,000 GPS, or Global Positioning System, is a constellation system of 29 Earth-orbiting satellites 10 00:00:37,000 --> 00:00:43,000 that were originally designed by the U.S. military in the 1970s as a navigation system. 11 00:00:43,000 --> 00:00:46,000 Twenty-nine sounds like a lot of satellites. 12 00:00:46,000 --> 00:00:47,000 Why do you need so many? 13 00:00:47,000 --> 00:00:52,000 It takes 24 satellites to provide global coverage, leaving five spares. 14 00:00:52,000 --> 00:00:59,000 The orbits are arranged so that at any given time, anywhere on Earth, there are at least four satellites visible. 15 00:00:59,000 --> 00:01:02,000 Why do four satellites need to be visible? 16 00:01:02,000 --> 00:01:06,000 Your GPS receiver needs four satellites in order to determine its own location. 17 00:01:06,000 --> 00:01:09,000 How does our GPS locate itself? 18 00:01:09,000 --> 00:01:13,000 By using a simple mathematical principle called trilateration. 19 00:01:13,000 --> 00:01:17,000 I'm not sure simple and trilateration should be used in the same sentence. 20 00:01:17,000 --> 00:01:19,000 What is trilateration? 21 00:01:19,000 --> 00:01:25,000 It's kind of tricky to explain in three-dimensional space, so let's start with a two-dimensional example. 22 00:01:25,000 --> 00:01:30,000 Let's say you're totally lost somewhere in the U.S. and your GPS is not working. 23 00:01:30,000 --> 00:01:33,000 Like us yesterday when we couldn't find our geocache. 24 00:01:33,000 --> 00:01:41,000 As you are trying to find where you are, a friendly person tells you that you are 1,000 kilometers from Boise, Idaho. 25 00:01:41,000 --> 00:01:42,000 Do you know where you are? 26 00:01:42,000 --> 00:01:46,000 No. I could be 1,000 kilometers in any direction from Boise. 27 00:01:46,000 --> 00:01:55,000 Exactly. Now let's say another friendly person comes by and tells you that you're 1,110 kilometers from Minneapolis, Minnesota. 28 00:01:55,000 --> 00:01:57,000 Do you know where you are? 29 00:01:57,000 --> 00:01:59,000 Not yet, but I'm getting closer to my location. 30 00:01:59,000 --> 00:02:04,000 That's right. Now you have two choices, but you still don't know where you are. 31 00:02:04,000 --> 00:02:11,000 Finally, another friendly person informs you that you are 990 kilometers from Tucson, Arizona. 32 00:02:11,000 --> 00:02:13,000 Now do you know where you are? 33 00:02:13,000 --> 00:02:17,000 It looks like I'm in Denver, Colorado, which of course I will be soon. 34 00:02:17,000 --> 00:02:23,000 The same concept works in three-dimensional space, but instead of circles, you need to think in terms of spheres. 35 00:02:23,000 --> 00:02:27,000 And the satellites are the friendly people telling you how far you are from a place. 36 00:02:27,000 --> 00:02:36,000 That's right. If you know your distance from Satellite A, you could be anywhere on a huge imaginary sphere at that radius. 37 00:02:36,000 --> 00:02:45,000 If you know your distance from Satellite B, you can overlap the first sphere with the second sphere and they intersect in a perfect circle. 38 00:02:45,000 --> 00:02:51,000 So if you know the distance to a third satellite, you get a third sphere which intersects with the circle at two points. 39 00:02:51,000 --> 00:02:57,000 Very good. And the Earth acts as the fourth sphere, so you can eliminate the point in space because you're on Earth. 40 00:02:57,000 --> 00:02:59,000 So do you only need three satellites? 41 00:02:59,000 --> 00:03:08,000 An approximate position can be found with three satellites, but to improve accuracy and get precise altitude information, four or more are better. 42 00:03:08,000 --> 00:03:11,000 How do GPS receivers know how far they are from the satellite? 43 00:03:11,000 --> 00:03:19,000 They analyze the high-frequency, low-powered radio signals from the GPS satellites and calculate the time the signal traveled. 44 00:03:19,000 --> 00:03:21,000 Do satellites have stopwatches? 45 00:03:21,000 --> 00:03:25,000 No. The satellites need to be more accurate than a stopwatch. 46 00:03:25,000 --> 00:03:32,000 The satellites use a very accurate atomic clock, which produces exact time-coded signals. 47 00:03:32,000 --> 00:03:35,000 And what happens if a satellite malfunctions? 48 00:03:35,000 --> 00:03:41,000 It's possible. Again, that's why we have 29 satellites when only 24 are needed, leaving a few spares. 49 00:03:41,000 --> 00:03:43,000 It helps to have extra. 50 00:03:43,000 --> 00:03:48,000 True. Here at NASA, we use GPS to determine the position of aircraft and satellites. 51 00:03:48,000 --> 00:03:53,000 And we also are developing a system to perform remote sensing of the environment. 52 00:03:53,000 --> 00:03:56,000 All of these tasks require precision information. 53 00:03:56,000 --> 00:03:59,000 Spare satellites make sure we get the data we need. 54 00:03:59,000 --> 00:04:03,000 Cool. NASA's always doing amazing things. 55 00:04:03,000 --> 00:04:09,000 Mr. Ganell, what would happen if your GPS was only receiving a signal from one or two satellites? 56 00:04:09,000 --> 00:04:11,000 Would you get incorrect results? 57 00:04:11,000 --> 00:04:18,000 No. Usually, your GPS device will let you know that it doesn't have enough satellites to calculate an accurate position. 58 00:04:18,000 --> 00:04:20,000 Sounds like we need to do some more research. 59 00:04:20,000 --> 00:04:22,000 Thanks, Mr. Ganell. 60 00:04:22,000 --> 00:04:25,000 You're welcome. And good luck with your geocaching.