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How Does GPS Work
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Fourth segment of NASA Sci Files The Case of the Technical Knockout explaining how GPS receivers work with satellites to determine an exact location. This segment explains the concept of trilateration in two and three dimensions.
Be sure to look for the answers to the following questions.
00:00:00
How do GPS receivers detect their distance from satellites?
00:00:04
What is the electromagnetic spectrum?
00:00:08
What is static electricity?
00:00:11
When you see this icon, the answer is near.
00:00:13
Dr. D's friend, Mr. Gnau, works at NASA Langley Research Center.
00:00:17
He develops new instruments for global positioning systems and has agreed to meet with us.
00:00:21
If anyone can help us learn more about GPS, he can.
00:00:26
GPS, or Global Positioning System, is a constellation system of 29 Earth-orbiting satellites
00:00:30
that were originally designed by the U.S. military in the 1970s as a navigation system.
00:00:37
Twenty-nine sounds like a lot of satellites.
00:00:43
Why do you need so many?
00:00:46
It takes 24 satellites to provide global coverage, leaving five spares.
00:00:47
The orbits are arranged so that at any given time, anywhere on Earth, there are at least four satellites visible.
00:00:52
Why do four satellites need to be visible?
00:00:59
Your GPS receiver needs four satellites in order to determine its own location.
00:01:02
How does our GPS locate itself?
00:01:06
By using a simple mathematical principle called trilateration.
00:01:09
I'm not sure simple and trilateration should be used in the same sentence.
00:01:13
What is trilateration?
00:01:17
It's kind of tricky to explain in three-dimensional space, so let's start with a two-dimensional example.
00:01:19
Let's say you're totally lost somewhere in the U.S. and your GPS is not working.
00:01:25
Like us yesterday when we couldn't find our geocache.
00:01:30
As you are trying to find where you are, a friendly person tells you that you are 1,000 kilometers from Boise, Idaho.
00:01:33
Do you know where you are?
00:01:41
No. I could be 1,000 kilometers in any direction from Boise.
00:01:42
Exactly. Now let's say another friendly person comes by and tells you that you're 1,110 kilometers from Minneapolis, Minnesota.
00:01:46
Do you know where you are?
00:01:55
Not yet, but I'm getting closer to my location.
00:01:57
That's right. Now you have two choices, but you still don't know where you are.
00:01:59
Finally, another friendly person informs you that you are 990 kilometers from Tucson, Arizona.
00:02:04
Now do you know where you are?
00:02:11
It looks like I'm in Denver, Colorado, which of course I will be soon.
00:02:13
The same concept works in three-dimensional space, but instead of circles, you need to think in terms of spheres.
00:02:17
And the satellites are the friendly people telling you how far you are from a place.
00:02:23
That's right. If you know your distance from Satellite A, you could be anywhere on a huge imaginary sphere at that radius.
00:02:27
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.
00:02:36
So if you know the distance to a third satellite, you get a third sphere which intersects with the circle at two points.
00:02:45
Very good. And the Earth acts as the fourth sphere, so you can eliminate the point in space because you're on Earth.
00:02:51
So do you only need three satellites?
00:02:57
An approximate position can be found with three satellites, but to improve accuracy and get precise altitude information, four or more are better.
00:02:59
How do GPS receivers know how far they are from the satellite?
00:03:08
They analyze the high-frequency, low-powered radio signals from the GPS satellites and calculate the time the signal traveled.
00:03:11
Do satellites have stopwatches?
00:03:19
No. The satellites need to be more accurate than a stopwatch.
00:03:21
The satellites use a very accurate atomic clock, which produces exact time-coded signals.
00:03:25
And what happens if a satellite malfunctions?
00:03:32
It's possible. Again, that's why we have 29 satellites when only 24 are needed, leaving a few spares.
00:03:35
It helps to have extra.
00:03:41
True. Here at NASA, we use GPS to determine the position of aircraft and satellites.
00:03:43
And we also are developing a system to perform remote sensing of the environment.
00:03:48
All of these tasks require precision information.
00:03:53
Spare satellites make sure we get the data we need.
00:03:56
Cool. NASA's always doing amazing things.
00:03:59
Mr. Ganell, what would happen if your GPS was only receiving a signal from one or two satellites?
00:04:03
Would you get incorrect results?
00:04:09
No. Usually, your GPS device will let you know that it doesn't have enough satellites to calculate an accurate position.
00:04:11
Sounds like we need to do some more research.
00:04:18
Thanks, Mr. Ganell.
00:04:20
You're welcome. And good luck with your geocaching.
00:04:22
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- Idioma/s:
- Niveles educativos:
- ▼ Mostrar / ocultar niveles
- Nivel Intermedio
- Autor/es:
- NASA LaRC Office of Education
- Subido por:
- EducaMadrid
- Licencia:
- Reconocimiento - No comercial - Sin obra derivada
- Visualizaciones:
- 881
- Fecha:
- 28 de mayo de 2007 - 15:34
- Visibilidad:
- Público
- Enlace Relacionado:
- NASAs center for distance learning
- Duración:
- 04′ 27″
- Relación de aspecto:
- 4:3 Hasta 2009 fue el estándar utilizado en la televisión PAL; muchas pantallas de ordenador y televisores usan este estándar, erróneamente llamado cuadrado, cuando en la realidad es rectangular o wide.
- Resolución:
- 480x360 píxeles
- Tamaño:
- 26.72 MBytes
